Washer with Nuts !!

GA! You can make yer choice between scoring a 3p washer or a £300 machining.

Yep. They also help account for centering tolerances when mating two surfaces, like attaching a cover to a chassis which could require numerous holes to be aligned.

I don't know about your part of the world, but here we usually use washers because some half-wit in the machine shop drilled out the part's holes too big.

Male washers tend to be stronger. Anyway, what would you 'tighten' if you used a washer without nuts??? . . . . Bbb. . :-) .

Good question. To understand the answer you need to consider the purpose of the bolt in the first place.

The bolt is there to ensure that two surfaces remain clamped together. The bolt, therefore, must be capable of overcoming the forces that are attempting to separate the two surfaces. You know from your materials class that when you pull a metal rod at both ends, you develop a reaction force within the metal rod. The shank portion of the bolt undergoes the same type of stress when you "pull" it at both ends by advancing the nut.

Just as the force induced in the metal rod can be related to the elongation of the rod (assuming elastic deformation) so can the force induced in the bolt. Quite simply, you need to elongate the bolt to induce enough reaction force to overcome the separation forces trying to pull the two surfaces apart (plus any factor of safety.)

So what does this have to do with the washer? Well, have you ever tried to measure the elongation of the bolt? It is highly impractical bordering on improbable. Newer ultrasonic technology makes this possible - but try to translate all of this to Joe Six-pack or Bob the mechanic.

This is where torque and hence washers come into play.

The bolt is elongated by turning the head of the head of the nut. The amount of elongation needed is directly proportional to the induced force in the bolt that is required. It doesn't change. You need whatever elongation is needed as dictated by the required force and the material properties of the bolt.

The amount of friction needed to induce the elongation, however, can vary.

The bolt tension is equal to the normal force that the bolt head or washer experiences. The coefficient of friction, multiplied by the normal force gives you the amount of friction the bolt head or washer must induce. The resultant diameter upon which the torsion in the bolt head or washer creates to overcome the frictional force is directly related to the distance from the centerline of the bolt. The greater the resultant diameter, the less friction that will need to be applied in order to generate the required bolt elongation and hence the required clamping force in the bolt.

When you use a washer you move the resultant diameter upon which the frictional force acts out further away from the centerline of the bolt. The net result is that less torque torque is required to generate the clamping force that is needed by the bolt to keep the two surfaces from separating.

The other advantage of using a washer is that it gives you more control over the assumptions used when generating a torque specification. As you can see, if any one of the key assumptions is violated (e.g. coefficient of friction, resultant diameter where frictional force is acting) the amount of clamping force generated in the bolt can be off.

That is why it is very important that you use the exact washer or lubricant specified by the manufacturer when they give you a torque specification. If you do not use a washer where one was used in the calculation, you will not generate enough clamping force in the bolt. Same thing goes if you do not use proper lubrication. Of course, you could have the problem in the other direction if you use too big of a washer or use a lubricant with a much lower coefficient of friction. In these cases you would likely plastically deform the bolt. You might have enough clamping force to do the job this time, but if the bolt were ever loosened and then re-tightened, you would likely not have enough clamping force.

I hope this helps.

Regards,

Paul