Cam Tappet body Cylinder Ball check valve Plunger Plunger spring Push rod socket Push rod Push rod shroud tube Oil pressure chamber Oil supply chamber Oil hole Figure 1-30. Hydraulic valve tappets. Figure 1-31. Rocker arms.  1-22 A bearing is any surface which supports, or is supported  by, another surface. A good bearing must be composed  of material that is strong enough to withstand the pressure  imposed on it and should permit the other surface to move  with a minimum of friction and wear. The parts must be held  in position within very close tolerances to provide efficient  and quiet operation, and yet allow freedom of motion. To  accomplish this, and at the same time reduce friction of  moving parts so that power loss is not excessive, lubricated  bearings of many types are used. Bearings are required to  take radial loads, thrust loads, or a combination of the two.  An example of a radial load would be a rotating shaft being  held or contained in one position on a radial plane. Thrust  load would be the rotating shaft being contained from moving  axially along the shafts axis. These radial and thrust loads  Each valve is closed by two or three helical springs. If a single  spring were used, it would vibrate or surge at certain speeds.  To eliminate this difficulty, two or more springs (one inside  the other) are installed on each valve. Each spring vibrates  at a different engine speed and rapid damping out of all  spring-surge vibrations during engine operation results. Two  or more springs also reduce danger of weakness and possible  failure by breakage due to heat and metal fatigue. The springs  are held in place by split locks installed in the recess of the  valve spring upper retainer or washer, and engage a groove  machined into the valve stem. The functions of the valve  springs are to close the valve and to hold the valve securely  on the valve seat. The rocker arms transmit the lifting force from the cams  to the valves.  [Figure 1-31]  Rocker arm assemblies are  supported by a plain, roller, or ball bearing, or a combination  of these, which serves as a pivot. Generally, one end of the  arm bears against the push rod and the other bears on the  valve stem. One end of the rocker arm is sometimes slotted to  accommodate a steel roller. The opposite end is constructed  with either a threaded split clamp and locking bolt or a tapped  hole. The arm may have an adjusting screw, for adjusting  the clearance between the rocker arm and the valve stem tip.  The screw can be adjusted to the specified clearance to make  certain that the valve closes fully. Valve Springs Rocker Arms Bearings