dont do that,
every new builder of a certain age that comes into this sport do something similar, and no matter how many times people tell them not to, they all do.

the result? broken motor/gearbox, and wasted event as you break in the first fight.

This happend 12 months ago with some new builders, 2 years ago with others, and will no doubt continue. I hope you are the type to listen to out advice, which is...

do not weld spinners direct to shafts, but drill motors will work fine if used to power the spinner using belt or friction drive for example.