Preload has a huge effect on elastomeric materials. Both tangent and secant stifness change with static preload. Some materials soften, but most of the elastomers generally harden.

When using elastomer pads, usually, the pad producer provides a chart giving the dynamic stifness as a function of force applied (which is generally provided by the weight of the machinery).

Does it worth? Well it depends on what you need to do. Generally, preload on pads can be increased by adding mass between the machin pedestal and the pad itself using blocks of steel or iron. Sometimes it's useful, but you increase stifness together with mass.. hence the system gets tuned nearby the original frequency. In fact, this is usually done to limit the static deflection when loading the machine itself with process material, or to compensate for static misalignment.

To conclude, I wouldn't reccomend to preload mechanically (let's say, with a clamp) the elastomer pad, since creep is relevant for this material. So if you apply preload by imposing a certain displacement, after a while (it can be weeks) your preload force disappear. So with elastomer pads it is important to apply preload by means of constant forces.

Thanks, Alberto. You've said  "So with elastomer pads it is important to apply preload by means of constant forces.", what about constant forces that come from initial tension of bolts that clamp the pad? 

I think Alberto's last paragraph adequately addresses the why bolting does not provide a long-term solution for applying preload.

Yes, it has.

In a special case preload is very useful to achieve the full functionality of elastomeric pads => at railway tracks inserted between rail and concrete sleeper preloaded by the clamp at the railfoot to achieve a higher distribution of the axle loads over several sleepers.

It is observed that the stiffnes of the pads will change over the years but mainly due to the summed axle loads than to the preload induced by the clamps.

Preloads can be important when you are aiming for a nearly linear dynamic movement of resulting vibrating system. As long as the reduced hight - under preload plus machine loads -  is less than 10% of the unloaded thickness of the elastomeric pad, this can be achieved.

Coming back to the clamps at the railfoot - it is obvious that the vertical vibration of the rail is very non-linear, as the upper limitation of the rail movement is given by the very rigid clamps and the downward limitation is given by the increasing stiffnes of the elastomeric pads.