Nuts and Bolts!

Nuts and Bolts are so often misused and damaged, generally caused by over tightening, mixing threads, and poor lubrication.

There are two general principals for choosing a thread, for soft materials(Aluminium, Copper, Bronzes, Cast Iron) it is usual to use a deep coarse thread, Whitworth, UNC, or the coarse Metric equivalent. In steels and harder materials a fine, self locking thread is more suitable, BSF, UNF, and the finer Metric thread. The term self locking thread is used for the finer threads because the helix angle of the thread is smaller and able to withstand an axial load without tending to unwind. The coarser threads have a larger helix angle and in some circumstances the applied load will cause them to unwind or undo.

Over Tightening It is very easy to over tighten a fine thread, and to combat this the length of a spanner is controlled to reduce the amount of torque that can be applied and even that cannot prevent some enthusiastic tightening to the point of thread failure. In practice the feel of a fully tightened nut should be firm to the grip of the spanner. In the case of a box spanner with a tommy bar through it where two hands can be used, the tommy bar should be set central in the spanner and this will halve the amount of torque each hand can apply. A stretched thread generally indicates an over tightened nut. The downside of a stretched thread is that the integrity of the thread is compromised and will not support the correct load.

All threads should be lubricated prior to fitting. There are two reasons for this, one is to ensure that the threads run freely and secondly to ensure that they are tightened correctly. A dry thread has two consequences first that the two components(the nut and the bolt) are generally of the same material and therefore "pick up" during the process, and second all quoted torque values(if you use a torque wrench) are for lubricated components. This is particularly important when tightening a long stud, if a nut has picked up it will twist the stud. The lubrication of the thread also applies to the washer beneath the nut because this will give the wrong reading for a torque wrench too, and the wrong feel to the spanner.

An exception to the rule to lubricate is the fitting of a steel bolt into cast iron. Cast Iron has a high carbon content in the form of graphite which acts as a lubricant. It doesn't prevent corrosion between the bolt and the cast iron so some sort of assembly paste is a good solution. When fitting a lubricated bolt into a blind hole in cast iron it is possible that the hydraulic pressure build up in the void as the bolt is tightened will be high enough to damage the cast iron. To avoid this a shallow saw cut down the thread on the bolt will relieve the pressure.

Mixing Threads It is easy to mix threads if you keep a box of odd nuts and bolts, as most people do if they work on their own cars. The simple test is to see if the nut will go on by hand on a clean thread. If it only goes one to one and a half turns there is a difference in pitch. If it runs on easily and the nut wobbles on the bolt, there is a difference in diameters and there may also be a pitch difference. Be wary of mixing Unified(UNC, UNF) and BSF and BSW, in many sizes the pitch is the same but the angle of the thread is different. The right way to check is either with a thread gauge or, for external threads, put two threads together, side by side so that they interlock and see how well the match.

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