Rooster's Revenge

In the spring of 1998, we witnessed an epidemic of commercial aerial cam unit failures in the field. These off-the-shelf cams were relatively new to the automotive industry, replacing the time-honored practice of engineering and constructing home-made aerial cams.

We discovered that the cams were failing because the dies were engineered with no cam pad for stripping the cam steels. The slide return spring had to do all the work. In many cases, the spring was inadequate and the cam failed.

I personally contacted every supplier of commercial aerial cam units in the industry world-wide seeking an equation to quantify slide return force in aerial cams. The response was universal: there was none.

My personal mantra is find a way or make one. I had to make one.

I created an equation to quantify slide return force in aerial cams. This equation works regardless if there is a cam pad or not.

The equation for aerial cam slide return force is:

• Fasr = [Fs / cos(α + β)] + {(0.00981 • ms) • [0.20 • (cos β - sin β)]}

where:

• Fasr = aerial cam slide return force (kN)
• Fs = stripping force (kN)
• α = upper driver angle from horizontal (degrees)
• β = work angle from horizontal (degrees)
• ms = working slide mass (kg)
• 0.20 = coefficient of friction
• 0.00981 = force due to gravity

NOTE: if there is a cam pad, then the stripping force (Fs) required by the aerial cam slide is equal to ZERO.

For example, assume an aerial cam has a working slide with 40 kg mass with an upper driver angle of 30° from horizontal. The work angle is 20° from horizontal and 4.65 kN stripping force is required. There is no cam pad.

The required slide return force is:

• Fasr = [Fs / cos(α + β)] + {(0.00981 • ms) • [0.20 • (cos β - sin β)]}
• Fasr = [4.65 / cos(30° + 20°)] + {(0.00981 • 40) • [0.20 • (cos 30° - sin 30°)]}
• Fasr = (4.65 / 0.6428) + (0.3924 • -0.1541)
• Fasr = 7.17 kN slide return force

If the aerial cam slide return spring does not have the required force capability, then either a supplemental pressure system or a cam pad will need to be added to the cam.