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The greatest tool-retention holding power of the Lucas PTE system occurs when the area of contact on the outer cone of the Collet equals the area of contact of the inner cone of the Collet. At any point other than this, a possibility exists that one part will begin to 'bed' thru the layer of hardness into the mating part and allow the mechanism to loosen.
The Lucas mechanism, however, was NOT originally designed at the point of greatest surface area. It is designed for approximately equal width of contact on the outer cone and the inner cone. I suspect that it was designed in this manner due to the amount of excessive calculation required for equal area and the difficulties involved in performing those calculations manually - as they were surely done when the mechanism was originally designed. (I have the advantage of spreadsheets).
The cutting force required to extract a clamped tool is greater than the force applied by the spring washers. This is due to the need of the cutter pressure to 'back drive' the 3-degree differential angle of the Collet (which has a 37-degree outer angle and a 40-degree inner angle - resulting in a 3-degree differential angle). The force generated by the 'back drive' must, additionally, be greater than the force provided by the spring washers. The amount of resistance to tool extraction depends more on the necessity to 'back drive' the 3-Degree angle than on the amount of spring force that must be overcome by the 'back drive'.
Machinery's Handbook states that the formula for determining this force is:
P=W * Sin (3-Degrees)
P=W * 0.05234
Where P= the 'back drive' force and W= the 'squeezing' force applied to the Collets 3-Degree differential angle due to the cutting pressures. Due to the nature of the formula, only approximately 5% of the 'squeezing' force becomes a 'back driving' force against the spring washers.
The Collet is in the design position (equal contact width) when the inner bore of the Collet is at the 'Trial Rod' size. Therefore, the Drawback Collet Adapter (draw stud) position when properly 'set' to the PTE Gauge must place the Collet at the design position for maximum holding force.
It is the nature of springs (with certain exceptions like the Negator) that the amount of stored mechanical pressure increases as the deflection increases. Therefore, the point of maximum PTE spring pressure is obtained at 100% compression (flat) and the point of minimum spring pressure is obtained with the springs completely released. The greatest point of spring pressure in an assembled Lucas PTE is developed at the Eject position and spring pressure becomes progressively lower as the Collet draws further 'in' and the springs release. Please note that the point of maximum tool holding power does NOT occur at the point of greatest spring pressure.
Many shops make use of a 'gauged' tooling arbor that is equipped with a hydraulic pressure gauge to read mechanical force pulling 'outwards' on the Drawback Adapter. This 'gauged' tooling arbor reads the pressure stored in the spring washers and it's reading is NOT indicative of the holding power of the Proprietary Lucas PTE system. The Lucas proprietary PTE system was not designed to use the gauged arbor. The gauged arbor was designed for use with other tooling retention systems.
The correct point of maximized tool retention may be determined with certainty by purely mechanical methods as described in the so-called "Pry Test". Do NOT use a 'gauged' tooling arbor!