Types of drive belts: SPZ, XPZ, SPA, SPB, ribbed and timing belts explained

A drive belt is one of the cheapest parts in a machine, yet when it slips or snaps the whole drive stops. The tricky part is that at first glance every belt looks the same: black rubber, a trapezoidal cross-section and some code printed on the side. In practice, though, the difference between an SPZ and an SPA belt, between SPB and XPB, or between a ribbed and a timing belt decides whether a drive runs for years or for weeks.

This guide walks through all the main belt types you will meet in industry, in the workshop and on vehicles: classical V-belts, narrow wedge belts SPZ, SPA, SPB and SPC, their cogged versions XPZ, XPA, XPB and XPC, V-ribbed (Poly-V) belts and synchronous timing belts. For every profile we list the dimensions, standards, marking system and typical applications so you can order the right belt first time.

How a belt transmits power: two basic groups

Before comparing profiles it helps to know that every belt works in one of two ways.

• Friction belts transmit power through friction between the belt flanks and the pulley groove. This group includes classical and narrow V-belts, V-ribbed belts and flat belts. A small amount of slip is always present, so the ratio is never perfectly exact, but the drive absorbs shock loads well and is easier to fit.
• Synchronous (timing) belts transmit power by positive engagement of belt teeth in pulley teeth. There is no slip and the ratio is exact, but the drive requires more accurate geometry and correctly set tension.

A V-belt works on the wedge principle: under load it is drawn into the groove and its flanks are pressed against the pulley walls, so it transmits far more force than a flat belt of the same width. The belt wedge angle is 40 degrees, while the pulley groove is machined slightly narrower (usually 34, 36 or 38 degrees depending on diameter) so the bent belt seats correctly.

Classical V-belts: Z, A, B, C, D and E

Classical V-belts are made to ISO 4184 (DIN 2215) and are the oldest industrial standard. They are relatively wide compared with their height, roughly in a 1.6 : 1 ratio.

• Z (10) – 10 × 6 mm
• A (13) – 13 × 8 mm
• B (17) – 17 × 11 mm
• C (22) – 22 × 14 mm
• D (32) – 32 × 20 mm
• E (40) – 40 × 25 mm

Intermediate sections of 20 × 12.5 mm and 25 × 16 mm are also available, along with small 5 × 3, 6 × 4 and 8 × 5 mm sections for light-duty mechanisms.

Marking: the number in a classical belt code is the inside length Li in millimetres, for example 17 × 2000 Li. The same belt often also carries a nominal length Ld and an inch code such as B 79.

Where they are used: agricultural and garden machinery, older fans and pumps, transport equipment and spare parts for existing machines. New drives are almost always designed around narrow wedge belts because they transmit more power in the same space.

Narrow wedge belts: SPZ, SPA, SPB and SPC

Narrow wedge belts are made to DIN 7753 part 1 and ISO 4184. Compared with classical belts they are narrower and taller (a width to height ratio of about 1.2 : 1), so a larger share of the flank area carries the load. The result is up to roughly 50 percent more transmitted power in the same space, with smaller pulleys and smoother running.

• SPZ – 9.7 × 8.0 mm, common datum lengths from 512 to 3550 mm
• SPA – 12.7 × 10.0 mm, lengths from 647 to 5350 mm
• SPB – 16.3 × 13.0 mm, lengths from 1250 to 9000 mm
• SPC – 22.0 × 18.0 mm, lengths from 2000 to 16,500 mm

SPZ belt

The smallest wedge section, 9.7 × 8 mm, with a recommended minimum pulley datum diameter of 63 mm. Used on smaller fans, pumps, compressors, machine tools and woodworking machines, wherever pulleys are small and the power per belt is a few kilowatts. The related American section is 3V.

SPA belt

A 12.7 × 10 mm section with a recommended minimum pulley diameter of 90 mm. SPA is the workhorse of industry: piston compressors, fans, mixers, agricultural machines and sawmills. It covers a wide power range while keeping pulley sizes reasonable.

SPB belt

A 16.3 × 13 mm section with a recommended minimum pulley diameter of 140 mm. It is intended for demanding drives such as large compressors, crushers, quarry and mining equipment, large fans and pumps, usually in sets of several belts on multi-groove pulleys. The related American section is 5V.

SPC belt

The largest wedge section, 22 × 18 mm, with a recommended minimum pulley diameter of 224 mm. It is reserved for heavy, high-power drives: crushers, mills, separators and large industrial fans.

Marking: for SP sections the number in the code is the datum (pitch) length, written as Lw, Ld or Lp, for example SPA 1550 Lw. It is not the inside length, so the two must never be mixed up when ordering.

XPZ, XPA, XPB and XPC: cogged wedge belts

The letter X in front of the section means the belt has transverse notches on its underside, usually combined with raw (uncovered) flanks. These are not synchronous timing belts: XPZ and SPZ run in the same groove and share the same cross-section, but the X version bends far more easily.

Practical advantages of the cogged design:

• Smaller pulleys. XPZ allows a 56 mm pulley instead of 63 mm, XPA 71 mm instead of 90 mm, XPB 112 mm instead of 140 mm and XPC 180 mm instead of 224 mm.
• Less heat build-up. The notches reduce bending stress, so the belt runs cooler and lasts longer, especially on small pulleys and at high speeds.
• Higher power and speed. Quality EPDM versions handle belt speeds up to around 50 m/s, against roughly 30 m/s for conventional wrapped belts, and manufacturers quote a multiple of the power rating.
• Wider temperature range. EPDM cogged belts work from about −40 to +130 °C, while wrapped belts are usually limited to around +70 °C.

Dimensions normally match the parent SP section: XPZ 9.7 × 8 mm, XPA 12.7 × 10 mm (9 mm with some manufacturers), XPB 16.3 × 13 mm and XPC 22 × 18 mm (16 mm in some ranges). For that reason XPZ can usually replace SPZ with no modification, while the reverse swap is not always possible if the drive was designed around small pulleys.

V-ribbed (Poly-V) belts

The V-ribbed belt, also known as Poly-V, Micro-V or multi-rib belt, combines a flat belt with a V-belt. The thin back gives excellent flexibility while a row of longitudinal ribs provides V-belt style grip. It therefore transmits high power in a small space and tolerates bending in both directions.

Sections to ISO 9982 (DIN 7867) differ by rib pitch:

• PH – rib pitch 1.60 mm
• PJ – rib pitch 2.34 mm
• PK – rib pitch 3.56 mm
• PL – rib pitch 4.70 mm
• PM – rib pitch 9.40 mm

Key benefits are operation on very small pulleys, belt speeds up to about 60 m/s, high transmission ratios, the option of back-bending idlers in serpentine drives, and quiet, almost vibration-free running. Standard versions operate roughly from −30 to +80 °C.

Marking: the industrial code 1168 PJ-6 means a length of 1168 mm, section PJ and six ribs. The automotive industry reverses the order, so 6PK1168 means six ribs, section PK and a length of 1168 mm. The safest way to confirm the rib count is to count them on the old belt.

Where they are used: accessory drives on vehicle engines (alternator, power steering pump, air conditioning compressor, water pump), household appliances and washing machines, power tools, conveyor rollers and industrial drives with large ratios.

Synchronous timing belts

A timing belt transmits power by tooth engagement rather than friction. It cannot slip, needs no high pre-tension, loads the bearings less than a V-belt drive and allows exact positioning. In return it demands the correct length, good alignment and properly set tension, because a skipped tooth means an immediate loss of synchronisation.

Trapezoidal inch sections to ISO 5296

• MXL – pitch 2.032 mm
• XL – pitch 5.08 mm
• L – pitch 9.525 mm
• H – pitch 12.70 mm
• XH – pitch 22.225 mm
• XXH – pitch 31.75 mm

The code 420H150 means a length of 42.0 inches (1066.8 mm), section H and a width of 1.50 inches (38.1 mm). This is the oldest industrial type; new designs generally use curvilinear sections because they carry more torque.

Curvilinear HTD and STD sections

HTD (High Torque Drive) uses a rounded tooth that spreads the load more evenly. Standard pitches are 3M, 5M, 8M, 14M and 20M, where the number is the pitch in millimetres. STD or STS sections (S1.5M, S2M, S3M, S4.5M, S5M, S8M, S14M) use a parabolic tooth and run more quietly at high speed. The code HTD 8M-1200-20 means section 8M, length 1200 mm and width 20 mm.

Metric trapezoidal T and AT sections

Polyurethane belts with steel cords are made in T2.5, T5, T10 and T20 sections and in the reinforced AT5, AT10 and AT20 sections. They are used in automation, linear axes, conveying and machine building. The code 50 T 10 - 1960 means a width of 50 mm, section T10 and a length of 1960 mm.

Materials: rubber timing belts with glass cords are the industrial standard and run from roughly −20 to +100 °C. Polyurethane belts with steel cords cope better with oils, greases and fuels and are used in food processing and clean rooms, roughly from −30 to +80 °C.

How to measure and order the right belt

• Copy the full code from the old belt. It contains the section and the length and is the fastest, safest source of information.
• If the code is illegible, measure the cross-section. Top width and height identify the section: 9.7 × 8 is SPZ or XPZ, 12.7 × 10 is SPA or XPA, 16.3 × 13 is SPB or XPB, 13 × 8 is classical A and 17 × 11 is classical B.
• Check which length is meant. SP and XP sections are specified by datum length Lw (Ld), classical sections by inside length Li, and the outside length La is always the largest figure. Mixing these up is the most common ordering mistake.
• For ribbed belts count the ribs and read the section; for timing belts count the teeth and measure the width and pitch.
• Check the pulley too. Belt and groove must match: XPZ fits an SPZ groove, but a classical A belt does not belong in an SPA groove.

Fitting, tensioning and maintenance

• Never lever a belt over the pulley rim. Always reduce the centre distance or release the tensioner. Forced fitting breaks tensile cords and the damage is invisible from outside.
• Align the pulleys. The generally recommended limit for V-belt drives is about half a degree; more misalignment rapidly wears both belt flanks and grooves.
• Tension correctly. A well proven rule of thumb is 16 mm of deflection per 1000 mm of free span under the specified test force. For accurate work use a tension gauge or an acoustic frequency meter.
• Re-tension after run-in. A new belt settles during the first hours of operation, so tension should be checked after a short running period and then periodically.
• Replace the whole set. Mixing an old belt with a new one means the new belt carries nearly all the load. Use matched sets on multi-groove drives.
• Do not use belt dressing. Sprays and resins mask the symptom briefly and accelerate wear. Slip means insufficient tension or a worn groove.
• Inspect the pulleys. A dished or mirror-polished groove means the belt sits too deep and loses grip; such a pulley should be replaced together with the belt.
• Protect the drive. Oil, fuel, solvents, UV radiation and excessive temperatures drastically shorten belt life.

The most common selection mistakes

• Ordering an SPA belt with the length read as inside instead of datum length, so the belt ends up around 40 mm too short or too long.
• Replacing a classical A belt with an SPA belt without changing the pulley, even though the groove profiles differ.
• Fitting a standard SPZ belt to a drive designed for XPZ with a 56 mm pulley, which leads to overheating and early failure.
• A ribbed belt of the correct length but with the wrong rib count or wrong section (PJ instead of PK).
• A timing belt of the correct pitch but the wrong number of teeth, because the length was estimated with a tape instead of counted.

Frequently asked questions

Can I fit an XPZ belt instead of SPZ?

As a rule, yes. XPZ has the same cross-section, runs in the same groove and is more flexible and thermally more favourable thanks to the notches. The reverse is not always true: if the drive uses a pulley below the recommended minimum for SPZ, a standard wrapped belt will fail quickly.

What is the difference between SPA and SPB belts?

SPA measures 12.7 × 10 mm and needs a pulley of at least 90 mm, while SPB measures 16.3 × 13 mm and needs at least 140 mm. SPB transmits considerably more power per belt but requires a larger drive. They are not interchangeable because the grooves differ.

What does the code 6PK1600 mean?

It is a V-ribbed belt with six ribs, section PK (3.56 mm rib pitch) and a reference length of 1600 mm. The industrial notation for the same belt would be 1600 PK-6.

Is a ribbed belt the same as a timing belt?

No. A ribbed belt has longitudinal ribs and transmits power by friction, so slight slip is possible. A timing belt has transverse teeth and transmits power by positive engagement, without slip.

Can I mix an old and a new belt in one set?

No. The old belt has already stretched, so the new one carries most of the load and fails quickly. Always replace all belts at the same time.

How long does a drive belt last?

Service life depends on tension, alignment, pulley diameters, temperature and load. A correctly set up industrial drive typically runs for several thousand hours, but poor alignment or low tension can cut that by more than half.

Conclusion

Choosing a belt comes down to three questions: which section does the pulley require, how much power must be transmitted, and how much space is available. Classical sections remain a sensible choice for simple and older machines, narrow SPZ, SPA and SPB sections cover most industrial drives, their XP versions solve small pulleys and high speeds, ribbed belts suit compact drives with high ratios, and timing belts are the answer wherever synchronous, slip-free transmission is required.

If you are unsure which section you need, measure the cross-section of the old belt, copy its code and compare it with the tables in this guide. You can browse the full range in the belts category, and for sizes that are not in stock simply contact us with the code from your old belt.