MRO Today - Think you know drive belts?

Think you know drive belts?
Many plant pros don't. So, here are answers to those frequently asked questions.

by Todd Sellden

Though belt-drive applications may seem routine, some areas are often misunderstood, as witnessed by the many questions belt manufacturers receive.

To put you on the right track, here are answers to some of the most commonly asked questions.

Question: What causes belt-drive vibration and how is it corrected?
Answer: Drive belts experience vertical and lateral vibrations when their natural frequencies coincide with the resonant frequency 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. Position this device roughly one-third of the span distance from the larger pulley.

If this doesn't work, consider changing other drive parameters to reduce the amplitude of vibration or alter its frequency. These include span length, belt type, misalignment, inertia of driving or driven machinery, pulley diameter and weight, speed, and the number of belts.

To reduce lateral vibration, increase flexural rigidity in the lateral direction. Accomplish this by using joined belts - two or more belts held together with a high-strength 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 tension it carefully. Undertensioning may cause a synchronous belt to jump teeth (ratchet).

Question: What causes a squealing belt?
Answer: Belt slip, often due to undertensioning, is usually the cause of a squealing V-belt.

When a new belt replaces one belt in a multiple-belt 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 multiple-belt 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 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. This condition usually lasts only a few seconds, but it can cause heat buildup, which reduces belt life.

If you suspect belt slip and heat buildup, turn off the drive and place a gloved hand on the belt to feel if the belt is excessively hot.

Grit, oil and grease cause belts to slip. Therefore, keep drive components clean. And, don't use belt dressing. This only masks the real problem: inadequate tension.

Large-pitch, wide synchronous drives may generate noise at high speeds. Excessive or deficient belt tension can cause this, which prevents the belt teeth from smoothly entering or leaving the sprocket grooves. As a result, alignment requirements are tighter for synchronous belts than standard V-belts.

When inspecting a problem drive, review all components. Non-belt sources (bearings, guard vibration and loose mounts) may be the source of the problem.

Question: Do cutting fluids or oil affect belts?
Answer: Occasional oil or grease splatter normally doesn't affect standard belts. But, a large amount of oil or grease on a V-ribbed or V-belt will cause slips. In such a case, shield the belt, or use a synchronous belt, which is less affected and able to operate under such conditions.

Extensive exposure to petroleum products also causes rubber to swell and the adhesion between belt components to break down.

All stock belts made by members of the Rubber Manufacturers Association (RMA) are reasonably oil- and chemical-resistant. When evaluating a belt's resistance to chemicals, consider the type of chemical, concentration, exposure time, type of belt, temperature and humidity.

Belts containing polyurethane compounds are more resistant to some chemicals.

Question: How is static conductivity determined and why is it important?
Answer: RMA bulletin IP3-3 explains how to test for static conductivity by passing an electric current of specified voltage through a section of belt while measuring the belt's resistance to conduct the current. A resistance of 6 megawatts or less prevents measurable static voltage buildup, thus preventing a potentially hazardous static discharge.

The RMA bulletin applies to new, clean belts. However, older belts can collect debris or become worn and damaged, giving it infinite resistance. This enables static charge buildup. Thus, in a hazardous environment, additional protection is recommended, such as grounding the entire system to ensure against accidental static spark discharges.

Also, a static conductive brush or similar device bleeds off static buildup on the belt.

Question: What is the normal shelf life of a drive belt?
Answer: When belts are properly stored, you shouldn't detect a significant change in performance for up to eight years.

Proper storage means protecting the belt from moisture, temperature extremes, direct sunlight and high ozone levels. Store the belt in its original package and avoid sharp bends or crimping that could damage the belt. Also, avoid bending or hanging belts on anything with a diameter less than the smallest recommended pulley diameter for that cross section.

Machines using belt drives sometimes stand idle for prolonged periods (six months or longer). Relax the tension on such belts during idle periods and store the equipment in an environment consistent with belt-storage guidelines. If this isn't possible (i.e. equipment stored outdoors in a cold environment), remove the belts and store them separately.

Question: What is the acceptable temperature range for most belt drives?
Answer: There is no well-defined temperature limit that ensures satisfactory performance. However, a properly applied belt generally yields acceptable service within an ambient temperature range of minus-30 to 140 degrees F.

When a rubber belt operates with excessively high internal temperatures, the adhesion between belt components breaks down, causing premature failure.

Ambient temperature, time of exposure, ventilation and drive design affect internal belt temperature. Tests indicate V-belt service life is cut in half for every 36-degree increase in ambient temperature. And, for every 2-degree increase in ambient temperature, there is a 1-degree increase in the belt's internal running temperature. Thus, each 18-degree increase in internal belt temperature cuts belt life in half.

Question: What are the primary causes of heat buildup in belt drives?
Answer: A belt drive's operation generates both internal and external heat. Belt flexing as it moves around the pulleys causes internal heat. Slippage between, for example, a V-belt and sheave, creates external heat.

Pulley diameter, load, belt flexing, belt type, maintenance, ambient temperature and air cooling are among parameters that affect belt operating temperature. To counteract adverse effects on belt temperature, apply these guidelines:
-- Use the largest pulley diameter possible. This reduces internal heat buildup due to small-radius bending. Plus, it reduces belt tension and bearing loads, increases air flow, and increases belt contact area, all of which minimize belt slip and heat buildup.
-- Follow proper installation procedures. V-belts require a run-in period and retensioning to ensure proper seating and non-slippage.
-- Install a belt guard that allows good ventilation. If additional measures are necessary, use forced ventilation or finned pulleys to dissipate heat and reduce heat buildup.
-- Select a flexible belt type to reduce heat buildup. Synchronous, V-ribbed and molded notched V-belts provide good solutions to temperature buildup, particularly for small-diameter pulleys.
-- If heat is a suspected problem, use pulleys made of steel or similar materials that conduct heat away from the belt. Plastic materials don't conduct heat from the belt.

Todd Sellden is the power transmission product application manager for Gates Rubber Company.

This article appeared in the June/July 2000 issue of MRO Today magazine. Copyright, 2000.