REFLECTIONS

A Brief Word

| August/September 1996

Fig. 1: The Inclined Plane Cam.

Gerry Scells

In this issue you'll find a full page ad regarding our tour to Australia next February and March. As you'll note from the ad, the folks in Australia are certainly setting out the welcome mat.

This writer has long entertained the notion of going to Australia (and New Zealand). Some parts of this world we have no interest in whatever, but for some reason Australia has always held a bit of fascination. We're told that lots of folks who visit New Zealand find it so beautiful that they want to live there! Anyway, we've initially set things up for an entourage of 80 people, and from the queries we've already had, we look for this tour to sell out quickly. Thus, if you're at all interested, write or call for the tour brochure and booking forms.

Have you ever wondered about the various cam gears used on engines? Looking at different engines at a show, you'll see many kinds of cam gears. All cams of course, are a function of the inclined plane, which we all learned about in school, and as is shown in Figure 1. However, the most commonly used cam on gas engines is the tangent or properly, the tangential cam, shown in Figure 2. Especially for the benefit of model makers, simple directions for laying out this cam are as follows:

First, draw out the base circle, B. Usually this is three or four times the lift of the valve; if the valve has a lift of  1/2 inch, then the base circle will be 1 1/2 to 2 inches. Now, outside of the base circle, draw another circle that represents the clearance between the cam and the push rod roller. This is usually about 1/64 of an inch.

The next step is to draw two radii representing the angle of the valve opening. Figure 2 shows a design of 116 degrees, which is about ordinary for most gas engines. The cam is operating at half the crankshaft speed, so this gives 232 degrees of crank travel. From the two points where the radii cut the cam circle, draw tangents as shown. Finally draw the arc D across these tangents at a height which equals the valve lift. The corners of these lines are usually rounded. The points where the roller R begins and leaves the cam circle are shown at R in Figure 2.

A few engines having a symmetrical cam (a cam which is the same on both sides of its center), may be reversed. For instance, we've seen several Galloway engines that were reversed, mainly for a curiosity, at the shows. This is possible only when the cam is symmetrical. On engines where another small lobe has been added for the purpose of operating an ignition timer or other device, reversing the engine may not be possible.