Bolt Torque, Safety Wire, and Thread Locker
There are not many redundant bolts on the critical systems of a vehicle. This being the case, it is very important that you take proper care to tighten all bolts correctly and give them the proper attention to make sure that they have not loosened. This post is just meant to outline some of the schemes used to secure bolts and keep them fastened.
Generally, the purpose of a bolt is to clamp two or more pieces together, this clamping
force is produced when a bolt is tightened and a strain is produced within the bolt. When a bolt is
torqued it stretches slightly and the female and male threads create an ever tightening interference fit.
When the wrench is removed, this strain remains and will exert a clamping force on the joint, this is
what will prevent the parts from moving relative to each other and the bolt from backing out. The
level of strain (stretching) in the bolt is the critical value when determining how much to tighten a
bolt, the trouble is that in most joints the stretch of the bolt is impossible to directly measure. It is
possible to calculate the strain in a bolt using the torque applied to it, but the amount of friction
(therefore the torque required to produce a given strain) depends on the thread class, lubrication,
material, and plating. Many tables exist that contain the recommended torque for a given size and
class of bolt and some even note what kind of plating they are for. These tables are generally a good
guide but be aware that the table may not necessarily apply to your specific application. Whenever
possible, torque bolts to the manufacture’s recommended value. Our 60pt Prep Sheet contains most
of the critical torque values for the Lotus Elise/Exige. More info about the benefits of the 60pt Prep
can be found at a previous blog post HERE.
A properly torqued bolt is not only much less likely to come loose; it also has a much longer
fatigue life than one that is not tightened enough. Bolts usually break not from a one-time overload
but from fatigue, so torquing bolts and keeping them tight will go a long way to keeping your
vehicle working properly.
A quality torque wrench should be in your tool box.
Paint Marker, Safety Wire and Threadlocker
Sometimes properly torquing a bolt is not enough to keep it from coming loose. The first step to keeping a bolt tight is to inspect it at reasonable intervals. The easiest way to inspect a bolt is to put a bit of paint marker on the head of the bolt and the part it is screwed into, this will allow you to quickly inspect the tightness of a bolt with just a visual inspection.
Threadlocker is a common way to keep your bolts tight as well. There are many formulations and strengths available for different applications. The most common ones used in our applications are Loctite #243 which is a blue medium strength compound and Loctite #271 a red high strength compound. When using thread locker, make sure both surfaces are clean or it will not adhere properly. When dealing with thread lockers, more is not necessarily better a little dab will do you and excess locker will just squeeze out and make an unsightly mess. One final tip is to never use red Loctite on small bolts that may have to be removed one day; you will probably strip the bolt out before you remember that you used red Loctite on it and need to heat the fastener before you try to remove it.
Safety wire is another way to keep bolts in their place, it provides a mechanical means of bolt retention. There are a couple different philosophies when it comes to safety wire. Some prefer to put safety wire on very tight to stop a bolt from loosening. It is my opinion that safety wire is only there to prevent the bolt from coming all the way out, it buys you enough time to detect the loose bolt before it causes a catastrophic failure. Additionally, in some applications you cannot pull the safety wire tight because the bolt much rotate, an example of this is how TMI wire the bolts on the steering knuckles of the Ariel Atom. Whichever your philosophy is on the subject, it is always advisable to use the proper pliers to twist the wire and to route the wire in such a way that if the bolt loosens, the wire will tighten. Safety wire requires drilled bolt heads so it is very time intensive to implement if bolts are not readily available predrilled. We generally do not safety wire any bolts unless they comes safety wired (like on the Atom), a combination of proper torque, threadlocker, and frequent inspection are our recipe to fastener care.
|This is how NOT to safety wire a bolt. Note the sloppy wire and lack of paint marker.|
|This bolt is properly safety wired and paint marked. It also happens to be torqued to spec.|
Hopefully this post has been informative and shown the importance of bolt care. An entire book could be written about the design and care of the bolted joint and in fact one has! An extremely good resource if you would like to learn more is a book called Nuts, Bolts, Fasteners & Plumbing Handbook by Carroll Smith.