Fastener Quality Act Information

Fastener Quality Act Information
This link to the National Institute of Standards homepage on the Fastener
Quality Act as an aide to individuals who need detailed and complete information on the
Act. Click here to have the most detailed information on the Fastener Ouality Act at your
fingertips.


Proper Tightening of Fasteners
Threaded fasteners are tightened for the obvious reason of clamping parts together and
transmitting loads. In gasketed joints, the purpose is to prevent leakage. In other joints,
the clamping force is developed to prevent the parts from separating or shaking loose.
The proper amount of tightening (or pre-load) is important. If the fasteners are too tight
they may break - either during the tightening itself or when the working load is added to
the pre-load in applications such as gasketed joints. If too loose, the fastener will shake
loose in vibration. Often overlooked, but equally important, is the tendency of fasteners
subjected to cyclic loading to fail from fatigue if not sufficiently tightened.
In normal joints, the clamping force should equal the working load. In gasketed joints, it
should be sufficient to create a seal.

EFFECT of FRICTION
When torque is applied to a fastener it (1) overcomes friction to turn the fastener and (2)
stretches the fastener - however slightly - to develop the clamping force. The latter is
considered the useful part of the torque.
It has been estimated that between 50% and 80% of the applied torque is needed to
overcome friction. As a result, failure will occur in the fastener before the axial strength,
determined by tensile testing the fastener, can be reached.
It is evident then, that the better the lubrication on the fastener the more of the torque
energy will be converted into actual clamping force.
The type of lubricant used has a definite effect on how much of the torque is needed to
overcome friction. Molybdenum disulfide, wax, and white lead are good lubricants;
cadmium and silver are fair, and machine oil is considered poor.
Published torque values are for average conditions. If the bolts are coated with a good
lubricant, threads help maintain a more consistent torque-tension relationship. Other
factors affecting friction are hardness of the material (generally, the harder it is, the less
friction) and type of material (aluminum is less "sticky" than steel but more so than cast
iron).


DETERMINATION of TORQUE
All fastener materials are slightly elastic and must be stretched a small amount to develop
clamping force. As a rule of thumb, a stretch of .001 inch per inch length of a fastener
develops about 30,000 psi clamping force in steel and 15,000 in titanium.
The best way to determine the correct torque is to run tests on the particular joint by
tightening five bolts until they just begin to yield. The optimum torque is 80% of this
value.
Another method is to tighten to 50% to 80% of the ultimate tensile strength on steel parts.
Pre-loading to 50% to 60% of the strength of the fastener is used because of the many
variables. The fastener should not be loaded above its torque yield strength, and the lower
values will generally keep the loads below this point. However, the more accurate the
method of controlling tightness the more of the strength of the fastener can be utilized.