Jockey Pulley Tensioner

It carries no drive, only tension — so it needs to spin with almost no friction.

1. Cut a 60 mm idler pulley from 18 mm ply and bore it 22 mm for a 608 bearing.
2. Press the bearing in and check the wheel spins freely for several seconds when flicked.
3. Crown its face slightly if it will run against a flat belt.
4. Fit an M8 × 50 hex bolt through the bearing as its axle.

Why the bearing matters more here than elsewhere. A jockey pulley runs constantly and contributes nothing but drag. A stiff idler is a permanent power loss and a source of heat, which is why even cheap production tensioners get a proper sealed bearing.

Arm length and pivot position decide how constant the tension stays.

1. Cut an arm 140 mm between hole centres from 6 mm aluminium flat bar.
2. Pivot one end to the baseplate with an M6 × 35 hex bolt, M6 flat washers × 2 and an M6 nylon insert lock nut, set for free swing.
3. Mount the idler at the other end.
4. Position the pivot so the arm sits roughly PERPENDICULAR to the belt span when tensioned.

Why perpendicular. With the arm at right angles to the belt, the spring force converts most efficiently into belt tension, and small swings of the arm change that geometry least. Set the arm nearly in line with the belt and the tension varies wildly for a small movement — the same reason a toggle's force changes so sharply near dead centre.

Three ways to push the idler into the belt, with genuinely different behaviour.

1. Spring: hook a compression or extension spring from the arm to the base. Tension falls slightly as the belt stretches and the arm swings.
2. Weight: hang a mass on an extension of the arm. Tension stays CONSTANT regardless of arm position.
3. Screw: fit an M6 × 60 hex bolt as an adjuster against the arm and lock it. Tension is fixed until you adjust it again.
4. Build the spring version first, then try the others.

The tensioner does two jobs. Measure both.

1. With the jockey removed, measure the wrap angle on the small pulley with a protractor.
2. Load the output until the belt slips and record the torque.
3. Fit the jockey, press it into the slack span, and measure wrap again.
4. Repeat the slip test.
5. Now stretch the belt — or fit a slightly longer one — and confirm the jockey takes up the slack with no adjustment.

Put the jockey on the SLACK span, never the tight one. The tight side is already carrying the drive tension; pressing on it just adds load to bearings and belt without gaining anything. The slack side is where the surplus length appears, so that is where it must be taken up. Getting this wrong is a common and quietly damaging mistake.

History. Idler and jockey pulleys are as old as belt drives themselves and appear throughout line-shaft practice, often as a weighted arm hanging on the slack side. The automatic spring tensioner in every modern car's serpentine belt drive is the same device with a damper added — and its arrival is what allowed one long belt to drive every accessory, because a single tensioner can maintain the whole run.

Its siblings solve tension differently: moving the motor on slotted feet is simplest and needs re-doing as the belt stretches. A crowned flat belt drive needs less tension in the first place, because it tolerates slip. A timing belt needs correct tension too but cannot be tensioned by slip. The jockey is the answer when the shaft centres are fixed and the tension must not be a maintenance job.

One caution: a spring tensioner and a belt make a mass-spring system, and it can resonate. If a drive howls or the belt visibly flutters at one particular speed, the fix is usually damping rather than more tension — which is exactly why automotive tensioners include a friction damper.