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
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    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


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