Cyndi Nyberg
Former EASA Technical Support Specialist

There are two main types of load that act on the bearings of a motor – radial and axial.

• Radial – A radial load is defined as a load that is applied perpendicular to the shaft. An example of a radial load would be an overhung load, such as with a sheave.
• Axial – An axial load, also referred to as thrust, is a load that acts parallel to the shaft on which the bearing is mounted. Just the rotor weight of a vertically mounted motor will cause a downward axial load on the bearing.

The type and magnitude of the load will determine what type of bearing should be used in the application. If the wrong type of bearing is used, it could lead to a motor failure in a short period of time.

This article will address many of the common types of bearings used in electric motors, the types of load for which they are designed, their applica- tions, and their enclosures.

Horizontal motors
As the ball bearing is loaded, the rolling elements contact the inner and outer races at a single point. That helps it to turn with relatively little friction, but with very little contact area holding that load. Too much radial or axial load will overload it. See Figure 1. The relative radial and thrust load capacities of several bearing types as compared to a deep- groove ball bearing are shown in Table 1.

Cylindrical roller bearings
In general, roller bearings can support heavier radial loads than ball bearings. In these bearings, the rolling elements are cylinders, so they contact the inner and outer races in a line. This distributes the load over a larger surface area, so the bearing can handle a higher load than a ball bearing.

However, this type of bearing is not designed to handle thrust loading. Cylindrical roller bearings are suited for belted applications, where the load is predominately radial.

Tapered roller bearings
Tapered roller bearings are designed for combined radial and axial loads. They have a high radial load capacity and a relatively high thrust (axial) capacity, usually about 60% of the radial load capacity.

Spherical roller bearings
Spherical roller bearings are typically used on larger machines, have high load capability and may be grease or oil lubricated. They are typically used where the nature of the load is such that alignment to the driven equipment changes, for example when the base is not rigid.

Thrust bearings
A thrust bearing is designed to handle axial loads in one direction only and subsequently are used for purely axial loads.

Vertical motor bearings
Vertical motors need bearings that are designed to handle axial loads, also called thrust, sometimes in both directions. Depending on the magni- tude of the load, there are several types of vertical motor bearings used.

A majority of vertical motors are used on centrifugal pumps. In a pump, downthrust includes the weight of the rotor, pump shaft, impeller and the liquid being pumped. Upthrust occurs momentarily when the pump is first energized.

Bearings for thrust
The construction differences between horizontal and high-thrust vertical motors are primarily dictated by the differences between radial bearings and thrust bearings. Radial bearings are designed primarily to handle radial loads. The deep groove, ball bearing is a modification capable of handling moderate axial loading. The thrust bearing is designed to handle axial thrust load only in one direction and small radial loading in proportion to the axial load.

Angular contact ball bearings
These bearings are used in many vertical pump applications. The amount of thrust load that the bearing is capable of carrying is determined by the angle of contact between the balls and the cage. Often, these bearings are stacked for increased thrust capability, although stacking two in tandem does not double the load capacity.

The normal high-thrust bearing in vertical hollow shaft motors is the angular contact, ball bearing type. A typical type is shown in Figure 2. This bearing was developed specifically for pump service, having a high-contact angle of up to 40 degrees. With such high-contact angles, the bearing must have a considerable thrust applied to maintain proper operation. A motor running no load will often sound noisy because the thrust bearing balls skid under this condition.

The deeper the pumping depth, the more thrust load capability is required for the bearing. An easy way to obtainadditional thrust capacity is to stack two or more bearings in tandem. By using two or more thrust bearings in tandem, the rolling elements in each will be smaller than using a single bearing; the larger the rolling ele- ments become, the less efficient they are in carrying the load.

Spherical roller bearing for vertical motors
At even higher thrust load require- ment, when ball bearing size becomes excessively large, the spherical roller type is used. A typical construction is shown in Figure 3. Again, the rollers will have a much larger contact area than balls and are arranged to be self- aligning. Roller axis is at 45 degrees to the shaft and the bearing can carry both moderate- and high-thrust loads.

The rollers must be guided, however, and therefore develop more friction than the ball type. At high speeds, water cooling is required.

Lubrication is also more critical, andthe bearing must also be preloaded or it may separate due to hydrodynamic forces if no thrust is present. A mini- mum down thrust, based on bearing size, is required during operation.

Enclosures
Ball, roller and thrust bearings can have several different enclosures. The bearing may be open, where there is no enclosure for the balls or rollers, and the cage. A shielded bearing can have single or double shields, meaning that there can be a shield on the inboard or outboard side, or both. The purpose of the shield is to keep contaminants out of the bearing and retain grease inside the bearing. A sealed bearing provides an even better barrier than a shielded bearing in applications where contamination is an issue. The type of enclosure is not typically changed unless dictated by the end user or the application.

Sliding plate bearings
For still greater thrust loads, on larger machines, the sliding plate (e.g., Kingsbury) type is available, shown in Figure 4. These are used when the required thrust or size exceeds roller bearing capability. Basically, this bearing has two sliding plates separated by an oil film. This is a very old bearing type; it is very expensive, has high losses and requires water cooling. It is not good for frequent starting under thrust loads. At higher speeds, it has leakage problems.

The life of this bearing is theoreti- cally infinite. However, in real life operating conditions, they require extensive maintenance, particularly at higher speeds. Sliding plate bearings are often found in applica- tions like low speed hydrogenerators, where they are started once a season, then shut down during the winter, allowing for maintenance.

Sleeve bearings
Sleeve bearings (Figure 5) have been used in almost all sizes of electric motors since motors were invented. Although most motors now have ball bearings for economic reasons, sleeve bearings are still used in fractional horsepower motors, as well as in large motors where the desired bearing life cannot be achieved with rolling-element bearings. The limiting factors in larger motors are the diameter of the bearing and the speed of its rolling element.

Sleeve bearings do not work well with radial loads or belted applications.

The principle of the sleeve bearing is simple, although the construction, assembly and repair are not. Sleeve bearings are oil lubricated, whereas ball and roller bearings can be either grease or oil lubricated.

Figure 5 shows a typical sleeve bearing. The oil rings located in the ring slots bring oil to the top of the bearing.

Oil is then deposited into the oil distribution groove. The oil rings rotate and deposit the oil to maintain a film between the babbit and the motor shaft. This film is only a few thousandths of an inch thick. There is always oil available in the distribution groove as the shaft is turning. The oil exits through the drain groove and back into the reservoir where it is circulated.

Categories: Bearings, Design/theory/application

Tags: Bearings

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