The first roller bearing, it can be said, was invented over 3000 years ago when man first discovered that heavy objects could be moved with greater ease if they were rolled on logs rather than pushed along the ground. The steel bearings, which are familiar to us today, did not come into being until the 1800's when iron and steel became readily available and machines were invented that could mass produce parts accurately and hard enough to construct satisfactory bearings.

The purpose of a bearing is to reduce friction and wear. No bearing is completely without friction and they do wear with use, however, very little friction is produced when starting and running, and if properly installed and maintained, they can outlast the life of the driven machine.

Bearings are used to centrally support the rotating elements of a motor (rotor and shaft) while providing the motor user with a relatively rigid support for the output shaft. As such, the bearing becomes the connection point between the rotating and the stationary elements of a motor.

The most conventional connection is a ball bearing in small and medium size motors. The reasons are multiple. First, ball bearings are mass produced, therefore relatively inexpensive while readily available. Secondly, the ball bearing provides very reliable performance and life when used in a wide variety of applications and also, the self-aligning characteristics of a ball bearing allow the mass production of motor components without the need for prohibitive, selective or matched assembly.

General Bearing Information
Bearing Standards

The dimensions and tolerances for anti-friction bearings have become very standardized. The standards govern inside diameter, outside diameter, width and the precision to which these dimensions are machined. These uniform standards have helped to reduce manufacturing cost, lower selling price and also provide an alternate source of supply for the bearing buyer.

The organizations which have developed their standards throughout the world are the International Standards Organization (ISO) which is worldwide, the Anti-Friction Bearing Manufacturer Association (AFBMA) which is very familiar to us in North America, the Japanese Industrial Standards (JIS) in Japan, and the Deutsche Industrial Normen (DIN) in Germany.

Most ball bearings are designed to fit into one of four series of standard dimensions for the bore and outside diameter. The following figure demonstrates the increase or decrease in load carrying capacity in relation to the proportions of the bearing bore and O.D. for standard single row ball and roller bearings by series classification. The width dimension may vary depending on the application or requirements for sealing devices, extra radial load capacity and extra grease reservoir. Sealed and double row bearings are an example of width variance.

Comparison of bearings with the same bore diameter

Comparison of bearings with the same outside diameter

Duty Ratings

A - Extra light duty
B - Light duty
C - Medium duty
D - Heavy duty

AFBMA also has a standard radial clearance tolerance established for ball bearings. Radial clearance is defined as the total diametrical movement of the unclamped ring when a specified radial load is reversed. Radial ball bearings are usually supplied with a specified radial clearance, which will normally diminish during mounting because of interference fits with the shaft and house. After mounting, the bearing should have a small radial clearance remaining. The AFBMA standards are designated by a suffix after the bearing number as follows:

C2 - Radial clearance less than standard
No suffix - Radial clearance is standard
C3 - more than standard (Reliance Standard)
C4 - more than C3

Internal Fit

The drawing shown above shows predesignated clearance between the balls and the rings.

Points to remember

  1. Bearings are normally a press fit on the shaft of the rotating unit. The degree of press stretches the inner ring O.D. - this can compress the balls and reduce life.
  2. Rotational caused heat can expand the balls and rings. To eliminate precompression of balls radial clearance should be allowed.
  3. Allows slightly more misalignment without causing ball-ring compression.

Four levels of dimensional precision for anti-friction bearings have been established by the AFBMA. They include tolerances for bore, outside diameter, width and radial runout and are as follows in order of low to higher precision:

ABEC-1   tolerances suitable for general commercial applications such as appliances, automobiles, and construction equipment
ABEC-5   tolerances suitable for use in machine tools and high speed electric motors.
for measuring and recording instruments and other precision equipment applications.

Note: See AFBIVIA Nomenclature regarding standards.

Bearing Types
Single Row, Deep Groove Ball Bearings

This is sometimes called a Conrad bearing and is listed by AFBMA as type BC (single row radial contact without filling slot). This is the most commonly used bearing in small and medium size a-c induction motors and the most popular type for all ball bearing applications.

The Conrad bearing is assembled by offsetting eccentrically the inner and outer rings to allow the insertion of balls (see figure below). The Conrad type bearing therefore has uninterrputed ringways (no filling slot) which permits excellent bearing performance under light to moderate radial loads, relatively moderate thrust loads, or combined radial and thrust loads.

Bearing Nomenclature

Points to remember:

  1. Somewhat self-aligning; allows a minor misalignment (1/4 degrees) without affecting the bearing operation and life.
  2. The bearing is capable of radial and/or moderate thrust loads regardless of direction of load.

Single Row, Deep Groove Ball Bearing

Single Row, Maximum Capacity

This bearing is AFBMA type BL (maximum capacity ball bearing with filling slot). It contains a maximum complement of balls which are inserted by means of a filling slot in the inner and outer ringways (see figure below). The addition of these extra balls permits heavier radial loads in moderate speed applications but with only light thrust loads since the balls will be damaged by contact with the filling slot if the angle of contact or axial load is too great. This type of bearing is available in sizes corresponding to the Conrad type, but usage of this bearing is decreasing in general and is not used in Reliance motors.

Points to remember:

  1. The additional balls increase fatigue capacity over non-filling slot type.
  2. Radial load capacity is greater than Conrad type.
  3. Bearing is not capable of thrust loads nor angular misalignment.
  4. Since bearings are symmetrical, slot direction is difficult to maintain at assembly.

Max. Capacity Filling Slot Type, Single Row Ball Bearing

Angular Contact Ball Bearings

The Angular Contact Ball Bearing is designed to support a thrust load in one direction or a thrust load combined with a radial load. A steep contact angle, assuring the highest thrust capacity and axial rigidity, is obtained by a high supporting shoulder on the inner ring and a similar high shoulder on the opposite side of the outer ring. These bearings can be mounted singularly or, when the side surfaces are flush ground, in multiple, either face-to-face or back-to-back for all combinations of thrust and radial loading. Flush ground bearings can also be tandem mounted to permit sharing heavy thrust loads in one direction among two or more bearings.

Points to remember:

  1. Increased thrust capacity over Conrad or Maximum capacity type bearing.
  2. Thrust must be in the direction of the thrust shoulder. Thrust in an opposite direction can drive balls over the counterbore shoulder or cause ball damage, and failure.
  3. Increased cost of bearing in comparison to single row, non-filling slot.

Angular Contact Ball Bearings

Double Row Self-Aligning Ball Bearings

The self-aligning ball bearing, with two rows of balls rolling on the spherical surface of the outer ring, compensates for angular misalignment resulting from errors in mounting . . . shaft deflection . . . and distortion of the foundation. It is impossible for this bearing to exert any bending influence on the shaft ... a very important consideration in many applications requiring extreme accuracy, at high speeds. Self aligning ball bearings are recommended for radial loads and moderate thrust loads in either direction.

Double Row, Self-Aligning Ball Bearings

Ball and Roller Thrust Bearings

The ball thrust bearing is designed for thrust load. The load line through the balls is parallel to the axis of the shaft . . . resulting in high thrust capacity and minimum axial deflection. Flat seats are preferred ... particularly where the load is heavy.. . or where close axial positioning of the shaft is essential.

The spherical roller thrust bearing combines a very high load-carrying capacity (heavy thrust loads or combined loads which are predominantly thrust) with complete self-alignment . . . and can operate at relatively high speeds under heavy loads.

Ball and Roller Thrust Bearings


Basic Bearing Comparison Guide Single Row Conrad Type Max Capacity Double Row Angular Contact Cylindrical Roller
Radial Load Capacity 2 1 1 2 1
Thrust Load Capacity 3 3 2 1 4
Combined Load Capacity 2 3 2 1 4
Life Expectancy 1 2 1 1 1
Operating Speeds 1 2 3 1


Compensation for Misalignment 3 3 3 4 3
Grease Capacity 3 3 2 3 3
Sealing Effectiveness 1 3 2 4 4
Shaft and Housing Rigidity Under Moment Loads 3 3 1 4 4



Good 2
Fair 3
Unacceptable 4


Cooler Bearing Operating Temperatures - Open bearing (non-shielded) construction (1) minimizes friction, allowing cooler bearing operation.

Positive Lubrication/Relubrication in any Mounting Position - Exclusive grease channeling window (2), with minimum grease path entry (3), channels grease directly into bearing track and avoids premature relief out shaft bore or drain plug.

Minimizes Corrosion - Small clearance on either side of grease window uniformly distributes grease to both inboard and outboard reservoirs (4) to protect bearing surfaces during motor storage, idle times and start-up. Bearing system is completely greased during motor assembly.

Restricts Inboard Contaminants - Inner bearing cap (5) with anti-churning vanes (6) and close running shaft tolerances (7) minimizes contaminant entry into bearings, and grease migration into motor.

Prohibits Overgreasing During Lubrication/Relubrication - Grease relief port (8) accurately indicates completion of lubrication/relubrication. (If port is plugged during lubrication, PLS design will relieve grease along the shaft (9).

Bearings: Sealed And Shielded
Sealed Bearing

The construction of this bearing is shown below. It has a flexible nonmetallic member secured to the outer ring and makes a light contact on the inner ring.


  1. Entry of contaminants is less likely than an open bearing.
  2. No regular relubrication is necessary nor possible.


  1. 'The bearing life is restricted to the lubrication packed between the seals of the bearing.
  2. Sizes available are restricted due to the excessive heating on larger sizes.
  3. Maintenance requires replacement of the bearing.
  4. Higher Friction Losses.
Shielded Bearing

A shielded bearing is also a variation of the Conrad bearing and is very similar to the sealed type bearing except that the shielded bearing has metallic rather than non-metallic shield. The metal member is secured to the outer ring with a close running clearance to the inner ring. A shielded bearing can be relubricated.


  1. Retains the lubricant at the rolling elements regardless of the chamber fill.
  2. Provides relubrication to the balls by the Slinger feeding of inner ring.
  3. Eliminates large particles from getting into the rolling elements at installation and in operation.
  4. Higher Friction Losses over open but typically less than Sealed Bearings.


Excessive pressure with no relief provided can force the shield against the cage or balls, thereby, eliminating regreasability or causing immediate failure.

Shielded bearings are used on some Reliance motors.

Pre-load Washer

A pre-load washer, sometimes referred to as a wavy washer, is used to provide an axial thrust load by moving the outer ring axially to engage the balls and insure good contact. This is done to prevent or minimize ball skidding rather than rolling because of a loose internal fit, especially when the motor is running in a cold state. Skidding tends to prevent ideal full film lubrication which may cause excessive wear of the balls and rings and therefore early bearing failures.

The thrust pressure provided by a preload washer is not significant enough to adversely affect the bearing life and as a motor gets hotter in a running state, the pre-load is overcome by the thermal expansion which insures the ball to ring contact at all times.

In a horizontal position, the magnitude of the thrust should be great enough to move the complete rotating unit - motor and shaft - to insure both bearings are making contact.

Inner Cap

An inner cap is provided on most Reliance motors. It is a retainer which fits over the shaft and bolts to the inside of the motor frame to hold the bearing in place. In some cases the cap also acts as a bearing housing with the bearing mounted in the cap which then fits into the end frame of the motor.

The cap does provide some measure of protection in that it restricts grease from being forced into the motor and winding, and acts as a barrier which restricts any failed bearing components from striking the winding. With small framed motors, due to the small likelihood of the above mentioned problems occurring and the relatively small repair cost to correct these problems, most motor manufacturers do not put inner caps on their small motors.

Bearing Life

The bearing life for Reliance standard motors is 17,500 hours (belted) or 100,000 hours (coupled) L10 minimum life which is based on the highest rated speed of the motor and a radial load consistent with the smallest dia. belt sheave as defined in NEMA Mg. 1-14.43a. L10 is a statistical expectation of bearing fatigue life. It represents a point at which the 10% rate of failure can be expected for a given bearing run at the same speed (RPM) with a constant load. A normal statistical distribution of failure would graphically look:

The point at which 10% failure occurs (point A is defined as L10 hours.)

Point B is the median/50% life and is approximately five (5) times as many hours as the L10 life.

Obviously, many bearings exceed this life. It should be pointed out that the L10 calculated life is limited to the fatigue endurance life.

Factors of lubrication, temperature, contaminants, etc., are not considered in this process.

Using this basis for comparing motors or for predicting service life can be very misleading since most bearings fail due to other reasons as discussed below.

Note that the bearing life figure for Reliance standard motors is a minimum design calculation and almost all Reliance motor bearings will have a life exceeding this figure unless the bearing is subject to abnormal conditions. The method of arriving at the bearing design life is very conservative and most motors will have a minimum life many times the tabulated values.

The term B10 and L10 are used interchangeably however, L10 is the current and correct designation.

Causes Of Bearing Failures

Almost without exception, the published life of a bearing is the L10 life based on the fatigue failure of rings or rolling elements. Actually, it is very unusual to find a bearing which has failed from this cause.

Confining this discussion to electric motor bearings the most common causes of failure are as follows:

  1. Contamination
    1. Rusting or corrosion
    2. Dirt
    3. Abrasive material
    4. Water (grease washout)
  2. Inadequate lubrication
  3. Excessive temperature
  4. Misalignment
  5. Brinelling
    1. False
    2. True
  6. Bearing fits
    1. To housing
    2. To shaft
  7. Improper assembly practices
  8. Electric currents
  9. Faulty bearing - storage or manufacturing error
  10. Overloads

Some common causes of bearing failures are discussed in more detail below.

Due to fatigue: The metal of the bearing ring way is continually flexing under the extremely high pressure of the balls rolling in the ring way. This flexing eventually causes "spalling" which is the flaking of small metal particles from the ring way. This condition will spread rapidly until failure.

Due to dirt or other foreign matter: Dirt and other hard particles such as metal and chips from abrasive wheels can get into the bearing ring ways and be squeezed between the balls and ring way. This will cause roughness of the ring and ball, and will eventually cause failure. If the particles are the consistency of a very fine dust, they will act like a lapping compound and cause excellerated wear of the rings and ball.

Due to overload bearings: An overloaded bearing can cause premature spalling, breakdown of lubrication, excessive heat and failure of the bearing. Such an overload can be caused by too tight a fit either on the shaft or in the housing, or by axial thermal expansion of the shaft.

Due to misalignment: The misalignment of a bearing will impose overloads on the bearing at points 180° apart from each other and will cause excessive wear on the ball cage which will usually be the first part to fail. The misalignment bearings will run hot and noisy.