Don't Overlook Your Belts
For many decades, power transmission belts have been used as an efficient, economical way to transmit power from a prime mover to the driven device. Advances in belt and sheave design have led to the 5V profile that many refrigeration systems manufacturers use today. Proper drive component selection, inspection and maintenance will extend compressor life.
Drive Component SelectionWhen drive components are selected at the factory, they are matched to the operating conditions that are specified when the order is entered. The length of the belts required is influenced by two factors. The first is the sheave and pulley sizes needed to achieve the correct compressor speed for the specified operating conditions. The second is the shaft center-to-center distance between the motor and compressor.
The number of belts installed on the drive is based on the horsepower required by the compressor at the specified operating conditions and the horsepower-carrying capacity of the belts. The horsepower that is transmitted per belt is very important to the life and efficiency of the drive components. If the horsepower transmitted through the drive is greater than the capacity of the belts, the belts will slip. Conversely, if the horsepower transmitted through the drive is less than the minimum capacity of the belts, the belts have a tendency to flap, causing vibration, belt stretching and, if allowed to continue, rolling over in the groove.
If it seems that I'm stressing the need for accurately specifying operating conditions -- I am. To derive the benefits of a smoothly running drive system, the drive components must match the operating conditions. If correct conditions are not given, selection of the correct drive components can be, at best, a shot in the dark, often resulting in a drive system suffering excessive wear and vibration due to over- or under-belting. Many times, an order has been entered with "standard operating conditions" -- an ammonia high-stage compressor operating at 185 psig discharge and 0oF (-17oC) suction -- when actually the exact operating conditions are quite different.
Other problems appear when operating conditions are changed in the field due to a change in processes or to take advantage of energy savings. Drive components that were sized originally for a specific service are now over- or undersized, resulting in slippage, vibration and wear.
A fictional example will illustrate this point. Suppose Sub-Zero Cold Storage Co. has several high-stage compressors that have been running problem-free for years. One day, Sub-Zero's local utility presents a plan to reduce operating costs by reducing Sub-Zero's head pressure by 30 psig in the winter, thereby saving 10 percent on horsepower. While this change will reduce Sub-Zero's pow-er demand, it also changes the operating conditions. One unintended -- and unanticipated -- result is the reduced horsepower requirements of the compressors will cause the horsepower-per-belt loading to drop. At the lower loading, the compressor is overbelted, which will result in belt flapping and possibly rolling over in the grooves. The lesson in this example? If large changes in operating conditions are needed, check with your supplier before any change is attempted.
First-Time InstallationsIf belts are being installed for the first time, the following items should be taken care of before the belts are installed.
Level Equipment. The compressor, motor and base should be level. Having the shafts level and in the same plane before starting the alignment process will help speed alignment.
Finish Piping. All piping must be finished and supported properly, and any piping stress must not be permitted to act on the compressor frame. The base must be secured to the floor and grouted.
Check for and Correct a Soft Foot. One of the most overlooked machine problems, soft foot describes the condition where a machine's foot deflects a small amount when the hold-down bolt is tightened. Like a chair with one leg too short, a system with a soft foot will introduce a twisting force to the compressor frame and base assembly, potentially reducing the life of compressor components and creating a potential source for vibration.
The compressor should be checked for a soft foot and shimmed accordingly. Checking for a soft foot always has been associated with the alignment of direct-connected units. However, eliminating a soft foot in belt-driven compressors is just as important.
Inspect Sheave and Pulley. The sheave and pulley should be checked for paint and foreign matter in the grooves. Any foreign material will reduce the horsepower-transmitting ability of the belts and lead to accelerated wear of the belts, pulley and sheave.
Be Sure Lubricants and Antiseize Compounds are Absent. Absolutely no lubricants or antiseize compounds should be used in the installation of the motor sheave and bushing. As supplied by the manufacturer, the bushing should not have any lubricant applied to it or the sheave bore. The applied lubricant will be trapped between mating surfaces of the sheave and bushing, or bushing and shaft. When the bushing is tightened, the resulting hydraulic pressure of the lubricant trapped between the mating surfaces can exceed 5,000 psi and will result in a cracked sheave or bushing. Lubricant or antiseize usually is applied to the shaft for ease of disassembly. However, it works in reverse: As the antiseize is trapped, it creates a sliding layer between the parts. The parts are now free to move independently while the unit is in operation, creating fretting corrosion -- the result of which is the parts welding themselves together.
Belt Inspection and ReplacementIf the belts are being inspected or replaced on the unit, the following procedures should be used.
Use a Gauge. A groove-wear gauge should be used to check the condition of the sidewalls and width of the groove. Excessive wear is not permitted as this will lead to problems in achieving correct belt tension and loading, contributing to excessive belt and sheave wear.
Cleanliness is Important. Clean any foreign matter that has accumulated in the grooves.
Inspect for Signs of Wear. The sheave and pulley should be inspected for cracks and other damage that could affect the integrity of the drive components. In addition, the sheave and pulley should be inspected for excessive runout, indicating bent components. Bent pulleys or sheaves will wear the groove sidewalls unevenly because the belt changes its angle of engagement as the pulley turns.
The groove bottom also should be checked for polishing as this indicates the belt is not riding correctly in the groove. Polishing results when the belt or groove has worn to the point that the belt is actually rubbing on the bottom of the groove. This effectively changes the pitch diameter of the sheave or pulley for that groove, causing that belt to operate at a different speed than the rest. This will result in excessive heat and wear to the drive components.
Use Identical Components and Match Existing Belt Configuration. When drive components are replaced, it is necessary to replace them with identical components. Some people mistakenly think that if a compressor flywheel has open grooves, the manufacturer made a mistake and those have to be filled. So, they fill the flywheel grooves with belts. But this is incorrect. To reduce manufacturing costs, compressor manufacturers machine one or two flywheels that can be used on a wide range of systems, depending on the number of belts installed. Depending on the designed belt-load considerations, some of the grooves on the flywheel may not utilized.
Inspect Old Belts Before Discarding. Even if the belts are not going to be reused, they should be inspected for abrasion, tearing, separation or checking that would indicate possible damage to drive components or alignment problems. They also should be inspected for sharp bends and kinks while they are slack, as this indicates internal damage due to incorrect installation techniques.
Clean Before Reusing. If the belts are going to be reused, only soap and water should be used to clean the belts and other components. Belt dressing should never be used on drive components as it will attack the elastomer used in construction of the belts.
Change in Sets. Belts always should be changed in sets to minimize length variations between used and new belts.
Belt MatchingHeadquartered in Washington, the Rubber Manufacturers Association (RMA) publishes rules that provide the belt manufacturing industry a tolerance against which all belts are manufactured. You may have seen numbers on the belt such as 48, 50 or 52. These group numbers classify certain length belts in that group range.
The number 50 is the ideal length for a specific belt size. All belts in that group have a maximum length tolerance of 0.300" between shortest and longest belts -- for example, a 120" long belt would vary 119.85 to 120.15" in length. Numbers less than 50 (49, 48, 47 and so on) represent groups of belts with a less-than-ideal length. Numbers greater than 50 (51, 52, 53 and so on) represent groups of belts with a greater-than-ideal length. These groups also have a tolerance of 0.300" between the longest and shortest belts in the group.
The RMA match number system was developed in the 1980s when belt manufacturers decided it was not economically feasible to keep matched belt sets. Prior to its development, belt suppliers produced matched sets of belts that were manufactured as a drum and cut off in sets that were kept together.
While easier for belt manufacturers, the RMA match number system presents a problem for both belt users and equipment manufacturers. If you work with these belts on a regular basis, you probably have seen the results of the decision to use the RMA match number system: belt groups that will not tension evenly, leading to excessive wear and vibration. And, belt manufacturers say there has been an increase in belt-related problems.
One possible alternative is to use aramid-fiber-banded belts. Aramid-fiber-reinforced belts are designed to be more resistant to stretching than nylon-reinforced belts and actually have a tendency to shrink when exposed to moisture. Aramid-fiber belts have proven their worth in holding their tension longer and having a higher resistance to wear than standard nylon-reinforced belts.