Torque vs sliding force in clamped joint

[sprintcar]

Any spreadsheet calculators or shortcuts to calculate resistance to sliding force in a clamped joint? Steel on steel (clean and dry) in a machined way, multiple bolts in a row along the axis of applied force.

Force --> [ o o o ]

We used the clamp force generated by all the bolts, the contact area of the machined surfaces and friction coefficient of dry steel on steel. The calculated value was well above the axial force, but the part still moved. I think the machined area needs to be more localized to the bolt/washer area instead of the entire plate contact area. Any suggestions?
Thanks! Keep the wheels on the ground
Bob
showshine@aol.com

 

[vonlueke]

Regardless of any particular localized contact area assumed, I think the total possible slip resistance per bolt, R, will be the bolt preload P (computed per Eq. 1 in

http://euler9.tripod.com/fasteners/preload.html

) multiplied by the kinetic coefficient of friction (COF) between the two plates, mu. For clean, dry, smooth steel plates, kinetic COF between plates might be about mu = 0.20 for slightly rough plates, or about mu = 0.17 or 0.16 for smooth, dry steel plates.

Next, consider the joint applied tensile load F. The simplifying assumption (in so-called nominal stress design) that loads are evenly distributed in a lap joint is not really true, so you can get away with such an assumption only if you use a sizable factor of safety against slip in your calculations of, say, FS = 1.70 or 2.00. Thus, in above example for n bolts in line, applied shear load per bolt could be roughly estimated as V = FS*F/n; so if R > V, i.e., if mu*P > FS*F/n, it indicates joint does not slip (for joint load F applied statically, meaning gradually).

 

[vonlueke]

Clarification regarding above COF values. I think kinetic COF between clean, dry, smooth steel plates might be about mu = 0.20 for slightly rough, squeaky-clean steel plates, about mu = 0.17 or 0.16 for smooth, squeaky-clean steel plates, or about mu = 0.15 for smooth, so-called "dry" steel plates, where "dry" in quotation marks means "having the slightest amount of residual machine oil from manufacturing" (a common condition). Also, for two, clean, unmachined, unpainted, standard structural steel members in the typical so-called "clean mill scale" condition as manufactured (notice it's fairly rough), a typical kinetic COF value might be about mu = 0.34.