• Should I use a washer under the bolt head?
  • What kind of washer? Flat? Spring / lock? Taper?
  • Should I use a lock nut?
  • Should the lock nut be another full sized nut, or a half nut or a specialised self locking nut?

Lots of questions arise when trying to decide how to assemble your nuts, bolts and washers, apart from what materials and sizes to use. The questions are quite straightforward, but perhaps the answers are not.

It all depends on use I suppose.

We tend to think that we can use a flat washer under the head of the bolt to help spread the load under the bolt head and nut face. But they can move during tightening and cause problems in that the washer can then rotate with the bolt head during tightening which can then affect the torque tension relationship.

That said,  if used correctly and carefully in a non critical situation they should do a perfectly adequate job and help prevent deformation of the hole.

Nuts and bolts generally work loose as a result of transverse loadings causing slippage. If your application has this kind of movement then you need to take care. Often spring / lock  / split washers are used, but it can be shown in tests that using these types of washers can in effect be worse than not using them at all (See here for a great video showing this test)

An alternative is to use the two nut system. Often utilising one full nut and one half nut. Again, this can have problems. The traditional thinking is that you put the full nut on and torque appropriately, then put the half (lock) nut on and tighten it up to the full nut. But it can be shown using tests that it may be better to put the half nut on first – IF YOU APPLY THE TORQUES CORRECTLY, but that is not easy. (An explanation of the tests is here)

So if not using lock washers what else? Well there a wide range of thread locking systems, that can be classed as either :

  • Free spinning (plain bolts with circumferential teeth that lock into the bearing surface)
  • Friction (where distorted threads, or plastic inserts ‘jam up’ the thread to prevent loosening
  • Chemical (Use of adhesives)

Graphskill offer flat washers and spring washers : http://bit.ly/mJuAZl

We have two styles of Friction self lock nuts, Aerotight and Prevailing torque: http://bit.ly/msmI47

If you want to read more about this fascinating subject then this is a great place to start: Bolt Science – A great organisation dedicated to all aspects of bolt fastening issues and problems.

They discuss

  • Tightening methods
  • Failure examples
  • Torque Tightening
  • Tighten the nut or the bolt
  • And much more