V-Belts

What is a V-belt?

A V-belt is a power transmission belt with a trapezoidal, or V-shaped, cross section. It runs in matching V-shaped grooves on pulleys, transferring power from a motor or engine to a driven machine. As the belt is pulled into the groove under tension, its sides wedge against the groove walls, which gives a strong grip and lets it transmit high power without slipping.

A standard V-belt has four main parts:

• Tension cords, usually polyester, which run along the length of the belt and carry the load
• A rubber compound body, which supports the cords and gives the belt its V shape
• A base or compression section, which transfers the load to the sides of the belt
• On wrapped belts, a fabric cover around the outside, which protects the belt and controls grip.

The wedging action is what makes V-belts so effective.

• The harder the belt is pulled, the more it wedges into the groove, so grip increases with load.
• This lets V-belts transmit more power than a flat belt of the same width, while running quietly and absorbing shock.

V-belts are identified by their section, which sets the width and depth of the belt, and their length.

Why are V-belts essential?

V-belts are one of the most widely used ways to transmit power in machinery. They are simple, low in cost, and easy to fit, and they can connect shafts some distance apart, change speed between the motor and the machine, and absorb shock loads that would damage gears or couplings. If a machine jams, a V-belt can slip rather than break something more expensive.

Because V-belts are made to international standard sizes, replacements are easy to find, and belts from different reputable makers are interchangeable when the section and length match.

Typical applications include:

• Electric motor drives on pumps, fans, and compressors
• Agricultural machinery, such as mowers and harvesters
• Air conditioning and ventilation units
• Machine tools, saws, and workshop machinery
• Conveyors and material handling equipment
• Engines and generators
• Garden and grounds care equipment
• Washing machines and household appliances

The main types of V-belts

• Classical V-belts (Z, A, B, C, D) - The traditional V-belt sections, in a range of sizes from the small Z section up to the large D section. They are widely used on older and general machinery.
• Wedge belts (SPZ, SPA, SPB, SPC) - Narrower, deeper belts that transmit more power than a classical belt of similar width. They allow smaller, lighter drives and are common on modern machinery.
• Raw edge cogged belts (XPZ, XPA, XPB, XPC) - Wedge belts with cut sides and notches, or cogs, on the inside. The cogs let the belt bend easily around small pulleys, and the raw edges give extra grip, so they run cooler and more efficiently.
• Fractional horsepower belts - Light-duty V-belts for small motors, domestic appliances, and garden equipment.
• Banded V-belts - Several V-belts joined together along the top, which stops individual belts twisting or jumping off the pulleys on drives with heavy or pulsing loads.
• Double V-belts - Belts with a V shape on both sides, used on drives where the belt bends both ways around the pulleys, such as some agricultural machinery.

How to choose the right V-belt?

• Section - The belt section must match the pulley grooves. Check the existing belt markings, or measure the top width of the belt or the groove.
• Length - Choose the correct length for the drive. Check whether the length is given as inside, outside, or datum (pitch) length, as these differ for the same belt.
• Classical or wedge - Replace like with like, as classical and wedge belts need different pulley grooves. Some wedge belts will fit classical grooves of the matching size, but always check first.
• Power and speed - For high power or small pulleys, choose a wedge or raw edge cogged belt, which can carry more power and bend more easily.
• Matched sets - On drives with more than one belt, fit a matched set of new belts, so they share the load evenly.
• Conditions - For hot, oily, or dusty conditions, check the belt is suitable, as heat and oil shorten belt life.

Essential references and specialised ranges

How the numbering works:

The letters show the belt section, and the numbers show the length.

Classical belt sections and their approximate top widths:

Z = 10mm, A = 13mm, B = 17mm, C = 22mm, D = 32mm

Wedge belt sections and their approximate top widths:

SPZ = 9.7mm, SPA = 12.7mm, SPB = 16.3mm, SPC = 22mm

The number after the section is the length:

Classical belts are often marked with the inside length in inches, for example A50 is an A section belt with a 50 inch inside length.

Wedge belts are marked with the datum (pitch) length in mm, for example SPA1250 is an SPA section belt with a 1250mm datum length.

Raw edge cogged wedge belts use an X in the section, for example XPA1250.

Suffixes:

X = raw edge cogged belt, for example XPA or XPZ

Some manufacturers add their own codes for belt type, such as banded belts or special compounds.

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about V-belts

What is the difference between a V-belt and a wedge belt?

A wedge belt is a type of V-belt with a narrower, deeper section. It transmits more power for its width than a classical V-belt, which allows smaller, lighter drives.

How do I measure a V-belt?

Measure the top width and depth of the belt to find the section. For the length, read the markings on the belt if possible. If not, measure the outside length with a tape around the outside of the belt, or around the pulleys with a new belt fitted, and check which length system the replacement uses.

Why should belts be replaced as a matched set?

On drives with several belts, new and old belts stretch differently, so a mix of new and old belts will not share the load evenly. Replacing all the belts together gives even tension and longer life.

How tight should a V-belt be?

The belt should be tight enough not to slip under load, but not so tight that it overloads the bearings. As a general guide, the belt should deflect about 16mm per metre of span when pressed firmly at the midpoint. Check the tension again after the first few hours of running, as new belts bed in and stretch.

How do I fit a V-belt?

Loosen the motor mounting or tensioner so the belt can be fitted without force. Never lever a belt over the pulley edge, as this can damage the cords. Fit the belt into the grooves, check the pulleys are aligned, then tension the belt to the correct setting.

How do I identify the belt I need?

Check the section and length marked on the old belt. If the markings are worn away, measure the top width and depth to find the section, and measure the length, or use the pulley sizes and centre distance to work it out.

How can I tell if a V-belt needs replacing?

Warning signs include:

• Squealing or chirping, especially when starting
• Cracks across the inside of the belt
• Frayed, glazed, or shiny sides
• The belt sitting low in the groove or touching the bottom
• Slipping, or the belt jumping off the pulleys
• Missing chunks or exposed cords