Belt types and sections

How a V-belt works

A V-belt transmits power by friction between its sides and the walls of the pulley groove. It does not run on the bottom of the groove. As tension pulls the belt down into the V, the wedge multiplies the side force: with a groove angle of about 38°, the effective friction is roughly three times that of a flat belt on the same pulley. That is why a V-belt can carry high power at modest tension, and why a belt that has worn down until it touches the groove bottom suddenly starts slipping.

Classical V-belts (Z, A, B, C, D, E)

The original trapezoidal V-belt, with a top width to height ratio of about 1.6. Still found on most older South African plant, farm machinery, pumps and compressors. Classical sections follow DIN 2215 / ISO 4184 (metric) and RMA/MPTA IP-20 (inch, where A, B, C are equivalent in size).

• Strengths: forgiving, cheap, widely available, tolerate some misalignment and shock.
• Limits: lower power per belt than wedge belts; drives need more belts or wider pulleys.
• Typical belt speed limit: about 30 m/s.

We supply classical belts in the Turboflex V-belt range, wrapped and raw-edge cogged.

Narrow wedge belts (SPZ, SPA, SPB, SPC and 3V/5V/8V)

Narrow (wedge) belts are deeper relative to their width (ratio about 1.2). The cord line sits higher and more of the section is supported by the groove walls, so the same width carries considerably more power than a classical belt. SP sections follow DIN 7753 / ISO 4184; the inch-based 3V, 5V and 8V sections follow RMA/MPTA IP-22.

• Strengths: high power density: fewer belts, narrower pulleys, lower shaft load for the same power.
• Typical belt speed limit: about 40 m/s (check the maker for raw-edge types).
• Watch out: SP belts must run in SP-profile grooves. Classical and narrow belts of similar top width (e.g. B and SPB) are not interchangeable without checking the groove.

Section dimensions

Nominal dimensions. The datum width is the width at the belt's datum line, which sits at the pulley's datum diameter: the diameter used for all speed and length calculations.

Minimum pulley diameters and mass per metre are typical values used in drive calculations; the maker's catalogue governs. Banded and cogged versions allow smaller pulleys in some sections.

Wrapped vs raw-edge cogged

Banded belts (joined V-belts)

Two to five (or more) V or wedge belts joined by a tie band across the top. The tie band stops individual belts turning over or jumping off under pulsating loads.

Use on: saw head-rigs, chippers, crushers, hammer mills, piston compressors, long-centre or vertical-shaft drives, and any drive where single belts have turned over in the grooves.

Double-V (hexagonal) belts

A hexagonal section that drives on both faces. Used on serpentine drives where pulleys sit on both sides of the belt: harvesters, some agricultural machines and multi-shaft drives.

Poly-V (ribbed) belts

A thin flat belt with longitudinal V-ribs (profiles PH, PJ, PK, PL, PM; ISO 9982). Combines the flexibility of a flat belt with the grip of V-ribs.

• Use on: compact high-ratio drives, small pulleys, high speeds, fans, bottling and packing machines, machine tools.
• Watch: alignment tolerance is tighter than for V-belts (about ¼°). Specify by profile, number of ribs and effective length, e.g. PJ 1016 × 8 ribs.

Variable-speed belts

Wide, thin belts that run on variable-pitch (variator) pulleys, where the pulley halves move apart or together to change the running diameter and therefore the speed ratio. Specified by top width, angle and length (inch RMA or metric DIN series). Common on agricultural harvesters, older process-line variators and some feeders.

Timing (synchronous) belts

Toothed belts that mesh with toothed pulleys: no slip, constant ratio, about 98 % efficiency held over the belt's life (US DOE). Used where timing matters (packing machines, packhouse sizers, indexing, printing, automation) and increasingly to save energy on steady drives.

Specify as: pitch length or number of teeth, profile, width, e.g. 1600-8M-30 = 1 600 mm pitch length, 8 mm pitch HTD, 30 mm wide. Use the timing belt calculator.

Round PU belts (Polycord)

Round polyurethane cord, welded to length on site. The workhorse of fruit packlines: singulators, roller tables, sizer drives and light conveyors. Available in smooth and rough surfaces and a range of diameters and hardnesses.

• Set length by stretch: a round belt is typically cut a few percent shorter than the path length; follow the maker's stretch figure for the diameter and hardness.
• Weld with a proper welding blade and clamp so the joint is square and not undercut. Most round-belt failures are poor welds.
• Keep spare cord and a welding kit at the line for the season.

Flat and conveyor belts

Flat belts (modern polyamide or elastomer constructions) run efficiently at high speed on crowned pulleys: tangential drives in textile mills, folder-gluers, packaging. Light conveyor belts carry product on packing and food lines, in packhouses and warehouses; specify width, length, top surface (smooth, grip, food-grade), joint type (finger splice, mechanical fasteners) and carcass.

Choosing a type

Identifying a belt

1. Read the markings on the back of the old belt (photograph them in good light; turn the belt to find a clean section).
2. No markings? Measure the top width and height with a vernier and compare with the section table. A height of about 0.6 × width → classical; about 0.8 × width → narrow wedge.
3. Length: measure the old belt's outside length with a tape (it will have stretched), or better, measure the pulleys and centre distance on the machine and use the identify-a-belt calculator.
4. Check the groove with a groove gauge to confirm classical vs SP profile.
5. WhatsApp us the photos and numbers; we'll match it.

Reading belt markings