LO 9-6 Identify and explain the purpose, construction, and operation of the camshaft
1. Camshafts
LO 9-06: Identify and explain the purpose, construction, and operation of the camshaft. |
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Camshafts Camshafts are critical components affecting engine performance because they control the gas dynamics or engine breathing characteristics Figure 47. They are commonly made from nodular cast iron. More recently, hollow camshaft tubes are made with eccentric lobes that are pressed on the shaft. This construction technique helps to reduce engine weight and inertia forces in the camshaft exaggerated by torsional vibration. Solid camshaft lobes may be occasionally induction hardened or even nitrided to withstand high unit loading. Cam lobe profiles are ground to create quieting ramps that minimize valve velocity when seating valves Figure 48. Valve timing (opening and closing in relation to crank position), valve lift, duration, and lobe separation are established by the cam profile. If a camshaft is shared between different power ratings of the same engine model, offset keyways on the cam gear may change valve timing. Cam bushings are used to provide a bearing surface for camshaft journals Figure 49. |
FIGURE 09-47: A typical diesel camshaft. Note the three lobes per cylinder. One is for the injector, and the other two are for the valves. |
FIGURE 09-48: Nomenclature of a camshaft lobe. The closing ramp slows the valve velocity as it reaches the valve seat. |
FIGURE 09-49: In this low-mounted, in-block camshaft, the cam bushings provide a bearing surface for the camshaft journals. |
FIGURE 09-50: Timing marks on the geartrain are used to synchronize the valve operation with the piston position. |
All camshafts require correct synchronization with the engine crankshaft to ensure that the valves open at precisely the correct time in relation to piston position. Timing marks are stamped into the geartrain and are aligned during assembly to establish correct camshaft-to-engine timing Figure 50. A low-mounted camshaft will typically use a single camshaft gear that rotates at half the engine speed to engage the engine’s crank gear.
If a helical-cut gear drives the camshaft, a thrust washer is placed behind the cam gear to prevent the gear from wearing into the cylinder block or head.
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