The Effect of Tappet Clearance
Now for a small dose of pragmatism. Anyone who has seen a New-In-Box model four-stroke will probably have noticed the little bag of tools that accompanies it. One of these tools is a feeler gauge that is used to set the tappet clearance. Generally, this is performed by loosening the tappet adjusting screw in the rocker and adjusting its position until the feeler gauge slips firmly but freely between the rocker nose and valve top. Obviously this introduces some "slop" into the train of parts that actuates the valve.
This clearance is very necessary in order to account for thermal expansion of components, principally the valve itself. If this is not done, expansion may lift the valve off its seat—not good for hopefully obvious reasons. The cylinder will also expand, although in most geometries, this will tend to compensate for valve expansion. But keeping the valve head firmly on its seat when not "on the cam" is of prime importance, so the tappet clearance is very necessary. The amount of clearance depends on many factors, but given the sizes and materials typical of our model engines, 0.002" to 0.005" is common.
Introducing tappet clearance effectively places a gap between the cam-follower and the cam. In the diagram here, the gap induced by the tappet clearance has been exaggerated for clarity. Recall that in designing our cam, we specified the opening angle so that the flanks would be tangential to the base circle at the points defined by the cam opening angle, under the assumption that the cam follower would faithfully and exactly follow the cam profile and lift would commence at the desired points (the green lines).
By adding tappet clearance, there will now be a delay before the follower contacts the flank (the red lines). In the diagram above, a clearance of 0.010" has produced the rather extreme reduction of almost 50° in the cam opening angle. This represents a 37% reduction from the design figure. If clearance in the drawing—which depicts the Westbury Kittywake exhaust cam—were reduced to the required 0.004", the actual reduction in the cam angle would still be some 14% or 20° with the cam opening angle being 114.7° compared to the 135° we thought we had.
One cure for this problem is to relieve or undercut the base circle by the required tappet clearance. Note we are not reducing the base circle. The cam flanks must still be tangential to the base circle in order to provide the required opening angle. The relief is introduced by a cutting a tangent between the end of the cam flanks on the base circle radius and the relieved base circle (as shown in this diagram). How accurate do we have to be? Well, at model sizes, it's all a bit approximate unless you are a serious performance builder—and hence unlikely to be reading this for more than idle curiosity .
The actual tappet clearance will vary with the engine's operating temperature, and to a degree, so will the cam durations. But from the figures shown above, the need to introduce an allowance for the tappet clearance in the base circle while maintaining the correct location of the flank ends should be obvious.
There is another cure: increase the "distance" between the open and close points to compensate for the tappet clearance. This is not as "hit and miss", if you'll pardon the pun, as it sounds, especially if you can model the thing in a CAD package. All the usual problems of juggling flank radii and nose radius to achieve the new cam angle still apply, and the acceleration figures produced by CamCalc will go out the window as they assume tangential cam contact starting at the flank to base circle intersection point. In both cases, there will be a hammering effect as the clearance is taken up. That procudes mechanical noise, which some believe adds to the charm of the running engine!
Source Code for the Curious
For the curious, and in the spirit of the Open Source community, the links below provide access to the Java source code for the command-line program that generates the table, and the Applet wrapper used to execute it from this HTML page (use your browser's "view source" if you want to see how the applet is executed).
The approach used in the CamCalc program was derived from an article written by Mr Roderick Jenkins that appeared in in Strictly Internal Combustion magazine, Volume 8, Number 18, dated Dec 1990/Jan 1991. Back issues of SIC are available from the editor/publisher.
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