Split cotters

[Michael GilliganParticipant @michaelgilligan61133]

Thank you for your valuable contribution, Ian

… it came like the proverbial breath of fresh air

[ or was it perhaps just wind ? ]

MichaelG.

[CirclipParticipant @circlip]

Hope the weather is good in Wales this weekend, my next door neighbour has gone fishing with his mates.

Regards Ian.

[duncan webster 1Participant @duncanwebster1]

What a performance to add a clip from a CAD drawing, but here it is. As you will see I've drawn the clearance exaggerated, but the principle holds, it contacts first at the outboard end

 

Edited By duncan webster on 26/06/2021 10:22:25

[Martin KyteParticipant @martinkyte99762]

Posted by Michael Gilligan on 25/06/2021 21:35:57:

As promised, Martin : Here’s my version of the clamping geometry

… with the bar shown smaller diameter, for clarity.

Assume that the bar is ‘grounded’

• Tightening the clamp screw moves the two pads
• That in turn moves the whole block sideways
• Clamping then takes place at three lines [running into the page] with locations roughly indicated by the tips of the red arrows.
• Because the clamp radius and the bore in the block are [in reality very slightly] larger than the bar radius, there can be no other contact … it’s just those three lines.

With appropriate dimensions; these movements, and the clearances created, will be small : but they do say “a miss is as good as a mile”

MichaelG.

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Edited By Michael Gilligan on 25/06/2021 21:38:42

Hi Michael

I don't disagree with what you say and when the pad bolts are machined in situe when the main bore is bored then the arrangement will be pretty much as you describe. However even when machined as just stated there must be some clearance in the pad bolt bore itself and tightening the bolt may well move the line if contact quite possibly inwards.

To return to the situation of the OP that of the front bar of the Quorn it is impractical to machine in situe and a fixture is made to generate the pads seperately. In fact it cannot be guarenteed that the offset of the pad bore to the main bore in the fixture is exactly the same as that in the finished clamping system. In consequence the actual relationship of the curve on the pads may not be absolutely concentric with the main bar and it is anyones guess where the contact line will actually end up when fitted.

Taking the above statement to be the case. We should design for worst case which is when the line of contact is at the inner most end of the pad as this is the point of shallowest taper. Therefore if we use Duncan's formula of G = D/8 wherever the line of contact ends up there can be no wedging action and the cotter will always self release.

As I said I agree with your analysis for a perfect assembly, but even in that case parts wear and the wear on the pad will tend to eventually conform it to the radius of the main bar whereby your slightly larger radius on the pad disappears and contact is possible towards the inner ends of the pads. I'll ignore flexibility and material compression.

In practice locking is encountered at times. I have made a number of pad bolts and it can happen. My initial post was intended to show that offset was there just to allow clearence for the bolt and had nothing to do with the clamping action. This got us onto what does matter to clamping and to the subsequent debate.

So my conclusion is that whilst lockup will not normally happen if the system is made perfectly as you describe and is in an unworn condition, to ensure that it can never happen when that is not the case, having a Gap equal to one eigth of the main bar diameter is good practice, especially when the pads have not been machined in situe in the first instance.

regards Martin

Edited By Martin Kyte on 26/06/2021 11:15:07

[david bennett 8Participant @davidbennett8]

Posted by Michael Gilligan on 26/06/2021 06:01:08:

Posted by david bennett 8 on 26/06/2021 01:24:46:

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What makes you think it was addressed to you? It was a general comment on the thread.

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Only the fact that it immediately followed my post.

Thanks for the clarification

MichaelG.

Edited By Michael Gilligan on 26/06/2021 06:24:01

I think you will find my post was timed BEFORE yours.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by david bennett 8 on 26/06/2021 13:26:50:

I think you will find my post was timed BEFORE yours.

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That’s rather odd, David … I do see your point

It appears to be timed before, but displayed after

… no idea how or why !!

MichaelG.

.

[Martin KyteParticipant @martinkyte99762]

PS I have realised I cannot spell situ

:O)

Martin

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Kyte on 26/06/2021 14:47:57:

PS I have realised I cannot spell situ

:O)

Martin

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No problem, Martin … it's the thought that counts !

Thanks for this morning's note … I do take your point about wear of the pads having the potential to change the geometry, and I must confess that I had only considered the 'as made' condition.

MichaelG.

[derek hall 1Participant @derekhall1]

Hi everyone,

As the person who started this thread I am been following the comment with interest and thanks again for all of your input.

I "think" we have at the answer to the question!

But here is another question, what about the material of the cotters?. I have always made them from brass to lock onto steel shafts, but Hemingways mention the cotters should be made of steel for the Quorn, as steel is more effective at locking. While this maybe true I will probably make mine out of brass for my Quorn as I have not had any issues with brass cotters and steel shafts on other equipment…

Regards

Derek

[Martin KyteParticipant @martinkyte99762]

Hi Derek

I made mine from steel for what it's worth. The coefficients of friction are given as 0.51 brass to steel and 0.5 to 0.8 steel to steel. I mentioned earlier that I can get the front bar to lock by finger tightening the cap head screw (used for a temp try out) so with the ball handles it will just require nipping up. My preveous version had way too much movement on the ball handles with the slit casting design. Because of the limited space for the handles I feel that the cotters will eliminate the need for larfe swings of the handles. I lapped my front bar casting so the bore is only a few tenths bigger than the bar. I'm sure if you use brass it won't make much odds, you can always swap to steel later as you will have the fixture to machine another set if the fancy takes you.

Don't get too put off but all the theoretical chit chat that myself and others have indulged in, they aren't hard to make and do tend to work straight out of the box as it were.

regards Martin

Edited By Martin Kyte on 26/06/2021 22:20:27

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I had a look at the arrangement on the table of an old pillar drill that I have. The pillar is a solid 3 inch diameter steel bar and the steel cotter with the hole for the bolt is 0.996 inches in diameter and 1.4 inches long and I assume the other one is the same, and as it will only come out by removing the table, I didn't measure it. The distance between the pillar and the cotter bolt is 3 / 16 inches, the bolt being half an inch in diameter and the hole for the cotter I removed is 1.004 inches, thus giving an 8 thou clearance between the two. There doesn't appear to be any noticeable wear on the cotters or the hole they are in and the table has no noticeable slop on the pillar, the table being a fairly heavy cast iron one. the distance between the two cotters when they are in contact with the pillar is approximately a quarter of an inch without being clamped up. The photo below bears evidence to Duncan Webster's CAD drawing that the outboard end makes contact first and there doesn't seem to be much hard contact to most of the area.

Regards Nick.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Nicholas Farr on 26/06/2021 23:45:52:

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Hi, I had a look at the arrangement on the table of an old pillar drill that I have. […]

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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.

[Nicholas FarrParticipant @nicholasfarr14254]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for that, Nick … Much appreciated

MichaelG.

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Nicholas Farr on 26/06/2021 23:45:52:

Hi, I had a look at the arrangement on the table of an old pillar drill that I have. The pillar is a solid 3 inch diameter steel bar and the steel cotter with the hole for the bolt is 0.996 inches in diameter and 1.4 inches long and I assume the other one is the same, and as it will only come out by removing the table, I didn't measure it. The distance between the pillar and the cotter bolt is 3 / 16 inches, the bolt being half an inch in diameter and the hole for the cotter I removed is 1.004 inches, thus giving an 8 thou clearance between the two. There doesn't appear to be any noticeable wear on the cotters or the hole they are in and the table has no noticeable slop on the pillar, the table being a fairly heavy cast iron one. the distance between the two cotters when they are in contact with the pillar is approximately a quarter of an inch without being clamped up. The photo below bears evidence to Duncan Webster's CAD drawing that the outboard end makes contact first and there doesn't seem to be much hard contact to most of the area.

Regards Nick.

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.

[david bennett 8Participant @davidbennett8]

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

[Howard LewisParticipant @howardlewis46836]

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

[Peter Cook 6Participant @petercook6]

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?

[John BaronParticipant @johnbaron31275]

Posted by Howard Lewis on 03/07/2021 17:32:27:

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

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.

[Mark RandParticipant @markrand96270]

Posted by John Baron on 03/07/2021 18:09:17:

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.

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

[Martin KyteParticipant @martinkyte99762]

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

[Howard LewisParticipant @howardlewis46836]

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

[John BaronParticipant @johnbaron31275]

Posted by Howard Lewis on 04/07/2021 08:43:34:

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.

[Howard LewisParticipant @howardlewis46836]

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

[Charles LamontParticipant @charleslamont71117]

My MK2 Quorn kit is waking from a long, long sleep under the bench. A tiny amount of machining has been done. I am intending to use split cotters, and I want to run this argument by the forum for comment:

I intend to use back-to-back cotter pairs in most of the situations where the assembly is not disturbed in use, such as the workhead base, rocking lever, and the rotating base clamp in the tilting bracket.

However, the problem with a double cotter arrangement sitting in a through bore is that there is nothing holding it in place when you need to remove the assembly in question. Imagine you have one of those jobs where you need to turn the wheelhead upside down. You lift the heavy unit off, and half way through turning it over the cotter assembly drops out of the wheelhead collar, causing numerous expletives.

So in locations where the clamp is going to be disengaged from its bar, I intend to use a single clamp pad, either from the handle side or the back depending on the location. Even then, once it is loose, the ball hande and pad are free to rotate out of position or the pad can move axially, partially blocking the hole and meaning you have to poke a finger down the hole to realign the pad before you can put the unit back on its bar (or a mandrel in the toolholder). So as with GHT’s revised design of the Pillar Tool, I intend to use a keway in the clamp pad and a pin in the housing to stop the pad rotating.

The problem of the pad blocking the hole by moving axially could be prevented by some postive mechanical arrangement or a spring between the pad and the bottom of its chamber. (I have done some work on that, but don’t want to preempt anyone else’s ideas).

[Author]

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