Belt and motor manufacturers recommend that one not do so but we all
know it's done all the time. The following link has some guidelines
that are useful background knowledge.
The upshot is the measurement is the force required to deflect the belt
the proper distance at the midpoint of the span. The nominal
deflection is the span/64 and there's a table of force vs motor size
and rpm. Roughly for 1 hp motor the recommended force is 1-2 lb from
1000 to 3500 rpm.
The tension should be no greater than that required to prevent slip.
One way to observe slip if one has a timing light is to mark a spot on
the belt and use the light to observe it during operation using the
driver pulley to synchronize the light.
Not knowing anything about the application, anything more specific is
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