Because a crown makes the belt track itself, and without it a flat belt has nothing to keep it on the pulley.

A V-belt sits in a groove and a timing belt has flanges and teeth. A flat belt has neither - it simply lies on the pulley face, and any misalignment or asymmetry would walk it off the edge.

Crowning solves it. The pulley is made slightly larger in diameter at the centre than at the rims, so the belt sees a small hill. A belt running off-centre finds the tension on one side slightly higher, and that difference steers it back towards the crown. The effect is continuous and self-correcting.

The practical consequences are that at least one pulley in the drive must be crowned, that the crown must be correct for the belt width and speed, and that a belt which persistently runs to one side indicates a shaft alignment error or a damaged crown - not a belt that needs guiding with a flange.

Higher speed, longer centres, better efficiency and much less bending loss.

A V-belt transmits by wedging into a groove, which multiplies the friction but also means the belt is compressed and released as it enters and leaves - and that flexing generates internal heat. A thick V-belt bending around a small pulley loses meaningful energy that way.

A flat belt is thin and flexible. It bends around a pulley with very little internal hysteresis, so efficiency is high and stays high at small pulley diameters. There is no wedging to limit speed, so flat belts run at speeds that would destroy a V-belt.

They also tolerate very long centre distances without the whip that troubles V-belts, and they are quiet.

The V-belt keeps the advantages of higher torque capacity per unit width at moderate speeds, simple tensioning, and tolerance of poor alignment - which is why it still dominates general industrial drives.

By adjusting centre distance or a tensioning idler, at a tension calculated from the transmitted power - and joined either endless or with a mechanical or welded splice.

A flat belt transmits by friction, and friction requires tension. Too little and the belt slips, generating heat and polishing the pulley. Too much overloads the shaft bearings and shortens belt life. Manufacturers specify tension as an elongation percentage, which is measured by marking a length on the belt and stretching it to a target - a far more reliable method than judging by feel.

Endless belts, made as a continuous loop or spliced in the factory, give the smoothest running and the longest life and are preferred where the drive can be dismantled to fit them.

Where it cannot, belts are joined on the machine with mechanical fasteners or by a hot or cold splice. Splices are a discontinuity and are usually the point where a belt eventually fails, so a well-made splice matters.

In conveying and handling far more than in classic power transmission.

Live roller conveyors, package and parcel handling, and belt-driven roller beds use them heavily, because a thin belt can drive many rollers from beneath with minimal loss. Folder-gluers, printing and paper converting machines use them for transport and for high-speed drives.

Textile machinery, tangential spindle drives and any application needing very high speed with low vibration remain classic uses.

They are also used where the belt must double as the working surface - carrying product as well as transmitting motion - which no other transmission belt does.

What has largely disappeared is the line-shaft application flat belts were invented for. Individual motor drives replaced it, and for general industrial power transmission the V-belt and timing belt took over on cost and convenience.

Because the two faces do different jobs - one grips the pulley, the other often touches the product.

The belt is a laminate. Its core is a tension member, usually polyamide or polyester, which carries the load and determines the belt's strength and stretch. Covers are bonded to one or both faces.

The driving face needs friction against the pulley, so it typically carries an elastomer or leather-substitute cover with a high coefficient of friction.

The other face depends on the application. On a conveyor it may need to grip product, in which case it is also high friction. On a live roller drive or an accumulating conveyor it may need to slide, so a low-friction fabric or PTFE-loaded cover is specified.

Covers are also chosen for oil resistance, temperature, food contact approval and antistatic properties. Specify both faces separately - assuming they are the same is a common ordering error.

Almost always alignment or pulley geometry, and almost never the belt itself.

The crown steers the belt to the highest point on the pulley. If the belt still walks off, something is overriding that.

Check shaft parallelism first: two shafts that are not parallel steer the belt continuously toward one side, and even a small angular error is enough. Check that the pulleys are on the same plane and not offset axially. Check the crown itself - a worn, damaged or incorrectly machined crown loses the steering effect, and a pulley that has been reversed or replaced with a flat one will not track.

A belt with uneven tension across its width, from a bad splice or from damage, will also steer, but this is much rarer.

Fitting flanges to contain a wandering belt treats the symptom and destroys belt edges. Find and correct the alignment instead.

From the effective tension the belt can carry, the coefficient of friction, and the arc of contact on the smaller pulley.

A friction drive can only transmit as much force as the friction between belt and pulley allows before slipping. That depends on the belt tension, the friction coefficient of the cover, and how far the belt wraps around the pulley - the arc of contact.

Arc of contact is the term most often overlooked. A drive with a large ratio wraps the small pulley through much less than half a turn, and capacity falls sharply. Increasing the wrap with an idler is frequently the cheapest way to fix a slipping drive.

Belt speed matters too: for a given power, higher speed means lower force, so a fast drive needs a narrower belt. Flat belts tolerate high speeds well, which is why they suit high-speed drives.

Work from the manufacturer's selection data using power, speed, ratio, centre distance and the arc of contact.