Though belt-drive applications may seem routine, some areas are often misunderstood, as attested to by the many questions received by belt manufacturers. To put you on the right track, here are the answers to the belt-drive questions most commonly asked.

Q. What is the maximum speed that a drive belt can safely handle?
A. For most drives, pulley rim speed is the limiting factor, rather than the belt. This limiting speed depends on the pulley material and design, Table 1 and Figure 1.

Stock pulleys made of iron are statically balanced for rim speeds up to 6,500 fpm. A pulley running at more than 6,500 fpm may cause vibration, noise, poor bearing life, and high fatigue stresses. Therefore, pulleys that exceed 6,500 fpm should be dynamically balanced as described in Mechanical Power Transmission Association (MPTA) bulletin No. SPB-86. In some cases, depending on pulley size or application requirements, dynamic balancing may be required for pulleys operating at less than 6,500 fpm. For speeds outside of the normal ranges, consult the manufacturer.

Q. What causes belt-drive vibration and how can it be corrected?
A. Drive belts experience both vertical and lateral vibration when their natural frequencies coincide with resonant frequencies of connected equipment.

Belt tension can affect the amplitude of this vibration. Therefore, to correct the problem, first check for proper tension. A common method to control vertical vibration uses a restraining device (metal rod or idler pulley) placed perpendicular to the belt span and close to or lightly touching the belt. This device should be positioned roughly 1/3 of the span distance from the larger pulley.

If this does not work, consider changing other drive parameters to reduce the amplitude of vibration or alter its frequency. Such parameters include span length, belt type, misalignment, inertia of driving or driven machinery, pulley diameter and weight (inertia), speed, and the number of belts. In some cases (where original unit was oversized), it may be possible to downsize the drive by reducing the number of belts or belt width, and increasing the static tension to alter the belt’s natural frequency so it doesn’t coincide with the excitation frequency of the machinery. When it can be done safely, it is preferable to reduce the static tension to keep the operating belt tension below the belt’s natural frequency range.

To reduce lateral vibration, increase flexural rigidity in the lateral direction. This can be accomplished by using joined belts, Figure 2, which consist of two or more belts held together with a highstrength band that prevents the belts from bending sideways and keeps them running straight into the pulley grooves even under severe pulsating or shock loads. A wider synchronous belt can increase lateral rigidity, but should be tensioned carefully. Undertensioning may cause a synchronous belt to jump teeth (ratchet).

Q. What causes a squealing belt?
A. V-belt squealing is usually caused by belt slip, often due to undertensioning. When a new belt replaces one belt in a multibelt drive, the new belt may be tensioned properly, but all of the old ones are undertensioned. To avoid this problem, replace all belts in a multibelt drive at the same time, and with belts of the same construction from the same manufacturer. Belts from different manufacturers, although identified as being similar, may not be of the same size or construction.

Replace worn sheaves, which can lead to noise and belt rollover, as well as worn or damaged belts.

Sudden, high startup torques or peak loads also cause belt slip. Usually, this condition lasts only a few seconds. But, it can lead to heat build-up, Figure 3, which reduces belt life. If belt slip and heat build-up is suspected, turn off the drive and place a gloved hand on the belt to feel if the belt is too hot.

Grit, oil, or grease causes belts to slip. Therefore, keep the drive components clean. And don’t use belt dressing. This only masks the real problem of inadequate tension. (Also, see the following question on cutting fluids.)

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