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To sum up, the fundamental problem of the designer is to use the simple tension test data and relate them to the strength of the part , regardless of the stress or the loading situation.
It is possible for two metal to have exactly the same strength and hardness, yet one of these metals may have a supeior ability to aborb overloads, because of the property called ductility. Dutility is measured by the percentage elongation which occurs in the material at frature.
The usual pding line between ductility and brittleness is 5 percent elongation.Amaterial having less than 5 percent elongation at fracture is said to bebrittle, while one having more is said to be ductile.
The elongation of a material is usuallu measured over 50mm gauge length.siece this did not a measure of the actual strain, another method of determining ductility is sometimes used .after the speciman has been fractured, measurements are made of the area of the cross section at the fracture. Ductilitycan then be expressed as the percentage reduction in cross sectional area.
The characteristic of a ductile material which permits it to aborblargeoverloads is an additional safety factot in design. Ductility is also important because it is a measure of that property of a material which permits it to be cold-worked .such operations as bending and drawing are metal-processing operations which require ductile materials.
When a materals is to be selected to resist wear , erosion ,or plastic deformaton, hardness is generally the most important property. Several methods of hardness testing are available, depending upon which particular property is most desired. The four hardness numbers in greatest usse are the Brinell, Rockwell,Vickers, and Knoop.
Most hardness-testing systems employ a standard load which is applied to a ball or pyramid in contact with the material to be tested. The hardness is an easy property to measure , because the test is nondestructive and test specimens are not required . usually the test can be conducted directly on actual machine element
Virtually all machines contain shafts. The most common shape for shafts is circular and the cross section can be either solid or hollow (hollow shafts can result in weight savings). Rectangular shafts are sometimes used ,as in screw driver bladers ,socket wrenches and control knob stem. A shaft must have adequate torsional strength to transmit torque and not be over stressed. If must also be torsionally stiff enough so that one mounted component does not deviate excessively from its original angular position relative to a second component mounted on the same shaft. Generally speaking,the angle of twist should not exceed one degree in a shaft length equal to 20 diameters. Shafts are mounted in bearing and transmit power through such device as gears, pulleys,cams and clutches. These devices introduce forces which attempt to bend the shaft;hence, tha shaft must be rigid enough to prevent overloading of the supporting bearings ,in general, the bending deflection of a shaft should not exceed 0.01 in per ft of length between bearing supports.
in addition .the shaft must be able to sustain a combination of bending and torsional loads. Thus an equivalent load must be considered which takes into account both torsion and bending .also ,the allowable stress must contain a factor of safety which includes fatigue, since torsional and bending stress reversals occur. For fiameters less than 3 in ,the usual shaft material is cold-rolled steelcontaining about 0.4 percent carbon. Shafts ate either cold-rolled or forged in sizes from 3in. to 5 in. for sizes above 5 in. shafts are forged and machined to size .plastic shafts are widely used for light load applications . one advantage of using plastic is safty in electrical applications, since plastic is a poor confuctor of electricity..
Components such as gears and pulleys are mounted on shafts by means of key. The design of the key and the corresponding keyway in the shaft must be properly evaluated. For example, stress concentrations occur in shafts due to keyways , and the material removed to form the keyway further weakens the shaft. If shafts are run at critical speeds , severe vibrations can occur which can seriously damage a machine .it is important to know the magnitude of these critical speeds so that they can be avoided.