Whats the proper way to finish a v-belt pulley groove? I have a few used pulleys and it looks like the belt was spinning in them - the insides of the V are very smooth and shiny. Seems like using them as is will contribute to belt slipping.

Maybe a light sandblasting?

The smooth/shiny appearance is normal. You can lightly scuff them with sandpaper (by hand) if you like, but even with belts that don't slip, they'll be back to the polished finish before too long.

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Too many people who were born on third base actually believe they've hit a triple.

what is important is not the finish but the profile. the sheave needs to support the taper fully without allowing the inner flat to bottom in the valley. if this happens the pressure against the tapered sides will drop dramatically and slippage and wear of the belt and pulley both will increase. if the sheave is worn the new belt will wear very quickly to conform to the worn profile and if the other pulleys do not match, as is often the case, the belt will tear apart very quickly.

some grooving is tolerable but if there is excessive grooving the pulley must be replaced.

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