Thread: rear main seal
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Old 12-16-2000, 10:45 AM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Your concept of angle of rotation torqueing is wrong. Naturally all torque should be applied perpendicular to the bolt shaft, but the purpose of torque to yield fasteners and application has nothing to do with this.

To understand this type of torque one must understand the concept of yield; inelastic deformation. While hard to feel in most applications (without measuring devices), I'll give a simple example. Take a coat hanger and straighten it. Now bend it at a point. First bend it only far enough so that when released it goes right back to its original shape. Now bend it further so that it doesn't go fully back. In this case yield has taken place; inelastic deformation; it won't spring back. Try this slowly and you will find that the amount of force necessary to continue deformation goes down once you have exceeded the point of elastic deformation.

The process of slip (inelastic deformation) is unique in metals and takes place through a unique process called dislocation propagation (at least it was 27 years ago when I got my B.S in Metallurgical Engineering UofF 1974). It makes metals different from everything else.

Back to the point. The bolts used in these situations are often neckdown bolts (have a thin area designed to stretch (inelastic deformation). They are designed to stretch so that all of their ability to spring back is used up (no sure of why all that is important but it is what is being used). SOOO, when you are torqueing one of these bolts the 45Nm is measured during the elastic deformation phase. The designer then knows that further turning will start to stretch the bolt and the torque necessary to do this is LESS and as such is no longer a good measurement of the amount of stretch. The angle of rotation is an exact measure because the bolt has so many threads per inch/whatever. These bolts have a minimum thickness and are often used only once (BMW and VW head bolts are two examples). Most MB applications can be verified by checking a minimum thickness although if they have a newly designed bolt I would change them all.

BTW, back to the cloths hanger. For those who have tried to cut one by bending it back and forth till failure, you will notice that the darn thing keeps changing the point which is bending. This is a good example of "work hardening". In practice the lines of dislocation start pinning the propagation centers causing the force to slip to go up. The joint work hardens and the hanger starts flexing right next to the spot where it is still soft.

[Edited by stevebfl on 12-16-2000 at 10:49 AM]
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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