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| split cotter dimensions |
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| russ57:
;-) sorry matthew, i should have noted the cotter bit was way down the page. its the best description i've found. Joe, good idea, i'll draw it out and see what i can gain. I have enough trouble drilling centrally, so i could end up with offset anyway! regards russell |
| mattinker:
Russell, here's a .pdf on split cotters, I don't know if it'll help, I found it interesting when making my split cotters. Regards, Matthew |
| andyf:
Russell, going back to your original post, I'm sure a 2mm crescent would work fine. But if you want to increase the area of contact between the two sides of the cotter and the bar they are gripping, the 4mm hole through the 8mm dia. cotter could be offset by (say) 1mm from centre, allowing you to make the crescent up to 3mm deep. Andy |
| russ57:
I should measure properly, but owing to a slight 'miscalculation ' the crescent ended up only 1 mm or so, clamps fine. I think the most important factor is the fit between the clamp body and the shaft to be locked. However, wear over time will also be a consideration. Hopefully next one will be slightly more accurate! |
| Lew_Merrick_PE:
I have been following this discussion for several days now, but have had my back to the wall with a project release schedule closing in while my customer makes those wonderful last minute changes. The project was accepted for release yesterday. I learned this technique under the name split collet, so please bear with me if I slip up in my nomenclature. The first thing to remember is that what you are making is an anti-rotation clamp. The clamp works because of the resisting moment developed by the area of contact and coefficient of friction between the parts through the force applied by the locking feature. Only those areas wherein the collet/cotter actually intimately contact the shaft can apply a resisting moment to it. The better the fit smoother the finish between the collet/cotter and the shaft, the greater the intimate contact over which the coefficient of friction will work. The force of the locking element should really be nearly irrelevant as the loads applied to such a mechanism should be fairly low in proper design practice. Thus, assuming the fit is good (boring or reaming the holes makes good sense rather than depending on the smoothness and roundness of a drilled hole), the issue almost always boils down to surface finish and coefficient of friction between the collet/cotter material and the material of the shaft being clamped. Whereas leaded steels machine really nicely, their relative coefficient of friction can become a real issue in such an application. A trick to remember is that high-zinc aluminum (i.e. 6061 alloy) has quite a high coefficient of friction with respect to steel. Does this help? |
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