Figure 12-24.-L-head valve operating mechanism.camshaft rotates, the cam lobe moves up under the valvetappet, exerting an upward thrust through the tappetagainst the valve stem or a pushrod. This thrust over-comes the valve spring pressure as well as the gaspressure in the cylinder, causing the valve to open. Whenthe lobe moves from under the tappet, the valve springpressure reseats the valve.On L-, F-, or I-head engines, the camshaft is usuallylocated to one side and above the crankshaft; in V-typeengines, it is usually located directly above thecrankshaft. On the overhead camshaft engine, such asthe Murphy diesel, the camshaft is located above thecylinder head.The camshaft of a four-stroke cycle engine turns atone-half engine speed. It is driven off the crankshaftthrough timing gears or a timing chain. In the two-strokecycle engine, the camshaft must turn at the same speedas the crankshaft so that each valve may open and closeonce in each revolution of the engine.In most cases the camshaft will do more thanoperate the valve mechanism. It may have extra cams orgears that operate fuel pumps, fuel injectors, the ignitiondistributor, or the lubrication pump.Camshafts are supported in the engine block byjournals in bearings. Camshaft bearing journals are thehugest machined surfaces on the shaft. The bearings areusually made of bronze and are bushings rather than splitbearings. The bushings are lubricated by oil circulatingthrough drilled passages from the crankcase. Thestresses on the camshaft are small; therefore, thebushings are not adjustable and require little attention.The camshaft bushings are replaced only when theengine requires a complete overhaul.FOLLOWERS.— Camshaft followers are the partsof the valve-actuating mechanism (figs. 12-24 and12-25) that contact the camshaft. You will probably hearthem called valve tappets or vale lifters. In the L-headengine, the followers directly contact the end of thevalve stem and have an adjusting device in them. In theoverhead valve engine, the followers contact thepushrod that operates the rocker arm. The end of therocker arm opposite the pushrod contacts the valve stem.The valve adjusting device, in this case, is in the rockerarm.Many engines have self-adjusting, hydraulicvalve lifters that always operate at zero clearance.12-20
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business