What value of vibration would be required to overcome a known torque?

[kcme2005]

I am using a 10-32 1/4" set screw with a nylon patch (for its "self-locking" properties) to hold a wire in place. This wire is non-moving, but the entire assembly can be subject to vibration. Value, direction, etc of possible vibration is unknown.
We are concerned about the set screw loosening with vibration over multiple uses due to loss of prevailing torque. The set screw may need to be moved or adjusted through the products life, probably no more than 2 or 3 times, but we are looking at 5 times as a worst case situation.
According to the set screw manufacturer the "first on" prevailing torque is 18 in-lb, the "first removal" prevailing torque is 3 in-lb min, and the "fifth removal" prevailing torque is 2 in-lb min.
I know the "first on" prevailing torque is the necessary torque to turn the screw in the first time. Is the "first removal" prevailing torque the necessary torque to remove, or "unlock", the screw the first time?
With a known mass of the set screw at 0.029oz, is there a way to calculate the vibration value at which the "removal" prevailing torque would be overcome and the set screw could loosen?
I have looked through some of my old books, but have been unable to find anything that really applies to this.
Any suggestions would be appreciated.

 

[MintJulep]

"is there a way to calculate the vibration value at which the "removal" prevailing torque would be overcome and the set screw could loosen?"

Well, not really. At least not without introducing so many assumptions for various values that the result becomes meaningless.

 

[CoryPad]

The first removal torque is the value recorded while loosening the nut for the first time after it had been tightened for the first time.

There is no calculation. In general, preloading a screw to 50 % of its proof load has been found to make the screw resistant to loosening from vibration.

For actual cyclic externally applied forces, the initiation of loosening has been found to result from transverse displacement between the joined components, so the goal is to provide enough preload that the generated friction forces are higher than the applied forces.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SincoTC]

As a last resort, how about applying a penetrating thread locking compound on subsequent adjustments. Maybe even on first assembly as well to help dampen any vibration which is always very difficult to predict, especially if there is any variation in frequency

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756

 

[tygerdawg]

Or you could do initial install with breakable threadlocker. On subsequent adjustments, your procedure would be to discard existing set screw and re-install a new one with new breakable threadlocker.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[SincoTC]

It seems from your original post that you're using this setscrew rather in the manner of securing a wire in a terminal block, so you're not able to apply much pressure on the wire, therefore, there is little more than the nylon insert's installation torque to resist vibration.

As an alternative to thread locking compounds, if there is a suitable face, could you use a longer setcrew and fit a locknut. This would enable you to set a higher torque without crushing the wire (use a full nut and a hex-socket set set screw to get the highest torque the nut can safely take)

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756