How to test the gripping force of a chuck?

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awemawson:
Pekka, although I've seen that Youtube video, the one I'm thinking of isn't youtube but a manufacturers demo one. It shows a device measuring the force and remotely monitoring it by wireless transmission on a hand held display. As the operator increases the spindle rpm the display shows the gripping force almost halving at top speed given a constant operating pressure :bugeye:

Phil, my machine hasn't got the facility to change pressure under program or panel control - it's a 19 mm spanner on the regulator :ddb: So really I'm only wanting to check it statically. I have to say that the little 3 cylinder hydraulic device looks quite easy to make, and even accepting that it won't be calibrated it will be a very useful comparative device. Apparently as the operating ramps in the chuck degrade the grip reduces rapidly, though to be honest it probably is a 'non problem' with my low usage.

Lew_Merrick_PE:
OK, the chuck jaws have a given mass (measured in either kg or lbm) and a specific center of mass (also called center of gravity). If you have these to values, the acceleration they undergo is given as a = v²/r where r is the distance in m or inches from the center of the spindle to the center of mass. v is the rpm of the spindle * r * 2 * pi / 60 and will end up in either m/sec or in/sec. Thus, your acceleration ends up as: a = 4 * pi² * rpm² * r / 3600.

1 lbm X 1 g = 1 lbf. Thus, if you are using lbm and inches, your force per chuck jaw will be: F = ma = (M lbm * 4 * pi² * rpm² * r / 3600) / 386.09 -- where 386.09 is 1 g in inches/sec².

The caveat here is that though a kg is supposed to be a measure of mass, the unit kgf, which is 1 kg X 1 g has become quite common in recent years and, if your measure was in kgf, then the acceleration needs to be divided by 9.807 m/sec² to turn it into a measure of gravities. I work with a number of European-based companies that regularly supply "mass" values in kgf -- and often do not distinguish between mass and force which leads to major confusion (especially as the "proper" measure of force in the metric system is that worthless measure the Newton -- which is why the kgf has come into usage)!

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