Does Length of thread have an Impact on the Torque Applied 1

[Mechotron]

Hi,

Machinery's Handbook gives a formula for calculating the torque which is not relating to the length. I believe this assumes that there is minimum length of thread engagement available, which is calculated from the same machinery's handbook. I have a situation where i dont have the minimum length of thread engagement, so i dont believe that the formula in the machinerys handbook would apply. Is there any way of calculating the torque for short length of engagement or is there a formula for calculating the torque using the length of engagement as a parameter.

Mechotron

 

[OTHGWayne]

A properly torqued fastener is normally one where the tension load applied to the fastener stesses it to somewhere around 80% of the yield strength. That percentage can fluctuate some but 80% is reasonable. That stress is normally based on the tensile area of the thread to get the appropriate amount of stretch, your machinery's handbook will have charts and equations for tensile area. Once you have gotten to the point where you have the proper load, then check the thread shear stress to make sure that the applied torque load will not shear off the threads. Again you will find in your mach. handbook equations for thread shear based on length of enagagement. Also note that there are different shear area equations based on male/female threads as well as material choices. By using these equations and the allowabe shear stress of your materials, you can determine the length of engagement you require.

While it is true that if length of engagement is fixed in your application you can reduce the torque to reduce the shear stresses. That however may not provide enough tensile load to keep the joint tight, so be careful. That is also why you should do the torque calcs and the shear calcs seperately. The torque load is important for a reliable joint, don't base it on a thread that might have insufficient engagement. And before you suggest it, a threadlocker should not be used for a substitute to a properly torqued joint.

 

[desertfox]

Hi mechotron

The principle reason for the engagement length is to make sure that the bolt fails in tension before the internal thread strips.
What you need to do is calculate your shear area for the internal thread and using the tension you are putting on your bolt calculate the shear stress and make sure it doesn't go over yield.
Have a look at this site it may help

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 28.pdf

regards

Desertfox

Regards

 

[Mechotron]

One more question is Shear stress of a fastner 0.57% of yield or 40% of Yield.

 

[desertfox]

Hi Mechotron

Page 27 of the link I gave in the last post shows using 50%
of the tensile stress for the shear stress.
I should of added also yesterday that the thread engagement
also ensures the external thread doesn't strip as well as the internal.

regards

desertfox

 

[unclesyd]

Here is a paper by Dose that may help clear up the tension/shear/engagement relationship.

http://www.hexagon.de/dose/dose-1e.pdf

 

[tbuelna]

Threaded fasteners are best tightened to produce a specific strain value in the body of the fastener. This can be accomplished using torque-angle techniques, strain gauges, PLI washers, ultra sound, or bolt stretch measurement. Torquing to strain values is much more reliable than using wrenching torque, because wrenching torque is subject to highly irregular sliding friction values in the threads and fastener faces.

As for the number of threads in engagement required for a specific load, you can get a rough estimate by calculating the relative root shear areas of the engaged threads and divide it into the load (P/A). The actual capability of the thread is much more difficult to establish, since things like fatigue, creep and stress concentrations can have big effects. Also, the end threads in a threaded joint tend to carry more load, and will be the first to fail, so it's a good idea to carefully blend the thread runout in a highly loaded fastener.

A good rule of thumb is to have at least 5 complete threads in engagement for axial loads with fasteners (nut and bolt) of equivalent strength. For shear fasteners, you can get away with fewer engaged threads (3 maybe ?). Having too many threads in engagement can be a problem because the pitch tolerance stack-up over a long thread length will cause the fastener to seize up at installation.

Finally, when calculating your bolt stresses, be sure to use the combined load effects of both tension and torsion due to tightening.