Quote:
quote:Originally posted by dgc333
I would like to put an engineers twist on your torque wrench comments.
My first job out of college was developing torquing specs for jet engines at Pratt & Whitney Aircraft. What I learned in that 6 month project is that torquing a threaded fastener is about the most inaccurate thing you will ever do with a measurement tool.
In simplest terms the purpose of torquing a fasterner to ensure that you are appling more force (in pounds) to hold the the parts together than will be applied in use which trys to seperate them. If you accomplish this the joint will not loosen. You can mathematically calculate what the clamping force will be for a given torque knowing the fasterner diameter and thread pitch. The error comes in because the calculation assumes no friction in the threads. For a given torque reading the actual clamping force can vary as much as 40% depending on the conditions of the threads, finish on the threads, plating, and/or lubrication used. The best you can hope for is about a 10% variation fastener to fastener. This is the reason for manufacturers going to torque to yeild head bolts. Once you yeild the bolt the clamping force now becomes a function of the fastener size and material.
The variation in accuracy from the least to most accurate torque wrench is about 1-2% and the cheap beam type tend to me more accurate than the expensive micrometer type. And even the least accurate is almost an order of magnitude more accurate than the other variations that will effect the loading in your threaded joint.
NOTE: I am not recomending a cheap torque wrench over a high quality one (you will never regret buying a good tool) but if you can't afford a good one and you are an occasional home mechanic a $25 wrench will serve you fine. A $25 dollar one just will not stand up to the day to day use from a professional mechanic.
|
This reminded me of my old missile working days with the USAF.
I used to connect some parts to another (They are still secret) and the manual said:
Torque to 100 PSI +/-1, hit the coupler ring 6 times, retorque to 100 PSI +/-1 and repeat unti it retains the torque....
I loved it, hitting a real explosive missile with a hammer...[?]
Ian