Variable Speed Belts

What are variable speed belts?

Variable speed belts are wide V-belts designed to run on variable speed pulleys, also called variators. The sides of these pulleys can move closer together or further apart, which makes the belt ride higher or lower in the groove. This changes the effective size of the pulley, and so changes the speed of the driven machine smoothly, without stopping or changing pulleys. Variable speed belts are wider and flatter than standard V-belts, so they can move a long way up and down the pulley groove and give a wide range of speeds.

A standard variable speed belt has these main features:

• A wide, relatively shallow V-shaped section, much wider than it is deep
• A defined side angle, such as 28° or 30°, to suit the variable speed pulleys
• Strong tension cords, which carry the load and resist stretching
• A tough rubber body that resists wear and slip as the belt moves in the groove.

The wide section is what gives variable speed belts their range.

• The wider the belt, the further it can travel up and down the pulley groove, and the bigger the range of speeds the drive can give.
• The belt must also be stiff enough across its width not to buckle as the pulley sides squeeze it, while staying flexible enough to bend around the pulleys.

Variable speed belts are sized by their top width, height, and length.

Why are variable speed belts essential?

Many machines need to run at different speeds for different jobs, such as feed rates on agricultural machinery, cutting speeds on machine tools, or line speeds on conveyors. A variable speed belt drive lets the speed be adjusted smoothly and precisely while the machine is running, giving exact speed control and a wide range of speed changes with a simple, mechanical drive.

Because the belt must grip well while moving across the pulley face, variable speed belts are built for high power transmission with good resistance to abrasion and slip. Choosing the correct belt keeps the drive running smoothly across its full speed range.

Typical applications include:

• Combines, harvesters, and agricultural variator drives
• Machine tools, drills, and lathes with variable speed drives
• Industrial variable speed drive units
• Conveyors with adjustable line speeds
• Mixers, agitators, and processing machinery
• Woodworking machinery
• Textile and printing machinery
• Scooter, quad, and small vehicle transmissions

The main types of variable speed belts

• Narrow variable speed belts - Smaller sections, such as 13mm to 24mm wide, for light machinery, small variators, and compact drives.
• Medium variable speed belts - Sections such as 30mm to 40mm wide, for general industrial and agricultural variable speed drives.
• Wide variable speed belts - Large sections, such as 50mm to over 80mm wide, for heavy-duty agricultural and industrial drives with wide speed ranges and high power.
• Wrapped variable speed belts - Belts with a fabric cover, which protects the belt and gives a controlled grip.
• Raw edge and cogged variable speed belts - Belts with cut sides and, on cogged belts, notches on the inside. The cogs help the belt bend around small pulley diameters, and the raw edges give a strong grip.

How to choose the right variable speed belt?

• Width - The top width must match the variable speed pulleys, as it controls how the belt sits and moves in the groove.
• Height - Choose the same height, or depth, as the original belt, so it sits correctly in the pulleys.
• Length - Match the length exactly, and check whether it is given as inside length, as this is common for variable speed belts.
• Angle - The side angle of the belt, such as 28° or 30°, must match the pulley groove angle.
• Construction - Choose a cogged or raw edge belt for small pulleys and high power, or a wrapped belt where a smooth, controlled grip is needed.
• Temperature - Check the belt suits the operating temperature. Many variable speed belts are designed for around -30°C to +75°C.

Essential references and specialised ranges

How the numbering works:

Variable speed belts are usually referenced by their dimensions, in mm:

Top width x height x inside length

For example:

32x10x775 = 32mm top width, 10mm height, and 775mm inside length

54x16x1800 = 54mm top width, 16mm height, and 1800mm inside length

13x5x468 = 13mm top width, 5mm height, and 468mm inside length

The inside length is the length measured around the inside of the belt. Some manufacturers use the outside or pitch length instead, so always check which length is given.

Suffixes:

Li = inside length

La = outside length

Some manufacturers add codes for the belt angle or construction, such as raw edge or cogged.

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

FAQ - All you need to know about variable speed belts

What is the difference between a variable speed belt and a V-belt?

A variable speed belt is a type of V-belt with a much wider, shallower section. This lets it move further up and down the groove of a variable speed pulley, giving a wide range of speeds. A standard V-belt is narrower and deeper, and is designed for fixed pulleys.

How does a variable speed drive change speed?

The pulley sides move closer together or further apart. When they move together, the belt is pushed outwards to run on a larger diameter. When they move apart, the belt drops lower to run on a smaller diameter. This changes the ratio between the pulleys, and so the speed.

Can I use a standard V-belt on a variable speed drive?

No. A standard V-belt is too narrow and the wrong shape for variable speed pulleys. It would not give the correct speed range and could slip or be damaged.

Why is the belt angle important?

The sides of the belt must match the angle of the pulley groove, so the belt grips evenly across its sides. A belt with the wrong angle will wear unevenly, slip, and may not change speed smoothly.

How do I fit a variable speed belt?

Set the variable speed pulleys to their widest opening, or the lowest belt position, so the belt can be fitted without force. Fit the belt into the grooves, check it sits correctly in both pulleys, then reset the drive and tension it as the machine maker recommends. Check the drive changes speed smoothly through its full range.

How do I identify the belt I need?

Check the size marked on the old belt, or the part number in the machine’s manual. If the markings cannot be read, measure the top width and height of the belt, and its inside length, and note the side angle if it is known.

How can I tell if a variable speed belt needs replacing?

Warning signs include:

• Slipping or loss of power, especially at the ends of the speed range
• The drive not reaching its full speed range
• Squealing or chirping from the pulleys
• Cracks, fraying, or glazed shiny sides
• The belt narrowing or wearing unevenly across its width
• The belt sitting lower in the pulleys than it should