Design of Snap Fits

A major attraction of plastics to designers is the ease with which fast assembly mechanisms can be incorporated into the end-product. A very good example of this is the snap fit. A typical design is shown in Fig. 2.22 although there are many variations. Snap fits exploit the fact that thin plastic sections can undergo relatively large flexural deflections for a short period of time and exhibit complete recovery. The design of snap fits is straightforward and does not involve creep curves since the time-scale of the deflectionlstress is small.
The point that will be illustrated here is that in a real design situations it is necessary to choose combinations of dimensions which provide the necessary function but which do not overstress the plastic. In the following example a set of design curves are developed to show how the different combinations of dimensions might be selected.
 PLASTICS
ENGINEERING
Third Edition
R.J. Crawford, BSc, PhD, DSc, FEng, FIMechE, FIM
Department of Mechanical, Aeronautical
and Manufacturing Engineering
The Queen’s University of Belfast

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CORROSION

Corrosion removal deals with the taking away of mass from the surface of materials by their environment and other forms of environmental attack that weaken or otherwise degrade material properties. The complex nature of corrosion suggests that the designer who is seriously concerned about corrosion review a good readable text such as Corrosion Engineering by Fontana and Greene [35.1].
Included in this chapter are many corrosion data for selected environments and materials. It is always hazardous to select one material in preference to another based only on published data because of inconsistencies in measuring corrosion, lack of completeness in documenting environments, variations in test methods, and possible publishing errors.These data do not generally indicate how small variations in temperature or corrosive concentrations might drastically increase or decrease corrosion rates. Furthermore, they do not account for the influence of other associated materials or how combinations of attack mechanisms may drastically alter a given material’s behavior. Stray electric currents should be considered along with the various attack mechanisms included in this chapter. Brevity has required simplification and the exclusion of some phenomena and data which may be important in some applications.
The data included in this chapter are but a fraction of those available. Corrosion Guide by Rabald [35.2] can be a valuable resource because of its extensive coverage of environments and materials.
Again, all corrosion data included in this chapter or published elsewhere should be used only as a guide for weeding out unsuitable materials or selecting potentially acceptable candidates. Verification of suitability should be based on actual experience or laboratory experimentation. The inclusion or exclusion of data in this chapter should not be interpreted as an endorsement or rejection of any material.

Milton G. Wille, Ph.D., P.E.
Professor of Mechanical Engineering
Brigham Young University
Provo, Utah

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Polymer Categories, Acetal (POM)

Acetal polymers are formed from the polymerization of formaldehyde. They are also known by the name polyoxymethylenes (POM). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s when DuPont developed Delrin.1 Homopolymers are prepared from very pure formaldehyde by anionic polymerization, as shown in Fig. 1.4. Amines and the soluble salts of alkali metals catalyze the reaction.2 The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldhyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stability is copoly merization with a second monomer such as ethylene oxide. The copolymer is prepared by cationic methods.3 This was developed by Celanese and marketed under the tradename Celcon. Hostaform is another copolymer marketed by Hoescht. The presence of the second monomer reduces the tendency for the polymer to degrade by unzipping.4 There are four processes for the thermal degradation of acetal resins. The first is thermal or base-catalyzed depolymerization from the chain, resulting in the release of formaldehyde. End capping the polymer chain will reduce this tendency. The second is oxidative attack at random positions, again leading to depolymerization. The use of antioxidants will reduce this degradation mechanism. Copolymerization is also helpful. The third mechanism is cleavage of the acetal linkage by acids. It is, therefore, important not to process acetals in equipment used for polyvinyl chloride (PVC), unless it has been cleaned, due to the possible presence of traces of HCl. The fourth degradation mechanism is thermal depolymerization at temperatures above 270°C. It is important that processing temperatures remain below this temperature to avoid degradation of the polymer.5 Acetals are highly crystalline, typically 75% crystalline, with a melting point of 180°C.6 Compared to polyethylene (PE), the chains pack closer together because of the shorter C O bond. As a result, the polymer has a higher melting point. It is also harder than PE. The high degree of crystallinity imparts good solvent resistance to acetal polymers. The polymer is essentially linear with molecular weights (Mn) in the range of 20,000 to 110,000.7 Acetal resins are strong and stiff thermoplastics with good fatigue properties and dimensional stability. They also have a low coefficient of friction and good heat resistance.8 Acetal resins are considered similar to nylons, but are better in fatigue, creep, stiffness, and water resistance.9 Acetal resins do not, however, have the creep resistance of polycarbonate. As mentioned previously, acetal resins have excellent solvent resistance with no organic solvents found below 70°C, however, swelling may occur in some solvents. Acetal resins are susceptible to strong acids and alkalis, as well as oxidizing agents. Although the C O bond is polar, it is balanced and much less polar than the carbonyl group present in nylon. As a result, acetal resins have relatively low water absorption. The small amount of moisture absorbed may cause swelling and dimensional changes, but will not degrade the polymer by hydrolysis.10 The effects of moisture are considerably less dramatic than for nylon polymers. Ultraviolet light may cause degradation, which can be reduced by the addition of carbon black. The copolymers generally have similar properties, but the homopolymer may have slightly better mechanical properties, and higher melting point, but poorer thermal stability and poorer alkali resistance.11 Along with both homopolymers and copolymers, there are also filled materials (glass, fluoropolymer, aramid fiber, and other fillers), toughened grades, and ultraviolet (UV) stabilized grades.12 Blends of acetal with polyurethane elastomers show improved toughness and are available commercially. Acetal resins are available for injection molding, blow molding, and extrusion. During processing it is important to avoid overheating or the production of formaldehyde may cause serious pressure buildup. The polymer should be purged from the machine before shutdown to avoid excessive heating during startup.13 Acetal resins should be stored in a dry place. The apparent viscosity of acetal resins is less dependent on shear stress and temperature than polyolefins, but the melt has low elasticity and melt strength. The low melt strength is a problem for blow molding applications. For blow molding applications, copolymers with branched structures are available. Crystallization occurs rapidly with postmold shrinkage complete within 48 h of molding. Because of the rapid crystallization it is difficult to obtain clear films.14 The market demand for acetal resins in the United States and Canada was 368 million pounds in 1997.15 Applications for acetal resins include gears, rollers, plumbing components, pump parts, fan blades, blow-molded aerosol containers, and molded sprockets and chains. They are often used as direct replacements for metal. Most of the acetal resins are processed by injection molding, with the remainder used in extruded sheet and rod. Their low coefficient of friction make acetal resins good for bearings.16 

Modern
Plastics
Handbook
Modern Plastics
and
Charles A. Harper Editor in Chief
Technology Seminars, Inc.
Lutherville, Maryland
McGraw-Hill
New York San Francisco Washington, D.C. Auckland Bogotá
Caracas Lisbon London Madrid Mexico City Milan
Montreal New Delhi San Juan Singapore
Sydney Tokyo Toronto

High-Molybdenum Alloys13

High-molybdenum stainless and nickel alloys are welded with an overmatching filler metal. This is necessary to maintain corrosion resistance in the weld metal at least equal to the base metal. The reason is that molybdenum and chromium segregate as the weld metal solidifies from the melt. This leaves local areas with high and low molybdenum content. Pitting corrosion can start in the low-Mo areas, with the pits eventually growing even into metal with high molybdenum content. This occurs in alloys ranging from 316L to C-276, for the most part being more severe at higher alloy contents This matter began to receive attention when the 6% Mo stainless steels came on the market. If any of these 6% Mo grades are welded without filler metal, the result is a weld bead that may be as low as 3% Mo in areas. The end result can be that this weld has only the pitting corrosion resistance of 317L stainless. In the case of tubular products autogenously welded in production, a high-temperature anneal is used to homogenize the metal. In addition, a small amount of nitrogen, 3–5%, is added to the torch gas. Fabrications of thin sheet, which cannot be annealed after welding, should have this nitrogen addition to minimize the loss of corrosion. Even so, because thin-sheet welds solidify more quickly, the segregation is less severe. In normal fabrication of a 6% Mo grade, alloy 625 (ERNiCrMo-3) filler metal is used. The weld metal contains 9% Mo. After welding, segregation causes some areas to have as little as 6% Mo. The result is that the alloy 625 weld bead has approximately the same corrosion resistance as the 6% Mo base metal. Higher alloy weld fillers, such as ERNiCrMo- 10 or ERNiCrMo-14, may also be used, though the benefit may be more theoretical than real. ERNiCrMo-4 is not suggested, as it has 5% less chromium than does AL-6XN, for example. Since the mid-1980s nearly all of the 6% Mo alloy fabrications have been made, and put into service, using a 9% Mo weld filler. ERNiCrMo-3 weld filler is widely available and is appropriate for welding lower alloys such as 317L, 317LMN, and 904L for chloride service. The problem of reduced weld bead corrosion resistance from molybdenum and chromium segregation exists with most of the 13–16% Mo nickel alloys as well. Filler 686 CPT (ERNiCrMo-14) does appear to be markedly less susceptible to this effect than other high-molybdenum alloys. 

STAINLESS STEELS
James Kelly
Rochester, Michigan
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
2006 by John Wiley & Sons, Inc.
 

Line types and thicknesses

The standard ISO 128:1982 gives 10 line types that are defined A to K (excluding the letter I). The table in Figure 3.4 shows these lines. The line types are 'thick', 'thin', 'continuous', 'straight', 'curved', 'zigzag', 'discontinuous dotted' and 'discontinuous chain dotted'. Each line type has clear meanings on the drawing and mixing up one type with another type is the equivalent of spelling something incorrectly in an essay. The line thickness categories 'thick' and 'thin' (sometimes called 'wide' and 'narrow') should be in the proportion 1:2. However, although the proportion needs to apply in all cases, the individual line thicknesses will vary depending upon the type, size and scale of the drawing used. The standard ISO 128:1982 states that the thickness of the 'thick' or 'wide' line should be chosen according to the size and type of the drawing from the following range: 0,18; 0,25; 0,35; 0,5; 0,7; 1; 1,4 and 2mm. However, in a direct contradiction of this the standard ISO 128-24:1999 states that the thicknesses should be 0,25; 0,35; 0,5; 0,7; 1; 1,4 and 2mm. Thus confusion reigns and the reader needs to beware! With reference to the table in Figure 3.4, the A-K line types are as follows. The ISO type 'A' lines are thick, straight and continuous, as shown in Figure 3.5. They are used for visible edges, visible outlines, crests of screw threads, limit of length of full thread and section viewing lines. The examples of all these can be seen in the vice assembly detailed drawings. These are by far the most common of the lines types since they define the artefact. The ISO type 'B' lines are thin, straight and continuous, as shown in Figure 3.6. They are used for dimension and extension lines, leader lines, cross hatching, outlines of revolved sections, short centre lines, thread routes and symmetry ('equals') signs. 

Engineering Drawing for Manufacture
by Brian Griffiths
· ISBN: 185718033X
· Pub. Date: February 2003
· Publisher: Elsevier Science & Technology Books


 

Manufacturing Data and Knowledge

Machining methods (also referred to as machining practices) provide CAPP with the knowledge, expertise, and procedures that a human process planner uses. These methods may be based on sound scientific principles, experimental results, experience, or preferences established within a particular machining context. They also may be generic and applicable over a wide range of machining problems or specific to a single one. The challenges in using machining methods within CAPP fall into the following categories: • Identification and retrieval • Implementation • Maintenance • Customization Identification and retrieval are concerned with understanding how a human process planner applies experience and techniques to make decisions when generating process plans: What decisions are being made? What characteristics of the situation are being recognized by the planner that trigger these decisions? The main challenge here stems from the fact that human planners do not necessarily follow a consistent strategy in applying methods. The process often requires complex trade-offs of information from several sources. When one of these sources is experience, the basis of the applied method can be difficult to verify. Thus, identification and retrieval of methods are not just a bookkeeping task. Rather, it requires the cultivation of an attitude toward process planning based on a sound methodology for applying machining methods. Methods implementation requires an approach that is general enough to capture information from very different sources while at the same time is simple enough to provide a maintainable, noncorruptible environment. Rule-based expert systems have been the most commonly adopted implementation strategy among CAPP system developers. Because the need to update or add new methods always exists as more information becomes available or as new methods are applied to more applications, maintenance of the knowledge base becomes a key concern. As changes are made, the integrity of the information needs to be preserved. One problem occurs when new methods are added that conflict with old ones. The system needs to include a strategy for resolving such conflicts. One approach that has been used extensively with expert systems is to place the onus on a knowledgable engineer to avert such problems. However, as the size of the knowledge base grows, the cost of employing dedicated personnel for this task becomes prohibitive. Finally, creating off-the-shelf CAPP systems with the methods included is a difficult if not impossible task. This is because it is unlikely that the system developer can capture all the desired methods from all potential users during system development. Thus, while a system may come with some generic, widely accepted methods, it must include a facility to allow new methods customized to each context to be added to the system.

THE MECHANICAL
SYSTEMS
DESIGN
HANDBOOK
Modeling, Measurement,
and Control
OSITA D. I. NWOKAH
YILDIRIM HURMUZLU
Southern Methodist University
Dallas, Texas
CRC PRESS
Boca Raton London New York Washington, D.C.

BASIC LINKAGE CONCEPTS

Kinematic Elements A linkage is composed of rigid-body members, or links, connected to one another by rigid kinematic elements, or pairs. The nature of those connections as well as the shape of the links determines the kinematic properties of the linkage. Although many kinematic pairs are conceivable and most do physically exist, only four have general practical use for linkages. In Fig. 3.1, the four cases are seen to include two with one degree of freedom (f = 1), one with f = 2, and one with f = 3. Single-degree-of-freedom pairs constitute joints in planar linkages or spatial linkages. The cylindrical and spherical joints are useful only in spatial linkages. The links which connect these kinematic pairs are usually binary (two connections) but may be tertiary (three connections) or even more.A commonly used tertiary link is the bell crank familiar to most machine designers. Since our primary interest in most linkages is to provide a particular output for a prescribed input, we deal with closed kinematic chains, examples of which are depicted in Fig. 3.2. Considerable work is now under way on robotics, which are basically open chains. Here we restrict ourselves to the closed-loop type. Note that many complex linkages can be created by compounding the simple four-bar linkage.This may not always be necessary once the design concepts of this chapter are applied. 

Richard E. Gustavson

Technical Staff Member
The Charles Stark Draper Laboratory, Inc.
Cambridge,Massachusetts
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)

 

Standard Test Method for Water and Sediment in Crude Oil by Centrifuge Method

This standard is issued under the fixed designation D 96; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval. This method has been approved by the sponsoring committees and accepted by the Cooperating Societies in accordance with established procedures. This test method has been adopted for use by government agencies to replace Method 3003 of Federal Test Method Standard No. 791b. Annex A1 is under revision and will be included in subsequent revisions to this standard. 1. Scope 1.1 This test method covers the centrifuge method for determining sediment and water in crude oil during field custody transfers. This test method may not always provide the most accurate results, but it is considered the most practical method for field determination of sediment and water. When a higher degree of accuracy is required, the laboratory procedure described in Test Methods D 4006, D 4377 or D 473 should be used. NOTE 1—Water by distillation and sediment by extraction are considered the most accurate methods of determining sediment and water in crude oils. As such, these methods should be employed to resolve differences in results from variations of this procedure or between this procedure and other methods, or in the case of a dispute between parties. 1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 2. Referenced Documents 2.1 ASTM Standards: D 235 Specification for Mineral Spirits (Petroleum Spirits) (Hydrocarbon Drycleaning Spirits)2 D 362 Specification for Industrial Grade Toluene2 D 473 Test Method for Sediment in Crude Oils and Fuel Oils by the Extraction Method3 D 846 Specification for Ten-Degree Xylene2 D 1209 Test Method for Color of Clear Liquids (Platinum- Cobalt Scale)2 D 3699 Specification for Kerosine4 D 4006 Test Method for Water in Crude Oil by Distillation4 D 4057 Practice for Manual Sampling of Petroleum and Petroleum Products4 D 4177 Practice for Automatic Sampling of Petroleum and Petroleum Products4 D 4377 Test Method for Water in Crude Oils by Potentiometric Karl Fischer Titration4 E 1 Specification for ASTM Thermometers5 E 542 Practice for Calibration of Volumetric Ware6 2.2 API Standards:7 Manual of Petroleum Measurement Standards Chapter 8, Sampling Petroleum and Petroleum Products Chapter 10, Sediment and Water 3. Summary of Test Method 3.1 Known volumes of crude oil and solvent (water saturated if required) are placed in a centrifuge tube and heated to 60°C 6 3°C (140°F 6 5°F). After centrifugation, the volume of the sediment-and-water layer at the bottom of the tube is read. NOTE 2—It has been observed that for some waxy crude oils, temperatures of 71°C (160°F) or higher may be required to melt the wax crystals completely so that they are not measured as sediment. If temperatures higher than 60°C (140°F) are necessary to eliminate this problem, they may be used with the consent of the parties involved. If water saturation of the solvent is required, it must be done at the same temperature. 4. Significance and Use 4.1 A determination of sediment and water content is required to determine accurately the net volumes of crude oil involved in sales, taxation, exchanges, inventories, and custody transfers. An excessive amount of sediment and water in crud.

---------------------
1 This test method is under the jurisdiction of Committee D-2 on Petroleum
Products and Lubricants and is the direct responsibility of Subcommittee
D02.02.OB on Sediment and Water (Joint ASTM-JP).
Current edition approved March 25, 1988. Published December 1988. Originally
published as D 96 – 63T. Last previous edition D 96 – 73 (1984).e1
2 Annual Book of ASTM Standards, Vol 06.04.
3 Annual Book of ASTM Standards, Vol 05.01.
4 Annual Book of ASTM Standards, Vol 05.02.
5 Annual Book of ASTM Standards, Vol 14.03.
6 Annual Book of ASTM Standards, Vol 14.02.
7 Available from American Petroleum Institute, 1220 L St., Northwest, Washington,
DC 20005.

An American National Standard
British Standard 4385
American Association State
Highway Transportation Standard
AASHTO No. T55

AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
 

Three dimensional illustrations using isometric and oblique projection

Isometric projection
Figure 6.1 shows three views of a cube in orthographic
projection; the phantom line indicates the original
position of the cube, and the full line indicates the
position after rotation about the diagonal AB. The cube
has been rotated so that the angle of 45° between side
AC1 and diagonal AB now appears to be 30° in the
front elevation, C1 having been rotated to position C.
It can clearly be seen in the end view that to obtain
this result the angle of rotation is greater than 30°.
Also, note that, although DF in the front elevation
appears to be vertical, a cross check with the end
elevation will confirm that the line slopes, and that
point F lies to the rear of point D. However, the front
elevation now shows a three dimensional view, and
when taken in isolation it is known as an isometric
projection.
This type of view is commonly used in pictorial
presentations, for example in car and motor-cycle service
manuals and model kits, where an assembly has been
‘exploded’ to indicate the correct order and position of
the component parts.
It will be noted that, in the isometric cube, line AC1
is drawn as line AC, and the length of the line is reduced.
Figure 6.2 shows an isometric scale which in principle
is obtained from lines at 45° and 30° to a horizontal
axis. The 45° line XY is calibrated in millimetres
commencing from point X, and the dimensions are
projected vertically on to the line XZ. By similar
triangles, all dimensions are reduced by the same
amount, and isometric lengths can be measured from
point X when required. The reduction in length is in
the ratio
isometric length
true length
= cos 45
cos 30
= 0.7071
0.8660
= 0.8165 °°
Now, to reduce the length of each line by the use of an
isometric scale is an interesting academic exercise,
but commercially an isometric projection would be
drawn using the true dimensions and would then be
enlarged or reduced to the size required.
Note that, in the isometric projection, lines AE and
DB are equal in length to line AD; hence an equal
reduction in length takes place along the apparent
vertical and the two axes at 30° to the horizontal. Note
also that the length of the diagonal AB does not change
from orthographic to isometric, but that of diagonal
C1D1 clearly does. When setting out an isometric
projection, therefore, measurements must be made only
along the isometric axes EF, DF, and GF.
Figure 6.3 shows a wedge which has been produced
from a solid cylinder, and dimensions A, B, and C
indicate typical measurements to be taken along the
principal axes when setting out the isometric projection.
Any curve can be produced by plotting a succession of
points in space after taking ordinates from the X, Y,
and Z axes.
Figure 6.4(a) shows a cross-section through an extruded
alloy bar: the views (b), (c), and (d) give alternative
isometric presentations drawn in the three principal
planes of projection. In every case, the lengths of
ordinates OP, OQ, P1, and Q2, etc. are the same, but
are positioned either vertically or inclined at 30° to
the horizontal.
Figure 6.5 shows an approximate method for the
construction of isometric circles in each of the three
major planes. Note the position of the points of
intersection of radii RA and RB.
The construction shown in Fig. 6.5 can be used
partly for producing corner radii. Fig. 6.6 shows a
small block with radiused corners together with
isometric projection which emphasises the construction
to find the centres for the corner radii; this should be
the first part of the drawing to be attempted. The
thickness of the block is obtained from projecting back
these radii a distance equal to the block thickness and
at 30°. Line in those parts of the corners visible behind
the front face, and complete the pictorial view by adding
the connecting straight lines for the outside of the profile.
In the approximate construction shown, a small
inaccuracy occurs along the major axis of the ellipse,
and Fig. 6.7 shows the extent of the error in conjunction
with a plotted circle. In the vast majority of applications
where complete but small circles are used, for example
spindles, pins, parts of nuts, bolts, and fixing holes,
this error is of little importance and can be neglected.

Manual of
Engineering Drawing
Second edition
Colin H Simmons
I.Eng, FIED, Mem ASME.
Engineering Standards Consultant
Member of BS. & ISO Committees dealing with
Technical Product Documentation specifications
Formerly Standards Engineer, Lucas CAV.
Dennis E Maguire
CEng. MIMechE, Mem ASME, R.Eng.Des, MIED
Design Consultant
Formerly Senior Lecturer, Mechanical and
Production Engineering Department, Southall College
of Technology
City & Guilds International Chief Examiner in
Engineering Drawing

READ MORE.......

Circular & Total Runout

Geometric Tolerancing Geometric Tolerancing is used to specify the shape of features. Things like: •Straightness •Flatness •Circularity •Cylindricity •Angularity •Profiles •Perpendicularity •Parallelism •Concentricity •And More... Geometric Tolerances are shown on a drawing with a feature control frame. Runout is specified on cylindrical parts. It is measured by placing a gage on the part, and rotating the part through 360 degrees. The total variation is recorded as the runout. • Circular runout is measured at one location. • Total Runout is measured along the entire specified surface. Principles of Engineering Drawing Thayer Machine Shop

Gear Types

The most common types of gears are illustrated in Figs. 5.16 to 5.25. Other available types are generally modifications of the basic gears shown. Gear nomenclature and definitions can be found in ANSI/AGMA 1012-F90, Gear Nomenclature, Definitions of Terms with Symbols.1 Spur gears A spur gear has a cylindrical pitch surface and teeth that are parallel to the axis. Spur gears operate on parallel axes (Fig. 5.16). Spur rack A spur rack has a plane pitch surface and straight teeth that are at right angles to the direction of motion (Fig. 5.16). Helical gears A helical gear has a cylindrical pitch surface and teeth that are helical. Parallel helical gears operate on parallel axes. Mating external helical gears on parallel axes have helices of opposite hands. If one of the mating members is an internal gear, the helices are of the same hand (Fig. 5.17). Single-helical gears Gears have teeth of only one hand on each gear (Fig. 5.18). Double-helical gears Gears have both right-hand and left-hand teeth on each gear. The teeth are separated by a gap between the helices (Fig. 5.19).Where there is no gap, they are known as herringbone gears. Wormgearing Includes worms and their mating gears. The axes are usually at right angles (Fig. 5.20). Wormgear (wormwheel) The gear that is the mate to a worm. A wormgear that is completely conjugate to its worm has a line contact and is said to be enveloping. An involute spur gear or helical gear used with a cylindrical worm has point contact only and is said to be nonenveloping (Fig. 5.20). Cylindrical worm A worm that has one or more teeth in the form of screw threads on a cylinder. Enveloping worm (hourglass) A worm that has one or more teeth and increases in diameter from its middle portion toward both ends, conforming to the curvature of the gear (Fig. 5.20). Double-enveloping wormgearing This is comprised of enveloping (hourglass) worms mated with fully enveloping wormgears (Fig. 5.21). Bevel gears These are gears that have conical pitch surfaces and operate on intersecting axes that are usually at right angles (Fig. 5.22). Miter gears These are mating bevel gears with equal numbers of teeth and with axes at right angles (Fig. 5.23). Straight bevel gears These have straight tooth elements which, if extended, would pass through the point of intersection of their axes (Fig. 5.24). Spiral bevel gears These have teeth that are curved and oblique (Fig. 5.24). Hypoid gears Similar in general form to bevel gears, hypoid gears operate on nonintersecting axes (Fig. 5.25). 

Standard Handbook of Plant Engineering (3rd Edition)

By: Rosaler, Robert © 2002 McGraw-Hill
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)

Design Procedure for Crankshaft

The following procedure may be adopted for designing a crankshaft. 1. First of all, find the magnitude of the various loads on the crankshaft. 2. Determine the distances between the supports and their position with respect to the loads. 3. For the sake of simplicity and also for safety, the shaft is considered to be supported at the centres of the bearings and all the forces and reactions to be acting at these points. The distances between the supports depend on the length of the bearings, which in turn depend on the diameter of the shaft because of the allowable bearing pressures. 4. The thickness of the cheeks or webs is assumed to be from 0.4 ds to 0.6 ds, where ds is the diameter of the shaft. It may also be taken as 0.22D to 0.32 D, where D is the bore of cylinder in mm. 5. Now calculate the distances between the supports. 6. Assuming the allowable bending and shear stresses, determine the main dimensions of the crankshaft. Notes: 1. The crankshaft must be designed or checked for at least two crank positions. Firstly, when the crankshaft is subjected to maximum bending moment and secondly when the crankshaft is subjected to maximum twisting moment or torque. 2. The additional moment due to weight of flywheel, belt tension and other forces must be considered. 3. It is assumed that the effect of bending moment does not exceed two bearings between which a force is considered. 

FIRST MULTICOLOUR EDITION

(S.I. UNITS)

[A Textbook for the Students of B.E. / B.Tech.,

U.P.S.C. (Engg. Services); Section ‘B’ of A.M.I.E. (I)]

A TEXTBOOK OF

Machine Design

R.S. KHURMI

J.K. GUPTA

What Are Titanium Alloys?

For purposes of this chapter titanium alloys are those alloys of about 50% or higher titanium that offer exceptional strength-to-density benefits plus corrosion properties comparable to the excellent corrosion resistance of pure titanium. The range of operation is from cryogenic temperatures to around 538–595 C (1000–1100 F). Titanium alloys based on intermetallics such as gamma titanium aluminide (TiAl intermetallic compound which has been designated ) are included in this discussion. These alloys are meant to compete with superalloys at the lower end of superalloy temperature capability, perhaps up to 700 C ( 1300 F). They may offer some mechanical advantages for now but often represent an economic debit. Limited experience is available with the titanium aluminides. Temperature Capability of Titanium Alloys Although the melting point of titanium is in excess of 1660 C (3000 F), commercial alloys operate at substantially lower temperatures. It is not possible to create titanium alloys that operate close to their melting temperatures. Attainable strengths, crystallographic phase transformations, and environmental interaction considerations cause restrictions. Thus, while titanium and its alloys have melting points higher than those of steels, their maximum upper useful temperatures for structural applications generally range from as low as 427 C (800 F) to the region of about 538–595 C (1000–1100 F) dependent on composition. As noted, titanium aluminide alloys show promise for applications at higher temperatures, perhaps up to 700 C ( 1300 F), although at one time they were expected to offer benefits to higher temperatures. Actual application temperatures will vary with individual alloy composition. Since application temperatures are much below the melting points, incipient melting is not a factor in titanium alloy application. 

SELECTION OF TITANIUM ALLOYS

FOR DESIGN

Matthew J. Donachie

Rensselaer at Hartford

Hartford, Connecticut

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

by John Wiley & Sons, Inc.


 

Nuts, bolts, screws and washers

ISO metric precision hexagon bolts, screws and nuts are covered by BS 3643 and ISO 272. The standard includes washer faced hexagon head bolts and full bearing head bolts. In both cases there is a small radius under the bolthead which would not normally be shown on drawings, due to its size, but is included here for completeness of the text. With an M36 bolt, the radius is only 1.7 mm. Bolts may be chamfered at 45° at the end of the shank, or radiused. The rounded end has a radius of approximately one and one quarter times the shank diameter and can also be used if required to draw the rolled thread end. The washer face under the head is also very thin and for a M36 bolt is only 0.5 mm. Figure 16.1(a) shows the bolt proportions and Table 16.1 the dimensions for bolts in common use. Dimensions of suitable nuts are also given and illustrated in Fig. 16.1(b). Included in Table 16.1 and shown in Fig. 16.1(c) are typical washers to suit the above bolts and nuts and these are covered by BS 4320. Standard washers are available in two different thicknesses, in steel or brass, and are normally plain, but may be chamfered. Table 16.1 gives dimensions of commonly used bolts, nuts and washers so that these can be used easily on assembly drawings. For some dimensions maximum and minimum values appear in the standards and we have taken an average figure rounded up to the nearest 0.5 mm and this will be found satisfactory for normal drawing purposes. Reference should be made to the relevant standards quoted for exact dimensions if required in design and manufacture. 

Manual of

Engineering Drawing

Second edition

Colin H Simmons

I.Eng, FIED, Mem ASME.

Engineering Standards Consultant

Member of BS. & ISO Committees dealing with

Technical Product Documentation specifications

Formerly Standards Engineer, Lucas CAV.

Dennis E Maguire

CEng. MIMechE, Mem ASME, R.Eng.Des, MIED

Design Consultant

Formerly Senior Lecturer, Mechanical and

Production Engineering Department, Southall College

of Technology

City & Guilds International Chief Examiner in

Engineering Drawing
 

Geometric Tolerancing

Geometric Tolerancing is used to specify the shape of features.

Things like:

•Straightness

•Flatness

•Circularity

•Cylindricity

•Angularity

•Profiles

•Perpendicularity

•Parallelism

•Concentricity

•And More...

Geometric Tolerances are shown on a drawing with a feature control frame.

The Feature Control Frame

This feature control frame is read as: “The specified feature must lie perpendicular within a

tolerance zone of 0.05 diameter at the maximum material condition, with respect to datum

axis C. In other words, this places a limit on the amount of variation in perpendicularity

between the feature axis and the datum axis. In a drawing, this feature control frame would

accompany dimensional tolerances that control the feature size and position.

Principles of Engineering Drawing

Thayer Machine Shop

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SECTION 3 SURFACE TEXTURE

3.1 SCOPE OF SECTION This Section provides information on the indication of surface texture on

mechanical engineeringdrawingsandsimilar applications.For a more complete understandingof surface

texture, reference should be made to AS 2536.

3.2 SYMBOLS

3.2.1 Basic symbol The basic symbol is shown in Figure 3.1. The dimensions of surface texture

symbols are shown in Figure 3.2. Sloping lines in the symbol are at 60° to the horizontal.

3.2.2 Modification to basic symbol The following modifications may be made to the basic symbol:

(a) The symbol to be used where machining is mandatory shall be the basic symbol with a bar

added, as shown in Figure 3.3.

This symbol may be used alone to indicate that a surface is to be machined without defining

either the surface texture or the process to be used.

(b) The symbol to be used when the removal of material is not permitted shall be the basic symbol

with a circle added, as shown in Figure 3.4. This symbol may be used alone to indicate that a

surface is to be left in the state resulting from a preceding manufacturing process.

3.2.3 Extension of symbols When special surface characteristics are to be indicated (see Clause

3.4), the symbols shown in Figures 3.1, 3.3 and 3.4 may be extended by adding a line of appropriate

length to the long leg, as shown in Figure 3.5.

Australian Standard

Technical drawing

Part 201: Mechanical engineering drawing

Accessed by WOODSIDE ENERGY LTD on 21 Nov 2001

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The application of welding symbols to working drawings

The following notes are meant as a guide to the method

of applying the more commonly used welding symbols

relating to the simpler types of welded joints on

engineering drawings. Where complex joints involve

multiple welds it is often easier to detail such constructions

on separate drawing sheets.

Each type of weld is characterized by a symbol

given in Table 26.1 Note that the symbol is representative

of the shape of the weld, or the edge preparation, but

does not indicate any particular welding process and

does not specify either the number of runs to be deposited

or whether or not a root gap or backing material is

to be used. These details would be provided on a welding

procedure schedule for the particular job.

It may be necessary to specify the shape of the weld

surface on the drawing as flat, convex or concave and

a supplementary symbol, shown in Table 26.2, is then

added to the elementary symbol. An example of each

type of weld surface application is given in Table 26.3.

A joint may also be made with one type of weld on

a particular surface and another type of weld on the

back and in this case elementary symbols representing

each type of weld used are added together. The last

example in Table 26.3 shows a single-V butt weld

with a backing run where both surfaces are required to

have a flat finish.

A welding symbol is applied to a drawing by using

a reference line and an arrow line as shown in Fig.

26.1. The reference line should be drawn parallel to

the bottom edge of the drawing sheet and the arrow

line forms an angle with the reference line. The side of

the joint nearer the arrow head is known as the ‘arrow

side’ and the remote side as the ‘other side’.

The welding symbol should be positioned on the

reference line as indicated in Table 26.4.

Sketch (a) shows the symbol for a single-V butt

weld below the reference line because the external

surface of the weld is on the arrow side of the joint.

Sketch (b) shows the same symbol above the

reference line because the external surface of the weld

is on the other side of the joint.

Manual of

Engineering Drawing

Second edition

Colin H Simmons

I.Eng, FIED, Mem ASME.

Engineering Standards Consultant

Member of BS. & ISO Committees dealing with

Technical Product Documentation specifications

Formerly Standards Engineer, Lucas CAV.

Dennis E Maguire

CEng. MIMechE, Mem ASME, R.Eng.Des, MIED

Design Consultant

Formerly Senior Lecturer, Mechanical and

Production Engineering Department, Southall College

of Technology

City & Guilds International Chief Examiner in

Engineering Drawing

Elsevier Newnes

Linacre House, Jordan Hill, Oxford OX2 8DP

200 Wheeler Road, Burlington MA 01803

Read more........

EVOLUTION OF A DESIGN

Most likely you have, right at this moment, at least one machine design project in progress. Maybe you were the originator of the design, but I suspect you inherited this design from others. I further suspect that you have already identified elements of the design you feel could be improved. You might be under pressure from customer service or marketing to respond to some need for change. In responding successfully, either to your own observations for change or to those of others, the design will evolve. Recognizing that the evolutionary design process is decidedly complex, with a seemingly random sequence of steps, the primary purpose of Standard Handbook of Machine Design is to make the information you need as readily accessible and usable as possible. As an example of how a design can evolve, and to provide perspective on how the information in this Handbook has traditionally been used, let me review for you a project I was given in my first job as a mechanical engineer. It involved the positioning of a microwave feed horn for a 30-ft-diameter antenna dish.The original gn (not mine, by the way) called for a technician to climb up onto a platform, some 20 ft off the ground, near the backside of the feed horn.The technician had to loosen a half dozen bolts, rotate the feed horn manually, and then retighten the bolts. This design worked quite well until several systems were sold to a customer providing telecommunications along the Alaskan oil pipeline. Workers were not really safe going out in below 0°F weather,with snow and ice on everything. As a result of their concerns for safety, this customer asked that we provide remote positioning of the feed horn from the nearby control room. The critical design requirement was that the positioning of the feed horn needed to be relatively precise. This meant that our design had to have as little backlash in the drive mechanism as possible. Being a young engineer, I was unaware of the wide variety of different drive systems, in particular their respective properties and capa- bilities. I asked one of the older engineers for some direction. He suggested I use a worm drive since it cannot be back driven, and loaned me his copy of Joseph Shigley’s book, Mechanical Engineering Design. He said that Shigley’s book (a precursor to this Handbook) had been his primary source of information about worm drives, and a wealth of other machine design information. As it turned out, the resulting design worked as required. It not only pleased our Alaskan customer but became a standard on all antenna systems. I did not get a promotion as a result of the success of this new design, nor did I receive a raise. However, I was proud, and, as you can surmise, still am. I credit this successful design evolution to the material on worm drives in Shigley’s book. And there is more to this story. The worm drive gearbox we ultimately purchased contained a plastic drive element. This allowed the backlash to be greater than what could be tolerated in positioning accuracy and did not provide the necessary strength to break the feed horn loose from a covering of ice.The original manufacturer of the gearbox refused to change this drive element to metal for the units we would be buying. If we made the change ourselves, they said, the warranty would be voided. However, after absorbing the wealth of information on worm drives in Shigley’s book, I felt confident that we could make this substitution without endangering the reliability of the unit. Also, because of Joseph Shigley’s reputation in the mechanical engineering community and the extensive list of references he cited, I never felt the need to consult other sources. Another aspect of this story is also important to note. In addition to the information on worm drives, I also used Shigley’s book to find comprehensive design information on the many other machine elements in the new design: gear train geometry, chain drives, couplings, roller bearings, bolted joints, welds, lubrication, corrosion, and the necessary stress and deformation calculations I needed to make. All this information, and much more, was contained in the First Edition of the Standard Handbook of Machine Design, which Joseph Shigley coauthored with Charles Mischke. Now in its Third Edition, this Handbook includes the information machine design engineers have come to trust.We hope you will find this information invaluable as you constantly strive to improve your designs, whether by your own initiatives, or for other reasons.
Thomas H. Brown, Jr., Ph.D., P.E.
Faculty Associate
Institute for Transportation Research and Education
North Carolina State University
Raleigh, North Carolina
Source: STANDARD HANDBOOK OF MACHINE DESIGN
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
 

Injection Forging—Process and Component

Injection forging is a process in which the work-material retained in an injection chamber is injected into a die-cavity in a form prescribed by the geometry of the exit (Fig. 1). The process is characterized by the combination of axial and radial flows of material to form the required component-form. In the 1960s, some interest was generated in injection upsetting [1]; it was developed with a view to extruding complex component-forms. The process configuration has since been the subject of research spanning fundamental analysis to the forming of specific components; branched components and gear-forms have been produced. The single-stage forming of such component-forms has been achieved by injection techniques; these forms were previously regarded as unformable by conventional processes. Currently, the nett-forming of some complex component-forms has been achieved by Injection Forging [2]. To date, several names have been used to describe this configuration—injection forming, injection upsetting, radial extrusion, side extrusion, transverse extrusion, lateral extrusion, and injection forging [2–22].

COMPUTER-AIDED DESIGN,
ENGINEERING, AND MANUFACTURING
Systems Techniques And Applications
VOLUME
V I
MANUFACTURING
SYSTEMS PROCESSES
Editor
CORNELIUS LEONDES
CRC Press
Boca Raton London New York Washington, D.C.

Modulus of Elasticity

There are different techniques that have been used for over a century to increase the modulus of elasticity of plastics. Orientation or the use of fillers and/or reinforcements such as RPs can modifl the plastic. There is also the popular and extensively used approach of using geometrical design shapes that makes the best use of materials to improve stiffness even for those that have a low modulus. Structural shapes that are applicable to all materials include shells, sandwich structures, dimple sheet surfaces, and folded plate structure.
Plastics
Engineered
Product
Design
Dominick Rosato and
Donald Rosato
Elsevier Ltd, The Boulevard, Langford Lane, Kidlington, Oxford OX5 lGB, UK
Elsevier Inc, 360 Park Avenue South, New York, NY 10010-1710, USA
Elsevier Japan, Tsunashima Building Annex, 3-20-12 Yushima, Bunkyo-ku, Tokyo 113,
Japan

Mechanical Properties of Metals (2)

6. Brittleness.

It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Cast iron is a brittle material.

7. Malleability.

It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The

malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron, copper and aluminium.

8. Toughness.

It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. It is measured by the amount of energy that a unit volume of the

material has absorbed after being stressed upto the point of fracture. This property is desirable in parts subjected to shock and impact loads.

9. Machinability. It is the property of a material which refers to a relative case with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. It may be noted that brass can be easily machined than steel.

10. Resilience. It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for

spring materials.

FIRST MULTICOLOUR EDITION

A TEXTBOOK OF

Machine

Design

(S.I. UNITS)

[A Textbook for the Students of B.E. / B.Tech.,

U.P.S.C. (Engg. Services); Section ‘B’ of A.M.I.E. (I)]

R.S. KHURMI

J.K. GUPTA

EURASIA PUBLISHING HOUSE (PVT.) LTD.

RAM NAGAR, NEW DELHI-110 055

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PRESS FITS

When one object such as a shaft is assembled to another by forcing it into a hole that is slightly too small, the operation is known as press fitting. Press fits can be designed between similar plastics, dissimilar plastics, or more commonly between a plastic and a metal. A typical example occurs when a plastics hub in the form of a control knob or gear is pressed on a metal shaft. The position is reversed when a plastics sleeve or bearing is pressed into a metal bore.

Press fits are simple and inexpensive but there are some problems to look out for. The degree of interference between the shaft and the hole is critical. If it is too small, the joint is insecure. If it is too great, the joint is difficult to assemble and the material will be over-stressed. Unlike a snap fit, the press fit remains permanently stressed and it is the elastic deformation of the plastics part that supplies the force to hold the joint together.

When plastics materials are exposed to permanent stress the result is creep. This means that as time goes by, the force exerted by the press fit becomes less, lthough not necessarily to a significant extent.

There are two other pitfalls for press fits. Manufacturing tolerances on the shaft and hole must be taken into account to see whether the two extreme cases remain viable.

And when the joint is made between dissimilar materials, an increase in temperature will change the degree of interference between the parts. Remember too, that at elevated temperatures the effect of creep will be greater.

One way of countering the effect of creep in a shaft and hub press fit is to provide a straight medium knurl on the metal shaft.

The plastics hub material will tend to cold flow into the grooves of the knurl, giving a degree of mechanical interference between the parts. The frictional effect is also greater because the surface area of the joint has been increased by the knurl.

DESIGN GUIDES

for

PLASTICS

Clive Maier, Econology Ltd

Plastics Design Group - Plastics Consultancy Network

British Plastics Federation

pdg

plastics design group

December 2004 Edition

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SIZE OF DRAWING SHEETS

2.4.1 Preferred sizes The preferred size of drawing sheets shall be the ISO-A series for which the designation and dimensions are as given in Table 2.1. Preferred size drawing sheets, with slightly wider borders to take account of preprinting considerations, shall have dimensions as given in Table 2.2. Such sheets shall be additionally designated by the prefix R, i.e. RA0, RA1, RA2, RA3, and RA4. Where drawing sheets of a greater length are required, they should be selected from and have dimensions in accordance with one of the series given in Table 2.3. Such sheets shall bedesignated A3 × 3, A3 × 4, A4 × 3, A4 × 4, and A4 × 5. 2.4.2 Non-preferred sizes The non-preferred size of drawing sheets shall be the ISO-B series for which the designations and dimensions are as given in Table 2.4. Non-preferred size drawing sheets, with slightly wider borders to take account of preprinting considerations, shall have dimensions as given in Table 2.5. Such sheets shall be additionallydesignated by the prefix R, i.e. RB1, RB2, RB3, and RB4. 2.4.3 Roll drawings Standard widths of roll drawings shall be 860 mm and 610 mm. Lengthsof the roll drawing sheets shall be determined to suit the requirements of the individual drawings. Australian Standard Technical drawing Part 101: General principles For history before 1992, see Preface. Second edition AS 1100.101—1992. Incorporating Amdt 1-1994 PUBLISHED BY STANDARDS AUSTRALIA (STANDARDS ASSOCIATION OF AUSTRALIA) 1 THE CRESCENT, HOMEBUSH, NSW 2140

Mechanical Properties of Metals

The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness. We shall now discuss these properties as follows:

1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called *stress.

2. Stiffness. It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.

3. Elasticity. It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.

4. Plasticity. It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.

5. Ductility. It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminium, nickel, zinc, tin and lead.

Note : The ductility of a material is commonly measured by means of percentage elongation and percentage

reduction in area in a tensile test. (Refer Chapter 4, Art. 4.11).

FIRST MULTICOLOUR EDITION

A TEXTBOOK OF

Machine

Design

(S.I. UNITS)

[A Textbook for the Students of B.E. / B.Tech.,

U.P.S.C. (Engg. Services); Section ‘B’ of A.M.I.E. (I)]

R.S. KHURMI

J.K. GUPTA

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Sections and sectional views

A section is used to show the detail of a component, or an assembly, on a particular plane which is known as the cutting plane. A simple bracket is shown in Fig. 8.1 and it is required to draw three sectional views.

Assume that you had a bracket and cut it with a hacksaw along the line marked B–B. If you looked in the direction of the arrows then the end view B–B in the solution (Fig. 8.2), would face the viewer and the surface

indicated by the cross hatching would be the actual metal which the saw had cut through. Alternatively had we cut along the line C–C then the plan in the solution would be the result. A rather special case exists along the plane A–A where in fact the thin web at this point has been sliced. Now if we were to cross hatch all the surface we had cut through on this plane we would give a false impression of solidity. To provide a more realistic drawing the web is defined by a full line and the base and perpendicular parts only have been cross hatched. Note, that cross hatching is never undertaken between dotted lines, hence the full line between the web and the remainder of the detail.

However, the boundary at this point is theoretically a dotted line since the casting is formed in one piece and no join exists here. This standard drawing convention is frequently tested on examination papers.

Cutting planes are indicated on the drawing by a long chain line 0.35 mm thick and thickened at both ends to 0.7 mm. The cutting plane is lettered and the arrows indicate the direction of viewing. The sectional view or plan must then be stated to be A–A, or other letters appropriate to the cutting plane. The cross hatching should always be at 45° to the centre lines, with continuous lines 0.35 mm thick.

If the original drawing is to be microfilmed successive lines should not be closer than 4 mm as hatching lines

tend to merge with much reduced scales. When hatching very small areas the minimum distance between lines

should not be less than 1 mm.

In the case of very large areas, cross hatching may be limited to a zone which follows the contour of the hatched area. On some component detail drawings it may be necessary to add dimensions to a sectional

drawing and the practice is to interrupt the cross hatching so that the letters and numbers are clearly visible.

Manual of

Engineering Drawing

Second edition

Colin H Simmons

I.Eng, FIED, Mem ASME.

Engineering Standards Consultant

Member of BS. & ISO Committees dealing with

Technical Product Documentation specifications

Formerly Standards Engineer, Lucas CAV.

Dennis E Maguire

CEng. MIMechE, Mem ASME, R.Eng.Des, MIED

Design Consultant

Formerly Senior Lecturer, Mechanical and

Production Engineering Department, Southall College

of Technology

City & Guilds International Chief Examiner in

Engineering Drawing

Elsevier Newnes

Linacre House, Jordan Hill, Oxford OX2 8DP

200 Wheeler Road, Burlington MA 01803

Read more........

The Blow Molding Process

THEORETICAL BACKGROUND Blow molding is a fabrication process to convert the raw materials (resin) into finished hollow containers (products). It is a manufacturing process by which hollow plastic parts are formed. Principle of the Most Basic Type of Blow Molding 1. Resin is melted by heaters and plasticized by an extruder. 2. Then it is extruded so that it forms a tube-shaped parison matching the size of the product as it passes through the die. 3. Air is then forced into the parison and press it against the inner walls of the mold. After the parison cools and become solid in the mold, the air is released. 4. Finally, the mold will open and the product is deflashed and ejected or taken out the machine. Higher Institute for Plastics Fabrication WORKBOOK for Extrusion Blow Molding Practical Course Prepared by Extrusion Blow Molding Department 1st Edition 2009

Safety factor

A safety factor (SF) or factor of safety (FS) (also called factor of ignorance) is used with plastics or other materials (metals, aluminum, etc.) to provide for the uncertainties associated with any design, particularly when a new product is involved with no direct historical performance record. There are no hard and fast rules to follow in setting a SF. The most basic consideration is the consequences of failure. In addition to the basic uncertainties of graphic design, a designer may also have to consider additional conditions such as: (1) variations in material property data (data in a table is the average and does not represent the minimum required in a design); (2) variation in material performance; (3) effect of size in stating material strength properties; (4) type of loading (static, dynamic, etc.); (5) effect of process (stress concentrations, residual stress, etc.); and (6) overall concern of human safety.

The SF usually used based on experience is 1.5 to 2.5, as is commonly used with metals. Improper use of a SF usually results in a needless waste of material or even product failure. Designers unfamiliar with plastic products can use the suggested preliminary safety factor guidelines in Table 7.3 that provide for extreme safety; intended for preliminary dcsign analysis only. Low range values represent applications where failure is not critical. The higher values apply where failure is critical. Any product designed with these guidelines in mind should conduct tests on the products themselves to relate the guidelines to actual performance. With more experience, more-appropriate values will be developed targeting to use 1.5 to 2.5. After field service of the preliminary designed products has been obtained, action should be taken to consider reducing your SF in order to reduce costs.

Plastics

Engineered

Product

Design

Dominick Rosato and

Donald Rosato

ELSEVIER

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Mold Setup & Parison Processing

Performance Objective Setup to generate good container welds with even, easy flash removal at target flash to container interface temperature. Mold Setup •Setup molds with carbon paper to identify even mold compression •Setup mold temperature control of 80F to 110F (25C-40C) at pinch areas to achieve container deflash interface temperature of 180F to 200F (80C to 95C) Processing Techniques •Use mold cracking timer to enable larger containers to vent upon mold opening •Use blow pin lift timer to allow container to release from mold •Insure that container releases easily from the mold –Observe for scrapes on container –Observe for flash retensionto container or mold during mold open and container extraction •Avoid heavy pinch terminations –Program parisonto light weight pinch area –Capture parisonin inflated region –Locate pinch terminations up on base to sidewall radiaway from base footprint –Option: Locate pinch terminations within the base pushup –Option: Locate one end of pinch termination within the base pushup and the other end on the container sidewall Processing Techniques •Pinch weld termination location 1.Locate pinch terminations up on base to sidewall radii away from base footprint 2.Locate pinch terminations within the base pushup 3.Locate one end of pinch termination within the base pushup and the other end on the container sidewall EASTMAN Extrusion Blow Molding Presentation.