Product Manager Seeking Bolt Clamping Force Help

[MarktheV]

Hello all,

First, I'm a PM working with contract designers/engineers, so please excuse my ignorant questions

I'm in the sporting goods field and working on a handlebar clamp system in which I have a concern with and looking for help.
My question is how can I verify if a metric M6x1 bolt will have greater clamping force compared to a M5x.8 bolt at a given torque value of 5Nm> Both being zinc-coated steel and threading into aluminum.

Issue is the current clamp system uses metric M5x.8 zinc-coated bolts, which is allowing the bar to slip/rotate. I've modified a version to use larger M6 bolts, which have improved clamping force at the same torque. My problem is confirming this mathematically.
I've tried to calculate axial bolt clamp force but am not able to asses the results. Bolt Details:
M6x1 Maj Dia= 5.974mm, Coe Friction value of .18 for dry thread zinc coating, 44.25in-lb torque, Yields 41.17 axial clamp force.

M5x.8 bolt Maj Dia=4.97mm, Coe Friction same at .18, same torque of 44.25in-lb/5N. Yields 49.46 axial clamp force

From this can I conclude the M5x.8 yields greater clamping force??

[Kelly_Bramble]

First, in your calculations you're mixing in and mm. The 6 and 5 mm bolts diameters should be converted into inches if you are going to use in-lbs torque in the calculations.

Clamp force:

5 mm (.1968") = 1,249 lbs

6 mm (.2362") = 951.98 lbs

Be aware that this is the clamp or axial tension force applied to the FASTENER. The actual force needs to be looked at in terms of psi on the parts related.

Also, the torque seems way high for the diameter of the screws.

See:

http://www.engineersedge.com/torque.htm
http://www.engineersedge.com/calcula...orque_calc.htm

and

AC+2000Paper517.pdf

[Kelly_Bramble]

First, I'm a PM working with contract designers/engineers, so please excuse my ignorant questions

Why are you involving yourself in the design engineering details as opposed to managing schedule, meetings, communications, purchasing, etc..?

[MarktheV]

Hi Kelly,


Thank you for catching the in/mm mistake.

In terms of involvement I am wanting to better understand so I have better product knowledge for producing later iterations of these parts. I also work closely with usage testing for consumers so having the experience helps me offer better design criteria. I understand that PM’s have different levels of involvement in different **********. I guess with bicycles PM’s are intimately involved!


Somewhere along the line before I was involved someone designed this clamp system using M5 bolts that aren’t performing adequately. In investigating I machined the system to use M6 bolts of the same material. In testing the clamps with a digital force gauge the M6 system held higher stress loads under the same 5Nm torque, so I want to better understand this on paper to convey the message.


Thanks again,

[Cragyon]

To use the M6 there was less material on the clamped parts because of the larger diameter threads and probably clearance hole for the screw. Less material on the clamped parts changes the deflection characteristics and the effective clamp force.

I doubt it held "Higher Stress Loads" but had more friction due to the changed deflections.

You should ask the engineers for a stress, deflection and friction analysis of the clamped design - it sounds marginal.

[Kelly_Bramble]

You should ask the engineers for a stress, deflection and friction analysis of the clamped design - it sounds marginal.

I think a quality of manufacturing investigation should also be conducted. Could be nothing is wrong with the design but manufacturing variables or non-conformance is the real challenge.