A = Area (square inches).
N =Number of revolutions per minute. n = Number of bolts in cap. n1 = Number of bolts in bracket base. P = Total pressure on bearing (lbs.). p = Pressure per square inch of projected area (lbs).
Machine surfaces taking weight and pressure of other parts in motion upon them are, in general, known as bearings. If the motion is rectilinear, the bearing is termed a slide, guide, or way, such as the cross slide of a lathe, the cross-head guide of a steam engine, or the ways of a lathe bed.
If the motion is a rotary one, like that of the spindle of a lathe, the simple word "bearing" is generally used.
In any bearing, sliding or rotary, there must be strength to carry the load, stiffness to distribute the pressure evenly over the full bearing surface, low intensity of such pressure to prevent the lubricant from being squeezed out and to minimize the wear, and sufficient radiating surface to carry away the heat generated by friction of the surfaces as fast as it is generated. Sliding bearings are of such varied nature, and exist under conditions so peculiar to each case, that a general analysis is practically impossible beyond that given in the sentence above.
Rotary bearings can be more definitely studied, as there are but two variable dimensions, diameter and length, and it is the proper relation between these two that determines a good bearing. The size of the shaft, as noted under "Shafts," is calculated by taking the bending moment at the center of the bearing, combining it with the twisting moment, and solving for the diameter consistent with the assumed fiber stress. But this size must then be tried for deflection due to the bending load, in order that the requirement for stiffness may be fulfilled. When this is accomplished, the friction at the bearing surface may still generate so much heat that the exposed surface of the bearing will not radiate it as fast as generated, in which case the l>earing gets hotter and hotter, until it finally burns out the lubricant and melts the lining of the bearing, and ruin results.
The heat condition is usually the critical one, as it is very easy to make a short bearing which is strong enough and amply stiff for the load it carries, but which nevertheless is a failure as a bearing, because it has so small a radiating surface that it cannot run cool.
The side load which causes the friction and the consequent development of heat, is due to the pull of the belt in the case of pulleys, the load on the teeth of gears, the pull on cranka and levers, the weight of parts,etc. If we could exert pure torsion on shafts without any side pressure, and counteract all the weight that conies on the shaft, we should not have any trouble with the development of heat in bearings; in fact, there would theoretically be no need of hearings, as the shafts would naturally spin about their axes, and would not need support.
It can be shown, theoretically, that the radiating surface of a bearing increases relatively to the heat generated by a given side load, only when the length of the hearing is increaead. In other words, increasing the diameter and not the length, theoretically increases the heat generated per unit of time just as much as it increases the radiating surface; hence nothing is gained, and heat accumulates in the hearing as before. This important fact is verified by the design of high-speed bearings, which, it is always noted, are very long in proportion to their diameter, thus giving relatively high radiating power.
Bearings must be rigidly fastened to the body of the machine in some way, and the immediate support is termed a bracket, frame, or housing. "Bracket" is a very general term, and applies to the supports of other machine parts besides "bearings." It is especially applicable to the more familiar types of bearing supports, and is here introduced to make the analysis complete.
The bracket must be strong enough as a beam to take the aide load, the bending moment being figured at such points as are necessary to determine its outline. It may be of solid, box, or ribbed form, the latter being the most economical of material, and usually permitting the simplest pattern. The fastening of the bracket to the main body of the machine must be broad to give stability; the bolts act partly in shear to keep the bracket from sliding along its base, and partly in tension to resist its tendency to rotate about some one of its edges, due to the side pull of the belt, gear tooth, or lever load, as the case may be. The weight of the bracket itself and of the parts it sustains through the bearing has likewise to be considered; and this acts, in conjunction with the working load on the bearing, to we usually think more readily of a stand as having an upright or inverted position with reference to the ground. The ordinary "hanger" is a good example of an inverted stand; and the regular "floor stand," found on jack shafts in some power houses, is an example of the general class.