How to reduce belt drive noise

V-belt, synchronous belt, roller chain and gear drives will all generate noise while transmitting power. Each type of system has its own characteristic sound. V-belt drives tend to be the quietest belt drives, and synchronous belt drives are much quieter than roller chain drives.

If noise is an issue in your facility, here are several design and maintenance tips that should be followed to achieve the quietest possible belt drive.

Noise: Noise is an unwanted or unpleasant sound that can be described with two criteria — frequency and decibel (dB) levels. Frequency is measured in Hertz (Hz). The human ear is capable of distinguishing frequencies typically from 20 Hz to 20,000 Hz. The human ear generally does not perceive frequencies higher than 20,000 Hz.

The noise level or intensity of noise is measured in terms of decibels (dB). The decibel has become the basic unit of measure for sound, since it is an objective measurement that approximately corresponds to the subjective measurement made by the human ear. Since sound is composed of several distinct and measurable parts and the human ear doesn’t differentiate between these parts, measuring scales that approximate the human ear’s reaction have been adopted.

Three scales — A, B, and C — are used to duplicate the ear’s response over the scale’s ranges. The A scale (dBA) is most commonly used in industry because of its adoption as the standard in OSHA regulations.

Noise described in decibels is generally perceived as the loudness or intensity of the noise. The loudest sounds that can be tolerated by the human ear are about 120 dB, and sound levels above 85 dB are considered harmful. The human ear is most sensitive in the range of normal speech — 500 Hz to 2,000 Hz. As a consequence, this range is the most common concern for noise control.

Frequency is most closely related to what the ear hears as pitch. High-frequency sounds are perceived as whining or piercing, while low-frequency sounds are perceived as rumbling. The combination of decibel and frequency describes the overall level of loudness to the human ear. One without the other does not adequately describe the loudness potential of the noise. For example, an 85 dBA noise at 3,000 Hz is going to be perceived as much louder than an 85 dBA noise at 500 Hz.

For comparison, here are some noise sources and their typical sound levels:

* Normal speech: 60 dBA

* Busy office: 80 dBA

* Textile weaving plant: 90 dBA

* Canning plant: 100 dBA

* Heavy city traffic: 100 dBA

* Punch press: 110 dBA

* Air raid siren: 130 dBA

* Jet engine: 160 dBA.

Proper installation and maintenance procedures, as well as some simple design alternatives, can reduce belt drive noise.

Belt drive tension and alignment: Noise is a warning sign that some problem exists with the belt drive. Squealing can be caused by a slipping v-belt. The belt may be undertensioned. If a new belt has replaced one belt on a multi-belt drive, then the new belt may be tensioned properly while all of the remaining old belts are now undertensioned. It is important to replace all belts at the same time.

On drives that use more than one belt, all belts should be supplied from the same manufacturer. Belts from different manufacturers, identified as being similar, may not be of the same size or construction. Worn and damaged belts should always be replaced.

Sheaves also should be inspected for wear that can lead to noise and belt rollover. Sudden high start-up torques or peak loads can cause the belt to slip. Most of the time, this condition will last only a few seconds.

Belt slip can lead to heat buildup in the belt and in extreme cases smoke and the smell of burnt rubber can be witnessed. If belt slip and heat buildup are suspected, a gloved hand can be placed upon the belts with the drive shut down to feel if the belts are too warm.

Grit, oil or grease will cause belts to slip. Cleanliness of the drive components should be a part of a regular maintenance program.

Larger-pitch, wider synchronous drives may generate noise at higher speeds. This can be the result of too high or too low belt tension, or misalignment, which will not allow the belt teeth to smoothly enter/exit the sprocket grooves.

Improper tension in synchronous belt drives can affect how the belt fits in the sprocket grooves. Proper tension minimizes tooth-to-groove interference, and thereby reduces belt noise. Check to make sure that the drive is properly tensioned by using tension measurement gauges.

Misaligned v-belt drives will be noisier than properly aligned drives, since interference is created at the belt’s entry point into the sheave.

Misaligned synchronous belt drives tend to be much noisier than properly aligned drives due to the even greater amount of interference that is created between the belt teeth and the sprocket grooves. Misaligned synchronous belt drives may cause belt tracking that forces the edge of the belt to ride hard against a sprocket flange. Misalignment causing belt contact with a flange will generate noise that is easily detected. Because of this, alignment requirements are tighter for synchronous belts than standard v-belts.

Noise barriers and absorbers: Sometimes, even properly aligned and tensioned belt drives may be too noisy for a work environment. When this occurs, steps can be taken to modify the drive guard to reduce the noise level.

Noise barriers are used to block and reflect noise. Noise barriers do not absorb or deaden the noise; they block the noise and generally reflect most of the noise back towards its point of origin. Good noise barriers are dense and should not vibrate. A sheet metal belt guard is a noise barrier. The more complete the enclosure is, the more effective it is as a noise barrier.

Noise barrier belt guards can be as sophisticated as a completely enclosed case, or as simple as sheet metal covering the front of the guard to prevent direct sound transmission. Noise absorbers are used to reduce noise reflections and to dissipate noise energy. Noise absorbers should be used in combination with a noise barrier. Noise absorbers are commonly referred to as acoustic insulation.

Acoustic insulation (the noise absorber) is used inside of belt guards (the noise barrier) where necessary. A large variety of acoustic insulation manufacturers are available to provide different products for the appropriate situation.

A combination of noise barrier (solid belt guard) and noise absorber (acoustic insulation) will provide the largest reduction in belt drive noise. While the noise reduction cannot be predicted, field experience has shown that noise levels have been reduced by 10 dBA to 20 dBA when using complete belt guards with acoustic insulation.

Reminder: When inspecting a problem drive, review all aspects of the drive. Noise can be caused from non-belt sources such as bearings, vibration of guards, misalignment, etc. When a belt drive is excessively noisy, the belt is often incorrectly blamed. It is easy to eliminate the belt as the problem by spraying it with soapy water while it is running. If the noise goes away, or decreases, then the belt is part of the problem. If the same noise is still present, the problem is likely due to other drive components.

This article was provided by the engineers at Gates Corp.