Shaft torque connnection - anaerobic, press, or both?

[strokersix]

I'm working with an application where a shaft extension with a socket is pressed onto a stub shaft and used to transmit torque. I know that relying on a press fit to transmit torque is not the best design but we've arrived at it for various reasons. The shaft diameter is nominally 5/8 inch and both materials are steel. The connection is currently designed for .0002 to .002 inch diametral interference over about 1/2 inch length. 10 lb-ft is the needed torque transmission. Experimentally we find that the connection will slip if the intereference fit is .0002-.0004 inches. Anaerobic adhesive (Loctite 609 or 680) applied before pressing strengthens the joint to an acceptable level.

The proposed production solution is to have both an interference fit and adhesive. My concern is the effectiveness of the adhesive may be compromised in an interference fit. I am ready to suggest either a .0005-.0015 interference fit OR a .0005-.0015 slip fit with adhesive but NOT BOTH. The slip fit carries an additional benefit of hand assembly instead of pressing. I would appreciate any advice or shared experience that would apply to this issue.

Thank you in advance.

 

[desertfox]

Hi strokersix

What sort of enviroment is this device working in ie:- atmosphere, temperature etc its difficult to give advice on such a matter with so little information.
I see nothing wrong with using friction to transmit your torque and obviously you need to increase your friction via intereference if the joint won't give you what you require.
If you use an adhesive you need to check its suitability for the application and also what sort of gap filling properties the adhesive has, this sort of information should be easy to get hold of from a adhesive supplier.
How di you arrive at the interference you currently have was it calculated? I have formula here to calculate torque capacity from intereference fits if you need them I can post them in here if you let me know.

regards desertfox

 

[MintJulep]

A few things to consider:

The strength of an adhesive only soloution is highly dependent upon workmanship. Can you rely on the parts being properly cleaned and handled?

A slip fit could allow some eccentricity over and above that allowed by the manufacturing processes of the parts.

Loctite bills the 609 product as an augment to press fit.

 

[strokersix]

Thanks for the comments.

These parts are used on agricultural equipment in outdoor environments. Over time I expect some corrosion will increase the torque capacity. That doesn't help on a brand new machine though.

desertfox: I must confess that I did not attempt to calculate torque capacity. I simply took my best guess as to what was manufacturable. There were no problems with these assemblies until we received parts that were outside the print tolerance. Now we are looking more closely at the design to avoid future problems. I would appreciate it if you would post the formula for expected drive torque capacity or send a link to where I can find the formula.

MintJulep: The last operation on the shaft extension is bore sizing so the steel is bare but with machining lubricant residue. If we go to adhesive only I would consider having the bore sized before plating. This would ensure a clean bore but would add uncertainty due to plating thickness.

 

[desertfox]

Hi strokersix


Here are the formula you need:-

Torque that can be transmitted by an interference fit
without slipping:-

T= f*Pc*3.142*d^2*L/(2)

where f=friction coefficient
Pc=contact pressure between the two members
d=nominal shaft dia
L=length of external member.

to calculate Pc for a given interference use the formula:-

Pc=x/[Dc*[((Dc^2+Di^2)/(Ei(Dc^2-Di^2))+....................
(Do^2+Dc^2)/(Eo*(Do^2-Dc^2))-((Ui/Ei)+ (Ui/Eo))]



where x = total interference
Dc=dia of the contact surface
Di = dia of inner member
Do= outside dia of outer member
Uo=poissons ratio for outer member
Ui=poissons ratio for inner member
Eo=modulus of elasticity for outer member
Ei=modulus of elasticity for inner member

This formula for Pc will simplify if the materials are the same.
Hope this helps

regards desertfox

 

[desertfox]

Hi strokersix

Forgot to mention that Di is zero when the inner shaft is solid ie:- when the inner shaft is hollow or a cylinder then Di has a value.

regards desertfox

 

[strokersix]

Thank you desertfox, I'll give it a go.

 

[desertfox]

Hi strokersix

Thanks for your reply let us know how you get on

regards desertfox

 

[strokersix]

Using a chart in my Marks' 6th ed. p. 8-59 for the interface pressure and a friction coefficient of .2, the calculated torque capacity matches what we are seeing experimentally. For rework of loose shafts we'll use adhesive. We have not yet decided what to do for new production. The friction coefficient of .2 seems low for pressed steel components. Perhaps cleaning the parts first to remove machining lubricant residue would help. Of course then the pressing operation may be more difficult. We'll see. Thank you for pointing me in the right direction!

 

[desertfox]

Hi strokersix

Thanks for letting us know how you got on and glad to be of service.

regards desertfox