Servicing Synchronous Drives

Synchronous belt drive systems are an ideal replacement for chain, gear and V-belt drives in the food handling industry. Typical applications range from bucket elevators and belt conveyors, to pumps, dough mixers, bottling machines and meat grinders.

In general industrial applications, synchronous belt systems provide long, dependable life with reduced downtime and virtually no maintenance when installed according to the manufacturer’s recommendations.

And, for food processors, synchronous belt drives are clean-running systems that require no expensive oil baths or lubrication. When combined with stainless steel sprockets, the synchronous belt drive system has the added advantage of being able to resist rust, chemicals, the buildup of contaminants and caustic washdown solutions.

BELT SERVICE LIFE

Most well-designed systems should last, on average, two to three years. However, there are several factors that influence how often a belt drive should be replaced. These include drive operating speed, drive operating cycle, the critical nature of the equipment, temperature extremes in the food processing area, environmental factors and accessibility of the equipment.

Since each drive system is unique, belt service life may be considered acceptable even if the belt only lasts a few weeks or months due to the harsh working environment.

INSPECTION PROCEDURES

Unlike roller chain drives, if a synchronous belt system is installed properly, and retains its proper alignment when the system is loaded, there is no reason to check the drive until the belt fails.

Before installing a replacement belt, Gates Corp. Power Transmission Product Application engineers suggest that the drive be inspected to avoid future performance problems. With the power shut off, locked and tagged, and the machine components in safe positions, remove the drive guard and inspect the belt for signs of unusual wear or damage.

Symptoms such as excessive belt edge wear, tooth shear, belt cracking and tensile break are clues that may indicate the need to correct alignment, or to adjust tension to the correct values. Synchronous belts are sensitive to misalignment and should not be used on drives where severe misalignment is inherent to the drive operation. Any improper tracking or severe sprocket misalignment will result in some reduction of belt life.

When a belt fails, the maintenance technician should inspect the sprockets to make sure they are not severely worn. If the sprockets have significant wear, the life of the next replacement belt will be significantly reduced. The sprockets need to be replaced at this time. In addition, they should be changed after every third replacement belt has reached its maximum service life.

DRIVE ALIGNMENT

Misalignment is one of the most common causes of premature synchronous belt failure. Depending on its severity, misalignment can gradually reduce belt performance by increasing wear, fatigue and premature tensile failure due to unequal tensile member loading. Or, it can destroy a belt in a matter of hours or days. While the forms of misalignment may be fairly well understood, accurate measurements and acceptable limits must be determined if maintenance technicians are to take corrective action.

Basically, any degree of misalignment, angular or parallel, will decrease the normal service life of a belt drive.

ANGULAR MISALIGNMENT

Angular misalignment has a severe effect on the performance of synchronous belt drives. Symptoms such as high belt tracking forces, uneven tooth/land wear, edge wear, high noise levels, and potential tensile failure due to uneven cord loading are possible. Also, wide belts are more sensitive to angular misalignment than narrow belts.

PARALLEL MISALIGNMENT

Parallel misalignment is generally not a critical concern with synchronous belt drives as long as the belt is not trapped or pinched between opposite flanges, and as long as the belt tracks completely on both sprockets.

Synchronous sprockets are designed with face widths greater than the belt widths to prevent problems associated with tolerance accumulation, and to allow for a small amount (fractions of an inch) of mounting offset. As long as the width between opposite sprocket flanges exceeds the belt width, the belt will automatically align itself properly as it seeks a comfortable operating position on both sprockets.

It is normal for a synchronous belt to contact the flanges in the system while operating. Synchronous belts rarely run in the middle of the sprockets without contacting at least one flange.

CHECKING ALIGNMENT

Misalignment of all synchronous belt drives should not exceed 1/4 or 1/16-in. per ft of centre distance. Misalignment should be checked with a good straightedge tool. The tool should be applied from the driveR to driveN sprockets and from driveN to driveR sprockets, along the outside face of both sprockets.

Misalignment will show up as a gap between the outside face of the sprocket and the straightedge. A more precise method is to use a laser alignment device that uses a reflected laser line to quickly identify common types of sprocket misalignment. Sprockets and shafts can be checked for tilting with a bubble level.

Sprocket misalignment may result from the motor shaft and driveN machine shafts not being parallel, the sprockets not being properly located on the shafts, or the sprockets being tilted due to improper mounting.

Drive misalignment also can cause belt tracking problems. Although some belt tracking is normal and won’t affect performance, optimum operation of the drive can only be achieved when the belt is contacting one flange in the drive system. When the belt contacts flanges on opposite sides of the sprockets in the system, the result can be undesirable parallel misalignment.

Improper installation of the bushing can result in the bushing/sprocket assembly being “cocked” on the shaft. This leads to angular misalignment and also increases the possibility of vibration. It is important to follow the instructions included with the bushing.

CHECKING BRACKETRY

Maintenance technicians should also check related components, such as brackets and platforms, for proper design and placement. These parts must be strong enough to withstand the peak forces exerted by the drive without bending or flexing.

Synchronous belts are sensitive to fluctuations in the centre distance that can be caused by inadequate bracketry. Because brackets and motor mounts used in food processing are often made of lightweight aluminum, rather than hardened steel, they can flex under load, causing misalignment and affecting components in the drive system. Centre distance variation of as little as 0.004-in. resulting from flexing motor supports can negatively influence overall drive performance.

With the drive shut off and safely locked out, structural rigidity can be checked by pushing the two belt spans toward each other and looking for relative movement in the structure. If movement of the motor or centre distance is noticed, the drive likely will have a structure that is insufficient for maintaining the expected service life of the belt. The structure should be reinforced to obtain the maximum performance from a synchronous belt drive. Whenever possible, a support should be fixed on both side frames of the motor, and be both horizontally and vertically adjustable.

The alignment of the drive should be checked both before and after belt tensioning, since belt tensioning can possibly move some components.

BELT TENSIONING

Proper belt installation tension is important to the optimum performance and longevity of a synchronous drive system. Unfortunately, a vast majority of synchronous drives are not tensioned properly.

A synchronous belt requires correct tensioning when installed, and should be retensioned after a 24-hour run-in period. Subsequently, retensioning is not necessary unless drive conditions are altered.

For synchronous drive systems, ideal tension is the lowest tension that properly seats the belt in the driveN sprocket on the slack side. The calculated tension range at
which belts should be installed depends on the drive components, and the load and speed of the drive. The belt manufacturer’s recommendations should be followed to determine the calculated installation tension values.

Due to system inefficiencies, belt drives are often carrying far less load than they were selected to carry. The two extremes of improper tensioning are under- and over-tensioning. Studies by Gates engineers show that most synchronous belt drive systems are drastically under-tensioned.

When a belt is under-tensioned, it will prematurely wear the belt teeth and possibly even ratchet (jump teeth) under heavy start-up loads, shock loads, or structural flexing. Ratcheting increases stress on the belt teeth, accelerates tooth/groove wear and reduces belt life.

Dynamic crimping — when the belt is not properly seating in the sprocket grooves — accounts for most under-tensioning failures. When this happens, the tooth acts as a lever arm that pinches the tension cord at a sharp angle. This pinch point dramatically weakens the tension cord, causing a failure point. The belt will then prematurely break. This break will occur straight across the belt (parallel to the teeth), and usually takes place a few weeks or months after installation, depending on the severity of under-tensioning.

Ratcheting can also result in potential damage to bearings, shafts, and other drive components.

If a belt is over-tensioned, it will wear in the land area (the area between the belt teeth). With the land areas gone, the teeth are very susceptible to falling off, but the belt may continue to run on the drive. Since the cord is directly exposed to the metal and the belt is able to slip, severe hardware damage can result. This can lead to premature belt failure. Over-tensioning also can damage bearings, shafts and other drive components.

TENSIONING TOOLS

Gates engineers do not recommend using an “experienced thumb” for determining proper belt tension. Rather, they suggest that maintenance technicians use mechanical or electronic tools. One inexpensive, easy-to-carry tool is a pencil-type spring force tension gauge that measures static belt tension by indicating force at a specified deflection of the belt span.

For extremely accurate belt tension measuring, a sophisticated electronic sonic tension meter works on the theory that a belt vibrates at a particular frequency based on the belt span length, belt width and belt type. To test the tension, simply strum the belt to set it vibrating, and the meter records the resulting oscillating sound wave as a tension value.

Approximately 24 hours after the drive has been aligned and tensioned, it should be rechecked for proper installation.

SPROCKET MAINTENANCE

A common misperception about synchronous belt systems is that sprockets never wear out. Gates application engineers report that a significant percentage of the belt drive problems they investigate can be traced to worn, nicked or cut sprockets.

A sure sign of sprocket wear is abnormal belt wear and belt service life that progressively worsens with each belt that is installed. Most sprocket wear on an unprotected drive is due to abrasion caused by airborne particulate matter — such as powdered foods like flour, salt and sugar — in the vicinity of the drive. Nicks or cuts should be repaired immediately.

For applications requiring clean operating environments, such as food and beverage manufacturing, stainless steel sprockets and bushings should be used. Stainless steel is ideally suited for processes requiring optimal cleanliness and frequent high-pressure washdowns using caustic solutions and foam degreasers.

Although a synchronous belt drive system using stainless steel sprockets is more costly than a metal sprocket system, for comparison, the use of stainless steel roller chains and sprockets is four-to-five times more expensive than standard roller chain.

CONCLUSIONS

When installed properly with the correct alignment and tension, synchronous belt drives are virtually maintenance free. Synchronous belt drive systems require no lubrication. They do not give off the oil spray associated with roller chain drives. Clean operation makes them ideal for contamination-sensitive applications such as food handling and bottling.

For drives requiring optimal clean operating conditions, synchronous belt and stainless steel sprocket drive systems are designed to carry extremely high power ratings, and they resist corrosion and caustic washdown solutions.

Information for this article was provided by Gates Canada Inc.