PRODUCT DESIGN

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

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

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).