Frictional belt drives reliably transmit power from source to load without asking much in the way of upkeep. Their modest needs stem from the fact that belts elongate through normal use and wear. The same is true of chain and chain drives.

In a manufacturing environment, the cost of elongation can add up quickly. Consider a V-belt drive initially running at 96% efficiency. As the belt elongates, it begins to slip. This, in turn, accelerates belt and pulley wear and reduces drive efficiency by as much as 10%. It's also likely to increase downtime and raise operating and maintenance expenses.

Although elongation can't be prevented, it's easily treated with automatic tensioners. The simplest such devices employ a spring-actuated idler (or bearing surface) that maintains tension by taking up slack. In the case of rotary tensioners, the spring acts through a lever arm. In linear tensioners, the idler or bearing surface mounts on the end of a spring-loaded shaft.

Besides eliminating the need to manually retension drives, automatic tensioners reduce the risk of inadvertently over-tensioning drive components. They also reduce belt and chain whip on long-center drives, prevent drive components from snagging obstructions, reduce resonant frequency, and improve drive efficiency.

When selecting a tensioner, designers need to consider drive power, speed, torque, load, operating cycles, and available mounting space. One type of application where a spring-loaded tensioner should never be used is a reversing drive. The problem here is that when the drive reverses, the slack side becomes the tight side against which the tensioner's spring force may be inadequate to maintain proper tension. Automatic tensioners are also not recommended for use on synchronous belts unless the drive is intended strictly for motion transfer. If there's even the slightest question, consult the manufacturer.

QUESTIONS & ANSWERS

Q: When should I use a rotary tensioner instead of a linear one?

A: Sometimes mounting location, drive configuration, and other factors won't permit the use of one type or other. Rotary tensioners are found primarily on belt drives, although they can be used on chain drives as well. Linear tensioners are more often found on chain drives, especially tensioners that use a polymer wear surface in place of an idler sprocket.

Q: Can I use a tensioner on the backside of a belt?

A: Yes. Actually, a backside idler increases wrap angle and power transmission capacity, but it also causes a reverse or back bend in the belt that will reduce belt life.

Q: My chain drive appears to have excessive slack — will a tensioner help this problem?

A: Under normal conditions, chain slack shouldn't exceed 4% of the span. For unusual conditions such as heavy loads or frequent starts and stops, chain slack should be less than 2% of the span. Excessive slack usually occurs on drives with long spans. Typically, a span greater than 50 chain pitches will require a tensioner.

Keep it close

Tensioners work best and last longest when they don't have to contend with large overhung loads. The distance from the centerline of the idler to the face of the tensioner arm should not exceed the manufacturer's recommendations. This limits the load on the idler mounting bolt as well as tensioner arm and support bushing.

The best place for a tensioner on a belt drive is on the inside of the belt against the slack side. The idler should be located such that it provides nearly equal arcs of contact on both driver and driven pulleys. On a chain drive, the best location for the idler is outside the perimeter, on the slack side, an equal distance from driver and driven sprockets. When using a sprocket instead of a low-friction polymer wear surface as the idler, at least three idler teeth should engage the chain.