Torque effects on screw stress
Torque effects on screw stress
(OP)
I understand that putting torque on a fastener helps distribute external loads, but that doesn't seem to come across in my calculations for the stresses. When I calculate a bolt pre-load, I add it to the external tensile loads and can't seem to get a reasonable safety factor. How do you calculate what percentage of the external loads are taken up by the members being joined? At what point is it necessary to determine if a pre-load is necessary?





RE: Torque effects on screw stress
RE: Torque effects on screw stress
RE: Torque effects on screw stress
RE: Torque effects on screw stress
RE: Torque effects on screw stress
- the two joined surfaces must remain attached (no gap shall occur), or
- load fluctiation (fatigue) is significant
This because the load on the bold does not increase (so it does not elongates) until the external load reaches the bolt preload
RE: Torque effects on screw stress
Is it a single bolt with similar materials clamped? Is it a bolt pattern? A confined gasket or unconfined gasket? What are the materials being clampled? What bolt are you using?
With the above information I can give you the equations and coefficients that apply to your problem.
--Scott Wertel
scottw@interfaceforce.com
RE: Torque effects on screw stress
Preload is tensile in the bolt and compressive in the clamped flanges. When external load is applied, the ext load is carried by both, proportioned by the relative stifnesses. Upon joint separation, there is zero compressive load in the flanges and all of the preload + external load will then be carried by the bolt. See both Bickford (still the standard work on bolted joints) and the fastening and joining chap in Shigley's Machine Design.