DieselShip

Cams

The basic purpose of a cam is to convert rotary motion into reciprocating motion in order to actuate some mechanism. For an engine this usually means the operation of a valve or pump. A cam must be hard enough to withstand the considerable forces exerted upon it but it must also be reasonable resilient. For these reasons cams are generally made from surface hardened steel. The exception is the indicator cam which is usually made from cast iron as the loading are small.

Couplings are provided at each cylinder section of camshaft, these couplings being shrink fits with hydraulic adjustment capability. The advantage of having sections of camshaft is that it allows cams, couplings, to be removed and replaced more easily then would be the case with longer shaft sections.

The Sulzer engines employ a different method of cam fitment. A hub is keyed and shrunk onto the camshaft and the cam fits onto this hub being held in place axially with a nut. The cam is secured against rotation by means of radial teeth on both hub and cam, and since there are 360 of these teeth the cams may be altered in one degree steps. The profile of a cam, including the leading or rsing edge, the dwell period at the op, and the trailing or falling edge are all profiled to give the corect rate and duration of movement for the equipment they are operating .The rate of rise of the leading edge of the cam governs the speed at which the valve or pump operates. Too slight and operation may not be crisp, too steep and undue loading may occur.

Critically profiled cams , especially fitted for operating the fuel pump, may be used. In this the leading edge of the cam is critically profiled to give a requisite flow variation to suit engine makers fuel delivery requirements.

In the case of mechanically operated fuel valves on the Doxford timing block the lift only needs to be small and the cam profile may be designed to suit the rate of change required. With such a system there is no need to provide a usual cam needed. This insert is generally held into place by set-screws and slotted holes in the insert allows the cam to be adjusted.

Some followers do not run on the base circle of the cam, stops being used hold the follower clear. This is said to minimise wear and avoids problems due to the screw holding the cam insert in place.

By far the most common method for fixing cams is by hydraulically floating the cams onto the shaft. o-rings seals being provided for that purpose with the high pressure oil supplied from an external pump. When hydraulically floated the cam may be rotated into position.

author note:

on a large bore B&W one of the exhaust valve gear operating cams slipped causing severe engine running problems.No gear was on board for hydraulically floating the cam so the engineers managed to rig a system of chain blocks whereby they where able to drag the cam back into position as an emergency repair. Next port a makers representative oversaw proper repair. He never did accept that it was possible to move the cam by this method!

On a valve operated by direct contact with the cam or via a pushrod and rocker, there must always be tappet clearance in order to allow for thermal expansion of the valve during engine operation. That tappet clearance must be correct, too much and the opening period and timing can be altered, too little and the valve might not fully close.

Camshaft bearings for most large engines are of the white metal type. This not only allows for more convenient replacement and adjustment but also allows an oil wedge to build up, that oil wedge restricting the hammering effect on the bearing. Ball or roller races would be subject to considerable brinelling damage. Bearing weardown reduces the effective lift of both valves and pump plungers and so weardown must be corrected as soon as it reaches recommended limits.

Author note: Spalling damage was noted on what was believed to be the leading edge of cams on a daihatsu medium speed engine. Correspondence with the makers regarding the possibility of damage being caused by the follower slamming down on the trailing edge of the cam drew denials.

It was later found that the damage was actually on the leading edge of the cam. As the damage was so severe as to alter the profile of the cams repair was by replacement. On this engine the cams where mounted on individually sized tapers increasing in diameter away from the end the cams where fitted on. The cams where locked into position and jacked off by nuts fitted on threads located either side of the taper. An excellent system making adjustment to timing very simple.