On the press upstroke, the main pad must hold the stamping on the lower tool until the cam steels clear the stamping.

The simplest way to ensure the proper timing of the main pad when aerial cams are present is to calculate the main pad travel.

The equation for main pad travel with aerial cams is:

• Tpad = ESM + {[Tacs • sin(α + β)] / cos α }

where:

• Tpad = main pad travel (mm)
• ESM = engineered safety margin (mm)
• Tacs = aerial cam steel travel (mm)
• α = upper driver angle from horizontal (degrees)
• β = work angle from horizontal (degrees)

NOTE: if there are multiple aerial cams, then calculate for all and use the greatest result for the main pad travel. Also, if the direct cutting or forming requires more travel than the aerial cam(s), then the direct operation will drive the main pad travel.

For example, assume an aerial cam has cam steels with a work travel of 12 mm. The upper driver angle is 30° from horizontal. The work angle is 20° from horizontal. The engineered safety margin on the main pad is a minimum of 5 mm. The direct steels have 10 mm work travel.

The required main pad travel for this die is:

• Tpad = ESM + {[Tacs • sin(α + β)] / cos α }
• Tpad = 5 + {[12 • sin(30° + 20°)] / cos 30°}
• Tpad = 5 + {[12 • sin50°] / cos 30°}
• Tpad = 5 + (9.19 / 0.866)
• Tpad = 5 + 10.61
• Tpad = 15.61
• Tpad = 16 mm minimum

This tool requires a minimum main pad travel of 16 mm so the die operates properly with the aerial cam. Since the direct steels have only 10 mm work travel, the 16 mm minimum main pad travel will work fine.

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.