Torque Transmission Through Friction Coupling Assisted with Retaining Compound
Torque Transmission Through Friction Coupling Assisted with Retaining Compound
(OP)
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
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





RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
The formula you have quoted is the the standard shear stress equation for a tube in torsion, you need the preload force of the bolt between the two faces its clamping, whatever this preload force is, you then mulitply it by a friction coefficient say 0.2, from that you can then work out how much torque you can apply to the tube before the faces slide.
what happens if the torque is applied in a direction tending to undo the screw?
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
So I can calc frictional torque and I can calc shear torque of the bond...but how do I bring it altogether when I actually bolt the joint together with the 648 in place? Do I get something better than the frictional torque or something different?
Nereth1, I think you are right in a way, but unless I calculated it wrong, the shear torque of the compound alone was rather less than the frictional torque. So does it contribute nothing?
Regards,
Jon Reynolds
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
If you are using an adhesive it sounds like you are going for a permanent or semi permanent bond. It would be exceptionally easy to serrate these parts so they key together, or there are a lot of other ways to achieve the same. Alternately are there frictional compounds or coatings that can be used that can actually leverage the very high contact forces you will likely be able to generate with your threads?
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
Regards,
Jon Reynolds
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
The friction between the abutting faces is sliding friction and should be based on 0.2 * the bolt preload! that gives the frictional force needed to be overcome in order to get the surfaces sliding over one or other.
Once you have that friction force, I would assume the friction force acts at the mid radius of the two faces and use that mid radius to calculate my limiting friction torque.
Hope this makes sense
Desertfox
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
What I am questioning is whether adding a retaining compound (an adhesive) on that joint face would have a positive or negative impact on possible torque transmission.
Regards,
Jon Reynolds
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
Glue thickness = "very thick" --> glue
Glue thickness = "thick" --> glue
Glue thickness = "thin" --> glue
Glue thickness = "very thin" --> glue
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
Sorry I did miss your point however it's possibly because you mention using the surface area of contact, normally unless the pressure is very large, friction is independent of contact area and I don't see any sizes given.
Also in my first post I asked how you would prevent the screw from loosening if the torque was applied in an anti-clockwise direction?
As regards the compound being placed on the mating surfaces I would say not a good idea because you would need a gap between those surfaces which means your bolt isn't pulling the faces tight and I doubt whether the bond between the two surfaces will be as good as the frictional force as you yourself mention.
I would however be tempted however to put locking compound on your bolt or screw just prior to tightening.
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
Thanks all for replies.
desertfox, I agree, definitely will be putting thread lock on the thread. This seems to add a lot to the breakaway torque.
tbnelna, this sounds more like what I think might occur. I do not envisage an actual thickness of compound separating the two metal surfaces, especially as it is clamped up. I envisage it filling all the potential voids, effectively increasing the coefficient of friction of the joint from the basic metal on metal approximation.
Do you have any data/references to help me estimate this increase in coef.fric.?
Is there any danger of the compound breaking down leading to loss of preload?
Thanks.
Regards,
Jon Reynolds
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
Is it possible to place a annular groove in the mounting face about .002" deep? This would ensure a bonded layer which can be calculated. The remaining metal to metal contact (assuming the glue is squeezed out) could also give a calculated friction torque.
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
So, one can ADD the shear strength of the face bonded joint to the frictional strength of the metal on metal clamped joint.
This has been very interesting. Thanks for the input guys.
Regards,
Jon Reynolds
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
If it fills all small voids due to faces not being flat how will you know how much adhesive you have in the joint, in addition if they had said you cannot rely on adding the adhesive would you still buy some?
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
He also said, which is intuitive, that the larger the preload, if you are clamping the faces together, the less of a role the compound will play, as obviously the clamped friction capacity will increase with preload whereas the shear strength does not change. So it's a case of deciding whether you get any good returns or not.
In my case the joint is pretty small with relatively low preload, so I would benefit almost 100% from adding some compound. If it was high preload and the compound only contributed say 5% then perhaps one would not bother adding it.
Regards,
Jon Reynolds
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
I don't think I would rely on the adhesive personally, I would go with the friction which is also highly variable but I would do some practical tests on the joint to establish a coefficient of friction
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
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
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
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.
RE: Torque Transmission Through Friction Coupling Assisted with Retaining Compound
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?