First of all, the Garlok brand name represents is a good one. The Garlok belt is definately not a cheap one (produced in Paragould, Arizona, USA).
That being said, high initial belt stretch will depend on factors such as the type of carcass weave, the amount of fabric tensioning during belt manufacture, the type of material used in the warp strands, the tension used during belt rolling, et cetera. Generally, nylon warps will give substantially higher stretch than polyester warps. Loosely rolled belts will have a higher initial stretch than tightly rolled belts. Belts made with highly tensioned carcasses will stretch less than belts of the same carcass material produced under low tension. Etc., etc.
As far as pretensioning is concerned, it only helps to get rid of the minor stretch factors - such as belt roll tensioning, unless you apply a lot of force. Even then, the only part of the belt that gets Pre-Stretched is that section close to the weights - unless you prestretch for a long time (30+ days), which is too long to tie up equipment. The idlers, pulleys, and carcass weft strands all act as tension modifiers as you move away from the pretensioning wieghts (or force) and the initial stretching of the rest of the belt will start once you start to move it around the system. That is, in my opinion, pretensioning is of very limited value.
If you know that you have a belt that is likely to stretch past your take-up allowance, you would be better served to start off with mechanical fasteners and take them out once the rate of stretching has stabilized.
Hope this helps.
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