8.4 Installing Belt and Chain Drives

Belt drives

A belt drive transmits power by friction between the belt and the sheave groove — except for synchronous belts, which transmit by tooth engagement.

The V shape is the mechanism: as tension pulls the belt into the groove, the wedging action multiplies the normal force and therefore the friction. A belt should ride on the side walls of the groove and must never bottom out on the groove floor. A belt that is bottoming means the sheave grooves are worn out, and installing new belts in worn sheaves wastes both.

Sheave alignment

Check with a straightedge across the machined faces of both sheaves, or with a laser sheave alignment tool. Three errors must be distinguished:

• Offset (parallel) — the sheaves are on parallel planes but laterally displaced. Correct by moving a sheave on its shaft.
• Angular — the shafts are not parallel. Correct by moving the machine.
• Axial twist — one sheave is tilted. Usually a bushing or shaft seating problem.

Misalignment of even a fraction of a degree dramatically shortens belt life and pushes the belt against one groove wall, which produces one-sided sidewall wear.

Tensioning by deflection

The accepted field method:

1. Measure the belt span — the distance between the two points where the belt leaves the sheaves.
2. The target deflection is 1/64 inch per inch of span. A 32-inch span therefore deflects 32/64, or 1/2 inch.
3. Apply the manufacturer's specified deflection force at the center of the span with a belt tension gauge, and confirm that the belt deflects the target amount.
4. Run the drive, then re-tension after the first few hours of operation, when new belts have seated and taken their initial stretch.

Symptoms of getting it wrong:

Never pry or roll belts onto a sheave. Shorten the center distance, install by hand, then tension. Prying breaks the tensile cords inside the belt, and the belt will fail early with no visible external cause.

Always replace multiple-belt drives as a matched set. A new belt installed alongside a used, stretched belt carries far more than its share of the load and fails quickly, and the used belt slips.

Drive ratio

For a belt or chain drive, speed is inversely proportional to the diameter or tooth count:

$N_2 = N_1 \times \frac{D_1}{D_2}$

Worked example. A 1,750 rpm motor with a 4-inch sheave drives a 10-inch sheave. The driven speed is 1,750 × 4 ÷ 10 = 700 rpm, a 2.5:1 reduction.

Chain drives

Roller chain transmits power by positive engagement between the chain rollers and the sprocket teeth, so there is no slip and no speed loss.

Installation

1. Align the sprockets with a straightedge across the machined faces, the same as sheaves. Chain is even less tolerant of misalignment than belts, because a misaligned chain climbs one side of the tooth and wears the link plates.
2. Check the sprocket teeth. Hooked, pointed teeth mean the sprocket is worn and will destroy new chain. Replace sprockets and chain together as a set.
3. Fit the connecting link with the closed end of the spring clip facing the direction of chain travel, so the clip is not knocked off by contact.
4. Set the slack. On a horizontal drive, mid-span sag should be about 2 to 4 percent of the center distance. On a 40-inch center distance, that is roughly 3/4 inch to 1-1/2 inches. Vertical and steeply inclined drives are set tighter, about 1 to 2 percent, because gravity cannot hold the slack in the correct span.
5. Put the slack in the non-driving (slack) side of the chain wherever the layout allows.

Elongation and replacement

Chain does not "stretch" in the sense of the metal yielding. It elongates because the pins and bushings wear, lengthening the effective pitch. As the pitch grows, the rollers ride higher up the sprocket teeth toward the tips, engagement worsens, and the chain eventually jumps a tooth or breaks.

Measure by laying the chain under slight tension and measuring across a known number of pitches, comparing with the nominal length. Replace at roughly 3 percent elongation on adjustable drives, and at a tighter limit, often about 1.5 percent, on drives with no take-up adjustment or where the chain runs on hardened sprockets.

Lubrication

Chain lubricant must reach the pin and bushing interface, which means applying oil to the inside of the chain on the slack side, where the links are opening, and letting chain motion carry it in. Spraying the outside of the plates lubricates nothing that matters.

Guarding

Both belt and chain drives are exposed rotating hazards. A guard that fully encloses the drive is required, and it must be in place before the drive is started. A guard removed for a tension check and not replaced is the most common way this hazard actually injures people.