Variable Loads

Many machine parts are subjected to variable loads, that is, load varies in magnitude, as well as in its type, that is, tensile or compressive. Figure 10.1(a) shows a railway wheel rolling over the rail, which is supported on slippers. When the wheel is in between the slippers at Point C, the rail undergoes bending causing tension at its bottom, and when the wheel crosses and takes the position on the wooden slipper, the load on the rail at Point C is reduced. Thus, the load is changing with time.

Figure 10.1(b) shows a rotating shaft supported on two bearings at its ends with a load W in its middle. Points A and B are marked on the shaft in its position. Due to load, fibres of Point A will be in tension, while of Point B in compression. When the shaft turns by half revolution, Point A will take position on top and, hence, will be in compression, while Point B will be down and, hence, in tension. Thus, the stress varies from compression to tension and again to compression. Variable loads can be grouped as under:

a. Fluctuating loads Stress varies from some maximum value σmax to a minimum value σmin. There is some mean value of stress σm, which could be positive or negative, and some variable stress σa , as shown in Figure 10.2(a).

b. Repeated loads Stress varies from some maximum value σmax to zero and there is some mean stress σm, as shown in Figure 10.2(b).