I have a pillar drill with an intermediate pulley wheel, so the belts may be shorter than for one without. The mechanism slides back over two circular bars and can be “locked off” and the belt is a V belt that’s flat on four sides. Seems “crude” but it’s just fine.
I have it tensioned with both thumbs pressing back lightly without too much pressure, just to take up the slack. i’ve not had any problems.
For me, it’s too tight if it’s starting to stress the bearings and I can hear a change in speed or sound. It’s too loose if the belt has too much flex (whip) and isn’t straight e.g. appears bowed like a “plucked” guitar string. Seeing the belt running helps get a feeling for what’s about right.
If it was too tight it would prematurely wear bearings, affect accuracy and pull the spindle out of alignment and absorb (so not transmit) all the power and heat up - so not last as long. If in doubt less tension is better. Like the tension on the belt, don’t stress about it too much!
Always be prepared to switch to the right speed (find a guide for drill types, diameter and materials). Most PD have more than enough power so the belt tension won’t be affecting the result anyway.
