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

In summary, a customer is not following the qualified mounting instructions for a power module, using a different type of screw torqued to a lower value. However, this conversion does not seem to be the cause of the issue as the torque values are comparable. Other factors such as lubricant on the threads, compressive stress on the module, and the use of a self-locking fastener could be contributing to the problem.
  • #1
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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  • #2
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...
 
  • #3
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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1. What is screw torque conversion?

Screw torque conversion is the process of converting the rotational force applied to a screw into a measurable unit, typically expressed in Newton-meters (Nm) or foot-pounds (ft-lb). It is an important concept in engineering and is used to determine the amount of torque necessary to tighten or loosen a screw.

2. How do I calculate screw torque conversion?

To calculate screw torque conversion, you will need to know the screw's pitch and the force applied to the screw. The formula for screw torque conversion is torque = force x pitch. Make sure to use consistent units (e.g. Newtons for force and meters for pitch) to get an accurate result.

3. What is the importance of screw torque conversion?

Screw torque conversion is important because it allows engineers and scientists to accurately determine the amount of force needed to tighten or loosen a screw. This helps ensure that screws are not over-tightened or under-tightened, which can lead to mechanical failures or other issues.

4. Are there different types of screw torque conversion?

Yes, there are different types of screw torque conversion depending on the type of screw being used. For example, there are different conversion formulas for metric screws and imperial screws. It is important to use the correct formula for the type of screw being used to ensure accuracy.

5. What factors can affect screw torque conversion?

Several factors can affect screw torque conversion, including the type and size of the screw, the material it is being tightened into, and the lubrication of the screw. These factors can impact the amount of friction and resistance, which can affect the amount of force needed to achieve the desired torque.

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