Chuck jaw width: effect on gripping force?
Chuck jaw width: effect on gripping force?
(OP)
Everyone
I am involved in a workshop area where huge (up to 7 ton) rollers (at a steel plant) are reconditioned. This involves recentering after welding it up and heat treatment, followed by machining down to size.
The current cracked chuck of the recentering lathe has 4 jaws, each approximately 70mm (2.76’’) wide, 90mm (3.54’’) long (griping area). The jaws of the replacement chuck is much narrower, only 30mm (1.18’’). The machinist working for 20 odd years on the lathe refuses to use the new chuck and I must convince him otherwise.
We all know that the magnitude of the contact area between the jaw and workpiece should not affect the gripping force (friction force caused by the clamping force due to the friction coefficient between the jaw and work piece). The surface pressure obviously increases as the contact area is reduced, but the gripping friction force should remain unchanged if the clamping force stays the same. However, in the limit, if one uses 4 knife-blades (assumed to be indestructable) to clamp the workpiece, in theory the gripping force should remain unchanged, but practically there must be a limit (apart from the obvious exceeding of the work piece’s elastic stress limit).
Moreover, the driven rolls (most work pieces) have non-circular cross sections where only half of the jaw width is actually clamping to the workpiece, reducing the contact area further.
Can anyone advise me on the effect of the magnitude of the contact area between the jaws and work piece and the practical narrowest limit, or give any relevant information to either support the machinist’s view or persuade him to use the new chuck?
Regards
GJdW
I am involved in a workshop area where huge (up to 7 ton) rollers (at a steel plant) are reconditioned. This involves recentering after welding it up and heat treatment, followed by machining down to size.
The current cracked chuck of the recentering lathe has 4 jaws, each approximately 70mm (2.76’’) wide, 90mm (3.54’’) long (griping area). The jaws of the replacement chuck is much narrower, only 30mm (1.18’’). The machinist working for 20 odd years on the lathe refuses to use the new chuck and I must convince him otherwise.
We all know that the magnitude of the contact area between the jaw and workpiece should not affect the gripping force (friction force caused by the clamping force due to the friction coefficient between the jaw and work piece). The surface pressure obviously increases as the contact area is reduced, but the gripping friction force should remain unchanged if the clamping force stays the same. However, in the limit, if one uses 4 knife-blades (assumed to be indestructable) to clamp the workpiece, in theory the gripping force should remain unchanged, but practically there must be a limit (apart from the obvious exceeding of the work piece’s elastic stress limit).
Moreover, the driven rolls (most work pieces) have non-circular cross sections where only half of the jaw width is actually clamping to the workpiece, reducing the contact area further.
Can anyone advise me on the effect of the magnitude of the contact area between the jaws and work piece and the practical narrowest limit, or give any relevant information to either support the machinist’s view or persuade him to use the new chuck?
Regards
GJdW





RE: Chuck jaw width: effect on gripping force?
A few works about the elastic limit at the point of contact. In the both mentioned cases you have indentation so the maximum stress should not exceed the elastic limit of your work material (if you do not want to have marks from the jaws on your workpiece). However, in the second case this limit is achieved much faster (I mean, under mush lower clamping force).
Viktor
http://viktorastakhov.tripod.com
RE: Chuck jaw width: effect on gripping force?
RE: Chuck jaw width: effect on gripping force?
Barry1961
RE: Chuck jaw width: effect on gripping force?
The disclaimer, the jaws must be machined to the same diameter as the shaft to use the above calculation. If these are hard jaws, then you will only have line contact in one or two places and will probably mark the grip area.
RE: Chuck jaw width: effect on gripping force?
RE: Chuck jaw width: effect on gripping force?
RE: Chuck jaw width: effect on gripping force?
RE: Chuck jaw width: effect on gripping force?
You have 2 duties in solving this problem.
Your first job is solving the engineering problem. Find out why the old chuck cracked. Obtain whatever documentation you can find about that chuck. Get documentation on the new chuck as to its capacities and suitability for the work proposed.
Your second job is convincing the machinist of the new chuck's suitability and your own competence in assessing same.
You evidently don't have the complete blind trust of this machinist, his/her foreman or union representative. (Steel plant, I assume we're dealing with a union representative)
Do not try to stand there and bully him/her into submission with "rule-of-thumb" engineering. Provide documentation from the manufacturers to all parties involved. And then stand there with him/her while parts are being machined.
I think the machinist is rightly concerned after the failure of the old chuck. You are asking him/her to risk life and limb on your word. Make the effort and make your word count for something.
RE: Chuck jaw width: effect on gripping force?
I suppose I just needed some guidance in approaching this strange problem. I realize that it sounded like a stupid question, so I appreciate your professional opinions. For interest sake: Barry1961, the chuck is not half size from the one it will replace, it is just the contact end of the jaws taper down to a smaller contact area. In other words, the chuck is the same size, just the contact areas are half size.
Regards
GJdW