Screw Torque Conversion for Power Modules: EE vs. ME Perspective

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SUMMARY

The discussion centers on the torque conversion for mounting power modules, specifically comparing M6 8.8 screws torqued to 2.5-5.0 Nm against NAS 1802 screws torqued to 25 in-lbs (2.83 Nm). The customer’s use of a manual torque driver instead of the specified electronic driver with a breakaway clutch is likely contributing to the mounting failures, as inconsistent torque application can lead to excessive stress on the modules. Additionally, the potential use of lubricants on the threads, which is not specified in the mounting instructions, may further exacerbate the issue by increasing compressive stress on the plastic components of the modules.

PREREQUISITES
  • Understanding of torque measurement and conversion (Nm to in-lbs)
  • Familiarity with screw specifications (M6 8.8 and NAS 1802)
  • Knowledge of torque application tools (manual vs. electronic torque drivers)
  • Basic principles of stress analysis in mechanical engineering
NEXT STEPS
  • Research torque conversion methods and their implications on mechanical assemblies
  • Learn about the differences between manual and electronic torque drivers
  • Investigate the effects of lubricants on torque and compressive stress in fasteners
  • Study the characteristics and applications of self-locking fasteners
USEFUL FOR

Mechanical engineers, electrical engineers, quality assurance professionals, and anyone involved in the assembly and testing of power modules will benefit from this discussion.

Windadct
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Hello MEs -

I am an EE and have customer that insists in not following our qualified mounting instructions for a power module:

Our instructions M6 8.8 screw torqued to 2.5 min to 5.0 Nm Max.

Customer is using NAS 1802 torqued to 25 in lbs -->> Is this a suitable conversion?

Of course they are breaking the modules - they have 10 installed, 8 with mounting hole cracks, and we make about 100,000 of these a year. I have customers in my territory using 2-5K per year - and never a problem of this type. We are a high end German co, that does not mess around - the modules are rigorously qualified.

And of course - the customer is sure the modules are defective. They swear that the torque they are applying is correct - but they really have to be doing something to break the modules.

Also - they are using a manual torque driver - the type that clicks when the torque is reached, our spec is for electronic driver with break away clutch - much more consistent
 
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The two screws seems somewhat comparable. I'm assuming they're using a 1/4-28 screw out of this standard: http://www.av8design.com/Hardware/NAS1802.pdf.

Diameter: .250" (6.35 mm) vs. 6mm
Strength: ~160ksi (1100MPa) vs. 800MPa
Thread pitch: .036" (.914 mm) vs. 1mm

The screws are larger and stronger, but if they only torque to 25 in-lbf (2.83 N-m) they should be pretty close to your preload range. It could be they're over-torqued, or maybe they're using lubricant on the threads where you don't specify any. If the coefficient of friction in the joint is lower than expected from the calculation, a given torque will generate more preload force than expected, which might account for the problem too...
 
Hi Windadct,
Windadct said:
Our instructions M6 8.8 screw torqued to 2.5 min to 5.0 Nm Max.

Customer is using NAS 1802 torqued to 25 in lbs -->> Is this a suitable conversion?
Torque is a measure of rotational force.
T=Fr
Where T= torque
F= force
r= moment arm or radial distance from the location of the torque to the force.

So all you do to convert Nm to in lb is convert the units.
1 meter = 39.37 inches
1 Newton = .2248 lb

So 1 Nm = 8.850 inch lbs
5 Nm = 44.25 inch lb

So it looks like the 25 in lb (2.825 Nm) is not the issue.

(conversions for torque can also be found here: http://www.onlineconversion.com/torque.htm)

***

Edit: Couple more thoughts... for the torque you list, a stress analysis indicates the bolt stress is very low (on the order of 10,000 psi) but the area under the head of a bolt is roughly the same as the tensile area, maybe just slightly higher. So the compressive stress on your module, which I'm assuming is plastic, is roughly the same. That much compressive stress will break most unreinforced plastics. And as Mech Engineer mentions, adding a lubricant to the thread will increase that compressive stress substantially.

Another thought that occurs to me is whether or not your standard torque assumes a self locking fastener is used. A self locking fastener, such as one with a nylon insert to prevent loosening, also creates a torque on the fastener that resists tightening. In other words, if you put 5 Nm of torque on a self locking fastener, it's like you're increasing the friction tremendously so the final load on the part is reduced. If your company assumes a self locking fastener is being used and they're not using one, the result will be a much increased force on the part which could be causing the breakage issue.
 
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