Bearings
The function of bearings in centrifugal pumps is to keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. Bearings that give radial positioning to the rotor are known as radial or line bearings, and those that locate the rotor axially are called thrust bearings. In most applications, the thrust bearings actually serve both as thrust and radial bearings.
Types of Bearings Used All types of bearings have been used in centrifugal pumps. Even the same basic design of pump is often made with two or more different bearings, required either by varying service conditions or by the preference of the purchaser. In most pumps, however, either rolling element or oil film (sleeve-type) bearings are used today.
In horizontal pumps with bearings on each end, the bearings are usually designated by their location as inboard, or drive end, and outboard, or non-drive end. Inboard (drive end) bearings are located between the casing and the coupling. Pumps with overhung impellers have both bearings on the same side of the casing so that the bearing nearest the impeller is called inboard and the one farthest away outboard. In a pump provided with bearings at both ends, the thrust bearing is usually placed at the outboard end and the line bearing at the inboard end.
The bearings are mounted in a housing that is usually supported by brackets attached or integral to the pump casing. The housing also serves the function of containing the lubricant necessary for proper operation of the bearing. Occasionally, the bearings of very large pumps are supported in housings that form the top of pedestals mounted on soleplates or on the pump bedplate. These are called pedestal bearings.
Because of the heat generated by the bearing or the heat in the liquid being pumped, some means other than radiation to the surrounding air must occasionally be used to keep the bearing temperature within proper limits. If the bearings have a force-fed lubrication system, cooling is usually accomplished by circulating the oil through a separate water-to-oil or air-to-oil cooler. Otherwise, a jacket through which a cooling liquid is circulated is usually incorporated as part of the housing.
Pump bearings may be rigid or self-aligning. A self-aligning bearing will automatically adjust itself to a change in the angular position of the shaft. In babbitted or sleeve bearings, the name self-aligning is applied to bearings that have a spherical fit of the sleeve in the housing. In rolling element bearings, the name is applied to bearings, the outer race of which is spherically ground or the housing of which provides a spherical fit.
Although double-suction pumps are theoretically in hydraulic balance, this balance is rarely realized in practice, and so even these pumps are provided with thrust bearings. A centrifugal pump, being a product of the foundry, is subject to minor irregularities that may cause differences in the eddy currents set up on the two sides of the impeller. As this disturbance can create an axial hydraulic thrust, some form of thrust bearing that is capable of taking a thrust in either direction is necessary to maintain the rotor in its proper position.
The thrust capacity of the bearing of a double-suction pump is usually far in excess of the probable imbalance caused by irregularities. This provision is made because (1) unequal wear of the rings and other parts may cause an imbalance and (2) the flow of the liquid into the two suction eyes may be unequal and cause an imbalance because of an improper suction-piping arrangement.
Rolling Element Bearings The most common rolling element bearings used on centrifugal pumps are the various types of ball bearings. Roller bearings are used less often, although the spherical roller bearing (see Figure 86) is used frequently for large shaft
- FIGURE 86 Self-aligning spherical roller bearing (SKF USA, Inc.)
sizes, for which there is a limited choice of ball bearings. As most roller bearings are suitable only for radial loads, their use on centrifugal pumps tends to be limited to applications in which they are not required to carry a combined radial and thrust load.
Ball Bearings As the coefficient of rolling friction is less than that of sliding friction, one must not consider a ball bearing in the same light as a sleeve bearing. In the former, the load is carried on a point contact of the ball with the race, but the point of contact does not rub or slide over the race and no appreciable heat is generated. Furthermore, the point of contact is constantly changing as the ball rolls in the race, and the operation is practically frictionless. In the sleeve bearing, a constant rubbing of one surface over another occurs, and the friction must be reduced by the use of a lubricant.
Ball bearings that operate at an absolutely constant speed theoretically require no lubricant. No speed can be called absolutely constant, however, for the conditions affecting the speed always vary slightly. For instance, a motor with a full-load speed rated at 3,510 rpm might vary in speed over the course of a minute from 3,505 to 3,515 rpm. Each variation in speed causes the balls in a ball bearing to lag or lead the race because of their inertia. Consequently, a very slight, almost immeasurable sliding action takes place. Another limiting condition is that the hardest of metals suffers minute deformations on carrying loads, thus upsetting perfect point contacts and adding another slight sliding action. For these reasons, ball bearings must be given some lubrication.
Ball thrust bearings are built to carry heavy loads by pure rolling motion on an angular contact. As a thrust load is axial, it is equally distributed to all the balls around the race, and the individual load on each ball is only a small fraction of the total thrust load. In such bearings, it is essential that the balls be equally spaced, and for this purpose, a retaining cage is used between the balls and between the inner and outer races. This cage carries no load, but the contact between it and the ball produces sliding friction that requires lubrication.
Types and Applications Pump designers have a wide variety of rolling element bearings and arrangements to choose from. Ball bearings with their high-speed capabilities and low friction make them ideal for small and medium-size pumps, while roller bearings are more common in larger, slower speed pumps where a heavy capacity is required. Depending upon the specific bearing type, optional characteristics such as seals, shields, various cage materials and designs, and special internal clearances and preloads are available. Although several might be dimensionally acceptable, it is best for users to adhere to manufacturer recommendations to ensure optimum reliability.
The most common ball bearings used in centrifugal pumps are 1) single-row, deep-groove, 2) single-row, angular contact, and 3) double-row, angular contact ball bearings.
Sealed ball bearings are used in special applications such as vertical in-line pumps. Sealed prelubricated bearings require special attention if the unit in which they are
installed is not operated for long periods of time (such as stand-by units or units kept in stock or storage). The shaft should be rotated occasionally (see specific instruction manual directions) to agitate the lubricant and maintain a film coating on the bearing elements.
Self-aligning ball bearings (see Figure 87) are sometimes used for heavy loads, high speeds, long-bearing spans (large deflection angles at the bearings) and no axial thrust requirements. This bearing design acts as a pivot that compensates for misalignment and shaft deflection. For large shafts, the self-aligning spherical roller bearing (refer to Figure 86) is used instead of the self-aligning ball bearing, and it can carry both radial loads and axial thrust loads.
The single-row, deep-groove ball bearing (see Figure 88), sometimes referred to as a Conrad-type bearing, is the most commonly used bearing in centrifugal pumps, except for the larger size pumps. The Conrad-type design is recommended for use in centrifugal pumps because it can support either radial, axial, or a combination of radial and axial loads. This makes it ideal for the radial bearing in end-suction centrifugal pumps or as both the radial and thrust bearings in small pumps. The bearing design requires a careful alignment between the shaft and the housing. It is often used with seals or shields in grease-lubricated applications to help exclude dirt and retain lubricants within the bearing.
Angular contact ball bearings are commonly used in centrifugal pump applications to support axial loads or a combination of both axial and radial loads. Their axial stiffness and small operating clearances provide precise position accuracy for the shaft. Angular contact bearings are manufactured in a single-row design (see Figure 89), typically with a 40° contact angle, and also as a double-row bearing (see Figure 90), most commonly with a 30° contact angle.
Single-row, angular contact ball bearings support axial loads in only one direction when used singly. To support reversing axial loads or combined loads, single-row bearings must be mounted in a back-to-back or face-to-face arrangement where the contact angles oppose each other. Owing to its more rigid design, the back-to-back arrangement is generally recommended for centrifugal pumps, while the face-to-face arrangement is common when a slight misalignment is expected. When required to support heavy axial loads, single-row, angular contact ball bearings can be mounted in tandem where their contact angles are in the same direction. This arrangement must still be opposed with a third bearing in a back-to-back or face-to-face arrangement with the tandem pair when radial or reversing thrust loads must also be supported (see Figure 91). Depending upon the operating conditions of the pump, single-row, angular contact ball bearings typically operate with either a small clearance or a light preload.
Some applications exist where a high axial load occurs predominantly in one direction, but the thrust bearing must be capable of carrying occasional smaller axial loads in the
FIGURE 89 Single-row, angular contact bearing FIGURE 90 Double-row, angular-contact bearing
FIGURE 89 Single-row, angular contact bearing FIGURE 90 Double-row, angular-contact bearing
FACE-TO-FACE BACK-TO-BACK BACK-TO-BACK /TANDEM
FIGURE 91 Paired bearing arrangements (SKF USA, Inc.)
FACE-TO-FACE BACK-TO-BACK BACK-TO-BACK /TANDEM
FIGURE 91 Paired bearing arrangements (SKF USA, Inc.)
reversing direction. When this occurs, a typical back-to-back angular contact bearing arrangement can result in one bearing becoming nearly completely unloaded. In the most severe cases of axial unloading of angular contact bearings, skidding of the unloaded balls within the bearing races can occur. This skidding can result in bearing heating and subsequent damage, even failure, with time. To avoid ball skidding under light load or no-load conditions, standard angular contact bearing sets can be arranged for a light preload that will result in a sufficient load on the dynamically unloaded bearing to prevent skidding. Another alternative is to install a matched set of two angular contact bearings with different contact angles (see Figure 92). By utilizing an angular contact bearing with a lower contact angle (say 15 degrees instead of the normal 40 degrees), the unloaded bearing will have a lower requirement for an axial load and be more resistant to ball skidding. This means the bearing will run at a lower temperature.
The double-row, angular contact ball bearing (see Figure 93) is similar in design to a back-to-back pair of single-row, angular contact ball bearings, but in a narrower width package. Its ease of mounting, along with its low-friction operation, high-speed capability, and seal or shield availability, make it an ideal bearing for light- to medium-duty end suction centrifugal pumps and submersible pumps.
Lubrication of Antifriction Bearing In the layout of a line of centrifugal pumps, the choice of the lubricant for the pump bearings is dictated by application requirements, by cost considerations, and sometimes by the preferences of a group of purchasers committed to the major portion of the output of that line. For example, in vertical wet-pit condenser circulating pumps, water is the lubricant of choice, in preference to grease or oil. If oil or grease is used in such pumps and the lubricant leaks into the pumping system, the condenser operation might be seriously affected because the tubes would become coated with the lubricant.
FIGURE 92 Angular contact bearings with different contact angles (SKA USA, Inc.)
FIGURE 93 A double-row, angular-contact ball thrust bearing that is grease-lubricated and water-cooled
FIGURE 92 Angular contact bearings with different contact angles (SKA USA, Inc.)
FIGURE 93 A double-row, angular-contact ball thrust bearing that is grease-lubricated and water-cooled
Most centrifugal pumps for refinery services are supplied with oil-lubricated bearings because of the insistence of refinery engineers on this feature. In the marine field, on the other hand, the preference lies with grease-lubricated bearings. For high pump operating speeds (5,000 rpm and above), oil lubrication is found to be the most satisfactory. For highly competitive lines of small pumps, the main consideration is cost, and so the most economical lubricant is chosen, depending upon the type of bearing used.
Ball bearings used in small centrifugal pumps are usually grease-lubricated, although some services use oil lubrication. In grease-lubricated bearings, the grease packed into the bearing is thrown out by the rotation of the balls, creating a slight suction at the inner race. (Even if the grade of grease is relatively light, it is still a semisolid and flows slowly. As heat is generated in the bearing, however, the flow of the grease is accelerated until the grease is thrown out at the outer race by the rotation.) As the expelled grease is cooled by contact with the housing and thus is attracted to the inner race, a continuous circulation of grease lubricates and cools the bearing. This method of lubrication requires a minimum amount of attention and has proved itself very satisfactory. A vertically mounted thrust bearing arranged for grease lubrication is shown in Figure 94.
A bearing fully packed with grease prevents proper grease circulation in itself and its housing. Therefore, as a general rule, it is recommended that only one-third of the void spaces in the housing be filled. An excess amount of grease will cause the bearing to heat up, and grease will flow out of the seals to relieve the situation. Unless the excess grease can escape through the seal or through the relief cock that is used on many large units, the bearing will probably fail early.
In oil-lubricated ball bearings, a suitable oil level must be maintained in the housing. This level should be at about the center of the lowermost ball of a stationary bearing. It can be achieved by a dam and an oil slinger to maintain the level behind the dam and thereby increase the leeway in the amount of oil the operator must keep in the housing. Oil rings are sometimes used to supply oil to the bearings from the bearing housing reservoir (see Figure 95). In other designs, a constant-level oiler is used (see Figure 96).
Because of the advantages of interchangeability, some pump lines are built with bearing housings that can be adapted to either oil or grease lubrication with minimum modifications (see Figure 97).
Post a comment