Here is a replacement motor coupler.  This fits Busch / Hill-Rom vacuum pumps.  This one fits the 10 HP versions - RC-0250C.  If you are taking the time and expense to rebuild you pump, you might as well replace the coupler.  This is cheap insurance against a prolonged shut-down.

We repair pumps and have seen many pumps over the years, where the coupler failed, leaving the customer with a non-funtional pump.  It's the first thing I would recommend checking if your pump is not working.  With a spare motor coupler, it is a simple matter to unscrew the bolts and swap out the coupler in a matter of minutes. Contact me for additonal information or for other repair parts or filters.  I can also provide rebuilt pumps.  I have several of these couplers, in case you are interested and will gladly combine shipping.