Preloads

Preload is the level of stress within the bolt when the nut is tightened, with the assembly carrying no load. Adding a working load to the assembly, by increasing the pressure inside it for instance, increases the stress in the bolt once the preload stress has been balanced by the working load. When the working load is removed, the preload stress remains.

"increases the stress in the bolt once the preload stress has been balanced by the working load."

If it would be so all bolts would fail !

Do you mean bearing? it is quite different from fasteners .

However for the purpose of preload if you do mean it,

The fasteners(and bearings) are preloaded to a particular designed value. The pre-load means loading the components to the same direction as the final load is going to be.

Thus if it is a bearing, the pre-load is to reduce the clearance. If it is a fastener the pre-load is to tense them.

By this application of pre-load, the joints are stiffened.

In a stiff joint, the load is shared between the joint and the joiner( ie fastener/ bearings). Lower the stiffness, more and more of the % is taken by the fastener and more and more it is stiff, lesser percentahge is by the fastener.

Doing this, in case of a dynamic load, we are reducing the stress fluctuation on the fastener/ bearing and that reduces the fatigue on it.

Thus if a bolt is keeping the flanges together(zero stiffness- ideal, not physical, unless it is toltally loose) Any separating force will be totally taken by the fasteners and as it releases or a compressive force is applied again the total is transmitted to it. Where as in a fully stiff joint (not physically possible), whole of these variation is taken by joint, no additional stress passed on the fasteners.

In practice we work in between these two extremes, ensuring the fluctuation in the joint as well as fasteners stresses are well within the elastic and fatigue limits of both.

Almost similar (a bit different though , but much more complex) exercise is done in case of the bearings, where again the bearing stiffness, along with the load transmissions (amount, direction etc) and the end-plays are taken into account and the pre-load is decided.

"Thus if it is a bearing, the pre-load is to reduce the clearance."

Not only, the preloading of a bearing (balls or rollers) has a much more important reason. It will increase life expectancy due to the the fact that the ratio minimal stress/maximal stress comes nearer to 1. In not preloaded bearings the cycle is pulsating between zero and maximal load. An other aspect is sliding of balls/rollers. If the bearing is not preloaded in some positions the friction between separators and balls/rollers is higer than the one to the rings so that the balls/rollers slide on the ring. This is not good due to the minute flats it generates.

You are right- the avoiding of sliding movement is critical in the antifriction bearings.

there is a good article on this

http://www.bardenbearings.com/PDF%20CD/PB11.PDF

Almost all the principles are in use with us including the spring loading in a few applications.

also there is one bearing expected life curve - i don't know whether it is available in soft somewhere- it was discussed in the then Torrington seminar once - in early 90s. It was very informative.

clearance- preload- arrangement- life (on the roller brg area- balls we are not in the field )

A very good paper on preload in taperd bearings it shows a lot of similatrities with bolted assemblies.

http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=1_0_86

Here's a simple description: A bolted fastener acts as a "spring" when tightened. Preload is simply the amount of force trying to return the bolt to its original length.