Does Length of thread have an Impact on the Torque Applied
Does Length of thread have an Impact on the Torque Applied
(OP)
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
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





RE: Does Length of thread have an Impact on the Torque Applied
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.
RE: Does Length of thread have an Impact on the Torque Applied
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://ww
regards
Desertfox
Regards
RE: Does Length of thread have an Impact on the Torque Applied
RE: Does Length of thread have an Impact on the Torque Applied
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
RE: Does Length of thread have an Impact on the Torque Applied
http://www.hexagon.de/dose/dose-1e.pdf
RE: Does Length of thread have an Impact on the Torque Applied
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.