Troubleshooting Motor Bearings

Troubleshooting Motor Bearings

Alternating-current motors account for a high percentage of electrical repair work. A high proportion of failures are caused by faulty bearings. Sleeve and ball bearing failure can occur in both newer and older motors; but sealed motor bearings are much less prone to failure.

Types of Bearings

There are many types of motor bearings, with ball bearings being the most common. There are several different types of ball bearings used in motors:

- Open

- Single shielded

- Double shielded

- Sealed

- Double row and other special types

Open bearings, as the name implies, are open construction and must be installed in a sealed housing. These bearings are less apt to cause churning of grease, and for this reason are used mostly on large motors.

Single-shielded bearings have a shield on one side to exclude grease from the motor windings.

Double-shielded bearings have a shield on both sides of the bearing. This type of bearing is less susceptible to contamination, requires no maintenance, and does not require regressing. It is normally used on small- or medium-size motors.

Each bearing type has characteristics which make it the best choice for a certain application. Replacement should be made with the same type bearings. The following list of functions provide a basic understanding of bearing application, a guide to analysis of bearing troubles due to misapplication, and emphasize the importance of proper replacement.

Figure 7-1 shows several types of bearings used in electric motors. The following is a brief description of each:

Self-aligning ball bearings: Self-aligning ball bearings are used for radial loads and moderate thrust loads in either direction. This ball bearing, has 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—an important consideration in high-speed applications requiring extreme accuracy.

7-1 Various bearing types.

Single-row, deep-groove ball bearings: The single-row, deep-groove ball bearing will sustain, in addition to radial load, a substantial thrust load in either direction, even at very high speeds. This advantage results from the intimate contact existing between the balls and the deep, continuous groove in each ring. When using this type of bearing, careful alignment between the shaft and housing is essential. This bearing is also available with seals and shields, which exclude dirt and retain lubricant.

Angular-contact ball bearings: The angular-contact ball bearing supports a heavy thrust load in one direction, sometimes combined with a moderate radial load. A steep contact angle, assuring the highest thrust capacity and axial rigidity, is obtained by a high thrust-supporting shoulder on the inner ring and a similar high shoulder on the opposite side of the outer ring.

Double-row, deep-groove ball bearings: The double-row, deep-groove ball bearing has a lower axial displacement than the single-row design, substantial thrust capacity in either direction, and high radial capacity due to the two rows of balls.

Spherical-roller bearings: The spherical-roller bearing has maximum capacity, due to the number, size, and shape of the rollers, and the accuracy with which they are guided. Since the bearing is inherently self-aligning, angular misalignment between the shaft and housing has no detrimental effect, and the full capacity is always available for useful work.

Cylindrical-roller bearings: This type of bearing has high radial capacity and provides accurate guiding of the rollers, resulting in low friction that permits operation at high speed. The double-row type is particularly suitable for machine-tool spindles.

Ball-thrust bearings: The ball-thrust bearing is designed for thrust load in one direction only. The load line through the balls is parallel to the axis of the shaft, resulting in high-thrust capacity and minimum-axial deflection.

Spherical-roller thrust bearings: The spherical-roller thrust bearing is designed to carry heavy thrust loads, or combined loads, which are predominantly thrust. This bearing has a single row of rollers that roll on a spherical outer race with full self-alignment. The cage, centered by an inner ring sleeve, is constructed so that lubricant is pumped directly against the inner ring’s unusually high guide flange.

Tapered-roller bearings: Since the axes of its rollers and raceways form an angle with the shaft axis, the tapered-roller bearing is especially suitable for carrying radial and axial loads acting simultaneously. A bearing of this type usually must be adjusted toward another bearing capable of carrying thrust loads in the opposite direction. Tapered-roller bearings are separable; their cones (inner rings) with rollers and their cups (outer rings) are mounted separately.

The dos and don’ts for ball-bearing assembly, maintenance, inspection, and lubrication are shown in Figure 7-2.

7-2 Do’s and don’ts for ball-bearing assembly, maintenance, and lubrication.

Frequency of Lubrication

Frequency of motor lubrication depends not only on the type of bearing but also on the motor application. Small- and medium-size motors equipped with ball bearings (except sealed bearings) are greased every 3 to 6 years if the motor duty is normal. Severe applications (high temperature, wet or dirty locations, or corrosive atmospheres), may require more frequent lubrication.

Lubrication in sleeve bearings should be changed at least once a year. When the motor duty is severe or the oil appears dirty, it should be changed more frequently.

Lubrication Procedure

Cleanliness and using the proper lubricant are critically important when lubricating motors. Follow this procedure:

1. Wipe the bearing housing, grease gun, and fittings clean.

2. Take care to keep dirt out of the bearing when greasing.

3. Next, remove the relief plug from the bottom of the bearing housing. This prevents excessive pressure from building up inside the bearing housing during greasing.

4. Add grease, with the motor running if possible, until it begins to flow from the relief hole. Let the motor run 5 to 10 minutes to expel excess grease. Replace the relief plug and clean the bearing housing.

5. Avoid over-greasing. When too much grease is forced into a bearing, churning of the lubricant occurs, resulting in high temperature and eventual bearing failure.

6. On motors that don’t have relief holes, apply grease sparingly. If possible, disassemble the motor and repack the bearing housing with the proper amount of grease. During this procedure, always maintain strict cleanliness.

Testing Bearings

Two of the most effective tests are what might be called the “feel” test and the “sound” test. Perform the “feel” test while the motor is running; if the bearing housing feels overly hot to the touch, it is probably malfunctioning.

During the “sound” test, listen for foreign noises coming from the motor. Also, one end of a steel rod (about 3 ft long and 1.2 in. in diameter) may be placed on the bearing housing while the other end is held against the ear. The rod then acts as an amplifier, transmitting unusual sounds such as thumping or grinding, which indicate a failing bearing. Special listening devices, such as a transistorized stethoscope, can also be used for the purpose.

The troubleshooting chart in Figure 7-3 lists the most common problems with motor bearings.