Conveyor systems are used to move bulk products, raw materials, finished products, packages, and other objects from point A to point B. Depending on the industry, conveyors have many diverse designs and uses. In my experience, belt and roller conveyor systems are the most common type used in production, distribution, and fulfillment facilities. Depending on the magnitude of machinery noise, conveyor systems often set the background noise level throughout an area.

Belt conveyors, as shown in Figure 1, use a rubber or plastic compound combined with one or more layers of fabric. The belt is attached in a loop to two or more wheels, also known as rotors, which are driven by at least two synchronized motors, one on each end of the conveyor. The belt usually slides over a metal support bed or surface. Some belts are unsupported; in these systems, multiple idle rollers are added under the top belt.

Roller conveyors, as shown in Figure 2, employ a series of wide rollers connected by a narrow belt or chain, which is in turn powered by electric motors. Some operations use a smaller number of free-rolling conveyors that are driven manually or by gravity.

High noise levels (above 85 dBA) in conveyors are typically due to their design, build, and need for maintenance. Following are descriptions of each potential cause of excessive noise and steps to mitigate it.

Roller bearings are the greatest source of noise in a conveyor system. The most effective method of controlling bearing noise is reducing it at the source. Unless a customer specifies higher quality bearings and agrees to an increased cost, conveyor manufacturers commonly use commercial-grade ball bearings. The loose steel balls and excessive radial movement in this type of design result in elevated noise levels. Noise levels also increase proportionally with the speed of the conveyor system.

While quieter systems are available from manufacturers, they typically cost more, as they require tighter tolerances and more precision in their manufacturing. Therefore, it will be necessary for the end user—that is, the buyer—to determine if the noise reduction justifies the additional cost. In my experience, quieter conveyor systems typically cost 5 to 10 percent more than standard systems. But this premium pales in comparison to the cost of retrofitting equipment once in production, which is approximately 10 times the initial cost of a standard system.

Bearing precision is defined in industry guidelines published by the American Bearing Manufacturers Association (ABMA). In the appropriate standards, ABMA describes bearing precision using American Bearing Engineers Committee (ABEC) values of 1, 3, 5, 7, and 9. Lower ABEC numbers indicate less precision. To affect noise on the front end of the design or selection process, I recommend that bearings with an ABEC rating of at least 7 be specified for all new and replacement equipment.

Many types of rolling-element bearings are available, including various grades of precision and quality. Usually, bearing selection depends on load, life, and speed requirements; however, if low noise is a prime requirement, then ball bearings can offer distinct advantages over rolling-element bearing types.

The roller itself typically comprises either a hexagonal or cylindrical shaft (that is, an axle). Some roller shafts fit through holes in the side frame and are held in place by nuts. Others are spring-loaded on one or both ends, which allows the roller to be snapped into position; a similarly shaped but slightly oversized mating hole in each side frame accepts the shaft ends, holding it in place. This loose-fitting mounting method often leads to another significant noise problem: as the roller turns, rotating forces due to roller imbalance or intermittent loading cause the shaft ends to bounce around, producing an audible rattle. Aside from the noise considerations, the resultant forces on the shaft ends cause the frame to wear prematurely and tend to shorten the life of the roller. Furthermore, as the mounting holes in the side frames become worn, the noise level increases over time.

Even when precision or high-quality components have been selected, the physical installation of conveyors can affect the noise level. During machine assembly, bearings can be contaminated, damaged, or distorted. Some factors that can adversely affect noise output from conveyor systems are misalignment of bearings and roller shafts, over-tensioning of drive belts, and minor damage sustained during installation. Using a skilled and knowledgeable integrator, one that pays attention to meticulous build practices, is important to prevent noise on the front end of the project.

MAINTENANCE A critical component for sustaining low-noise operations is to vigilantly keep manufacturing equipment in optimal working condition. This is especially true for conveyor systems. No matter how well manufactured the equipment and how carefully it is installed by integrators, routine inspections and maintenance are mandatory. Most conveyor equipment generates increased noise levels when in need of adjustment, alignment, or routine repair. Also, conveyors will have extended life and run more reliably when routine maintenance is performed.

Chances are likely that industrial hygienists will be called upon to address noise from conveyor systems already in operation. The following comprehensive checklist and troubleshooting guide can help with this effort.

Maintenance and operating personnel should be trained to observe and listen for potential noise sources outside the norm for conveyors—or any other equipment. They should become familiar with the noise-generating mechanisms (bearings, rollers, drive belts, and so on) and with visual inspection procedures. Corrective action should be implemented as soon as practical or at the next scheduled maintenance cycle. The items discussed below should be part of an auditory and visual inspection.

First, check with each machine operator or material handler to identify any problems that may be causing excessive time to be spent at the machine component or conveyor section of concern. Check all machine controls for proper setting. All loose parts should be tightened or secured.

Rollers The following tasks should be performed while the system is in operation: • Walk around both sides of the system and note the condition of all moving components, as well as any unusual behavior. • Note points of material buildup. • Look for any signs of misalignment or improper belt tracking. • Check drive amperage requirements and compare amperages to previous levels for similar loads and conditions. (A rise in amperage above previous levels may indicate increased drag within the system, perhaps due to failed or unlubricated bearings. This should be further investigated when the system is at rest and electrically locked out.)

While the system is at rest and electrically locked out, check the following: • Confirm that components are in proper alignment. Make any necessary adjustments. • Inspect suspect components and take corrective action as appropriate. For example, if rollers are not revolving freely, lubricate or replace them. • Check for belt wear—especially at edges and splices. • Check for damage or wear at loading and transfer points. • Check clearances at chute and skirting areas. • Re-lubricate all bearings on pillow blocks (a type of mounting unit found in some conveyor systems) per the manufacturer’s schedule or the specific requirements of the installation. • Re-lubricate rollers per the integrator’s schedule or the specific requirements of the installation. • Replace motor brushes—the electrical contacts that conduct current—as specified by the manufacturer. • Correct any misalignment or imbalance of roller shafts. • Look for wear or deformation at the point of engagement between roller mounting holes in the conveyor frame and roller shaft ends.

Lubrication. For oil lubrication, most bearing manufacturers’ catalogs provide guidance on the selection of oil viscosity for operating conditions. This guidance is based on the thickness of oil film necessary to prevent surface-to-surface interactions within rolling contacts from adversely affecting bearing life. Because similar film thicknesses are required to minimize bearing noise, it is important to follow the manufacturer’s recommendation.

Greases contain a variety of additives to enhance performance. Some additives are in the form of solid particles that can create high levels of impulsive noise in rolling bearings. In high-speed bearings, these particles may even induce a type of distortion known as rolling-element-cage instability. (The cage is the component in a ball bearing that separates the balls and maintains spacing between them.) Quiet-running greases, free from particles, are available from several manufacturers and are recommended for noise-critical applications.

Ring-seating fits. Normal practice is for bearing inner rings to be made an interference fit on shaft diameters (meaning that one part is slightly larger than the part to which it is mated) and for bearing outer rings to be made a clearance fit in housing bores (meaning that space exists around the mating parts). The level of interference or clearance is controlled by dimensional tolerances of both the bearing rings and the shaft and housing seating. Bearing manufacturers’ catalogs contain recommended fits and seating tolerances.

Ring-seating fits have an important effect on bearing performance, including noise and vibration. The looseness of inner rings on rotating shafts allows small relative radial movements to occur between ring and shaft. These movements have a detrimental effect on rotor balance and manifest as a high level of vibration at running speed. Ring-seating fits are a compromise: excessive looseness is a source of wear, noise, and vibration, while extreme tightness of an inner ring on a shaft or of an outer ring in a housing can reduce bearing internal clearances or prevent outer rings from sliding to take up axial thermal expansion of the shaft.

Misalignment. Misalignment between the inner and outer rings of a rolling-element bearing can be a source of significant noise and vibration. Misalignment causes ball speed variation in ball bearings and roller skewing in roller bearings, both of which cause high rolling-element-cage forces and increase the risk of cage instability. Consult the manufacturer’s installation instructions for allowable limits of misalignment per bearing type.

Contamination. The oil film thickness generated between rolling elements and raceways (the grooves in which the balls move) is typically 0.3 μm. Particles of a size greater than the oil film thickness entering a rolling-element raceway create impulsive vibration, and they may even result in permanent indentation of the surfaces. This will raise bearing noise levels and reduce bearing life. To prevent contamination during bearing installation or replacement, all fitting and handling of bearings and adjacent machine parts should be performed in a clean environment (whenever practical) if low bearing noise is required.

If the existing bearings or coupling shaft alignment prove to be a noise problem, carry out the steps outlined below. Even a misalignment of two degrees can result in a 6–8 dBA increase in noise. In addition to the potential noise reduction, these items may improve bearing performance and life expectancy: • Verify the shaft alignment and correct any deficiencies. • Verify that the oil viscosity for the operating conditions meets the bearing manufacturers’ criteria. • Verify that the bolts holding the end cover in position have been carefully tightened to a set torque in the appropriate sequence. Otherwise, distortion of the end cover and bearing ring can result. • Replace bad or worn bearings as needed.

Bearings removed after operation or during inspections should be checked visually to evaluate whether operating conditions are satisfactory. If any abnormality is detected, investigate the cause and implement corrective action to ensure the failure is not repeated.

As a long-term solution, all concentric or squeeze-locking bearings should be replaced with bearings that employ a true taper-locking engagement with the roller shaft. The roller shaft will also need similar upgrading to tapered chucks.

MINIMIZING RISK The noise control options described in this article should improve the long-term life of conveyors and minimize noise exposure risk to workers. When a noise-producing problem is identified during a visual and auditory inspection, the problem should be corrected immediately if it involves only a minor malfunction or adjustment, and even if the equipment appears to be operating normally. If the problem requires more extensive attention, then it should be labeled or tagged at the problem location and be scheduled for service during the next maintenance round.