V-belts and roller chains

November 3, 2026  ·  Indumor

A classic scene at the parts counter: someone brings in the old belt, runs a tape around the outside and asks for "a six-footer". A used belt has stretched, it has sunk into the groove because it lost its flanks, and the outside measurement is none of the lengths a catalog lists. Ordering properly takes the section, which the top width tells you, and the length that section is specified by; if the belt is gone, the length is calculated from the pulley diameters and the center distance. Chains are much the same: the pitch is measured between pins and everything else comes from the ISO or ANSI table.

V-belts: classical and narrow sections

The classical sections of ISO 4184 and DIN 2215 are named with letters, Z to E, 10 to 40 mm wide. The narrow (wedge) sections of DIN 7753 carry the SP prefix: SPZ, SPA, SPB and SPC. For the same width the narrow belt is taller, carries more power and tolerates smaller pulleys, which is why new machines use it; the classical belt lives on in older motors and farm equipment. ISO 4183 defines the groove in pairs (Z/SPZ, A/SPA, B/SPB, C/SPC), so an SPA fits the pulley of an A. What changes is the height and the length that is specified, not the groove.

A belt has three lengths. The inside length L_i is the one in the code of a classical belt: an A68 is 68 inches around the inside, 1727 mm. The datum length L_d (formerly "pitch length") is that of the neutral line, the one in the formulas and the one marked on narrow belts. The outside length L_a is what a tape measure gives, and it is no good for ordering. The differences between the three depend on the section and are in the tables.

A classical belt is ordered by its inside length L_i, in inches in the code (A68 = 68 in = 1727 mm). The formulas use L_d = L_i + the difference in the table, which is approximate and varies slightly by maker.

A narrow belt is specified by its datum length L_d, marked on the belt (SPA 1800 = L_d 1800 mm).

The pulley: groove and angle

The groove does not have a single angle. Bending around a small pulley, the belt flattens at the top and its flanks close in; that is why ISO 4183 and DIN 2211 specify 34° for small diameters, 36° on the D and E sections and 38° on large pulleys. A pulley turned to 40° "because that is the belt angle" bears only at the edge: the belt overheats, rolls over and lasts half as long. The diameter quoted is the datum diameter d_d, measured at the datum line; the outside diameter is d_d plus twice b.

b_d: groove width at the datum line; b: height from that line to the rim (outside diameter = d_d + 2·b); e: groove spacing; f: rim edge to the center of the first groove. The A groove takes the SPA belt, and so on for each pair.

Tension is checked by deflection: 16 mm per meter of free span, pushing at mid-span with the force the maker specifies for that section. A slack belt slips and glazes the pulley; an overtensioned one kills the motor bearings, which cost more than the belt.

Belt length and center distance

With the two datum diameters and the center distance, the length comes from the formula; the other way round, with a belt of known length, C is solved for. The wrap angle on the small pulley has to exceed 120°, since that is the pulley with the least grip. The ratio is D over d, and because a V-belt slips 1 to 2 %, the output speed comes out slightly below the arithmetic (power and torque through the shaft are worked out in chapter 24). Belts run comfortably between 10 and 25 m/s; past 30 m/s the pulley needs checking, above all if it is grey cast iron.

Timing belts

A timing belt does not slip: the ratio is exact, as with a gear, so it goes wherever synchronism matters. There are three families that do not mix. The inch series of ISO 5296 (MXL, XL, L, H, XH, XXH), with trapezoidal teeth; the metric T of DIN 7721 and AT, a reinforced manufacturer profile; and the curvilinear HTD type (3M, 5M, 8M, 14M) of ISO 13050, which carries more for the same width. Pulley and belt have to be the same profile: a 5M will not run on a T5 pulley even though both have a 5 mm pitch.

The code gives length, pitch and width. An 800-8M-30 is 800 mm long (100 teeth of 8 mm) and 30 mm wide; a 225L050 is an L section, 22.5 inches long and half an inch wide.

The pulley has to match the belt profile; pitch alone is not enough (a 5M will not run on a T5 pulley). Inch widths are coded in hundredths: 050 = 1/2 in = 12.7 mm.

Roller chains: ISO 606 and ANSI B29.1

Two families coexist in Argentina: the European one of ISO 606 (B series, the old DIN 8187) and the American one of ANSI B29.1, which ISO 606 includes as the A series. The ANSI number gives the pitch in eighths of an inch: a 40 chain has 4/8, half an inch; a 60, 3/4. The 08B and the 40 share the 12.7 mm pitch but not the roller, the width or the pin, so neither links nor sprockets interchange. To identify a chain, measure three things: the pitch (better over 10 pitches, then divide), the roller diameter and the inner width between plates. With those the table names it.

Find a chain

Duplex and triplex chains repeat these dimensions in two or three strands. The 25 and 35 are rollerless: d₁ is the bushing. 08B and 40 share the 12.7 mm pitch and nothing else: links and sprockets do not interchange. Minimum tensile strength is per ISO 606; mass is approximate and varies by maker. 1 kN is 225 lbf.

Approximate sprocket outside diameter: d_a ≈ p·(0.6 + cot(180°/z)). Tooth form and tolerances are in ISO 606.

Sprockets, ratio, lubrication and wear

The small sprocket governs chain life. With few teeth the chain engages in jerks (the polygonal effect) and the roller hammers the tooth; 17 teeth is the recommended minimum at speed, 19 or 21 with shock loads, and 11 the absolute minimum for slow drives. The large sprocket should not exceed 114 teeth, because on a large sprocket any elongation makes the chain ride up. The rest is in the table: ratio, wrap, center distance and an even number of links, so no offset link is needed.

Chain speed: v = z·p·n / 60000 (m/s, with p in mm and n in rpm of the small sprocket). The maker's catalog has the diagram by pitch (ISO 10823).

A chain does not stretch: it wears at the pins and bushings, and the pitch grows. To know when to replace it, tension a run and measure 20 pitches. On a 12B they should come to 381 mm; at 392 mm (3 % more) the chain is due, and with large sprockets the limit is lower. A new chain on worn sprockets does not last: hooked teeth chew through it in weeks.

Frequently asked questions

How do I tell which V-belt section I have?

By the top width: 10 mm is Z, 13 is A, 17 is B, 22 is C. If a belt of that width is taller than usual (8, 10, 13 or 18 mm), it is a narrow section: SPZ, SPA, SPB or SPC.

How long is an A68 V-belt?

68 inches of inside length, 1727 mm. Its datum length L_d is 30 mm more, 1757 mm, and that is the figure that goes into the center distance formula.

When should a roller chain be replaced?

When it has elongated 3 % through pin and bushing wear: 20 pitches of a 1/2 in chain should measure 254 mm; at 262 mm it is due. With sprockets of many teeth the limit is lower.

Sources: ISO 4184 (classical and narrow V-belts; current edition 2025); ISO 4183 (grooved pulleys; current edition 2026); DIN 2215 (classical V-belts); DIN 7753-1 (narrow V-belts); DIN 2211 (pulleys for narrow V-belts); ISO 5294 and ISO 5296 (synchronous belt pulleys and belts MXL to XXH); DIN 7721 (metric T timing belts); ISO 13050 (curvilinear synchronous belts); ISO 606:2015 (roller chains and sprockets); ANSI/ASME B29.1-2011 (precision power transmission roller chains); ISO 10823 (chain lubrication). Belt and chain length from the geometry of the open drive; sprocket pitch diameter with d_p = p / sin(180°/z).