question about acme pins

[3kushn]

Your information is good, but per Conetip, I left out the important proviso that the same torque is being applied in the situation at hand (tightening cues by hand).

Tension is pretty directly related to clamp load.

As Conetip mentions, the compression of the faces causes clamp load. What he skimmed over is that the compression forces are also translated into sliding friction, thus for a lower friction interface, the torque applied will result in higher clamp loads (and more compression of the surfaces).

I think you have a fine understanding of properly installing bolts and controlling torque. The fact that we are dealing with cues and tool-less tightening kind of switches things backwards. You likely deal with installing fasteners that are engineered to supply the correct clamp load (i.e. flanged plumbing, iron beams). These fasteners are chosen such that the clamp load and the friction between the joined members resists the slipping of the two members and no shear force is applied to the fastener (most fasteners are very poor at resisting shear loads). In the cuestick world, we start with a torque (the amount that a person can be reliably expected to apply with his hands) and end up with a screw selected by being the largest that will work.

Maybe we should move this to the NPR Forum You are correct that I deal in engineered products and torque between a few in/oz to 58K ft/lbs in Assembly plants and heavy industry.

Just a couple of comments. We sometimes confuse/reverse things when talking about this stuff. I'm guilty too for sure.

I might suggest that tension and or clamp load causes compression in the thread. Can't have compression without tension.

I don't have ready proof of this but I'm thinking that even with the same applied torque you will get a higher clamp load with a fine pitch than course. I say this simply because of the difference in the inclined plane. Its much easier to walk up my driveway (fine pitch) than it is a mountain (course pitch). If my theory is has any validity we can climb higher with less effort in the shallower incline of the fine pitch which would create higher clamp. Maybe hogwash.

Conetip States: With cues, the wood is the weakest part of the equation and the joint face materials effects the torque when assembling a cue.

I would suggest here that the joint face materials effect the clamp load not torque. Torque its assumed remains the same in this discussion. I contend that friction overcomes applied torque. If I were stronger or tightened the cue with pipe wrenches I could achieve higher clamp till the joint failed. I doubt there's any measurable stretch in a cue pin. Extremely little.

Take a look at Layani's joint. His first cues did not have a screw at all. Problem was they were hard to get apart so he added a jack screw. The short screw also aids in fully seating the joint.

 

[fERrELZ]

When making a drive screw, the pitch absolutely matters when regarding torque.

What we have is a joining screw where the load is caused by two faces coming together. The clamp load climbs more quickly with a coarse screw, but will come out *basically* the same regardless of the pitch.

The absolute truth of the matter is that there are many factors that contribute to clamp load, but in theory the pitch doesn't matter...I spent several weeks working on this with a major aerospace company. I had several different setups with about $50000 worth of torque wrenches, calibration stations and loadcells...the formula works.

I agree that there is very little stretch in a joint pin in a cue, the other materials are going to fail first.

Joint face materials affect the clamp load, just as you say. If you have two materials with very high relative coefficients of friction, it will take more torque to overcome that friction to get the same clamp load.

Mostly I think we are saying the same thing in different ways. Here is a good document which explains things better than I can: http://www.pcbloadtorque.com/pdfs/engineering fundamentals.pdf

 

[bbb]

i appreciate that my thread has generated the knowledeable discussion it has generated
but for us people less scientifically oriented could you explain it simpleton terms so i can understand about the torque characteristics and what is the difference of the acme pin
thanks

 

[fERrELZ]

i appreciate that my thread has generated the knowledeable discussion it has generated
but for us people less scientifically oriented could you explain it simpleton terms so i can understand about the torque characteristics and what is the difference of the acme pin
thanks

From the point of view of a person playing with an acme pin, it will not be noticeably different than any other pin of the same diameter. It is sufficient for it's job, but is a bad choice from a theoretical standpoint and from a manufacturing standpoint.

I'd never buy one.

 

[conetip]

i appreciate that my thread has generated the knowledeable discussion it has generated
but for us people less scientifically oriented could you explain it simpleton terms so i can understand about the torque characteristics and what is the difference of the acme pin
thanks

I found some pdf files, if you give me your email I can email them to you, or go to the following links and download them. Hopefully they will all work.
The one I was really trying to find and thought I had down loaded had info on a Japanese guy who figured out the helix angle relative to the screw diameter that made the screw locking even under severe vibration. He also discovered/identified the helix angle that made fasteners tighten in certain situations and then they also failed.

But in a quick over view, the Acme threads have quite a high helix angle, the angle of the thread relative to it's center line, or also known as pitch angle. The steeper the angle, the quicker it moves when being rotated. The shallower the angle , the slower the movement. So the fine pitch can resist being driven to unscrew, while the coarser thread can more easily be a driver, or want to unscrew more easily.

Friction is your friend when it comes to preventing two components from coming loose.
An example of this is a sash clamp that I have. It worked OK ,but was very tight to try and tighten up. I put a very small amount of some slick 50 grease on the acme threaded shaft. The result, even though it did up tightly very easily, it would not stay clamped. It just slowly unwound to where there was effectively no clamp pressure. I had to tie the clamp handle for that job and have since removed the slick 50 from the nut and screw.

But all the various thread pitches and things, it may well be possible that the joints that have problems staying together, could be too smooth or some sort of lubricant has been applied, negating the necessary friction to keep the joint together.

As can be seen in the various pdf files, too much clearance in the thread form can also lead to the joint coming loose from vibration, but not in the linear plane, but in the transverse/lateral/ side ways, micro movement.
In this case, a piloted joint that prevents lateral movement of the joint will allow a loose thread to still function in keeping the joint together while not coming loose.
On my cues, the pilot keeps the joint concentric to within .001 inch. As a result of the close fitting pilot, absolute cleanliness is a must and joint protectors are important to keep it all very clean and the mating faces from getting damaged .

Lots of food for thought and no doubt discussion.

So there is some good reasons for some joint configurations.
Neil

http://www.scribd.com/doc/168155872/Vibration-Loosening-i-e-a-Us-01

http://www.ajaxfast.com.au/downloads/Technical noteFINE THREAD VS COARSE THREAD.pdf

http://perfectlockbolt.com/wp-content/uploads/2010/12/Brochure-V7.pdf

http://www.katonet.com/article/coarsevsfine.html

https://www.electronicfasteners.com/pdfs/longlok/handbook.pdf

http://www.boltscience.com/pages/Why_nuts_and_bolts_can_self-loosen.pdf

http://www.boltscience.com/pages/fastenerdatasheet.pdf

 

[GBCues]

Guys, this is a great thread (no pun intended) and shows the cooperative and informative spirit that this forum was created for (IMHO).
:thumbup:
Gary

 

[3kushn]

When making a drive screw, the pitch absolutely matters when regarding torque.

The clamp load climbs more quickly with a coarse screw, but will come out *basically* the same regardless of the pitch.

Let me try to explain this another way. I think you will agree that if we lubricate a fastener we produce higher clamp loads for a given torque. For those of interest that don't know this fact: Since a lubricated fastener is easier to turn its also easier to turn it further than what's possible in an un-lubricated screw with the same torque. The screw moved to 6:00 instead of stopping at 12:00 with X Torque value.

If we can assume for argument sake, that we take all friction out of the process ( virtual impossibility) the pitch angle of the finer pitch screw makes it easier to climb the hill (similar to lubrication) than the steeper pitch angle of the course thread screw. Therefore if its easier to climb the hill a much larger angle can be achieved. In other words we climb further up the hill which will give us a higher clamp.

From a practical point of view, the additional forces of friction in a fine pitch screw vs course, negate most if not all the additional rotation angle in the fastener with a given torque. Therefore I concede from a practical standpoint, I'm full of crap.

One last point. Gerhard Junker's famous study on causes of self loosening fasteners determined vibration to be minimal factor. Lateral movement of the clamped parts is the most significant factor of lost clamp. That was 1969 and we still talk about vibration.

 

[fERrELZ]

When making a drive screw, the pitch absolutely matters when regarding torque.

The clamp load climbs more quickly with a coarse screw, but will come out *basically* the same regardless of the pitch.

Let me try to explain this another way. I think you will agree that if we lubricate a fastener we produce higher clamp loads for a given torque. For those of interest that don't know this fact: Since a lubricated fastener is easier to turn its also easier to turn it further than what's possible in an un-lubricated screw with the same torque. The screw moved to 6:00 instead of stopping at 12:00 with X Torque value.

If we can assume for argument sake, that we take all friction out of the process ( virtual impossibility) the pitch angle of the finer pitch screw makes it easier to climb the hill (similar to lubrication) than the steeper pitch angle of the course thread screw. Therefore if its easier to climb the hill a much larger angle can be achieved. In other words we climb further up the hill which will give us a higher clamp.

From a practical point of view, the additional forces of friction in a fine pitch screw vs course, negate most if not all the additional rotation angle in the fastener with a given torque. Therefore I concede from a practical standpoint, I'm full of crap.

One last point. Gerhard Junker's famous study on causes of self loosening fasteners determined vibration to be minimal factor. Lateral movement of the clamped parts is the most significant factor of lost clamp. That was 1969 and we still talk about vibration.

You can't look at it like climbing a hill. When you are climbing or pushing something up a hill the applied force stays the same throughout the hill. This is akin to lead screw on a machine, which is why the pitch matters (whereas diameter does not) in the force calculation for a drive screw.

When you tighten a screw, it is like walking up a parabolic hill. The further you go, the harder it is to go any further. On the steeper parabola (i.e. coarse screw) you get to a maximum 'height' on the hill faster than on a shallower parabola. The heights, representing force, will be the same on either.

 

[conetip]

Just to mess with your mind, there is a thread form on oil drilling pipes that cleverly seats on both ends in such a way that the threads are loaded from both ends. So the 1st thread from each end shares the load, instead of just the 1st thread in a nut or standard cue joint situation.

Neil