Tightening Bolts and Relaxation

An article by George Lorimer, Retired;
formly of the G.M. Powertrain Fastener Lab.

George Lorimer

Did you know that the second you remove a wrench from a tightened bolt the clamp load starts to decrease?

So what ???

This points out the fact that even if you tighten, by whatever means, to a given clamp load it will not remain the same and can even decrease to a point that can cause unsatisfactory operation of a part or assembly. Let's consider for a moment, a headbolt. We have a piston that compresses a gas and air mixture before it gets ignited. When it ignites, there is a large increase in pressure inside the cylinder. This pressure converts to force by the area of the piston (force = pressure x area) and can be very high (for a 4" diameter piston and 600 psi gas pressure, the instantaneous force, is about 7,600 lbf). Now, not only do we have to hold the head on but also we have to seal the gasket to the head and block, so the gases stay where gases are supposed to be and to keep coolant where coolant belongs. In order to accomplish this we must have some minimum clamp load in each bolt holding the head down.

Through experience we have found that headbolt clamp load can vary +/- 40% when torque control is used. Let's examine a joint and see what results we can expect. We start with a clamp load of 12,000 lbf +/- 40%, so we can have 7,200 lbf - 16,800 lbf. Initially, now we have some relaxation to account for about 30 - 35% for headbolts (gasket creep - heat, etc.). So now we have the possibility of having a clamp load range from 5,040 lbf to 11,760 lbf and the real problem is that we don't know

Which bolt has what load?

The actual clamp load used in the example is not the important point. Rather that the needed clamp load should be determined early in the design game so that an adequate tightening strategy can be implemented. The end result is a joint that is together and has a satisfactory working clamp load during service. Some factors that influence relaxation of clamp load are:

1) Surface finish

2) Temperature (high and low)

3) Type of joint

  • Gasketed (soft)
  • Material of joint (hard or soft)
  • Combination of metals (steel and/or iron verses aluminum.)

4) Initial preload

5) Joint strength (spring rate)

6) Bearing area under head of bolt

7) Vibration

8) External loads to the joint, add infinitum

We can decrease the variation of clamp load by using a tightening strategy that allows closer control of the initial clamp load but the important issue is knowing what to expect out of a joint in terms of relaxation, in order to have adequate working joints for service.

I found that by conditioning a joint during the tightening process, the relaxation could be reduced by up to 50 % or more. One combination joint that this worked with was a cast iron/gasket/aluminum. The solution was to tighten to a preload torque and rotate for 90-110 degrees then wait for 2-3 seconds then loosen and immediately retighten to the preload torque and then a rotation of 90-110 degrees. The variation in the amount of degrees of rotation is due to the placement of the bolts and their working grip length.

If you consider a 4 cylinder engine with the intake on one side and the exhaust on the other. The heating forces vary widely from side to side and even from the area of coolant entry to the area of coolant return. A very difficult joint to control completely, however if you take all things into consideration success can be achieved in making a quality joint through fastening. And as a final note-all the bolts in the assembly did not require the same clamp load nor did they get them through tightening strategies. They did, however, keep the same relative clamploads by relaxing the same percentage via joint conditioning.

Home work.. Consider the hose clamp.

Successful fastening,

See also the second article by George on this topic More on Relaxation