Belts, chains, and couplings

Why it matters

Belts, chains and couplings carry power from a motor to the machine it drives. They are also among the parts you will adjust most often. A belt that is too loose slips, squeals and glazes; one that is too tight overloads bearings and shafts. A coupling that is a fraction of a millimeter out of line can halve the life of the bearings on both sides. Getting tension and alignment right is everyday mechanic work.

Belt drives

V-belts wedge into the grooves of pulleys (called sheaves). The wedging creates grip, so a V-belt does not need to be extremely tight. Synchronous (timing) belts have teeth that mesh with toothed pulleys, so they never slip and keep shafts in time. Flat belts and multi-rib belts are also common.

Speed through a belt drive follows the pulley diameters:

driven rpm = driver rpm × (driver diameter / driven diameter)

A 1,750 rpm motor with a 4 in sheave driving a 10 in sheave turns the fan at 1,750 × 4 / 10 = 700 rpm.

Tensioning a V-belt: the deflection method

The manufacturer's tables give the exact force, but the common field method is:

1. Measure the span: the distance between the points where the belt leaves one sheave and meets the other.
2. The target deflection is about 1/64 in per inch of span (about 1.6 mm per 100 mm). For a 32 in span, that is 0.5 in.
3. Use a belt tension gauge to push the middle of the span until it deflects that distance, and read the force.
4. Compare with the manufacturer's force range for that belt section and sheave size. Adjust the motor base and re-check.
5. New belts stretch and seat. Re-check tension after the first run-in period (often 24 hours of running).

Signs of wrong tension: squealing on start-up, a shiny glazed sidewall, belts flapping, or excess heat at bearings.

Sheave alignment

Even perfectly tensioned belts wear fast if the sheaves are out of line. Lay a straightedge (or a laser alignment tool) across both sheave faces. It should touch each face at two points. Check for:

• Offset: sheaves parallel but not in the same plane.
• Angular misalignment: shafts not parallel, so the straightedge touches only one side.

Replace multi-belt drives as a matched set. Mixing one new belt with old ones puts all the load on the tightest belt.

Chain drives

Roller chains are used where loads are high and slip is not allowed. Key checks:

• Slack: on a horizontal drive, the typical target is about 2 to 3 percent of the center distance, measured as up-and-down movement at the middle of the slack span. A 20 in center distance gives roughly 0.4 to 0.6 in.
• Wear (elongation): chain pins and bushings wear, so the chain gets longer. Measure a known number of pitches and compare with the manufacturer's wear limit, often around 1.5 to 3 percent. Replace a stretched chain together with worn sprockets.
• Lubrication: chains need oil in the pin joints, not just on the outside.

Couplings and shaft alignment

A coupling joins two shafts end to end, such as a motor and a pump. Rigid couplings need near-perfect alignment. Flexible couplings (jaw, grid, gear, disc and tire types) tolerate small misalignment, but tolerating it is not the same as liking it: misalignment shows up as heat, vibration and worn bearings and seals.

There are two kinds of shaft misalignment, measured in both the vertical and horizontal planes:

• Offset (parallel): shaft centerlines are parallel but displaced.
• Angular: shaft centerlines meet at an angle.

The usual sequence is:

1. Lock out and verify zero energy.
2. Check and correct soft foot: a motor foot that does not sit flat. Loosen one foot bolt at a time with a dial indicator on it; movement means shims are needed.
3. Do a rough alignment with a straightedge.
4. Do a precision alignment with dial indicators (rim-and-face or reverse-indicator methods) or a laser alignment system.
5. Move the motor, not the pump: the driven machine is usually piped in place. Add or remove shims for vertical corrections, then move it sideways for horizontal corrections.
6. Torque the bolts in sequence and take a final reading.

Alignment tolerances depend on speed: the faster the shaft, the tighter the tolerance. Use the coupling or laser manufacturer's table.

Common mistakes

• Prying a belt over a sheave instead of moving the motor in. It damages the belt cords.
• Tensioning by feel only. Use a gauge.
• Aligning a hot machine to cold readings without allowing for thermal growth.
• Skipping the soft foot check, which makes alignment readings change every time you tighten a bolt.

Safety

Belt, chain and coupling work is done only on locked-out equipment with guards removed under your site's procedure and refitted before restart. Nip points on belts and sprockets cause serious hand injuries. Your employer must train and authorize you first. See OSHA's machine guarding page: https://www.osha.gov/machine-guarding (Checked on 2026-10-23.)

Recap

• V-belt deflection target: about 1/64 in per inch of span, at the force in the manufacturer's table.
• Check sheaves with a straightedge for offset and angle.
• Chain slack about 2 to 3 percent of center distance; replace stretched chain and worn sprockets together.
• Coupling alignment: soft foot first, then offset and angular corrections in both planes.

Try it

Open the Do tab to practise the deflection and straightedge methods on a safe tabletop model.