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 produces mechanical noise, which some
believe adds to the charm of the running engine!
Source Code for the Curious
This program is a PHP translation of an earlier Java language version written
as a command line utility for personal use.
To make it suitable for a wider community, the utility was "wrapped" in a
Java Applet. While this worked and was easy for me, the numbers were effectively
"painted" to the graphics display, not printed. This meant the data could not
be cut and pasted from the screen, and some browsers chopped off the bottom
of the page when printing.
Enough requests to fix these limitations were received to do a rewrite.
For this, PHP was chosen. This means that the code MUST be served by a PHP
enabled server. The other choice would have been ECMA script (ne Javascript).
While this would have permitted off-line use totally inside a browser, the
PHP option was chosen as being simpler due to cross-browser script language
issues and the impact this has on testing.
If you want to see the PHP code, just do a "view source" on the pages involved,
or issue a "wget". For the Java source, see here.
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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