How to check V-belt tension and alignment

Tension a V-belt by force and deflection, not by feel: press the belt down at the middle of the span until it deflects 1/64 inch for every inch of span length, and the force it took to get there should land inside the range on the belt maker's chart for that cross section and small-sheave diameter. Alignment comes first, though — tensioning a misaligned drive just makes it fail faster.

Here is the whole sequence, in the order it has to happen.

Check alignment before you touch the tension

A drive can be perfectly tensioned and still throw a belt every three weeks. There are two errors to find, and they are different problems:

• Parallel (offset) misalignment — the sheaves are on parallel shafts but sit at different distances along them. The belt runs at an angle across both grooves.
• Angular misalignment — the shafts themselves are not parallel, so one sheave is cocked relative to the other. This is the one that eats belt sidewalls.

The no-tools check: lay a straightedge across the machined faces of both sheaves. On equal-width sheaves it should touch at four points — two on each sheave. Any gap tells you which error you have. A gap at one end of the straightedge on one sheave only is angular; an even gap across a whole sheave is offset.

The tolerance most manufacturers publish is half a degree or less, which works out to roughly 1/16 inch per foot of center distance. On a 4-foot center distance you get a quarter inch of total offset before you are out of spec, and less than that is better. A laser alignment tool made for sheaves takes the argument out of it and is worth owning if you have more than a handful of belt drives.

How to check V-belt tension

Measure the span — the free length of belt between the two sheaves where they leave contact. Then:

1. Divide the span by 64. That is your target deflection. A 32-inch span needs 1/2 inch of deflection.
2. Set a belt tension gauge, or a small fish scale, at the exact middle of the span, perpendicular to the belt.
3. Push until you hit the target deflection and read the force.
4. Compare against the belt manufacturer's deflection-force table for that cross section (A, B, 3V, 5V and so on) and the small sheave's pitch diameter. There is no universal number — the range for a 3V on a 3-inch sheave is nothing like a B on a 9-inch sheave.

A new belt is tensioned to the top of the range, or about a third above the used-belt value, because it will relax. A run-in belt should sit inside the normal range.

The step everyone skips

Retension after run-in. A new V-belt does most of its seating and stretching in the first 24 to 48 hours of operation, and it will drop out of the tension range in that window every time. Put it on the work order as a separate line item with its own date, or it does not happen. More belt drives fail from a missed run-in retension than from bad initial tension.

Signs the tension is wrong

Too loose:

• Squeal on startup or on load steps — that is slip, not a dry belt
• A glazed, shiny belt sidewall
• The driven shaft turning slower than the sheave ratio says it should
• A sheave you cannot keep your hand on, from friction heat

Too tight:

• Motor and driven-end bearings running hot
• Bearing failures six months down the line with no obvious cause
• Shaft fretting or, in bad cases, a cracked shaft at the keyway
• Belts that fail from the tension member outward rather than by sidewall wear

Over-tension is the more expensive mistake, and it is the more common one, because tight feels right. The bearing bill shows up long after anybody connects it to the belt job.

Check the sheaves, not just the belts

A worn sheave groove will destroy a brand-new belt in weeks. Two checks:

• Groove gauge. Seat the correct gauge in the groove and hold it to the light. More than about 1/32 inch of gap against the groove flank means the sheave is done. Replace it, do not run a new belt in it.
• Belt ride height. Drop a new belt into the groove. Its top should sit at or slightly above the sheave's outer diameter. If the belt sinks below the rim, the groove has worn and the belt is riding on its bottom, not its sides — which is where a V-belt gets all of its grip.

Also check for wobble. Put a dial indicator on the sheave face and turn it by hand; runout means a bent shaft, a sheave bored off-center, or a taper bushing that was pulled up unevenly. Tighten taper-lock bushing screws in a cross pattern, in stages, to the torque on the bushing — not until it stops turning.

Two things to never do

• Never pry a belt over the sheave with a screwdriver. It breaks tension cords you cannot see, and the belt fails early for reasons nobody traces back. Back the motor in on its slide rails or takeup instead.
• Never replace one belt in a matched set. A new belt carries almost the whole load next to stretched neighbors, and it will fail. Replace the set, and buy a matched set if the drive is more than two belts wide.

And skip belt dressing entirely. It is a slip additive on a friction drive; it collects grit and hides the actual problem, which is tension, alignment or a worn groove.

Putting numbers to your drive

If you are replacing a belt and do not have the part number, our V-belt length calculator gets you the pitch length from center distance and sheave diameters. The sheave ratio and speed calculator is the fast way to confirm the driven speed you should be seeing — useful for catching slip, because a drive that reads low on a tach with correct sheaves is slipping. The belt drive alignment and tension trainer walks the diagnosis with simulated symptoms.

When to call someone

Call if the same drive keeps eating belts, if you are replacing belts more than twice a year on one machine, or if a belt failure took a bearing with it. Repeat belt failure is almost always alignment, a worn sheave or a soft base — not the belt. Keystone works on-site across Southern California — Long Beach, the South Bay, Orange County and the Inland Empire — and belt drive inspection is part of every preventative maintenance route we run.