BY JOHN MALINOWSKI & DANIEL R. SNYDER
K C O T S L A T I G I D ©
CHOOSING AN ANTIFRICTION BEARING Selection of electric motor bearings for coupled and belted loads
HE DECISION BETWEEN WHICH
able for heavy belted loads (i.e., roller bearing), and the
antifriction bearing type to specify on
motor is directly connected, the bearing may prematurely
National Electrical Manufacturers Associa-
fail due to a lack of maintaining the required minimum
tion (NEMA) motors is not always easy or
radial load. Often, less than optimal bearing selection can
obvious. From an end user’s perspective, it would be sim-
work if special attention is given towards cleanliness, relu-
plest if each motor would only have one bearing configu-
brication, alignments, etc. This article discusses the rela-
ration available and have that bearing configuration be
tionships between these various factors and bearing
suitable for both direct-connected and belted loads. Unfor-
selection and helps the reader understand the tradeoffs
tunately, that is currently not the case. This is particularly
involved in the various applications of antifriction bear-
true on the larger higher-speed motors (larger than 125 hp
ings and suggests alternate solutions.
T
and faster than 1,200 r/min). If a bearing is selected that is optimized for direct connection (i.e., deep groove
History
ball bearing), and it is belted, the bear-
Within the pulp and paper industry, an electric motor
ing may prematurely fail due
may be connected to the driven load by two methods:
to a mechanical overload
coupled and belted. Some variations in this exist, such as
condition. On the other
inserting a quill shaft into a gearbox or using a vertical
hand, if a bearing is
pump motor that requires special analysis of radial and
selected that is suit-
axial loads; these will not be addressed in this article.
1077-2618/06/$20.00©2006 1077-2618/06/$20 .00©2006 IEEE
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A coupled load is where the motor shaft is usually meaning that clearances are 0.0001–0.0015 in tight. The connected by a flexible coupling to the driven load. This endplates of the motor have bearing bores machined to type of load presents no axial or radial load to the motor provide clearance or a “loose fit” with the bearing outside bearings except for the weight of the motor’s rotor and diameter. It is usually 0.0000–0.0023 in loose. Most shaft assembly (misalignment from mounting errors often, the drive end bearing is held captive in the endcan, however, add radial load). Antifriction (ball) or plate, and the opposite drive bearing is allowed some hydrodynamic (sleeve) bearings are commonly used suc- axial movement in that endplate to allow for thermal cessfully for a coupled load. growth or the shaft and rotor assembly. Often a spring The most common type of belted load is when a V- supplies an axial preload to the bearings to minimize groove pulley is mounted on the motor shaft and is con- noise and keep the balls loaded. nected to another pulley on the driven load by means of Since radial loads are relatively low on coupled loads, one or more belts held in tension. This type of load can shaft material selection can be of normal strength steel generate high radial loads on the motor shaft, mostly at such as AISI or SAE Grade 1137. Primarily torsional loads the drive-end bearing because it is closest to the applied are present. external loading. Depending on the magnitude of radial load, antifriction bearings (either ball or roller) are used Roller Bearing Configurations for belted loads. Like the motor with two ball bearings, the roller bearOn smaller motors (through about 100 hp in NEMA ing motor is built with a ball bearing and a cylindrical 404-5T frames), under normal conditions, a ball bear- roller or two self-aligning spherical roller bearings ing may be used for either coupled or belted loads mounted directly to the shaft of the motor. There is a (many times bearing L10 life is the keytypically a shoulder machined on the shaft, and the inner race of minimum of 50,000 h for belted loads). If the mini- the bearing is positioned directly against this shoulder. mum load requirements are met for the rolling bear- The roller bearings are mounted in a “press fit,” meanings, then the bearing life is often limited by the grease ing that clearances are 0.0005–0.0019 in tight. The lubrication life. As motors get larger than 100 hp, ball endplates of the motor have bearing bores machined to bearings may still be used on coupled loads, but belted provide clearance or “loose fits” with the outer ring of loads should be evaluated for load on the bearing shaft the bearing giving 0.0000–0.0023 in loose as well. and a roller bearing with higher load capacity and pos- Since roller bearings accommodate limited axial loads, sible misalignment capability selected, if required. the opposite bearing to the drive end bearing is held Roller bearings, however, require higher minimum captive in the endplate, and the drive bearing provides loading for operation than do ball bearings. some axial movement to allow for thermal growth or the shaft and rotor assembly. In the case of cylindrical roller Motor Construction bearings, this axial movement is accommodated within A requirement for two types of motors for different load- the bearing as opposed to the housing bores. Because ing conditions often causes problems in mills. If a 100+ roller bearings require higher minimum loads to funchp motor with ball bearings is used on a heavy belted tion than ball bearings, they are normally only used for load, the drive end bearing could fail after a short time belted or overhung loadings. from overloading. If a motor designed for heavy belting Motors requiring roller bearings with high overhung is used on a coupled load, there may not be enough radial loads require higher-strength shaft steel. Depending on loading to cause the rolling elements to roll. They will calculated radial shaft loads, the motor designer may select skate or skid on the race, causing high temperatures and grades such as AISI/SAE 1045 or 4140. sometimes rapid and traumatic failure. Underloading of a roller bearing, even with a belted load, can result in Toroidal Roller Bearing Configuration premature failure from the same skidding phenomena. A toroidal roller bearing provides self-aligning capabiliThis presents the challenge for a pulp and paper mill: ties and axial movement within the bearing, and it two motor designs are required depending on the requires lower minimum loading than other roller bearmethod of coupling to the load. Often these motors are ings. The toroidal bearing mounts directly to the shaft mixed in spare inventory, resulting in improper use and with an interference or tight fit of 0.0005–0.0022 in. early failure. A new toroidal roller bearing design does The outer ring is mounted to the endplate with a clearoffer some flexibility. These bearings may be capable of ance or loose fit of 0.0000–0.0023 in. It can provide operation under either a coupled or belted load, meet large axial movements within the bearing and, therefore, minimum load requirements, and provide adequate life, is a nonaxial load carrying bearing and must be mounted but they are not directly dimensionally interchangeable with a captive (or held) bearing capable of handling some with traditional bearings used in motors. thrust loading. Motors using a toroidal roller bearing offer advantages Ball Bearing Configurations in that they may be used on either a coupled or heavy beltThe motor is built with the bearings mounted directly to ed load. The shaft material selection must be for the the shaft of the motor with an interference fit. There is a worst-case condition of a heavy overhung load requiring shoulder machined on the shaft, and the inner race of the higher-strength shaft steel. Depending on calculated radial bearing is positioned directly against this shoulder. The shaft loads, the motor designer may select AISI or SAE bearings are mounted in an interference (or “press”) fit, grades such as 1045 or 4140.
The added width of a toroidal roller bearing may present challenges to the motor designer attempting to utilize existing endplate tooling to prevent the bearing from protruding inside the motor where it is unsupported. The loads from the bearing need to be directed to the strongest part of the endplate. Also, due to the additional heat produced when used with heavy belted loads, a cooling fan mounted on the shaft may be used. This may result in a non-NEMA “BA” dimension (shaft shoulder to foot mount holes). Table 1 illustrates a comparison of ISO radial capacity, required minimum loads, misalignment capabilities, and boundary dimensions for commonly used motor bearings.
Bearing Loads
selection. Most motor manufacturers can supply a worksheet that may be used to define the data required for the analysis.
Bearing Selection The purpose of using rolling bearings in electric machines is to support and locate the rotor, to keep the air gap small and consistent, and to transfer loads from the shaft to the motor frame. The bearings should enable high- and low-speed operation, minimize friction, and save power. The designer must consider many different parameters when selecting the bearing type and arrangement to meet the requirements of any particular motor application. The orientation of the motor and the drive coupling can have a significant impact on the final selection. Electric motors and generators use a wide variety of bearing types, including deep groove ball bearings, angular contact ball bearings, cylindrical roller bearings, taper roller bearings, spherical roller bearings, toroidal roller bearings, and spherical roller thrust bearings. The different bearing types are shown in Figure 1. In small horizontal machines, the most common arrangement consists of
The type of coupling connector used between the drive and driven unit will influence the loads on the motor bearings. There are two kinds of coupling drives: flexible and rigid. Good alignment is important in both cases; otherwise, additional forces may be induced into the bearing system to reduce service life. Proper alignment is particularly important with a rigid coupling, where there are typically three bearings on a shaft. When rigid couplings are aligned very accurately, by using laser aligning TABLE 1. BEARING CHARACTERISTC COMPARISON FOR A 9 5-MM equipment, the drive end bearing might SHAFT 449T FRAME, TEFC, 250-HP MOTOR AT 1,785 R/MIN. become relatively unloaded, the load being taken by the bearings on the nonBearing ISO Minimum Allowable drive end and the coupling shaft. In this Width Capacity Load Misalignment case, a deep groove ball bearing is norDeep groove 45 mm 35,730 lb 100 lb 2–10 min mally recommended at the drive end. ball bearing A belt or gear drive will often load the motor bearings more heavily than a couAngular contact 45 mm 40,450 lb 230 lb 2–10 min pling drive. Belt and gear drives therefore ball bearing often use cylindrical roller bearings at the Cylindrical roller 45 mm 87,640 lb 585 lb 4 min drive end. In applications where there are bearing heavy loads and a possibility of misalignSpherical roller 45 mm 95,500 lb 1,100 lb 120 min ment and/or shaft deflection, a self-alignbearing ing roller bearing should be considered. Any time a belted load is used on a large Toroidal roller 67 mm 137,000 lb 630 lb 30 min motor, the pulley data should be analyzed bearing to determine the appropriate bearing
Deep Groove Ball Bearing
Angular Contact Ball Bearing
Cylindrical Roller Bearing
Taper Roller Bearing
Spherical Roller Bearing
Toroidal Roller Bearing
Spherical Roller Thrust Bearing
1 Bearing types.
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two deep groove ball bearings. In largspeeds, bearing internal clearance or er or heavier loaded machines, roller preload, lubrication, and sealing. ROLLING bearings are typically used. In vertical The magnitude of the load is one of machines, deep groove ball bearings, the factors that usually determines the BEARINGS IN angular contact ball bearings, or sphersize of the bearing to be used. Generalical roller thrust bearings are typically ly, roller bearings are able to support ELECTRIC used, depending on the loads, speeds, heavier loads than similar-sized ball MACHINES temperature, and environment of the bearings. Ball bearings are mostly used application. where loads are light or moderate. For SUPPORT AND In many cases, however, several facheavy loads and where shaft diameters tors must be considered and weighed are large, roller bearings are usually the LOCATE THE against each other when selecting a more appropriate choice. bearing type, so that no general rules Cylindrical roller bearings, needle ROTOR, KEEP can be given. The most important facroller bearings, and toroidal roller beartors to be considered when selecting a ings can only support pure radial loads, THE AIR GAP standard bearing type and an appropriwhile other radial bearings can accomate choice are: modate some axial loads in addition to SMALL AND radial loads. ■ boundary dimensions CONSISTENT, Angular contact ball bearings can ■ magnitude and direction of loads support moderate axial loads at rela■ speed: fixed, variable, or high AND TRANSFER tively high speeds. For moderate and ■ required precision heavy axial loads acting in one direc■ shaft and housing material LOADS FROM tion, spherical roller thrust bearings ■ coupling, belt, or gear drive can be used. ■ horizontal or vertical mounting THE SHAFT TO THE A combined load comprises a radial ■ environment and an axial load acting simultaneously. ■ vibration level MOTOR FRAME. The ability of a bearing to carry an ■ noise level axial load is determined by the angle of ■ temperature contact or load action internal to the ■ required bearing life bearingthe greater the angle, the more suitable the ■ lubrication: grease versus oil, integral seals. A general overview of the standard bearing types, their bearing for axial loads. For combined loads, single- and design characteristics, and their suitability for the demands double-row angular contact ball bearings and single-row in motor applications is shown in the matrix in Table 2. taper roller bearings are most commonly used, although Other important bearing-specific criteria must be con- deep groove ball bearings and spherical roller bearings sidered when designing a bearing arrangement, including may also be suitable depending on the ratio of axial to load carrying capacity and life, friction, permissible radial loading. TABLE 2. BEARING PERFORMANCE CHARACTERISTICS.
Noise
Speed Capability
Radial Loading
Axial Loading
Combined Loading
Misalignment
Availability of Shields and Seals
Deep groove ball bearing
5
5
3
3
3
2
5
Angular contact ball bearing
5
5
3
4
4
2
2
Cylindrical roller bearing
5
5
5
2
2
1
1
Tapered roller bearing
3
3
4
4
4
1
1
Spherical roller bearing
3
3
5
2
3
5
3
Toroidal roller bearing
4
4
5
1
2
5
4
Spherical roller thrust bearing
3
3
2
5
4
5
1
Bearing Type
The numbers represent a relative scale from 1 to 5 with 1 being poor and 5 being excellent.
Single-row angular contact ball All ball bearings with integral bearings and tapered roller bearings can seals on both sides are filled with a BEARINGS only accommodate axial loads acting in grease of an appropriate quality and one direction. For axial loads of alterquantity based on the anticipated SHOULD ENABLE nating direction, these bearings must operating conditions and required be combined with a second bearing. service life. Because of this, the bearHIGH- AND When a load acts eccentrically on a ings are not designed to be relubriLOW-SPEED bearing, such as from an overhung cated in operation. sheave or pulley, a tilting moment will OPERATION, occur. Double-row bearings (e.g., deep Calculation of Bearing Load groove or angular contact ball bearThe loads acting on the bearing can MINIMIZE ings) can accommodate tilting be calculated according to the laws moments, but paired single-row anguof mechanics if the external forces FRICTION, AND lar contact ball bearings or taper roller (forces from power transmission, bearings are more suitable. work forces, or inertia forces) are SAVE POWER. Angular misalignments may occur known or can be calculated. When between the shaft and housing. Examcalculating the load components for ples are when the shaft bends (flexes) a single bearing, the shaft is assumed under the operating load, when the bearing seating in the to be a beam resting on rigid, moment-free supports housing is not machined to the same height, or when for the sake of simplification. shafts are supported by bearings in separate housings that Radial bearings are often subjected to simultaneousare too far apart. ly acting radial and axial loads. If the resultant load is Rigid bearings (i.e., deep groove ball bearings and cylin- constant in magnitude and direction, the equivalent drical roller bearings) cannot accommodate any misalign- dynamic bearing load can be obtained from the general ment or can only accommodate very minor misalignments, equation: unless by force. Self-aligning bearings (i.e., spherical roller bearings, toroidal roller bearings, and spherical roller thrust P = X Fr + YFa bearings) can accommodate misalignment produced under operating loads and also compensate for initial errors of mis- where P = equivalent dynamic bearing load, Fr = actual alignment resulting from machining or mounting errors. radial bearing load, F a = actual axial bearing load, X = The permissible bearing operating temperature in the radial load factor for the bearing, and Y = axial load facapplication limits the speed at which rolling bearings can tor for the bearing. be operated. Bearing types with low friction and correX and Y load factors can be obtained in bearing manuspondingly low heat generation inside the bearing are facturers’ catalogs. Except for vertical applications, beartherefore the most suitable for high-speed operation. The ings in electric motors are subjected to little if any axial highest speeds can be achieved with deep groove ball bear- loading, hence P = Fr . ings when loads are purely radial and with angular contact ball bearings for combined loads. This is particularly true Requisite Minimum Load of angular contact ball bearings or deep groove ball bear- If a bearing is to operate satisfactorily, it must always be ings with ceramic rolling elements. subjected to a given minimum load. This minimum Locating and nonlocating bearings generally support load ensures proper rolling element rotation, i.e., no shafts or other rotating machine components. Locating skidding, and enhances lubricant film formation in the bearings provide axial location for the machine component rolling contact areas. A general “rule of thumb” indiin both directions. The most suitable bearings for this are cates that loads corresponding to 0.02 times the dynamic those that can accommodate combined loads or can pro- radial load rating should be imposed on roller bearings vide axial guidance in combination with a second bearing. and loads corresponding to 0.01 times the dynamic Nonlocating bearings must permit shaft movement in radial load rating on ball bearings. The importance of the axial direction so that the bearings are not overloaded imposing this load increases where accelerations in the when, for example, thermal expansion of the shaft and bearing are high and where speeds are in the region of rotor assembly occurs. The most suitable bearings for the 75% of the permissible speed ratings quoted in the nonlocating position include cylindrical roller bearings. In bearing manufacturers’ catalogs. applications where the required axial displacement is relatively large and the shaft also may be misaligned, the Bearing Life toroidal roller bearing is the ideal nonlocating bearing. The bearing type and size to be used in a motor applicaThe selection of an integral seal can be of vital impor- tion can be initially selected on the basis of its load rattance to the proper performance of the bearing. A large ings in relation to the applied loads and the number of types and sizes are available for requirements regarding service life and reliability. The basic dynamic load rating C is used for calculations ■ deep groove ball bearings involving dynamically stressed bearings, for example, ■ angular contact ball bearings when selecting a bearing, which is to rotate under load. ■ spherical roller bearings It expresses the bearing load, which will give an ISO ■ toroidal roller bearings. ,
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281:1990 and ABMA Standards 9 and 11(1990) basic rating life of 1 million revolutions. The simplest method of life calculation is to use the ISO or ABMA equation for basic nonadjusted rating life, which is
,
,
,
,
,
Flat belt*
1.33
Timing belt**
0.9
Chain sprocket
0.7
Spur gear
0.75
Helical gear
0.85
The above multiplier is intended for use with conventional single-ply belts. When other then single-ply belts are used, the use of a larger multiplier is recommended. *
It is often necessary to install timing belts with a snug fit. However, tension should be no more than necessary to avoid belt slap or tooth jumping. **
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NEMA MG 1
NEMA MG 1 defines maximum allowable radial overhung load for P C horizontal motors with antifriction L10 = ball bearings in NEMA MG 1 Table P 14-1A. where L10 = basic rating life, millions ■ The limits are given and should of revolutions; C = basic dynamic not be exceeded. Bearing and shaft load rating; P = dynamic bearing failure constitute a safety hazard, load; and p = exponent of the life and safeguards suitable to each equation. application should be taken. For ball bearings, p = 3. For roller The figures in the NEMA table bearings, p = 10/3. show limits for loads applied at the For bearings operating at constant center of the N-W dimension and a speed it may be more convenient to reduction factor for loads applied at deal with a basic rating life expressed the end of the shaft. NEMA MG 1in operating hours using the equation 14.7.1 provides information on the mounting of sheaves (pulleys). P C 1 000 000 ■ In general, the closer pulleys, sheaves, sprockets, × L10h = or gears are mounted to the bearing on the motor 60n P shaft, the greater the load on the closest bearing. where L10h = basic rating, operating hours; and n = (The loading will be higher but the misalignrotational speed, r/min. ment less from shaft deflection). This will give ISO 281:1990/Amendment 2:2000 also makes progreater assurance of trouble-free service. visions for bearing manufacturers to recommend a suit■ The center of the belt, or system of V-belts, should able method for calculating the life modification factor not be beyond the end of the motor shaft (for adeto be applied to a bearing based on operating condiquate support for the sheave). tions. The ISO life modification factor aXYZ applies ■ The inner edge of the sheave or pulley rim should the concept of a fatigue load limit analogous to that not be closer to the bearing than the shoulder on the used when calculating other machine components. shaft but should be as close to this point as possible. a Furthermore, the life modification factor XYZ makes ■ The outer edge of a chain sprocket or gear should not use of the lubrication conditions, a factor for contamiextend beyond the end of the motor shaft. nation level to reflect the application’s operating con■ NEMA MG 1-14.7.2 further discusses the miniditions and performance level. mum pitch diameter for drives other than a V-belt. ■ To obtain the minimum pitch diameters for flatbelt, timing-belt, chain, and gear drives, the mulL10m = a1 aXYZ L10 tiplier given in Table 3 should be applied to the where L10m = adjusted rating life (at ∼10% reliability), narrow V-belt sheave pitch diameters in 14.41 for millions of revolutions; and aXYZ = life modification factor. alternating-current (ac) general-purpose motors or to the V-belt sheave pitch diameters as determined from NEMA 14.67 for industrial direct Standards Defining Bearing Performance To better clarify certain points, the authors have made current motors: (The multipliers do not improve or worsen L10 life. The tension in the belt will, but the size of the pulley diameter TABLE 3. LOAD TYPE MULTIPLIERS. has little influence in the bearing loading, as the location Drive Multiplier of the load on the shaft is the same).
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FOR HEAVY LOADS AND WHERE SHAFT DIAMETERS ARE LARGE, ROLLER BEARINGS ARE USUALLY THE MORE APPROPRIATE CHOICE.
comments in parenthesis in the following NEMA MG 1 and IEEE 841 standards.
IEEE 841
IEEE Standard 841-2001 addresses both bearing life and loading. Section 6 of that standard addresses mechanical features of the motor, namely that horizontal and vertical ball bearing and roller bearing manufacturing tolerance limits shall be in accordance with Table 4 of ABMA 20-1996. In addition, ball bearings used in horizontal motors shall have ABMA C/3 clearances. ■ Bearings are to be selected to provide an L10 life of 26,280 h minimum per ABMA 9-1990 or ABMA
11-1990, as applicable. L10 life calculations for vertical motors and horizontal motors mounted in a vertical position shall consider applicable thrust loading. L 10 life calculations shall be based on external side loads for NEMA belted application limits, in accordance with Section 14.42 of NEMA MG 1-2003, and internal loads defined by the manufacturer. For NEMA frame sizes above 445T, minimum sheave sizes in centimeters shall be defined by (1) or (2) and shall be applied per NEMA requirements for narrow width belts:
tion that must be defined by the users for analysis by the motor manufacturer.
THE SELECTION
Lubrication: Grease or Oil
OF AN INTEGRAL
If rolling bearings are to operate reliably and realize their full service life SEAL CAN BE they must be adequately lubricated. The function of the lubricant is to OF VITAL form a protective oil film that separates the bearing components and preIMPORTANCE vent metal-to-metal contact. The lubricant also protects the bearing and TO THE PROPER related components against corrosion. When grease is used as a lubricant, it PERFORMANCE can also help protect the bearing against contaminants like dirt, dust, OF THE BEARING. and water. Some important properties of a lubricant include viscosity, film forming ability, and consistency (for grease). The most important determi1/3 nants of the film thickness are , (1) Sheave diameter = 70.0 ( P/ N R) ■ rotational speed where P is rated is rated motor power (kW) and N ■ bearing temperature R is rated motor speed (r/min). ■ load ■ base oil viscosity ■ NLGI grade number or consistency for grease. 1/ 3 Sheave diameter = 63.5 ( P/ N , (2) R) The choice between grease lubrication and oil lubrication is chiefly determined by the following factors: where P is rated motor power (hp) and N ■ grease should be used in applications where the folR is rated motor speed (r/min). lowing requirements apply: Note that the two-pole belted applications are not – simplified maintenance recommended for motors above 19 kW (25 hp). – improved cleanliness (fewer leaks) ■ Antifriction bearings are acceptable where the dN – better protection against contaminants factor is less than 300,000. (The dN factor is the ■ oil lubrication should be used in applications where product of bearing size (bore) in millimeters and the normal operating temperatures are high as a result rated speed in r/min). of an external heat source or excess heat generated by ■ When direct coupled, the stabilized bearing temperthe machine or its bearings at high speed. ature rise at rated load shall not exceed 45 C (50 C Note that the friction in the bearing is generally lower on two-pole motors) as measured by a thermometer with grease than with an oil bath, provided that the or thermocouple on the surface of the bearing hous- appropriate type and amount of grease is used and that it ing as close to the outer race as possible. is supplied to the bearing in a suitable manner. The operating temperatures will be similar, however, since heat is ■ Bearings shall be suitable for, and supplied with, rust-inhibiting grease compatible with polyurea- not removed with grease. Oil lubrication should be used thickened grease. when the relubrication interval for grease is too short. Using the best and most suitable lubricant related to ■ Some modifications to motors may be required to accommodate oil mist lubrication. The manufactur- specific operating conditions is of crucial importance in er shall be consulted for oil mist lubrication. obtaining appropriate motor performance and reliability. IEEE 841–2001 requires some additional bearing sys- Areas of consideration should include the following: tem mechanical construction requirements: ■ bearing type and size ■ Bearings shall be regreasable without disassembly ■ operating temperature of the fan or fan covers and shall contain a reservoir ■ load equipped with outlet plugs that extend beyond the ■ speed range fan cover for elimination of purged grease. ■ operating conditions e.g., vibration levels, orientation of the shaft (horizontal or vertical) ■ Inner bearing caps shall be provided so that the entry of oil or grease into the motor interior is minimized. ■ cooling ■ Bearing and grease reservoirs shall be protected from ■ sealing efficiency entry of contaminants. Most end users request that ■ environment. replaceable rotating noncontact labyrinth seals be Grease Lubrication used for this. In section 3.2 of IEEE Standard 841-2001, abnormal Under normal speed and temperature conditions, the axial or side thrust is defined as an unusual service condi- bearings in electric motors are usually lubricated with ◦
◦
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grease. Grease has a number of ing prevent damaging electric curadvantages when compared to oil. It rentssometimes seen when using a BEARING allows simpler, more cost-effective motor with a PWM (pulse width modhousing and sealing designs while ulated or inverter) ac drivefrom passSELECTION offering better adhesion of the lubriing through the bearing. This is one of REMAINS A cant to critical surfaces and protecthe main reasons for using hybrid beartion against contaminants. ings in electric motors and generators. CONSIDERATION For small- and medium-sized ball High-speed electric motors use hybrid bearings where the grease life is bearings because they provide substanON LARGER longer than the expected service life tially longer service life due to lower of the bearings, one single filling of operating temperatures and longer NEMA FRAME grease is sufficient. The grease must grease life as well as lower friction than then be retained in the bearings and traditional all-steel bearings. MOTORS OF 125 prevented from escaping. Polyureaand urea-based greases are typically Conclusions HP AND ABOVE. furnished by most manufacturers of Bearing selection remains a consideration on larger NEMA frame motors of industrial motors. Lithium-based greases or oil are normally used for 125 hp and above. Different bearing roller bearings on larger motors. arrangements and solutions may be necessary depending The life expectancy of grease depends on several factors on the operating conditions. Bearing load capacity, miniincluding the type of bearing, the type of grease, the ori- mum loads, and lubrication methods all can influence entation and speed of the motor, and the operating tem- the proper choice. In addition, the external loading from perature of the bearings. For instance, roller bearings have pulleys or sheaves can lead to additional loadings as well shorter relubrication intervals than ball bearings. Other as misalignments within the bearing, thereby limiting factors must be considered, including the sealing arrange- life. A new toroidal roller bearing design may offer some ment, operating environment, and contamination. For advantages, but endplate modifications may be required. small ball bearings in standard motors, since normal The toroidal roller bearing is a self-aligning roller beargrease life usually exceeds the expected service life of the ing that combines the features of a cylindrical roller motor, these bearings are usually fitted with seals or bearing (internally adjusts for axial movements), the neeshields and lubricated for life. At the end of the bearings’ dle roller bearing (long rollers to maximize load capacity) life, they are simply replaced. and the spherical roller bearing (raceways based on Severe duty and IEEE 841 motors are often supplied spheres to accommodate misalignments). The user with open bearings and provision for regreasing regardless should evaluate the connection to the load and consult of the motor size. However, if the grease life is shorter with the motor manufacturer on belted loads to achieve than the expected bearing life, the bearings need to be an optimized solution. By utilizing this new toroidal relubricated while the grease is still performing satisfacto- bearing, it may now be possible for mills to stock one rily. This is usually the case on motors above 20 hp. When motor that is suitable for either coupled or belted loads. using high-performance greases, a longer relubrication interval and grease life may be possible. Oil Lubrication
References
Oil is typically selected as a lubricant when rotational speeds or operating temperatures make it impractical or impossible to use grease. In applications where there are high operating temperatures, recirculating oil systems are used to dissipate heat. Recirculating systems can also be used to remove and filter out contaminants. Oil lubrication requires more sophisticated seals and there could be a risk of leakage. In general, only large electric motors (above NEMA sizes) and high-thrust vertical pump motors are oil lubricated.
[1] Motors and Generators, NEMA Standard MG 1-2003. [2] IEEE Standard for Petroleum and Chemical IndustrySevere Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage induction MotorsUp to and including 370 kW (500 hp) , IEEE Standard 841-2001. [3] Rolling BearingsDynamic Load Ratings and Rating Life, ISO Standard 281:1990. [4] Load Ratings and Fatigue Life for Ball Bearings , ABMA Standard 91990. [5] Load Ratings and Fatigue Life for Roller Bearings , ABMA Standard 111999. (6) Rolling BearingsDynamic Load Ratings and Rating Life, Amendment 2: Life modification factor axyz,, ISO Standard 281:1990, Amendment 2:2000.
Alternate Bearing Materials Hybrid bearings have rings made from bearing steel and rolling elements made from bearing-grade silicon nitride. Silicon-nitride is a low-density, high-strength ceramic material that has a high degree of toughness and hardness with excellent insulating properties as well. When used as an insulator, the ceramic rolling elements in a hybrid bear-
John Malinowski (
[email protected]) is with Baldor Electric Company in Fort Smith, Arkansas. Daniel R. Snyder is with SKF USA Inc., Kulpsville, Pennsylvania. This article first appeared in its original form at the 2005 IEEE Pulp & Paper Industry Conference.
