Flat belts: Are they still around?

Electrical Apparatus, Jun 2004 by Nailen, Richard L

They still have their place, especially with small pulleys and high speeds

TO JUST ABOUT ANY MECHANIC TODAY, "BELT drive" means V-belt-or perhaps the newer timing or synchronous versions. Time was, however, when any industrial power transmission belt was a cumbersome, noisy, flat leather belt. Entire factories were powered by long overhead line shafts, with pulleys at frequent intervals from which long flapping belts hung down to run machines on the shop floor.

Such belts remained common as more machinery became powered by individual floor-mounted motors (Figures 1 and 2). But commencing in the 1930's, V-belts began taking over most industrial drives. Individual belts could readily be added or removed to vary the amount of power transmitted. Whereas flat belts had to depend on surface friction alone, some slippage being normal, the wedging action of a V-belt in its pulley groove greatly increased its torque capability. A narrower overall belt width permitted concentration of the pull closer to the motor drive-end bearing, reducing bearing load as well as shaft stress.

There were some disadvantages, too. At high speed, centrifugal force tended to throw the heavier V-belts out of their grooves. Alignment of driving and driven pulleys assumed greater importance. Flexing losses result in V-belt efficiency several percent below that of a modern flat belt (which can reach 98%). And the typical elastomeric V-belt is less resistant to attack by lubricants or ultraviolet radiation.

Hence, flat belts still have their place. Solid leather is no longer the material of choice, although often still used for outer surfaces because of its superior resistance to oil. A central core of nylon is common.

Flat and V-belts: similarities and differences

Before looking further at the contrast between flat and V-belt drives, let's review what the two types have in common:

* The method of power transmission is the same for both -friction between belts and pulleys. If the forces involved exceed the amount of friction that can be developed, slippage will occur between belts and pulleys.

* The total "belt pull" on the shaft is the sum of the tensions in the "tight side" and the "loose" or "slack" side of the belting. (See Figure 3.) For both types of drive, formulae exist to calculate those tensions based on pulley/belt dimensions, speed, and the power being transmitted.

* The effect on the driving motor will include a radial force on both bearings (particularly the one closest to the pulley) and a bending stress in the shaft (usually maximum at or near the drive-end bearing). The greater the force with which the driving and driven pulleys are held apart, the greater the belt tension, and the greater will be those forces on the motor.

Similarities end there. The differences are more numerous, and more important. They are:

* A flat belt drive involves only one belt. Most industrial V-belt drives consist of multiple belts.

* V-belts come in fixed lengths or loops. Although splicing methods exist for mending a broken belt, they are seldom used. Some flat belting may be supplied in rolls, cut to the desired length, and spliced into a loop in various ways, making it possible to repair or replace a belt without disassembling the drive as may be necessary for V-belt replacement.

* Flat belt drive pulleys are not grooved, because friction does not involve any wedging action of the belt within a tapered groove. Belt sides or edges take no part in power transmission.

* A flat belt drive must include some means of keeping the belt from sliding edgewise off the pulley, a provision not required in the V-belt drive because the grooves maintain belt position.

* Initial tension in a flat belt remains essentially unchanged once the belt is "run in." Although flat belt materials do experience some elongation under tension, the amount is minimal compared to the far more elastic V-belt.

Another significant difference is in the method of tensioning. The practice for V-belts is well-known. At initial setup, and during periodic maintenance checks, the rule of thumb is that a properly tensioned belt should deflect about 1/64'' per inch of span when a deflection force is applied in accordance with Table 1. Various makes of "tension testers" can also be used.

None of that applies to flat belts, for which tension is based upon measured elongation of the distance between pulleys. At installation, the pulleys are forced apart enough to stretch the belt initially by an amount the belt supplier recommends (about 2% for a leather-covered polyester-core belt).

A further percentage allows for the reduction of actual tension by the centrifugal force acting on the belt as it wraps around the pulley, which decreases both the friction and the contact angle between belt and pulley. This will vary with the type and construction of the belt. Figure 4 is typical.

Checking tension or "re-tensioning" during service is not normal practice for many modern flat belts, although such periodic "take-up" was common with older leather belting.