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| How to test the gripping force of a chuck? |
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| awemawson:
I've been installing a hydraulic chuck on the Traub CNC lathe, and it occurred to me that to have some means of testing gripping force would be mighty handy. I've only operated it on much reduced hydraulic pressure at the moment during testing, but even at 5 bar as opposed to it's intended 22 bar I cannot move a bar tapping axially with a copper hammer - but that's a bit of a crude test! Problem is there are too many variables. The jaws move a maximum of 3 mm, and being bored jaws are seated on serrations and bolted on, so themselves are movable. Only method that I can come up with at the moment is to turn up a slug of lead, or other very malleable metal, grip it with the chuck, release and examine the (presumable) deformation. Then do the same with a manual chuck of similar size and compare them. I suppose some sort of deformable tube filled with hydraulic oil and equipped with a suitable pressure gauge is another possibility? Any suggestions welcome. |
| awemawson:
:coffee: Google has found me this clever little gizmo: http://www.schmachtl.cz/web_get_img_data?aID=533906 I'd imagine there are three pistons the the jaws bear on with lip seals communicating with a central chamber that the gauge attaches to. Now can some mathematically versed member tell me how to relate the pressure read on the gauge, to the force on the three pistons. Piston area obviously influences it. Working in Imperial units if (say) we have a pressure of 100 lbs/sq in reading on the gauge, each piston contributes 1/3rd so 33.333.. lbs sq in per piston, so (to make calculation easy) if we have a piston area of 0.333.. sq in is that a force of 100 lbs on each jaw. Or is it more complicated than that? It's a bit like the 'Dead Weight' testers you can get to calibrate gauges I suppose, but I've never used one or seen how that they are set up :scratch: |
| Lew_Merrick_PE:
Andrew -- If your distribution pressure is (say) 100 psi, then each piston is pressurized to 100 psi. The piston area (i.e. cylinder bore) X the pressure (100 psi by your example) is applied to each piston. Thus, if the piston is ø.500 inch, then the piston area is (.25²*pi =) .1963 in² and, for a 3-jaw chuck, this would give you a total clamping force of (3 * 100 lbs/in² * .1963 in² =) 58.9 lbs. |
| awemawson:
Thanks Lew for your reply. It doesn't look to be beyond the capabilities of a Madmodder to make such a device :ddb: I suppose the issue is to size the pistons so that a standard gauge reads sensible figures. I'd imagine lip seals are available in a multitude of sizes and it's be a case of choosing one that works out sensibly mathematically and making cylinders and pistons to suit. Bleeding all the air from it might be fun. |
| Lew_Merrick_PE:
Andrew, Dual o-ring seals should be sufficient for most hydraulic pumps (3 ksi or lower pressure). I often design piston parts for pyrotechnic actuators that see something in excess of 60 ksi for short periods of time that allows me to design o-ring seals (30% compression) that use a single o-ring. In essence, you want a "gap" between the piston & cylinder of .004/.002 inches. I have (and am still "debugging") a spreadsheet for designing both static and dynamic o-ring seal glands from (SAE, NAS, and MS) standard o-ring sizes and materials. If you need a hand in such an effort, drop me a line. Basically, for a male gland piston/bore seal I need: Minimum ID of Bore, % of compression desired (which comes from SAE tables for desired pressure loads), and the o-ring material you would like to use. Everything else is (almost) automagic. This is a back burner project that moves ahead by fits and starts (and stops). |
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