TC BOLT
ASSEMBLY
Possible causes of bolt failures
The
functionality of tension control bolts depends on many
factors being designed, manufactured and handled properly.
If any of these elements are not adequate it can cause the
bolts to not meet the minimum tension or for a part of the
bolt to fail. Although there may be isolated issues where
only a few bolts are affected such as being left out in
the rain, there can be issues that cause mass amounts of
bolts to fail. If these issues are not caught before they
are used it can result in catastrophic failures and
damages. However, these issues are often hard to detect
and even if an issue is detected it can be extremely difficult
to pinpoint the exact cause.NOTE: The cause of the bolt failures in the pictures shown below have not been determined and are just posted under the most likely cause.
Material issues (too strong vs too weak)
The chemical composition of the material is clearly defined by ASTM (American Society for Testing and Materials) Standard. This is because the properties of metals varies in many ways depending on what chemicals are added and how much. For example, by adding more carbon to iron creates a strong but more brittle steel, whereas less carbon produces a steel that is not as strong but is more ductile (bendable). Both have their uses but if you accidentally use a material that is too brittle or ductile it can cause a lot of problems.Now let's look at how this can affect the tension control bolts. Consider if the material has too much carbon making it stronger but more brittle, the threads would be able to withstand a greater force, however, when it becomes too great, the threads will shear off and most likely to be a more instantaneous failure. This would most likely cause the measured tension to be much greater than the minimum requirement and can cause the threads to fail before the spline even shears off.
A similar scenario can happen if there is too little carbon. In this case you will most likely not reach the minimum tension. If the threads are too soft then they will fail first or the splice will shear off before reaching the proper tension.
Photo by UAF Contractor
Hot-dip Galvanization
Hot dip galvanization is specifically not allowed per the AISC and ASTM codes because it can weaken the surface of the material. In particular it can weaken the threads causing similar problems as discussed in the previous section. Since it is more isolated to the surface it is more likely to cause thread failures before the spline snaps off.
Mechanical galvanization
is permitted but it can still have a small effect on
the strength of the material. So if the bolts are not
expected to be exposed to the elements it is generally
preferred to go with non-galvanized bolts.
the geometry
The geometry of the bolts
is another very important feature of the bolts. There
are many geometrical elements to pretension bolts from
the shaft being centered on the head, the proper depth
and width of the groove for the sheer pain, to the
threads which are the most tedious of the dimensions.
The threads are very defined to the very last detail
in ASME (American Society of Mechanical Engineers)
Standard from the angle of the thread to the gap
between them. Improper geometry can cause a number of
different failures depending on the compromised
component.
Photo by UAF Contractor
Photo by UAF Contractor
Lubrication
Lubrication
may not seem like the a factor that could cause a failure
but you would be surprised. As we discussed on the Friction and Torque page,
friction is based on the friction coefficient between two
surface and when you add or remove lubrication you are
changing the friction. If you have too little then the
friction coefficient will be greater which can cause the
torque to increase before the proper normal force is
achieved causing the spline to snap off prematurely.
Per the ASTM F3125 11.3 coatings and lubrication is not
permitted to be added after final assembly lot testing.
Essentially this means that only the manufacturer is
permitted to add lubrication to the bolt assembly. Also,
because the lubrication plays an important role and can
potentially be affected by water it is important that the
bolt assemblies are properly stored and kept dry. 