Torque Transmission Through Friction Coupling Assisted with Retaining Compound

[JCReynolds79]

Hi all,

I have a tube, which has a bolt down the middle, which when tightened, pulls the end face of the tube against another face. This tube then has to transmit torque, via friction alone. I can workout the possible torque transmitted by firction, but what I am trying to figure out whether adding retaining compound (e,g. Loctite 648) between the mating faces of two abuting surfaces will actually help transmit more torque or not.

given the shear strength of the retaining compound, and the known axial load, can I calculate the torque capacity of the joint with the addition of the retaining compound? Is it additive or is it a case of whichever is strongest (friction or retaining compound shear strength) is the max value?

I think this is the formula for the shear strength of the joint, but I don't know whether the axial load needs to come into it or not...

T = t*PI(ro4 - Ri4)
2ro

T = torque; t = shear stress; ro = outer radius, ri = inner radius

Thanks in advance


Regards,

Jon Reynolds

 

[BrianE22]

I'd ask for something in writing on that. Maybe some tests they've run.

 

[JCReynolds79]

They're experience is gained from many years of experience in this field. They develop their own software to estimate joint strengths based on research carried out by universities.

One case study was a large electric motor, like large enough to power a digger type machine, which was bolted directly to the chassis on a flange with an array of bolts that would vibrate ans work loose. The manufacturer could have redesigned the joint but the easier, cost effective fix was to add a retaining compound to the joint face which prevented it from working loose again.

I think its like a lot of things that have to be taken with a pinch of salt.

Regards,

Jon Reynolds

 

[tbuelna]

Look at it like this. If you have a metal-to-metal interface at your flanged shaft connection, which is intended to transfer torque simply by the static clamping friction produced by fastener preloading, then for analysis purposes you would consider the static coefficient of friction that could possibly exist at the interface to be within a range of something like .10 to .30. But to be safe for analysis purposes you would use the minimum coefficient of friction of .10. On the other hand, if you were to apply an anaerobic adhesive compound at the clamped interface which ensured a consistent static coefficient of friction of .20 would always exist there, you would be able to show by analysis that the joint has much greater torque transfer capability.

It's not the shear strength of the compound that matters. Instead, it's the fact that the compound ensures a consistent and predictable coefficient of friction characteristic at the interface, which reduces the level of uncertainty that must be considered in the analysis approach.

 

[Tmoose]

If the clamped surfaces are "rough" and especially if they are non-ferrous I would be concerned that peaky high spots might embed or creep over time and from thermal loading, and THAT would reduce preload, which is what is really going to be doing most of the work providing torque transmission capacity.
Are there constraints keeping you tied to "relatively low preload?" Seems like cranking up the bolt torque would be the easy way to increase joint torque transmission capacity. Look at the torque spec for crankshaft and camshaft sprockets of any car. They are reliably transmitting 10-20 jerky HP from 1000-7000 rpm thru an interface under 2" diameter.

How is the drive tube centered, and are there radial loads on the tube? How does the torque get into the tube?