Flat Belt

What is a Flat Belt

A flat belt is a thin, wide, endless loop that drives one pulley from another entirely through friction across its flat contact face. There are no teeth and no wedge: the only thing holding the belt to the pulley is the normal force generated by belt tension wrapping around the pulley arc, multiplied by the coefficient of friction between belt and pulley surface. This makes the flat belt the conceptually simplest of all power-transmission belts, and also the one whose performance is most directly governed by friction physics rather than by tooth geometry.

The amount of power a flat belt can transmit depends on four factors: the velocity of the belt, the tension under which the belt is installed on the pulleys, the arc of contact between the belt and the smaller pulley, and the operating conditions. A flat belt drive therefore behaves differently from a roller chain or a gear: it can slip under overload, it cannot hold an exact synchronous phase, and it tolerates a degree of shaft misalignment that would destroy a toothed drive. Those same traits, controlled slip as overload protection and tolerance of misalignment, are why flat belts survived in specific niches.

Historically, flat belts of leather and woven fabric were the universal power-distribution medium of the nineteenth and early twentieth centuries. A single steam engine or water wheel turned a long overhead line shaft, and flat leather belts dropped from that shaft to drive each machine on the floor. The technology was so embedded in industry that, during the leather shortages after the First World War, leather drive belts were reportedly cut up to make shoes, halting some mills. The arrival of the individual electric motor, typically an AC motor, on each machine and, later, the V-belt for compact drives pushed flat belts out of general service.

The modern revival rests on synthetic traction layers. By bonding a thin, highly oriented polyamide, polyester, or aramid film between elastomer or leather friction covers, manufacturers produce endless flat belts that combine very low bending stiffness with very high tensile strength. These belts achieve efficiencies commonly quoted at 95 to 98 percent, with high-performance lines rated at or above 98 percent, against roughly 93 to 95 percent for a comparable V-belt drive. That few-percent efficiency edge, multiplied over continuous-duty machines running thousands of hours a year, is the commercial case for the flat belt today.

The scale a flat belt can reach is wide. Classical leather belts handled fractional to tens of kilowatts at modest speeds; modern film belts deliver hundreds of kilowatts at very high speed, with published figures of 373 kW transmitted at 51 m/s, and high-performance Extremultus lines rated up to roughly 1850 kW. The flat belt therefore is not a single product but a family that ranges from a small printer feed belt to a heavy main drive, unified only by the friction principle and the flat, crowned-pulley geometry.

Drive Types and Belt Construction

Two independent classifications matter when specifying a flat belt: the geometry of the drive layout, and the layered construction of the belt itself. Confusing the two leads to selecting a belt that the layout cannot use, for example fitting a one-sided power-transmission belt to a quarter-turn drive that flexes it the wrong way. The table below summarises the standard flat-belt drive layouts.

Open belt drive is the default: the two shafts are parallel and rotate in the same direction, with the driver pulling the belt from the lower (tight) side and returning it on the upper (slack) side. Keeping the slack side uppermost increases the arc of contact on both pulleys and is standard practice. Crossed belt drive reverses the direction of the driven shaft by twisting the belt into a figure eight; the belt surfaces rub where the strands cross, so belt speed is kept below about 15 m/s to limit wear. Quarter-turn drive connects shafts at right angles, and to keep the belt from running off, the receiving pulley face must be at least 1.4 times the belt width. Compound and stepped cone-pulley layouts relay power through multiple pulleys or provide a set of discrete speed ratios respectively.

Construction is the second axis. A traditional belt is built from leather plies or woven fabric and is specified by the number of layers, single, double, or triple ply, and by the hide weight, light, medium, or heavy. To build up thickness, strips are cemented or layered together. A modern high-performance belt instead is a sandwich: a thin oriented traction layer (the load-carrying film) bonded between friction covers. The Habasit A-3 belt, for instance, is a 3.4 mm thick belt with an acrylonitrile-butadiene-rubber (NBR) friction cover on the pulley side over a polyamide traction layer with two fabric plies and an impregnated-fabric reverse cover. The traction layer carries the tension; the covers provide grip and protect the film.

The practical distinction for the buyer is that traditional plied belts are made endless by a mechanical or cemented joint on site, whereas modern film belts are usually supplied endless or joined by a controlled thermal splice (Habasit specifies a Thermofix joint at 75 degrees for the A-3). A spliced film belt has near-uniform thickness and balance around its full circumference, which is what allows it to run smoothly at the high speeds discussed in Chapter 5. A mechanically laced belt is repairable in the field but introduces a thickness discontinuity that limits speed.

Traction-Layer Technologies

In a modern flat belt the traction layer, not the friction cover, defines tensile strength, elongation, and dimensional stability. Manufacturers offer the same belt family in several traction-layer chemistries so the engineer can trade modulus against shock tolerance. The table below compares the three mainstream high-performance traction layers against the classical leather and fabric constructions.

Polyamide film uses a sheet of highly oriented polyamide, the polymer family commonly known as nylon, as the tension member. Its great strength is forgiveness: polyamide belts absorb short-term shocks and overloads and tolerate the smallest pulley diameters because the thin film bends without fatiguing quickly. This is why the Habasit A-3, a polyamide belt, can run on a 125 mm minimum pulley and is marketed as abrasion resistant and forgiving in case of short-term shocks. The trade-off is slightly higher elongation than the alternatives, so polyamide belts need periodic re-tensioning monitoring on critical drives.

Polyester fabric traction layers, woven from polyester yarn, give a higher modulus and lower creep than polyamide. They hold their installed length and ratio more precisely, which suits high-speed drives where dimensional stability and balance dominate. Forbo Siegling builds its Extremultus polyester line on a thermoplastic traction layer reinforced with polyester fabric, marketed for high-efficiency, durable, shock-absorbing drives.

Aramid fabric is the highest-performance option. An aramid (para-aramid) warp gives a very high modulus and the lowest elongation, so an aramid-line belt carries the greatest pull for a given width and barely stretches in service. This is the choice for heavy-duty, high-power drives where width or shaft load must be minimised. Aramid belts cost more and are less forgiving of shock than polyamide, so they are reserved for duties that genuinely need the modulus.

Leather and woven fabric remain in service for restoration of historic line shafting and for niche textile, woodworking, and agricultural machinery. Chrome-tanned leather offers a high coefficient of friction and a forgiving, repairable belt, while plied cotton or fabric belts are inexpensive and field-serviceable. Both have lower modulus, higher and more variable creep, and lower speed ceilings than the film belts, which is why general industry moved on from them.

Materials, Friction, and Tension

Because a flat belt transmits power purely by friction, the relationship between the two strand tensions sets the whole drive. The governing law is the Euler-Eytelwein, or capstan, equation: at the verge of slipping, the ratio of the tight-side tension to the slack-side tension is T1 divided by T2 equals e raised to the power of the coefficient of friction multiplied by the wrap angle in radians. The useful pull, called the effective or peripheral force, is the difference T1 minus T2, and the transmitted power equals that difference multiplied by belt speed. Everything in flat-belt selection follows from raising the friction coefficient or the wrap angle so that a larger T1 minus T2 is available before slip.

Coefficient of friction depends on both the belt face and the pulley surface. Engineering references give roughly 0.25 to 0.35 for leather on a cast-iron pulley and around 0.4 for rubber on steel; worked design examples commonly adopt a value near 0.35 for leather. A higher friction coefficient lets a belt carry more load at the same installed tension, which is why the friction cover material, NBR, polyurethane elastomer, or chrome leather, is itself a selection parameter, not an afterthought. Damp, oily, or dusty pulley faces drop the coefficient sharply and are a common root cause of unexplained slip.

Centrifugal tension sets the upper speed limit. As the belt rounds a pulley at speed it generates a centrifugal tension equal to the mass per unit length times the square of the belt speed. This tension adds to the strand tensions but does not contribute to useful pull, so at high speed it eats into the friction reserve. The mathematics gives a clean design rule: maximum power is transmitted when one third of the maximum permissible belt tension is absorbed as centrifugal tension. Practically, leather and fabric belts have an economical band around 20 to 22 m/s, while light, balanced film belts push the limit far higher.

Drive layouts are grouped by speed accordingly. Light drives transmit small powers up to about 10 m/s, as in agricultural machines and small machine tools; medium drives run from 10 to 22 m/s; heavy drives operate above 22 m/s. The table below collects the friction-related material data an engineer needs at the selection stage.

Material density feeds directly into the centrifugal-tension calculation. Leather belt material is commonly taken at a density of about 1000 kg/m3 in design work; a lighter modern film belt of the same width and thickness generates less centrifugal tension at a given speed, which is part of why film belts run faster. The belt is finally installed at a defined initial tension (or initial elongation) so that, under full load, the slack side still retains enough tension to keep the belt gripping; that initial tension is set from the k1 percent figure described in the next chapter.

Key Specification Parameters

Reading a flat-belt datasheet is the core procurement skill, and it differs from reading a V-belt or chain catalogue because flat belts are tensioned by elongation, not by a part number that implies a fixed pitch length. The parameters that actually drive selection are belt thickness, k1 percent, nominal peripheral force, minimum pulley diameter, belt width range, operating temperature, and weight. Each is explained below, with the verified Habasit A-3 polyamide belt used as a worked reference.

k1 percent is the tensile force per unit width needed to elongate the belt by 1 percent of its length, in N/mm. It is the master sizing number: because flat belts are fitted at a target elongation, k1 percent times the chosen elongation times the belt width gives the static strand tension. The A-3 lists k1 percent equal to 12 N/mm after running in. Fitting a 100 mm wide A-3 at 1 percent elongation therefore sets roughly 1200 N per strand of static load before any peripheral force is added.

Nominal peripheral force per unit width is the recommended useful pull the belt can deliver, again in N/mm. For the A-3 this is 36 N/mm. Multiplying by belt width gives the maximum effective force T1 minus T2, and multiplying that by belt speed gives transmittable power. This is the number to compare belt to belt when sizing for a known torque.

Minimum pulley diameter caps how tightly the belt may bend. Below it, bending stress in the traction layer cuts life and raises flex losses. The A-3 specifies 125 mm both for normal running and for counter-flexion over a back-side idler. A practical guide for polyamide-core belts is to keep the pulley diameter at or above about 60 times the belt thickness.

Belt width and thickness follow preferred-number series. Per ISO practice, nominal flat-belt widths run in the R10 and R20 series, with standard values of 25, 32, 40, 50, 63, 71, 80, 90, 100, 112, 125, 140, 160 mm and upward to 600 mm. Preferred plied-belt thicknesses include 5 mm for widths of 35 to 63 mm, 6.5 mm for 50 to 140 mm, 8 mm for 90 to 224 mm, 10 mm for 125 to 400 mm, and 12 mm for 250 to 600 mm. The A-3 film belt, by contrast, is a constant 3.4 mm regardless of width, with seamless widths available to 1140 mm.

Operating temperature bounds the cover and traction-layer chemistry. The A-3 is rated for continuous service from minus 20 to plus 100 degrees Celsius. Exceeding the upper limit softens the elastomer cover and can delaminate the traction layer; running below the lower limit stiffens the belt and raises flex losses and noise.

Belt weight (mass per unit area) feeds the centrifugal-tension and shaft-load calculations. The A-3 weighs 3.5 kg/m2. Lighter belts generate less centrifugal tension at a given speed, raising the practical speed ceiling, which is one reason thin film belts displaced heavy leather at high speed. The remaining datasheet flags worth checking are antistatic equipment (the A-3 is antistatically equipped), food contact conformance (the A-3 is not food rated), and explosion-atmosphere (ATEX) suitability, which the A-3 explicitly has not been tested for and leaves to the user.

Selection Decision Factors

To turn the preceding chapters into a specific belt, work through the sequence below. Most selection errors come not from a single wrong number but from deciding the belt construction before the duty and geometry are fixed. These steps double as a fixed RFQ template.

1. Define the duty: transmitted power (kW), pulley speeds (rpm), and therefore belt speed (m/s), plus the load character, steady, shock, or reversing. Classify the drive as light (up to about 10 m/s), medium (10 to 22 m/s), or heavy (above 22 m/s) to set expectations on centrifugal tension.
2. Fix the geometry: centre distance, pulley diameters, and layout (open, crossed, quarter-turn, compound, or stepped). Confirm the smaller pulley meets the belt minimum diameter, and that any counter-flexion idler also meets the (usually larger) counter-flexion minimum.
3. Set the wrap angle: the arc of contact on the smaller pulley directly multiplies grip in the capstan equation. Keep the slack side uppermost on open drives and add an idler if wrap is marginal. Crossed drives stay below about 15 m/s; quarter-turn drives need pulley face width of at least 1.4 times belt width.
4. Choose the traction layer: polyamide for shock tolerance and small pulleys, polyester for dimensional stability, aramid for highest pull and lowest stretch, or leather and fabric for restoration and niche duties (Chapter 3).
5. Size width from peripheral force: divide the required effective force (T1 minus T2) by the belt nominal peripheral force per unit width (for example 36 N/mm for the A-3) to get the minimum width, then round up to a preferred R10 or R20 width.
6. Set installation tension: from k1 percent and the chosen elongation, confirm the static shaft load is within the bearing and shaft rating. Verify that under maximum load the slack side keeps enough tension to avoid slip, and that centrifugal tension does not exceed one third of maximum belt tension at top speed.
7. Match friction cover and pulley: select the cover (NBR, polyurethane, chrome leather) for the required friction coefficient and the environment (oil, abrasion, washdown), and confirm the pulley face is crowned for self-tracking on open drives.
8. Confirm environment and compliance: operating temperature window, antistatic requirement, food contact or ATEX needs, and the splice or joining method (for example Thermofix on film belts versus mechanical lacing on plied belts).

One last dimension that buyers overlook is serviceability and total cost of ownership. A flat belt drive must be re-tensioned and tracked over its life, whether by a movable motor base or a dedicated belt tensioner: budget for periodic elongation checks, confirm the splice can be remade in the field or that endless spares are stocked, and weigh the few-percent efficiency advantage of a film belt against its higher unit price over the machine's running hours. Established suppliers of endless film flat belts, Forbo Siegling (Extremultus polyester, aramid, and polyamide lines) and Habasit (polyamide A series and elastomer lines), along with Megadyne, Ammeraal Beltech, and Mitsuboshi, maintain calculation tools and technical support that shorten the selection and commissioning effort on larger drives.