Various Shapes of CAMS

Various Shapes of CAMS Points : various shapes of cams, problems of camshaft and its drive system, problems in internal combustion engines, ic, engine, auto diesel Referring to the three diagrams shown, it will be seen that each cam consists of a circular portion corresponding to the closed or ‘no lift’ period and a projecting portion, which gives the lifting action to the valve. The angular portion marked P shows the period of the valve opening. Since the cam operates at half engine speed the equivalent crankshaft angle to valve opening period will be 2P. The maximum lift of the valve is indicated in the diagram and is equal to the difference between the outer cam radius and the base (or no lift) circle radius.

The shape shown at (A) gives lift to the valve since immediately after the flat end of tappet rests on the flat pan of the cam, the curved peak portion lifts the valve quickly, but maintains the valve fully opened for a very short period: the use of large area valves of limited maximum lift is associated with this shape of rapid acceleration lifting and closing cam.

The sides of the cam can he curved, instead of flat, as shown at (B). This shape gives a more gradual opening (or acceleration) and therefore a greater effective opening and closing act ion than for (A), while retaining the same total opening period, the shape shown at (C) gives a very quick lift and fall to the valve, with a longer period maximum opening. This shape of cam is used for high output, e.g., racing engine but is apt to be noisy in action, it will be observed that it is necessary to use a roller-ended tappet for this shape of cam has marked influence on the power output of the engine; the best shape is that which gives the quickest (practicable opening and closing, consistent with the maximum allowable period of opening. The sides of the cam must not be too steep, however, or there will be too much noise at opening and closing, due to impact, too much side force on the tappet or rocker cam and the tappet will tend to “jump” or leave the cam during the opening period, thus necessitating the use of stronger valve springs to keep it in contact.