We must start our discussion of bearings with a brief word on friction. Friction is the resistance to relative motion between two bodies in contact. Sometimes this resistance is useful. For example, it is hard to walk on ice because there is very little friction between your shoes and the ice. In ordnance, however, friction is mostly unwanted. It takes effort to rotate trains of gears, to move levers and shafts, and so forth. Friction in these mechanisms adds to the effort required. Lubrication is one answer; this is discussed later in chapter 12. It coats the surfaces of moving parts in contact with one another and separates them by a fluid film. In some cases, though, this is not enough. Sometimes the speed of the moving parts or the load on them is so great that the oil film will be thrown out or ruptured. In this case, bearings (and lubrication) are the answer.

Plain (Sliding) Bearings

Most often the parts that rub together are made of steel. Sometimes the friction can be reduced sufficiently by simply inserting a strip of softer metal, such as bronze, between the two steel parts. This is the theory behind the plain, or sliding, type of bearing. In figure 4-10, you can see two applications.

Figure 4-10.-Example of plain (sliding) bearings.

In figure 4-10, view A, you see a gun slide weldment in which the gun and housing move in recoil and counterrecoil. The bearing strips (one is hidden) inside the slide are made of bronze and support the heavy housing as it moves forward and aft during firing. The barrel bearing, also bronze, supports the barrel in these movements. In view B, you are looking up at a sliding wedge type of breech mechanism. The breech operating shaft, in its rotating movement, raises and lowers the breechblock. Here, the steel operating shaft is kept from coming into direct contact with the steel gun housing and bearing caps (not shown) by the operating shaft bronze journal bearings. For ease in assembly and disassemble y, the bearings are in two parts. Notice the oil grooves cut into the inside of the bearing. These grooves distribute the lubricant around the shaft from a zerk fitting on the bearing caps.

The bronze bearing blocks, up inside the breechblock, transmit the rotating movement of the operating shaft to a vertical movement to raise and lower the breech. These are oblong bearings, riding in slanted blockways in the block. They are lubricated by means of oil holes drilled throughout their length.

Bearings with Roller Contact

The plain bearings we have just described will reduce friction. A much more efficient type, however, is that which inserts a rolling contact between the stationary and moving elements of the mechanism. The rolling elements are balls or rollers. The bearing assembly usually is made up of three parts: the rolling elements, a separator, and two races.

An ordnance application of roller bearings can be seen in figure 4-11, view A. Here, they are used to reduce friction between the rotating gun mount (base ring) and the stationary stand. The horizontal rollers support the weight of the mount. The upper race (roller path) is part of the base ring. The lower race is part of the stand. The separator keeps the roller bearings from getting canted and running into one another. The upright (radial) bearings reduce friction between the base ring and stand when a sideways force is exerted on the mount.

In view B, you see tapered roller bearings used in trunnions. In this application, they allow the trunnions to rotate freely but restrict sideways and up-and-down movement as the gun bucks during firing.

Figure 4-11.-Bearings with roller contact.