jedstress
Aerospace
- Jan 12, 2011
- 36
Hi all,
I am currently working on a problem which involves exposing a bolted joint to an increase in temperature (delta 250 C).
In the past I have dealt with typical pretorqued bolt joints (ambient temperature) where the required pretorque was approximated by working out the actual required joint clamp load based upon the external applied axial and shear force (friction coefficient usage). The min required pretorque was approximated based upon this minimum required clamp load to avoid joint slippage and separation (using machinery handbook text etc). As long as the external forces did not exceed the pretorque induced joint preload, the joint was assumed to have no slippage/separation.
However, I am struggling to work out how to account for the impact of thermal expansion in the above scenario. in the ambient temperature scenario, the external force has very small impact on the bolt preload (due to relief of joint compression...). But, in the case for a joint where the coefficient of thermal expansions for the plates and the bolt differ, and assuming that a bolt load due to thermal effect has been calculated, does this value need to be added to the bolt preload due to pretorque? Maybe I'm overthinking it and maybe that preload plus thermal load needs to be assessed against the bolt yield allowable and thats it.
Im trying to make sure that bolt strength isn't exceeded whilst also making sure that the bolted friction joint doesnt slip. For slippage to occur, I assume that the thermal expansion of the bolt would have to exceed the joint expansion? And to avoid that scenario, the bolt would have to be pretorqued enough such that the preload is greater than the required joint clamp load (friction joint) after deducting the relieving thermal load?
My apologies if I have totally misunderstood how to account for joint thermal loads. I would much appreciate it if someone could point me in the right direction regarding calculating the required pretorque. Please note that I am not too concerned with evaluating precise values for pretorque; I don't mind a simplified conservative approach.
Many thanks in advance
I am currently working on a problem which involves exposing a bolted joint to an increase in temperature (delta 250 C).
In the past I have dealt with typical pretorqued bolt joints (ambient temperature) where the required pretorque was approximated by working out the actual required joint clamp load based upon the external applied axial and shear force (friction coefficient usage). The min required pretorque was approximated based upon this minimum required clamp load to avoid joint slippage and separation (using machinery handbook text etc). As long as the external forces did not exceed the pretorque induced joint preload, the joint was assumed to have no slippage/separation.
However, I am struggling to work out how to account for the impact of thermal expansion in the above scenario. in the ambient temperature scenario, the external force has very small impact on the bolt preload (due to relief of joint compression...). But, in the case for a joint where the coefficient of thermal expansions for the plates and the bolt differ, and assuming that a bolt load due to thermal effect has been calculated, does this value need to be added to the bolt preload due to pretorque? Maybe I'm overthinking it and maybe that preload plus thermal load needs to be assessed against the bolt yield allowable and thats it.
Im trying to make sure that bolt strength isn't exceeded whilst also making sure that the bolted friction joint doesnt slip. For slippage to occur, I assume that the thermal expansion of the bolt would have to exceed the joint expansion? And to avoid that scenario, the bolt would have to be pretorqued enough such that the preload is greater than the required joint clamp load (friction joint) after deducting the relieving thermal load?
My apologies if I have totally misunderstood how to account for joint thermal loads. I would much appreciate it if someone could point me in the right direction regarding calculating the required pretorque. Please note that I am not too concerned with evaluating precise values for pretorque; I don't mind a simplified conservative approach.
Many thanks in advance