What exactly is a preload when associated with a bearing or ballscrew and how can I tell if the preload is correct?
This is a difficult question to answer because a preload becomes increasingly difficult to measure accurately after it passes zero preload when the variables become almost unmanageable. To give insight into these variables lets consider a simple precision roller bearing preloaded at the factory. It is then pressed into a tool bored housing and onto a ground shaft, each with an upper and lower tolerance. The shaft is then spun which causes friction between the preloaded surfaces, which induces heat and expansion, which again increases the preload even more. During a period of running, wear occurs decreasing the preload. Now what preload do you have?
There are ways to ensure that a preloaded device stays within efficient limits but many of the variables must be known.
In the case of a ball screw, consider that when the servo motor spins the ballscrew from stationary to 1000 rpm it requires the preloaded balls to instantly absorb the load exerted by the mass of the slide. But it is only the balls in the tracks responsible for taking the load in that direction that are affected. The balls in the opposite track momentarily become non-loaded. So what preload do you have on them?
To preload a device to perform flawlessly is like playing the violin, some can do it but most can't, and for those that can, it takes constant practice to play it well.