Chuck jaw width: effect on gripping force?

[GJdW]

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

 

[Viktor]

You asked a simple yet complicated question. Although theoretically there should be no difference, practically it is not quite so. If the jaws of your chuck are flat then you have only one point of initial contact which eventually become an area as the clamping force increases and workpiece deforms elastically with the growing contact stress. In practice, however, the jaw’s clamping area is not flat but a part of a cylinder of certain diameter. Hence, two different cases are possible in this situation. First, if the diameter of your workpiece is less that that of this cylinder then you have theoretically only point contact at the highest pint (middle) of the jaw. If otherwise, you have two points contact by the edges of the jaw. What I would recommend you to do is to draw in scale (using any CAD) the jaw and the workpiece and see what kind of contact you have now and what would happen with new jaws. I suspect that you might see the real picture which helps you to make an intelligent decision.

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

 

[ornerynorsk]

Is the new chuck rated up to that workload. I am a machinist also, and the first thought that would cross my mind is: the old chuck broke, the new chuck has smaller jaws . . . HELLO!!! So, you need to show your machinist that the chuck is rated for that workload.

 

[Barry1961]

I would have to agree with the machinist on this one. If the old chuck jaw failed and it was twice the size..... I would guess that if the jaw are half size there are other parts not as heavy duty also which may lead to deflection, deformation and slipping.

Barry1961

 

[EdDanzer]

When gripping a part the contact area must be large enough to not exceed the yield strength of either part. If the chuck is rated for the part weight then calculating the actual contact area is all that is left. If these are hard jaws then the shaft will probably be the softest part. If they are steel soft jaws then the yield should be at least 30,000 psi. A contact of 3.54 X 1.18 X 30,000 will support 125,316 lbs of weight, sufficient for this part. If the jaws are aluminum then you must do a hardness test to determine the yield strength.
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.

 

[EnglishMuffin]

When a guy in the shop says there is a problem, then 9 times out of 10, in my experience anyway, there's a problem! What he says the problem is, or his proposed solution of it, is often invalid. But somewhere in there, there's likely to be a problem - its just a case of quantifying it precisely. Echoing Barry1961's comments, it may just be the general wimpiness of the jaws themselves that is worrying this guy. I do know that there are a lot of dead and maimed people out there because large parts came out of chucks - it's a frightening thing to see and hear!

 

[ornerynorsk]

Englishmuffin, shame, shame, shame!!! There are plenty of us "lowly" machinists who are quite adept at pinpointing mechanical problems. As you know, behind every successful engineering team is a good machinist! Nuff said.

 

[EnglishMuffin]

Where did I say that there were not plenty of machinists who are adept at pinpointing problems? They frequently do not have the time or inclination to do this - it's not their job, but that doesn't necessarily mean that they are not potentially adept. And where did I use the term "lowly"? I would claim to be a reasonable machinist myself, but it's a funny thing - when I'm doing machining, I don't think like an engineer - I become an artisan, picking feeds and speeeds by instinct and feel, listening to the sounds the machine and process are making ("that doesn't sound right" etc) and making instinctive adjustments. I think machining is an art to some extent, like playing a musical instrument or driving a race car, and it's not necessaily the job of such practitioners to "pinpoint" problems. After all, if this particular operator had "pinpointed" the problem, assuming it exists, we wouldn't be having this discussion.

 

[funnelguy]

GJdW,
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.

 

[GJdW]

Thank you everyone.

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