Split cotters

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Thanks for the measurements, and the very informative photos, Nick

[ in the eternal quest for understanding ] Just one question, if I may :

Given the obvious absence of a picture of the second cotter … Is it ‘fully floating’ [as per the Duplex sketch] or is it guided/located in some way ?

MichaelG.

Hi MichaelG, the second cotter is fully floating, but the hole in the casting on that side is only about 10mm in diameter, I guess this is to allow air to escape for it to move OK.

Regards Nick.

Thanks for that, Nick … Much appreciated

MichaelG.

Hi, I've had a bit of a senior moment in the measurement between the pillar and the cotter bolt. Instead of it being 3 / 16 inches, it should in fact say 3 thou. Sorry if you were all scratching your heads about a big mistake.

Regards Nick.

Derek, a bit of practical advice here **LINK**

From a practical standpoint, split cotters would seem to prevent movement of the main bar, in two ways.

1 ) The two halves of the split cotter grip the bar like brake shoes, and being a close fit in their bore prevent rotary or linear movement of the bar.

2 ) By clamping against the bar, and having a gap between the ends, there would be a wedging action which exerts a force between the one side of the collets and their bore, forcing the bar outwards against the bore in which it is located. The frictional forces resulting will tend to prevent the bar from angular or linear movement.

If either of these hypotheses is correct, the split cotter does what is required of it, in preventing movement in any plane, once clearances have been taken up.

If anyone is so inclined, reference to the dimensions of a particular arrangement can allow the forces to be calculated, having made assumptions about the coefficients of friction of the materials and threads involved.

Howard

Is there any theory/rule of thumb about the best diameter for the cotter compared with the shaft it locks. All the examples shown have the cotter smaller than the clamped shaft. Is it just convenience, or is there something about the clamping action that suggests a particular ratio of diameters?

I want to lock a 10mm steel shaft, and was planning to use a 10mm brass cotter. Would that be too big, or just unnecessary?

No it doesn't prevent rotary motion, nor linear motion until the two halves are pulled together. As long as there is a few thou of gap between the halves when tightened they will firmly clamp whatever they are pressed against.

As far as a diameter for the cotter is concerned a larger area for the clamping surface simply means that the force required to lock a shaft is less than a small area.

This is 14 mm diameter cotter intended to secure the block to a 35 mm diameter post using an M6 screw. That gap is just under 2 mm.

Friction is a function of force, not area. A larger area won't make any difference.

Edited By Mark Rand on 03/07/2021 20:01:46

My reading of the action is that the cotter acts as an efficient force multiplier when the effective taper angle is shallow. Moving the taper inwards forces the main bar hard against the wall of the bore on the opposite side to the cotter. A steeper taper angle (as in a larger cotter) will reduce the mechanical advantage and require more force to be generated by the nuts or ball handles for the same grip. Just considering locking action cotters could be just plain wedges but curved surfaces are kinder to the bar and don't generate marks and dimples to the same extent a plain wedge would.

regards Martin

John Baron,

That was exactly the point that i was making, that when clamped to the bar angular and linear movement is prevented. The whole purpose for using split cotters, surely?

Howard

Yes I agree ! I misread your post then couldn't edit mine. I do apologise.

Hi John,

NO problem! Won't be the first time that I have misunderstood something, nor the last!

Would suggest not using brass for the split collets. The object is to grip the bar / quill, so the coefficient of friction between brass and steel will be less than steel against steel.

A M6 thread can exert quite a force, so a 10 mm diamter cotter should grip quite well (Assumes that the 10 mm refres to the cotter..

If the shaft to be locked is 10 mm diameter, the forces involved will be relatively low, so you are probably thinking in terms something more like a 4 mm cotter, with possibly a M3 thread. (Which is relatively big, possibly 2 mm or a good old 4 or 6 BA )

Howard