Chrysler is requesting a "Torque Strategy" for all Fastener Joints

#1
I was hoping someone can help with this. Chrysler is requesting a "torque strategy" would this just be part of the control plan or is it a separate document? And does someone have an example of what it would look like?
Here is the Chrysler requirement: "For all fastener joints, the Supplier shall have a defined torque strategy in place. For M6 and above, the Supplier shall develop a torque plus angle strategy unless explicitly told otherwise by an FCA Engineering representative. The torque plus angle strategy shall include error or mistake proofing such as lock out features in the event the torque and angle requirements are not met, etc"

Thanks!
 
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Jimbo3

#2
Re: Torque Strategy

It sounds like they want a plan (default) for all fasteners used. i.e. - if it is not specifically addressed in your print, what is the default torque for each given size of bolt, etc (M-6 default is always X, M-8 default is always Y , etc.). Our engineering has a dedicated document that addresses this issue.
 

ncwalker

Trusted Information Resource
#6
Unless your application is requiring you to torque your fasteners to a yield point, stay away from all of those. If you are just making an assembly, what they want to know is that your bolts are installed properly. This means two things - fully seated AND at the correct torque. In an automated environment, this is almost exclusively done with torque and angle because they're easily measured in the installation equipment.

Quick though experiment: Why isn't torque enough? I mean, when you fix your car, you get out your torque wrench and tighten to the torque and you're done ... right? The answer is a robot has a hard time determining that the bolt is seated. Imagine you have a blind hole and I drop a bearing ball in it. You install your bolt and the end of the bolt contacts the bearing before the head of the bolt contacts the shoulder. It will begin developing torque and at that point, you will hit your torque limit without the head of the bolt making contact. See?

There's usually two cases:

Case 1: A robot does the full install. The process is this:
a) You know how many turns you need to seat the bolt because math. So you spin this bolt the requisite number of turns (so you avoid my bearing ball in the hole scenario above).
b) IF that hurdle is met, and your are developing torque, you then tighten the bolt to the required torque. But as you do this, you also measure the ANGLE through which the bolt spins. So if you have, say, a 30 ft-lb requirement, you start tightening and the angle counter starts at, say, 2 ft-lb (because you know you have contact then). You continue to tighten up until your 30 ft-lb counting the angle all the way. THEN you compare this angle to a window that has an upper and lower limit. The spec is something like "torque to 30 ft-lb with a rotation between 390 and 430 deg." What this means is, once you pass the start window point (2 ft-lb) you should reach 30 ft-lb between 390 and 430 deg. (A little over a full turn). This ensures you aren't wringing out the threads when you tighten.

Case B: A human runs the bolts in, THEN a robot or machine finishes the torquing. Or, the human uses a gun for that matter. Process is the same, you just don't typically count the initial turns to run it in, relying on a human to notice the bearing ball situation. But, when the automatic tightening starts, you do indeed count the angle WHILE you are developing torque. And a human held gun can do this, though you aren't going to find it at Harbor Freight ... :)

That's what they mean. It goes on the control plan and you just put the torque/angle spec on it.

What should the spec be? I can't tell you that, that comes from design engineering and is based on: the bolt material, the material you are threading in to, the preload on the bolt you need, IF the bolt has loctite or lube on it, among other things.
 

ncwalker

Trusted Information Resource
#7
And I missed comment. It very much is NOT a default. It is driven by your design. Consider your lawn mower engine ... You have little M6 machine screws that are holding on little band clamps that hold your spark plug wires, or your fuel filter, attaching these to the block so they don't rattle. Those are probably default. But be careful - there's all kinds of tables in the Machinery's Handbook, which are usually steel bolt in steel thread. Your lawnmower block could be aluminum, that means different defaults.

Now consider the bolts that hold on your head. Those could be M10s, maybe M8s. I can assure you, THOSE are not default. (In a car, head bolts in newer models are actually torque to yield bolts, DO NOT REUSE).

I have no idea what your application is. But don't assume "use the defaults" is good for threads. Talk to your (or their) design engineer.
 

optomist1

A Sea of Statistics
Trusted Information Resource
#8
yep, good little treatise.....as part of the assembly process most if not all assembly points monitor (and store) Torque & Angle, (as well as perform key QA torque checks) as a check to ferret out any possible cross threading.....ultimately this info is key should there be any post manufacture fastener related issues.....I believe this info falls under the title of "regulatory requirements", if not just a good sound manufacturing practice.
 
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