nevile
it is most important to make sure that the allignment is spot on , any play and the vibration will soon shatter the rubber, a good idea is indeed to get an larger one than you need. take a look at fenners couplings, I believe if you have the figures output/revs ect you will source what you want.
roots blowers...... m. H alliday should be able to answer you their,
click on the pdf file