Re: [DML] Angle Torque & head bolts

["B Benson" <delornut@xxxx>]

Measuring torque with a torque wrench is referred to as a static torque
reading and as Mike points out it's affected by a variety of issues which
makes it's accuracy somewhat unpredictable. In the auto industry critical
joints are measured with a transducer built into the tightening tool which
measures dynamic torque and sometimes angle and is shut off upon reaching the prescribed specification via a computor. When a group of fasteners, such as head bolts, are being tightened the clamping pressure relaxes as the tightening sequence moves from one fastener to another. An example is the tightening of wheel nuts. If you tighten one at a time and the first is tightened to, say, 90nm, when the nut opposite it is brought to 90 nm the original tightened joint's clamping pressure will relax below what's specified. For that reason factory tightened joints like this are done with a pre-torque followed by a final torque. Angle is introduced when joints are extremely critical and clamping pressure must be uniform. In modern engines the head bolts are taken to a beyond yield point for maximum clamp load. Because they,ve been tightened beyond yield they need to be replaced after the head has been removed. Measuring angle is much more accurate than trying to achieve the somewhat variable static torque. The key to remember is a torque spec is given to achive a clamp load on the joint. The clamp load is the important thing. If the threads are rough a static torque may be right but the clamp load might be too light.

Bruce Benson



> Measuring torque on a fastener is an indirect method
> of measuring the amount of tension applied to
> the fastener. It's an approximation, since there are
> a number of factors that influence the torque reading
> on a wrench. > --
> Mike
>
>