Thursday, August 02, 2012

Does the bolt loosen slightly after you torque the bolt?

Continental IO520 with missing cylinder
25 miles out at sea and cylinder blows off but engine runs long enough to get him to the beach!

Yes, by a process called "embedment relaxation", fastener preload is reduced.  As each surface is pressed together, the high spots are crushed and deformed to form a surface capable of supporting the load. 

Surface Finish:
There are multiple surfaces in the joint: 

1. between the head of the bolt and the washer,
2. between the bolt washer and faying surface (faying surface is the surface of the object being fastened together),
3. between the two faying surfaces,
4. between the faying surface and nut washer, and 
5. between the nut washer and the nut face.

Each of these surfaces squeeze together. Any paint, sealants, nicks, or alignment errors are gradually crushed down to support the load. Bolt threads also embed. Threads are pulled in shear, slightly increasing the thread pitch. Nut threads are compressed and lose a little pitch. As embedment occurs, the surfaces press further together and reduce the bolt's clamping force (preload). Note thate relaxation occurs without any off-rotation of the nut.  

Any gaskets, sealant, paint, or lubricants, between the surfaces will extrude from between the surfaces. One reason not to use high poission's ratio (0.5) products, such as rubbers, in tension joints. 

How much preload is lost?
Under optimum joint conditions in a lab one can expect between 1 and 11 percent. Obviously then, in the field, more clamping force can be lost. Lockheed did a lab test, Report No. LR 25049 where they tested 1 inch by 12 UNJF thread size L-1101 engine pylon bolts. They used lab conditions, with hardened steel bushings. In a static joint with no load fluctuations, between 1 and 11% preload was lost. The greatest loss occurred in the first eight hours after installation.

How this effects the mechanic?
It's up to engineering to included embedment relaxation in the joint analysis and torque recommendation. The mechanic need not compensate for embedment relaxation but he does need to follow the torque recommendation (conditions of torque) carefully. Surfaces need to be clean and undamaged. 

It's also considered good technique to torque slowly to allow the surfaces to adjust to one another. Years ago when assembling engines we used a torque and hold technique where the full torque was applied and then held for 20 seconds to allow the joint to stabilize. One potential problem with snap style torque wrenches is that torque is immediately released at the torque point. If the joint hasn't stabilized and is still moving together then it hasn't been fully tightened; another reason to apply torque slowly.

What about multiple clicks of the torque wrench?
This technique is widely used by mechanics but does introduce some questions about its effectiveness. Once the nut stops rotating then it might take a lot more torque to break the nut free and get it moving again. The nut may not rotate not because the joint is fully loaded but because there is too much friction to break the nut loose. 

Just the opposite has been shown to occur where there was plenty of lubricant applied to the thread and nut surfaces so the nut was free to rotate. Subsequent application of click torque progressively tightened the joint far past the proper amount. This occurs because when you apply torque you apply a axial tension to the bolt - you stretch it, and you apply torsional tension to the bolt - you twist it. When you're wrench snaps and releases the twisting force, the bolt untwists. This untwisting occurs in the portion of the bolt where the torsional stress is highest - in the threads and causes a slight tightening of the nut onto the bolt. In effect, twisting stress is turned into axial strain.

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