For those that care, I managed to solve this problem by experimenting with several Hoover vacuum cleaner belts. One fit snug but when the motor revved up centrifical force caused the belt to actually lift free from the drive spindle. The belt is made of rubber, unlike the impregnated rubber weave found with the stock belt, and is designed to stretch substantially. (This tells you why the stock belt is designed to fit snug and not stretch much at all.) I eventually tried a Hoover belt that stretched and fit fairly tight and it worked fine. I am back to using the stock belt, but the Hoover belt is in my spare-parts box, ready to be used when the original belt wears out.I have a number of tools, and lately I have decided to stock up on spare parts (belts, brushes, guides, etc.), just in case down the line something gives way and the company informs me that the parts are no longer available. No use taking chances, so I am stocking up on some critical items right now for the future.
Anyway, I have a Delta SH100 table-top router/router that I tried to get a spare drive belt for and, wouldn't you know, on the phone the Delta people said the part was no longer available. If you go to the Delta web site the entire tool is no longer available.
My question is: does anybody here know of a source for the belt or something similar? Perhaps there is a substitute that is available from some other company or perhaps some parts outfit, somewhere, has a stockpile of parts. The fact that Delta probably had spare belts on hand at one time but now no longer does tells me that the belt might be prone to break.
I did remove the table from the unit and take a look at the belt. It is surprisingly thin and flexible (quite unlike a V-belt or segmented belt) and about an inch wide and drives the spindle shaft 3.5 times as fast as the motor rotates. It is about 15 inches long. The thing appears to be made out of some kind of rubber coated (both sides) woven fabric. It probably will last for years, but if I get a spare I know it will. Belts never break if you have a spare on hand.
Yes, I know that some people hate this particular tool (a replacement for the earlier Delta 43-505 model), but I have modified mine a bit and some of the problems others have had have not bothered me, anyway. The mods include my own design table extension (wood, of course) and a modification to the dust collector that makes it much more effective. I have also replaced the clunky fence with a single-piece version of my own design. The table works fine, although I wish it had variable speeds. It is a budget tool, but it works fine for the projects I have in mind.
In any case, I would like to have a replacement drive belt on hand, just in case the existing one breaks in the future. This experience with Delta tells me that my parts-stockpiling approach is not a bad idea. The bad thing is that I did not do this with this Delta shaper/router sooner.