As can be seen from the graph below (based upon fatigue tests),
even relatively small errors in the angle of the joint face
can have a significant effect on the fatigue performance.
By quantifying the loss of strength, decisions can be taken
as to what are acceptable tolerances for the design specification
such as what degree of joint face squareness is required.
Bolt fatigue problems stemming from an uneven joint surface are not uncommon.
Usually they orginate because of a manufacturing problem that resulted
in the joint surface not being square to the bolt hole. Frequently, the
problems stem from fabrications when the welding distortion is not adequately
An influencing factor relating the angularity error to the
fatigue endurance strength is the amount of clearance in the
thread. A large thread clearance allows the nut to rock slightly
on the bolt thread and so reducing the bending stress that
will be imposed on the bolt thread. The length of thread engagement
also has an influence on this effect.
Guideline values for the fatigue endurance limit have been
included in the latest version of the BOLTCALC
program. The program will determine the reduction in the fatigue
endurance limit that will be sustained by the bolt allowing
for the joint face angle, the thread tolerance and the length
So called self aligning nuts and washers are available, with
spherical bearing surfaces, to compensate for the effects
of joint face angularity. Such products need to be used with
care, since at small angles they will not completely self
align because of the effect of friction on the spherical bearing
surface preventing perfect alignment.