Clutch sliding and axial force from nut fastening

[thegasperus]

Dear fellow engineers!

I have designed my own (safety) clutch for some special machine. I used steel disc in the middle and on each side a ferod(some friction material) plated discs. So I was assuming it is single plated clutch. The two side discs are bolt tightened to middle disc with maedler locknut M60. The middle disc is bolted on the shaft body. I was doing some calculations (never did them before) that if the friction coefficient is 0.4, the axial force created from nut should be around 20kN - which that locknut should withstand. d1 is 61mm and d2 is 90mm - for the area. The outer discs cannot rotate on the right shaft. So the torque capacity should be around 600Nm, but the clutch slips now at cca. 300Nm. I thought that torque capacity could be increased by tightening the nut more, but the mechanics said that they cannot tighten it more. Where did I go wrong and how can I fix this problem if possible? Simplified sketch attached.

Torque cap. eq. I used from:

https://x-engineer.org/automotive-e...ing-devices/calculate-torque-capacity-clutch/

Thank you for your answers.

Best regards.

TheGasperus

 

[MintJulep]

Why don't you start by laying out your calculations?

 

[thegasperus]

Well the needed axial force is calculated from the final eq. for torque capacity in the link provided in the first post. I don't even know if I used this equation correctly.
T=(2/3)*k*((r1^3-r2^3)/(r1^2-r2^2))*Fa
The data:
d1=0,09m
d2=0,061m
T=600Nm - the torque at which I want my clutch not to slip.
k=0.4 - friction coefficient
______
Faxial=cca. 20kN

 

[hendersdc]

If the mechanics are saying they can't tighten the nut more is it possible that you have bottomed out the threaded portion of your bolt?

 

[thegasperus]

I am positive there is still enough thread on the shaft.

 

[Tmoose]

"I am positive there is still enough thread on the shaft.

what keeps the M60 threaded "bolt" from spinning when tightened?

 

[thegasperus]

There is slot on the right shaft up to the head and the shaft in fixed when tightening at the right end which sticks out. This is just simplifies sketch. But the sliding is between friction discs.