Clamp pressure

[ABQfm]

Hello
This may be simple but I am unable to calculate this.

I have a band clamp pressing a rubber seal tight. I have the torque applied on the band clamp as 5 N-m. How can I convert this torque value into a radial contact pressure acting on the seal surface? I have all the specifications of the clamp and with some friction coeff is there any straight forward formula that I can use?

Thanks for the help

 

[BobM3]

I'd try this for starters:

The radial pressure on the clamp produces a hoop stress in the clamp. Multiply the hoop stress by the clamp band cross sectional area. This gives you the tangential force on the hoop that the screw has to supply. You'd need to convert the torque on the screw to clamp load provided by the screw. I'd guess this may get you to within 25% of the real answer.

 

[GregLocock]

You need to know the pitch of the screw in the bandclamp.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[zekeman]

2piT/2=sigma/E*2pi*D*sigma*Ac/2
sigma/E*2PI*D=p
T=p*sigma*Ac
Ac=t*w
T=sigma*t*w*p
PD*w=2*sigma*t*w
P=2*sigma*t/D
P=2*T/(w*p*D)
P= radial pressure
Ac= band crossection =w*t
w band width
t band thickness
p screw pitch
sigma stress in band
D diameter of band



























































8d0

 

[israelkk]

To my best knowledge the pressure that the band creates on the seal is not constant due to the friction between the band and the seal which will change the local pressure exponentially over the seal periphery, similar to band brake analysis.

 

[zekeman]

quote"To my best knowledge the pressure that the band creates on the seal is not constant due to the friction between the band and the seal which will change the local pressure exponentially over the seal periphery, similar to band brake analysis."
------------------------------------------------------------
I doubt it. It's not the same problem since the drum is not spinning, where dynamic friction is the friction force equation, but not in this case. Try writing the equation explaining this.In equilibrium there is no dynamic friction and the static friction must be distributed in some unknown fashion around the hoop.Good doctoral question

 

[israelkk]

zekeman

The equations for spring clutch give the same exponential pressure distribution without any movement or slip. When calculating the spring clutch holding moment before slip you use the same equation with exponential pressure distribution .

The reason I think there is a friction factor is because the band strip it pulled tangent to the circle when torquing the band screw.

 

[ABQfm]

Thank you everyone for your suggestions. This is how I understood calculating the formula. (Please look at the attached picture).

Please let me know if my understanding does not make sense.

Thanks
FM

 

[BobM3]

Yes, that looks good. Seems like there should be an efficiency factor (friction and lead angle) though for the screw/circumferential load/torque/pitch relationships. Maybe it cancels out somewhere.

 

[rb1957]

yeah, i was a bit surprised with T = P*d too ... from the bolt preload equation T = P*d*0.2

 

[zekeman]

I believe you are missing a 2pi factor ( as I did) and that stress is the maximum and valid only at the clamping point.
Also, looking at Isaelk's comments there appears to be an exponential mu*@ falloff in stress as you go away from that point.

 

[BobM3]

Is there a torque transfered through the seal into the band clamp? If not, and the seal is torsionally compliant, then my guess is that there would not be much influence on the pressure due to the band moving circumferentially during tightening. I believe there's some nonuniformity in the pressure particularly near the screw. You (the OP) may be able to make a judgment by looking at the rubber under the tightened clamp and judging how uniform the radial displacement is and how much circumferential displacement of the rubber is occuring.

 

[Bribyk]

This sounds much easier to get some pressure indicating paper and test... Your worm gear friction is going to be tough to guess right.