Page 66 - Cam Design Handbook

THB2 8/15/03 12:48 PM Page 54

54 CAM DESIGN HANDBOOK

and stopping action and no abrupt change in acceleration, is the double harmonic curve.
If it can be manufactured with the accuracy necessary, it is suggested as the best basic
curve for the DRRD cam.

DRRD Cam—Asymmetrical Rise Return
In this type of cam, we may observe another difficulty at the point of maximum rise, as
shown in Fig. 2.16b. Using the simple harmonic motion curve as an example, we plot
the action with the rise occurring in a shorter time or cam angle than the return. At the
peak rise point, we observe an undesirable abrupt change in acceleration, and vibrations,
noise, and wear result. This abrupt acceleration may be ignored at moderate or lower cam
speeds.
Figure 2.17 shows the DRRD automotive racing car camshaft characteristic curves at
the initial stage of development. There is an undesirable dip in the negative acceleration
position. A final design had a smooth acceleration curve by modifying the exponents of
the polynomials.

Acceleration

Opening side Closing side

Dip

Nose
Cam angle
FIGURE 2.17. Automotive camshaft characteristic curves (preliminary design). (Courtesy Elgin Cam
Company, Redwood City, Calif.) (Note, final design will have no dip.)