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← ↑ → Anything with moving parts needs lubrication to reduce the amount of friction and heat generated between the parts. The purpose of lubrication is to reduce the amount of friction and wear on the mating parts and to carry away the heat generated by the friction. Fast-moving parts require constant protection with a large and continuous supply of lubricant. The slower-moving parts don’t require as much lubrication for proper protection. The speed of the parts and the type of service dictates the type and amount of lubrication needed for proper protection. A contaminated lubricant does not provide adequate lubrication. It also causes corrosion and deterioration of the components. FRICTION Friction may be described as the resistance that tends to oppose motion. Friction is not always bad. It’s the force that allows bolts and screws to hold machinery together. This type of friction is useful. The friction between bearings and gears in a washing machine transmission, for example, is not good. This type of friction causes wear, excessive operating costs, and high temperatures. This is the type of friction that we want to reduce with lubrication. Friction can also be caused by poorly aligned components such as pulleys and bearings. Components that are out of alignment will cause excessive wear, higher operating costs, and higher than normal heat generated at the friction points. EQUIPMENT DESIGN When engineers design appliances, they take into consideration the normal basic requirements of the types of bearings used, types of lubrication required, and the operating conditions of the equipment. Some of the factors are: 1. The equipment should be able to operate satisfactorily under a heavy load and extreme ambient conditions. 2. The bearing and gear system should be sealed in a shell with a permanent supply of the proper lubricant. The shell is sealed against dust and moisture to prevent contamination of the lubricant. 3. When hermetic sealing is not possible, convenient lubrication points are a part of the design. The manufacturer’s lubrication recommendations should be followed for the best performance, operation, and longevity. However, during the manufacture of these components, some degree of tolerance is allowed because every bearing and shaft obviously cannot be exactly the same dimension. When all of these tolerances are added together some of the bearings may be a little loose or a little closer than desired. Also, the shaft and bearings may not always be in complete alignment. Most manufacturers make limits on the amount of total tolerances allowable. It’s not practical or possible to completely eliminate these units from entering the field. These units will sometimes operate satisfactorily for years and others will start causing problems soon after being placed into service. The problem will usually appear as abnormal noise or poor operation because of tight, over heated bearings. Shaft Oil Fig. 1 Sleeve bearing. Oil film Fig. 2 Bearing lubrication methods. Grease fining Sometimes the unit is mishandled during shipment and/or installation causing the cabinet frame to warp, which in turn can lead to a misalignment of the bearings, belts, and other moving parts. If the customer uses an excessive amount of washing detergent or cleaning fluids, the bearing lubrication will dry out or be washed away. If the unit is placed in an excessively dirty area, dust may enter the bearings and contaminate the lubricant. The manufacturers are making every effort to eliminate these problems. BEARINGS Each different type of bearing receives a special type of lubricant for proper operation and longevity. The type of bearing and its use indicates the type and frequency of lubrication. Sleeve bearings--The sleeve bearing is of a porous, sintered-type construction. Sleeve bearings are usually pre lubricated by a special process. They don’t usually need any further lubrication. They contain about 15 to 20 percent of lubricant by volume. This lubricant is immediately available to the bearing on start-up, thus reducing friction at the time of start-up. See Fig. 1. When additional lubrication is needed, some bearings have an oil reservoir and a wick system. See Fig. 2. The oil is conducted from the reservoir to the bearing through the wick. In some designs, the wick actually touches the rotating shaft bearing surface. See Fig. 3. When the oil reservoir system is used, dust protection is very important. The type of oil most commonly used in this type of bearing is a non-detergent, SAE 20 or 30 weight type. Needle bearings--This type of bearing is usually assembled in an outer shell that is packed with the proper type of lubricant (usually oil or grease). The needles are almost in contact with the shaft bearing surface which serves as the inner bearing race. A small amount of lubricant is located between the needle bearings and the shaft bearing. The shaft bearing is made of hardened steel to permit long life of the shaft. Needle bearings offer less resistance to motion than sleeve bearings. To accurately check for wear, the bearings must be removed from the shaft. Before the shaft is moved through the bearing, make sure it’s clear of rust, burrs, and any other foreign matter that could possibly damage the needle bearing surfaces. Clean the shaft and check it for wear and/ or scoring. Check the needle bearings for wear and scoring. If either the shaft or the needles show wear, replace both of them. Be sure to prelubricate the bearings before operating the unit. Roller (ball) bearings This type of bearing rolls between an inner and an outer race. The bearing is self-contained. Most roller bearings are permanently lubricated at the time of manufacture. The entire bearing slips over the shaft and is fastened to the shaft with set screws to prevent movement on the shaft. The bearing has an almost metal- to-metal contact with the races. The lubricant serves as a moving surface between the metal parts and it carries away some of the heat of friction. These bearings have seals that prevent the loss of lubricant, as well as preventing dirt and moisture from entering the bearing races. When gears or sliding parts are used with the roller bearing inside a housing, an oil-type lubricant is preferred. The oil is usually heavy bodied with additives to obtain the specific properties that are needed for the particular application. When the assemblies are not in an enclosed housing, grease must be used rather than oil. Oilite wicking bearing Fig. 3 Oil wick touching shaft bearing surface. Bearing cup Bearing Oil type Oilite bearing Grease type Oil cup Plastic bearings--Du Pont has designed several types of plastic bearings that are well suited for light-duty self- lubricating applications. These bearings are made from materials such as Teflon, nylon, and Delrin. These materials are very resistant to corrosion, detergent, and water. They are, at present, very popular in small-appliance applications. LUBRICANT PROPERTIES The properties of a lubricant can be very technical if studied in detail. However, we will only present a very broad outline of the purposes of a lubricant, without getting too technical. There are four basic functions that a lubricant must perform: 1. To reduce the friction and wear of mating surfaces. 2. To carry away the heat produced by the friction of the moving parts that are being lubricated. 3. To seal out any contamination from the mating surfaces. 4. To prevent rust and corrosion from forming on the mating surfaces. The viscosity of an oil is a measure of how fast it will pour at a given temperature. Light-weight oils will run at very low temperatures. High-viscosity oils won’t be very fluid at low temperatures. When an oil is too light for the application, it will break down rapidly and not lubricate properly. An oil that is too heavy for the application will develop high temperatures and create a power loss because the film of oil cannot enter the friction area fast enough on a cold start-up. Oils that are designed to be used in a gear case are usually of the heavy-weight type and have additives such as antifoam, anti-wear, extreme pressure protection, corrosion inhibitors, and oxidation inhibitors. The oil used in refrigeration compressors is a special type that is wax-free, dehydrated, and will flow at the temperatures reached inside the evaporator. This oil must be compatible with the refrigerant used in the system and the compressor components. Never use anything other than refrigeration-type oil in a refrigeration compressor. Warning: Never over-lubricate ball or roller bearings. Too much lubricant inside the bearing can cause excessive churning, friction, and heat. These conditions will cause the lubricant to break down and eventually ruin the bearing. The space inside the bearing should never be more than 4 to 4 full of grease. Solid or sintered sleeve-type bearings are not generally damaged by over-lubrication. The damage that can occur in this instance is ruining what the oil drips onto. Bearing Failure A failing bearing will generally produce an unusual noise when the unit is operating. There will usually be play between the shaft and the bearing, especially when sleeve bearings are used. This play can be determined by trying to move the shaft in a sideways direction. In this instance, cleaning or lubricating the bearing will prove a waste of time. The noise may go away for a short period of time, but it will surely return. In the long run, the bearing will sometimes become frozen, causing an overload on the motor or causing the motor to completely stall. When attempting to make a decision about changing a bearing, the following factors must be considered: 1. The cost of the repair as compared to the cost of a new unit. 2. The general condition of the remaining parts. 3. The cost of the bearing. 4. The amount of time required to complete the job. Will the labor cost overshadow the cost of a new unit? 5. The availability of the proper tools to complete the job. If the decision to replace the bearing is made, the bearing should be taken to the supply house so that an exact replacement will be obtained. Bearing Replacement A bearing that is worn must be replaced in order to maintain its relative position to the remainder of the parts and to prevent overloading of the motor. Don’t replace a bearing on a scored or worn shaft or in a housing in which the bearing race has been damaged. This will only delay the replacement of the complete component at an additional cost to the customer. This will usually result in a dissatisfied customer and sometimes a lost customer. Running Clearance The running clearance is used in sleeve-type bearings. It’s the amount of space between the OD of the shaft and the ID of the bearing. This space is where the lubricant provides a protective film between the moving parts. This fit also allows for some expansion due to the heat build-up during operation. Press Fit Practices Without some form of friction, the bearing will turn in the housing and ruin it. The purpose of the shrink fit, press fit, or the interference fit (as they are all known) is to prevent turning of the bearing in the housing. This is accomplished by making the OD of the bearing just a bit larger than the hole in the housing. SHAFT SEALS When the housing has a shaft extending through it and the atmosphere is to be kept out and a liquid or a gas is to be kept in, a shaft seal is used. Some examples of shaft seal use are: 1. Where the crankshaft of a refrigeration compressor extends through the compressor housing. 2. The shaft extension where the impeller is connected to the shaft on a disposer or a water pump. 3. The shaft extension from the gearbox of a washing machine. Locating a leaky seal--Any time that there are signs of grease or oil under an appliance, a shaft seal should be suspected. The best and quickest way to determine the location of the leak is to clean the area and watch for an oil or grease drip or find where the grease or oil has been sprayed on adjacent parts from a spinning shaft. All of these conditions are indications of a leaking shaft seal. Shaft seals are used in washing machines to keep water and detergents out of the transmission. Oil and water dripping out of a relief hole in the gear housing indicate that there is a seal leaking into the transmission housing. A leaking shaft seal must be replaced. These devices won’t wear-in and repair themselves. The only way to solve the problem is to replace the shaft seal. It’s good practice to also install a new bearing because the water may have caused the bearing to wear excessively or a worn bearing may have caused the shaft seal to leak. Types of seals Basically, there are two types of shaft seals used on rotating shafts: 1. Face seal 2. Lip seal Face seals--These are usually two-part seals. One part is pressed into the housing and the other part is pressed onto the shaft. Each part has a mating surface that matches when the seal is properly installed. Be certain that the shaft and housing are completely clean. A small amount of oil or grease on the shaft and the hole in the housing will help in the installation of this type of seal. Be sure that the seal fits squarely into the housing hole and on the shaft. Any misalignment of these parts will allow the seal to leak. The seal faces must be kept absolutely clean. Don’t touch the sealing faces or a leak will probably occur. Seating Face-type seals have a very light fit on the face as well as on the shaft and in the housing hole. All of the seats must be clean and free of any foreign material. If there should be a loose fit of the seal to the seat, liquid or gas will leak at this point. In some applications a sealing compound is used to aid in the sealing process. Use the instructions on the compound package. A fit that is too tight may cause damage or distort the seal; sometimes the seal will wear excessively causing it to leak. As always, use an exact replacement part for the job. When removing the old seal, use caution not to dam age any of the sealing surfaces. Lip seals --The inner seal diameter has a smaller-diameter, flexible lip. The inner diameter is smaller than the shaft over which it’s placed. The seal is pressed into the housing with the outer shell against the edge of the housing hole. The inner flexible seal will make a pressure contact with the shaft when it’s inserted through the seal. When installing this type of seal, make certain that the liquid pressure will be on the underside of the lip, causing a tighter contact with the shaft as the pressure inside the housing is increased. As always, the shaft must be clean and free of all foreign material before inserting it into the seal. If the shaft surface is not glass smooth, polish it by either sanding or grinding it to the desired finish. When polishing the shaft, be sure that the polishing motion is around the shaft and not along it. Polishing along the shaft will cause the seal to wear much more rapidly. The lip seal is most satisfactory with either a hardened- steel or a stainless-steel shaft. If the shaft is not hardened, the seal will wear a groove in it. To be certain that the replacement seal is the proper one, use one with the correct part number for the unit being repaired. Seal leakage during storage Many times when a unit has been in storage for a long period of time, the seals will leak a bit. This is a normal occurrence. Any unit that has a shaft seal should be operated occasionally to keep the seals lubricated and seating properly. Shaft seal lubrication--Either lip or face seals are used on high-speed shafts to prevent leakage at that point. If they don’t receive the proper amount of lubrication, they will deteriorate and start leaking because of the friction heat caused by their contact with the rotating parts. O-Ring Seals O-ring seals are used on shafts that rotate relatively slowly or that have a slight sliding motion in their operation. They are also very popular in applications as stationary seals between parts in the place of gaskets. O-ring seals are designed to fit in a groove either in the shaft or bearing. A part of the seal is also forced into an area in the other part to be sealed. Thus, there is a seal between a shaft and a bearing or between two parts. 0- rings are made to very exact tolerances, so that a positive seal can be obtained. They are made from several different compounds which are dictated by their use. When an O-ring is used as a static seal, no lubricant is needed. However, when an O-ring is used on moving parts, some type of lubrication must be provided. Usually the fluid that is being sealed will lubricate it. There are special applications when the O-ring seal is lubricated by an oil socket next to the O-ring. Failure O-ring seals are also subject to leaks. The leaks are detected in the same manner as with any other seal; If an O-ring shows signs of leaking, it must be replaced. There is no way to repair a leaking O-ring. When a leaking O-ring is found, remove the O-ring very carefully. Examine the O-ring, the groove in which it fits, and the mating surface for any rough places, dirt, or foreign material that would cause a leak. The cause of the leak must be repaired or the new O-ring will also leak. Some of the most common causes of O-ring leakage are: 1. The O-ring was pinched during the installation. 2. The O-ring was damaged by sharp edges during the installation. 3. The O-ring was exposed to excessive pressure. 4. The O-ring was exposed to excessive temperature. 5. The O-ring was damaged by a worn bearing. 6. The O-ring groove finish was bad. 7. The O-ring was operating without lubrication on a moving surface. 8. The O-ring was exposed to the corrosive effects of contaminated oil. 9. The O-ring underwent abrasion of the surface be cause of contaminated oil. To install an O-ring, use the following procedure: When an O-ring needs to be replaced, be sure to correct the cause of the leak or it will only occur again. As with any other seal, the surfaces must be kept clean of all dirt, corrosion, burrs, and any other foreign material before and during the assembly process. Be sure to use the recommended replacement part. QUIZ 1. What is the resistance that tends to oppose motion known as? 2. What dictates the type and frequency of bearing lubrication? 3. What percent of lubrication, by volume, do sleeve bearings usually contain? 4. What is the purpose of the wick in a bearing? 5. What weight oil is generally used in sleeve bearings? 6. Does a sleeve or a needle bearing offer less resistance to motion? 7. How are most roller bearings lubricated? 8. Where is the bearing lubricant located to properly lubricate a bearing? 9. What is the measure of how fast a lubricant will pour at a given temperature known as? 10. Is a solid or sintered sleeve-type bearing generally damaged by over lubrication? 11. When a roller bearing is bad will there always be play between the shaft and the bearing? 12. Why must a worn bearing be replaced? 13. Is it good practice to install a new bearing on a shaft that shows a small amount of wear? 14. What is the key to successful bearing re placement? 15. What is the purpose of press fit bearings? 16. What do signs of grease or oil under an appliance indicate? 17. Will a leaking shaft seal wear in and repair itself? 18. Name the two basic types of shaft seals. 19. When polishing a shaft, what must be the polishing direction? 20. Where is an O-ring seal designed to fit? NEXT: Home top of page |