Why belts are used
A belt drive transmits power between two pulleys using a flexible loop. Compared with chains and gears, belts are quiet, need little or no lubrication, tolerate some misalignment and shock, and are relatively inexpensive. They also act as a mechanical fuse of sorts, since a belt may slip before more expensive parts are damaged, although that should not be relied upon as protection. The trade-offs are limited load capacity for their size, sensitivity to oil, heat and dirt, and gradual wear that needs regular attention.
Fans, compressors, pumps, conveyors and many general machines use belts, often between a motor and a driven machine.
V-belts
A V-belt has a trapezoidal cross-section that wedges into a matching groove in the pulley, called a sheave. The wedging action multiplies grip, so a V-belt transmits more power than a flat belt at the same tension. Power is carried by friction, so a small amount of slip is possible, and the speed ratio is therefore not perfectly fixed.
Several variations exist. Classical and narrow-section belts come in standard sizes, and several belts can run side by side in matched sets on multi-groove sheaves. Banded belts join belts together to prevent individual belts from flipping or whipping. Cogged or notched belts have grooves on the underside that improve flexibility around small pulleys and help dissipate heat.
When one belt in a set wears out, the usual advice is to replace the whole set. A mixed old and new belt set shares load unevenly, which can shorten the life of the new belts.
Timing belts
A timing belt, also called a synchronous belt, has teeth on the inside that mesh with a toothed pulley. Because the teeth engage positively, there is no slip, so the shaft speeds stay locked together. That makes them useful for indexing, positioning and applications where the timing between shafts matters. They also tend to run efficiently with lower tension than V-belts and need no lubrication.
They are less forgiving of misalignment than V-belts, and pulleys must be well aligned or the belt will track to one side and wear its edge against the flange. Timing belts are also sensitive to dirt that gets between the teeth, and to damage from sharp bends or tools used to pry them on.
Choosing between them
A short summary of how they generally compare:
| Feature | V-belt | Timing belt |
|---|---|---|
| Power transfer | Friction | Positive tooth engagement |
| Slip | Small amount possible | None in normal service |
| Speed ratio | Approximate | Fixed |
| Shock tolerance | Good | Moderate |
| Alignment sensitivity | Moderate | Higher |
The equipment designer chooses the type, and a drive should be replaced with the same specification unless a qualified person decides otherwise.
Why tension matters
Tension is the single most important variable in day-to-day belt care. A belt that is too loose slips, which creates heat, glazing and squealing, and wastes power. A belt that is too tight is worse in some ways. Excess tension increases the sideways load on the shafts and the bearings supporting them, speeding up bearing wear and possibly bending shafts over a long time. Over-tightening is one of the more common ways to shorten the life of a motor or fan bearing, which is why it features in fan bearing care.
Belt makers publish tension guidance for each belt type, often in the form of a specified deflection for a given force at the mid-point of the span. Gauges and sonic tension meters are available. The manufacturer’s method should be followed. Hand-feel is a poor guide.
New belts stretch slightly during their first hours of operation, so many guides recommend re-checking tension after a short initial run-in. Checking again at regular intervals is part of routine preventive maintenance.
Alignment
Pulleys must be aligned so that the belt runs straight into each groove. Two misalignments matter: angular, where the shafts are not parallel, and offset, where the pulleys are not in the same plane. A straightedge or string across the faces of the pulleys can reveal gross errors, and laser tools exist for belt drives. Misalignment causes uneven belt wear, noise and bearing load. The general ideas parallel those described under shaft alignment.
Typical wear signs
Inspect belts when the equipment is stopped and safe. Common signs include:
- Glazed, shiny sides, often from slipping.
- Cracks across the underside, from age, heat or small pulleys.
- Fraying or uneven edges, pointing to misalignment.
- Oil-soaked or swollen rubber, from contamination.
- Missing or sheared teeth on timing belts.
- Worn pulley grooves, which look shiny or shaped like a dish, and which can ruin new belts.
- Black dust near the guard, a sign of wear.
- Squeal on start-up, a typical sign of slipping.
Pulleys wear too, and checking the grooves with a gauge is common practice. Vibration from a belt drive, noise, and heat in the nearby bearings are also useful clues, and vibration analysis can help on important fans and pumps.
Safety
Belt guards protect against entanglement, and they must stay on whenever the machine runs. Before changing, tensioning or inspecting a belt, stop the equipment and follow the site’s lockout tagout procedures. Static electricity, belt dust and heat can matter in some hazardous areas, where special belts and rules may apply. Use trained personnel and follow the manufacturer’s manuals and local regulations. This article is general information, not engineering advice.