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| Axial play in threaded fasteners that are not preloaded or torqued |  | ||
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| Posted by: john2003 ® 12/17/2008, 18:45:48 Author Profile eMail author Edit |
Hi everyone, Are there any standards for maximum allowable axial play of threaded fasteners before they are preloaded or torqued down ? I'm just curious how much axial play in general there is for a 1/4-28 & 1/4-20 thread without any preload. I would guess it's within .005". Does thread class have a significant effect on axial thread play or clearance, or is that mainly just related to radial play / clearance ? I have a 1/4-28 bolt I'm using to move a small slider. I want to eliminate as much axial back & forth thread play as possible. The minimum amount of thread length engagement I have is 0.304". I would guess that to a point, having more thread length engagement reduces axial play. What is likely the minimum length of thread engagement I would need to insure the least amount of axial play ? I might be able to double the length of minimum thread engagement if it can significantly help. Due to limited space and assembly issues, it's difficult to preload the joint with a compression or Belleville spring in order to take up play so that it's not an issue when the load is reversed. Currently the adjusting bolt is threaded into aluminum but I may decide to use a steel threaded insert in the aluminum. A threaded insert with an internal nylon locking element would be nice as this would eliminate the axial play, but I have not found one yet. I thought of using a self locking adjusting bolt with a nylon patch on the threads. However, I may have to use a 1/4-20 thread to get into a self locking bolt. Here is what I found from a supplier... 1/4-20 thread zinc plated hex head bolt with nylon locking patch
1/4-28 hex head bolt (no nylon patches on 1/4-28 hex heads from the supplier I checked.
I suspect a nylon patch will go a long way towards taking care of any axial play. I'm still interested in any other options though. I'm also interested in learning what the maximum axial play is likely to be, and how the amount of thread length engagement and thread class may effect it. Any feedback would be appreciated. Thanks
Modified by john2003 at Wed, Dec 17, 2008, 20:56:13 |
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| : Axial play in threaded fasteners that are not preloaded or torqued | |||
| : Axial play in threaded fasteners that are not preloaded or torqued -- john2003 | Post Reply | Top of thread | Engineering Forum |
Posted by: randykimball ®  12/19/2008, 09:49:30 Author Profile eMail author Edit |
Yes indeed, the axial play in a thread pair is determined by the class of thread. The class of thread controls the tolerance in the pitch (effective) diameter of a thread. The following is an example of those pitch diameter tolerances. ("A" is external threads and "B" is internal threads) 1/4/20:
So with a little study you will see that a class 3 thread is fairly tight. A thread form is a 60° triangle so the lateral movement will be about 1/2 the difference in the pitch diameter of the male and female threads. IF... IF the threads are perfectly cut, the length of engagement has little to do with axial play. But, threads are not perfect so you will get some imperfect thread lead (threads per inch) which will result in a minor amount of length slack take up with distance, but I wouldn't count on it for any production model. I'm not sure of your load needs, but... Nylon tends to take a thread very well, has a decent life when threaded, AND tends to thread tight but still allow for one sweetheart of a screw rotation. I have used Nylon in sevral projects and as I recall there was never ANY lateral axial slack. In fact I can't recall any radial slop either. Nylon tends to hold firmly to screw threads like you want and yet allow for smooth thread rotation. But, I can't help but wonder if a screw is your best choice in this application. Screws are fasteners are intended to be tightened so that the threads center up and bind together. ..Or perhaps you need to apply Loctite to maintain a certain position of screw rotation. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Fri, Dec 19, 2008, 10:30:54 |
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| : : Axial play in threaded fasteners that are not preloaded or torqued -- randykimball | Post Reply | Top of thread | Engineering Forum |
| Posted by: john2003 ® 12/19/2008, 17:41:51 Author Profile eMail author Edit |
Hi Randy, Thanks for your reply. Is there any significant advantage of going with a 1/4-28 over a 1/4-20 thread as far as reducing axial backlash ? I could probably get away with a 10-32 adjustment screw but a 1/4" OD is preferable. This thing is adjusted infrequently, so I think a screw with a nylon patch would work well. Like you, whenever I have used a nylok nut or a screw with a nylon locking patch on it, it seems to take up all the axial play. I also tend to think that the nylon locking elements could take a fairly high load, just because they have to withstand the preload forces involved when the fasteners are torqued down, but this application will not have any preload forces, only the payload. The adjusting screw threads into a slider at on end, but on the other end, I have a nylok locking nut, so in order to assemble, I will have to run the nylok locking nut over the nylon patch on the screw threads. This only needs to be done once at assembly, so hopefully I can squeeze it by without damaging the nylon locking elements on either the nut or the screw / bolt. Ideally, I need to find a 1/4-28 X 1.25" long hex head bolt with a nylon patch or locking element. So far, I've only found them in 1/4-20 thread. If I have to, I can drill my existing taps out and install a 1/4-20 steel thread insert. A steel threaded insert with an (internal) nylon locking element is another option, but I have not found that yet either. If I can go that rout, I won't have the issue of running the locking nut over the nylon patch on the screw once at assembly. I will also try to get everything in the highest precision thread class I can get. Thanks again for your feedback. John P.S. Regarding the nylon patches on the screw threads, I prefer to lube the threads, do I need a special lube to prevent a reaction with, or deterioration of, the nylon ? Perhaps a silicone lube like Syl-glide ? Modified by john2003 at Fri, Dec 19, 2008, 17:55:45 |
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| : : : Axial play in threaded fasteners that are not preloaded or torqued -- john2003 | Post Reply | Top of thread | Engineering Forum |
| Posted by: randykimball ® 12/30/2008, 12:42:26 Author Profile eMail author Edit |
Sorry for the delay. I hope you still get a chance to read this. I haven't had much problem with lube on Nylon... but ... you don't need lube on Nylon...?? I don't think. As far as the 1/4-28 advantage... I'd tend to stay with coarse thread pitches in synthetic materials for the depth and size of thread grip. I'd expect the 28 pitch to give up some sooner. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. |
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| : : : Axial play in threaded fasteners that are not preloaded or torqued -- john2003 | Post Reply | Top of thread | Engineering Forum |
| Posted by: john2003 ® 12/19/2008, 23:56:43 Author Profile eMail author Edit |
Below, I have listed what I figured for the maximum axial play of 10-32, 1/4-20, and 1/4-28 screws taking 1/2 of the maximum difference between the internal and external pitch diameters. There is not much difference really. I could go with a 1/4-20 and a nylon patch if I can't find it in a 1/4-28. Pitch diameters, A is external B is internal... 10–32 UNF
1/4–20 UNC
10-32 class 2 axial play = 0.0039"
1/4-20 class 1 axial play = 0.007"
1/4-28 class 1 axial play = 0.00625
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