The correct tension can be calculated from the slack and the load, or the Bando tension meter can be used.
The optimum tension is the lowest tension at which the belts will not slip under full load.
Over or under-tensioning causes, respectively, damage to the shaft bearings and belt slippage.
After installation the drive should be run for 15 minutes to seat the belts before peak load is applied.
If they slip, tighten them. Check frequently during the first day of operation.