Part One of this article provided some background on screw compressors, then discussed the various components and capacity control methods. Part Two discusses maintenance of the screw compressor.
Higher than normal discharge temperatures could indicate an incorrect Vi (volume ratio) position, insufficient main oil supply, excessive suction line superheat, or an increase in the rotor end clearance.
If the oil temperature is too low, refrigerant may become entrained in the oil supply and enter into the bearing cavities.
An increase in full load amps while maintaining the same operating conditions could indicate a faulty bearing.
For compressors with anti-friction roller bearings for radial loads, clearance measurements can only be taken with a feeler gauge between the inner race and the top roller element. Do not pry the rotor up and down. Excessive radial movement of the rotor shaft could result in damage to the bearings.
Remember that preventive maintenance is proactive and can save both time and money.
Major repair categories: Major repair to compressors can usually be broken down into three categories. The first would be replacing bearings, O-rings, and fasteners. This type of repair is often associated with hours of operation rather than a known problem. Some manufacturers recommend this type of repair as a scheduled maintenance item. However, since the onset of vibration analysis, many end users now rely on vibration measurements to predict the need for rebuilding rather then operating hours. As discussed earlier, vibration analysis can often detect a problem before it is audible to the human ear.
The second category would be bearings, O-rings, and a few internal components. Again, this could be based on hours of operation and vibration readings as well as audible noise or compressor performance. Replaceable items could be balance pistons, slide valve, and slide stop.
The third category would include the first two groups as well as additional major components. Items such as rotors and major housings would be required as a result of a compressor failure. Compressors falling into this category will need to be evaluated to determine if rebuilding is a more financially advantageous decision than purchasing a replacement compressor.
Other critical items are bearings. Bearings are designed to handle specific loads, speeds, and conditions. Although the bearing manufacturer has a generic identification on their bearings, the identification may not always specify the exact bearing design. An example of this may be that the bearing clearance is not specified on the bearing code. As mentioned previously, it is strongly recommended to use the manufacturer’s designed shaft seal.
Once the correct seal is obtained, a few basic pre-installation techniques are recommended.
1. Ensure all compressor components are cleaned and ready for re-installation before removing the shaft seal from its package.
2. Once the seal is removed from the package, care must be taken to avoid contamination.
3. Never attempt to wipe the seal faces with anything other than a clean alcohol swab. Seal faces are lapped to two to three helium light bands. One helium light band is 11.6 millionths of an inch. Rubbing the faces with some other cloth may result in scratching the face.
4. The majority of shaft seals require coating the surfaces with oil prior to installation. Never wipe on the oil with a cloth or brush as both may either scratch the carbon or leave contaminants. The seal can be lubricated by either dipping the seal in clean refrigeration oil or by using a spray bottle.
5. Never use grease or products such as STP for either installation or lubrication. These products have a higher viscosity than refrigeration oil and may cause premature failure due to sludge formation.
Most shaft seals have an allowable oil leak rate; check with the manufacturer for the maximum allowable rate. Below is a list of possible causes for excessive shaft seal leakage.
1. Using the wrong type of oil.
2. Contaminants such as moisture, particles, or liquid refrigerant.
3. Mixing different types of oils.
1. Failure due to defective bearing seats on shaft and in housings: For the bearing to achieve its life expectancy, it’s essential that the shaft and the housing be geometrically true with the bearing.
2. Misalignment: Misalignment can occur between the compressor and motor or it can occur if the inner ring is seated against a shaft shoulder that is not square.
3. Faulty mounting practice: Failures under this item occur from abuse or neglect of the bearing before and during mounting. Besides mechanical damage as a result of abuse, allowing the bearing to become contaminated will result in a premature failure.
4. Damage due to improper fit: This occurs when the wrong bearing design is used. This typically occurs when deviating from the manufacturer’s specified bearing.
5. Inadequate or unsuitable lubrication: This occurs when using the wrong specification of oil or poor quality oil.
6. Vibration: A unit that is vibrating excessively during operation will result in premature bearing failures. Another type of vibration-related failure is called false brinelling. False brinelling occurs when the compressor is off for an extended period of time without the rotor shafts being rotated while a nearby machine is running. Micro-motion occurs between the rolling elements and inner raceway that will eventually cause the raceway to fatigue at the contact point.
7. Passage of electric current through the bearing: In certain electrical machining applications, there is the possibility that an electrical current will pass through the bearing. A few possible causes would be stray magnetic fields or through a welding operation on some part of the machinery with the ground attached so that the circuit is required to pass through the bearings.
8. Old age: Bearings are chosen during the initial design of a compressor based on “L10” life. L10 life is defined as the operating hours that 90 percent of a bearing population would be expected to meet or exceed before any wear surface develops a 1 mm square fatigue spall. The median life is approximately five times the L10 life. Good quality lubricant and a clean operating system can give operating service life well beyond the calculated L10 life. However, if compressors run long enough, the bearings would eventually be expected to fail from fatigue, or old age.
Anti-friction bearings typically come in two mounting types, loose fit and interference fit. Loose fit bearings are designed to slide into the bore or over the shaft without resistance. Interference fit bearings require some assistance for installation.
Four different mounting methods are used for anti-friction bearings, including cold mounting, hot mounting, oil injection, and slip fit. The most commonly used methods are cold mounting, hot mounting, and slip fit.
Cold mounting takes place when a bearing is frozen prior to installation. Freezing shrinks the bearing allowing it to be easily installed in the bore. Once the temperature of the bearing increases, it fits snugly in the bore. Cold mounting is also used to describe a bearing that is mechanically forced into position. Hot mounting is used for bearings that require interference fit. A typical application would involve the inner race of an anti-friction roller bearing. Heating expands the race allowing it to be easily slid into position. As the race cools it adheres to the rotor shaft. Slip fit bearings simply slide into place with normal hand force.
Yoder is a service technician with York Refrigeration/Frick in Waynesboro, Pa. For more information, visit www.yorkref.com.
Publication date: 03/10/2003