HSFG bolts in shear

[corus]

The number of High Strength Friction Grip bolts required to satisfy shear stress requirements through the tensile stress area is much less than the number of bolts required to prevent slip (using BS5950). These bolts are supposed to work by passing the load through the frictional forces between the two connections and not through the bolts. Does it matter if the surfaces slip?

corus

 

[dik]

If the bolts slip then they go into bearing and ususally have a greater load capacity. If the object is that the connection is slip critical, then the surfaces should reflect this and they shouldn't slip.

Dik

 

[CoryPad]

The owner/user of the structure has to decide if slip is allowed, and what to do if slip occurs.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Ussuri]

If you are unable to ensure the friction grip between the surfaces you should design the connection using normal bolts in bearing and shear.

One problem with friction grip bolts, especially a close bolt group, is getting enough torque on all the bolts to ensure the connection works. What can sometimes happen is as you torque up one bolt to the required rating, it loosens the already tightened adjacent bolt. You end up going round in circles trying to get all the bolts properly tightened.

 

[corus]

Thanks.
As a further basic question: Why would slip be critical? If it was a bridge that was bolted together for example, why would slip be critical as eventually the bolt must make contact with the clearance hole for shear to occur in the bolt. The paint might crack but if the slip occurred for some worst case scenario rather than normal operating conditions then would it be a problem? Are there any examples of when slip would be considered critical or even not desirable?


corus

 

[CoryPad]

In automobiles, many joints should not slip because geometry must be maintained (powertrain sealing, powertrain mounting, suspension).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[corus]

Reading from sites on slip critical connections, they refer to slip critical connections as those being subject to load reversals, ie. fatigue would be a consideration. Presumably this would not apply to an ultimate load case (considered as static loading) but to the lower loads arising from normal fatigue loading. Are the shear stresses considered for fatigue, or are the bending stresses of the bolt considered, presuming that the joint moves by the distance of the clearance holes? The design standards I've seen don't seem to clear on this case of a joint in shear.

corus

 

[Tmoose]

Even a few cycles of repeated slipping would fret and wear the surfaces in contact, reducing preload and clamping force, in a vicious cycle. Tight fitting keys and dowels are little protection against problems when the friction (from clamping) is not enough. Check any supplier of early VW and 4 cyl POrsche cranks and flywheels. 8 dowels does not prevent ruined cranks and flywheels when the 400 lb-ft gland nut clamping is not enough.