The proper tension for operating a V-belt drive is the lowest tension at which the belts will not slip at peak load conditions.For V-belts, after initial installation tensioning, a re-tensioning of the belt is recommended after a period of operation, usually one to two days. Belt tension should be checked periodically, about every three to six months.Under-tensioned belts can slip, generating heat that often results in cracking and eventual belt failure. Over-tensioned belts will create excessive stretching in the belt and reduce both belt and bearing life as the bearing loads will increase.Measuring belt tension by deflection evaluates the force needed to achieve a given belt deflection.A tensiometer is a tool that measures the force required to move the tensiometer plunger a given distance. This force can be compared to a table of recommended tension forces to determine the status of the belt.Measure the span length of the belt. Span length is the distance the belt spans between the sheaves. The desired belt deflection is 1/64 of an inch for every inch of belt span. For example, if the span length is 32 inches, the desired belt deflection is ½ inch. The thumb rule is that belt shall not deflect more than its thickness. If belts are deflecting more than thickness means they are loose and slip will occur. If any doubt please feel free to ask.