Press Fits, Bolt Torque and Friction!!

[tc7]

I am pressing a 30 inch O.D. x 2.75 inch thick steel sleeve into a mating steel cylinder, that has an average of (-.023 inch) diameter interference. This will be a field installation and I am using 12 each equally spaced 1/2-13UNC jacking studs/nuts (120ksi U.T.S) to pull this thing together (studs go through thru-holes in a flange on the sleeve and are threaded into holes in the mating hub. Nuts on the other end of the studs are tightened to draw the sleeve in). I have calculated a total installation force required of 162,320 pounds. I have used a friction coeficent of (.18) for this press force calculation. I based this calculation on the method described in Shigley's Machine Design text.

To determine the torque required on each jacking nut, I have used 1/12 of the total press force required and used a coef of friction of (.2). Based on other equations in Shigley, I have calculated an installation torque of 223 Ft lbs to draw the sleeve into position. This is a sum of thread torque plus torque to overcome friction between the face of the nut and flange.

NOW FOR THE QUESTION - It has been well discussed that 75-80% or more of fastener torque is lost in friction and does not result in useful axial bolt loading. So I fear that the installation bolt torques that I have calculated will not really provide the sleeve installation force that I have calculated. I am suspicious that the coef of friction values that I have used may be superficially low, but I can not find justification in the literature to use a higher value, or what the higher value should be. Please advise or comment that my approach is legitimate or outline a more appropriate method.

Thanks in advance for advice or referrals.
tc7

 

[yates]

0.2 is a fairly conservative friction coefficient for calculation purposes. You should be able to reduce this with lubrication. Since there does not appear to be any high temperatures involved with your pressing operation, I would suggest a Moly disulfide based grease, or a graphite-loaded grease. Make sure nut/flange surface is greased, as well as the thread. Typical figures for friction coeff. in bolts that have received a dry lube finish (Epoxy paint loaded with moly disulfide) friction coeffs. vary from 0.06 to 0.13 in torque/tension tests.