Sleeve Bearing

Coupling Sleeve Bearing Motors.

In order to prevent thrust from being transmitted to the bearings, a limited end float coupling should be used. The limited end float coupling is designed to have less end play than the total end play of the motor. Thrust that tends to separate the coupling is restrained by lips or coupling shoulders. Thrust that tends to push the coupling together is restrained by buttons on the shaft ends or by Micarta discs in the gap between shaft ends. The difference between the motor end play and the limited end float of the coupling represents the allowable tolerance in setting the motor. A motor built with ½ in. end play requires that the end float be limited to ³₁₆ in. Medium size motors having ¼ in. nominal end play require that free floating type couplings have their end play limited to ³₃₂ in. Small motors having ¼ in. end play or less seldom require limited end float couplings.

When installing the coupling, sufficient clearance, as specified by the coupling manufacturer, must be provided between shaft ends on the hubs of the flexible coupling. This is to allow for adequate thermal expansion of the shafts.

10.2.2 Lubrication principles (hydrostatic and hydrodynamic)

The differences between sleeve and antifriction bearings are also most apparent when considering lubrication. When load and relative sliding velocity are low, lubrication requirements may be minimal and indeed unnecessary. The only problem is to dissipate the heat generated, there being no circulated lubricant to aid the process.

Where loads are substantial, oil, water or even gas may be forced between the surfaces at sufficient pressure to balance the external load, and to separate them. This is known as “hydrostatic” lubrication.

When the closely conforming surfaces of a lower pair are slightly modified to produce a wedge-shaped gap filled with lubricant and when the surfaces are rotated, a pumping action will be generated within the bearing. This is called “hydrodynamic” lubrication.

15.1.1.1 Sleeve journal bearings

The basic shape of sleeve bearings is the cylindrical bearing, in which the cross-section of the bearing surface is a circle. Actually, this kind of bearing is not of use in steam turbines, since it is prone to causing instability, in particular oil-whip [5], and is has been replaced for a long time by other designs, i.e., by multilobe bearings or by TPJBs.

44.10.2 Bearings: sleeve (Babbitt)

End play or float

Practically all machines with journal or sleeve bearings have some end play or float. It is considered manageable if sufficient pressure can be applied to the end of the shaft during rotation to keep it firmly seated against the thrust bearing or plate. However, for large machinery or machinery that must be energized and ‘bumped’ to obtain the desired rotation, application of pressure on the shaft is often difficult and/or dangerous. In these cases, float makes it impossible to obtain accurate face readings; therefore, the indicator reverse method must be used as float has a negligible affect.

Bearing Cooling Is Not Usually Needed

Cooling water can be deleted from many sleeve bearings on centrifugal pumps and on small turbine drivers after experimentally verifying that oil sump temperatures do not exceed an operating limit of 180°F (82°C). This limit was found to be extremely conservative from a bearing-life point of view. If it is exceeded by a few degrees, more frequent oil sampling or oil replacement may be appropriate. A good synthetic lubricant may be ideally suited in this event and is easily cost-justified.

Since most general-purpose machinery is equipped with anti-friction bearings, attention is primarily directed to the significant maintenance credits which can result from eliminating cooling water from anti-friction bearings on pumps and small steam turbines. Experience shows that equipment life can actually be extended by removing cooling water from bearings. Cooling of bearing oil sumps invites moisture condensation, and bearings will fail much more readily if the oil is thus contaminated by water. Laboratory tests show that even trace amounts of water in the lube oil are highly detrimental. Hydrogen embrittlement on the steel granular structure can reduce the expected bearing life to less than one fifth of normal or rated values. Another reason for not cooling the bearing housings of pumps and drivers is to maintain proper bearing internal clearances. Hot-service pump bearings have often failed immediately after startup when the bearing housings were cooled by water. When it was recognized that high temperature gradients were responsible for reducing bearing clearances to unacceptably low values, a heating medium was introduced into the bearing bracket to heat the housing: The problem was solved.

6.2 Relative shaft displacements in the sleeve bearing housings

First, the relative shaft displacements in the sleeve bearings between the shaft journal and the bearing housings are calculated (node V to node B in Figure 4).

Figure 6 b) shows the calculated relative shaft displacements A_(p-p) and B_(p-p) for a horizontal magnetic force (φ_F = 90°). Again two maximum peaks occur, but the vibration behavior changes completely. The two highest peaks now occur at a rotor speed of 1710 rpm (3) with a value of 18.4 μm (peak to peak) and at a rotor speed of 2150 rpm (4) with a value of 16.8 μm (peak to peak). At both speeds the orbits of the rotor centre node W and of the stator centre node S are elliptical. Rotor and stator vibrate almost out of phase with one another and a significant bending of the rotor structure is again obvious. However, the major axes of the orbits of the rotor centre node W and of the stator centre node S have changed their direction, compared to Figure 6 a). They now lie very close to the horizontal direction. A direct comparison to the critical speed map in Figure 5 is here difficult, because the natural frequencies at intersection point III and IV are close together and a relative high modal damping is obvious (Figure 5).

Bearings

Motors use two types of bearings. These are ball bearings and sleeve bearings, (sometimes called a bushing). Sleeve bearings are normally, but not always, used on small fractional-horsepower motors up to one-half horsepower. Ball bearings are used on larger motors, but they may also be used on smaller motors.

Where there is end thrust on the motor, ball bearings are recommended. End thrust is present any time the motor and load are connected together with a belt, chain, or gears. End thrust causes abnormal wear on sleeve bearings, resulting in the interior circle becoming egg-shaped.

Sleeve bearings are less expensive than ball bearings, and they have a lower noise output. They can be used more effectively when the mechanical load is connected directly to the shaft. This includes such applications as fan, blower, and pump operations.

Ball bearings are of three basic types. They are the open, shielded, and sealed bearings. The environment in which they are going to operate determines which type is used. Sealed bearings can be used in all cases, whereas open bearings could not be utilized in areas where the bearing is exposed to dirt and corrosive materials. Under these conditions, shield or sealed bearings would be needed.

When the motor comes from the manufacturer, the bearings are packed in grease. Because of the relative high temperatures involved, the grease is usually rated at 150°C or above. Special grease is used by some manufacturers, and substitutes cannot be used without completely cleaning out the grease already in the housing. The different types of grease are not always compatible and will not mix well together. In some cases they will form a gummy mixture which will completely destroy the bearings. Manufacturer's specifications should be carefully followed when replacing the grease.

This problem is overcome if sealed bearings are used. These bearings come prelubricated and need no maintenance. The sealed bearing is designed for the life of the machine. They do fail, however, and should be checked during regular maintenance procedures. Sealed bearings are more expensive than other types.

The type of bearings used in motors should be selected carefully based on how the motor is going to be used. Consideration is needed in terms of the motor speed and if continuous or intermittent operation is proposed for the motor. How the motor is to be mounted is important. Horizontal, vertical, or some angular displacements all have different requirements in terms of the stress on the bearings.

These bearings have an extended or heavy shoulder on one end of the outer ring. A counterbored shoulder is used on the other end. With the application of heat, the counterbored shoulder allows more bearings to be inserted into the race than is possible with the standard radial deep-groove bearing. The extended shoulder allows angular contact between the shaft and bearing during times of high thrust.

Angular-contact bearings may be used singularly or may be stacked as shown in Figure 6-14. The heavy or extended shoulder on one end, and the counterbored shoulder on the other, provide for a high thrust in only one direction. The bearings may be mounted opposite to each other in a stack to provide protection in both directions.

When replacing bearings on one of these motors, the electrician should note the particular arrangement. It would be easy to make an improper application.

Angular-contact bearings used singularly will handle thrust loads of 8000 pounds at 1760 RPM. Used in tandem or stacked, thrusts of approximately 15,000 pounds can be applied without failure of the bearings.

For loads greater than 15,000 pounds, spherical-roller thrust bearings are used to carry the down-thrust. The lower guide bearing will carry the upthrust. Radial stability is insured by keeping the bearings in contact with the shaft under these conditions by a set of springs which push up against the lower race of the thrust bearings. Because spring pressure will be several thousand pounds, depending on design, considerable pressure is imposed on the guide bearing if the external down-pressure is not sufficient to overcome the spring pressure. It is important that consideration be given to the minimum pump total down-thrust requirement to insure that the springs are not loaded under normal operating conditions.

Spherical-roller thrust bearings are often water cooled. Cooling coils with water flowing through them are installed in the oil reservoir.

Special conditions can also influence the type of bearing to be specified for a motor. There may be end play limitations imposed by the pump seal or the possibility that the motor may be off-line for considerable periods of time. Other possibilities are that on-off cycles may be frequent, or that the motor must be reversed for each stop. There are many factors pertinent to any given installation that could influence the type of bearing. These special conditions should be discussed with the supplier.

Specifications requiring the motor manufacturer to meet unreasonable thrust requirements and extended life for the bearings should be avoided if at all possible. These specifications can be met with larger bearings and special cooling features. However, larger bearings cause a loss in efficiency, and oil foaming, vaporization, and leakage become more difficult to prevent.

Bearings eventually fail due to metal fatigue, even when operated under normal conditions. When specifying bearing life expectancy, the hours of operation and conditions under which the motor is going to perform must be considered. The lower cost of smaller bearings, and the necessity for replacing them more frequently, need to be balanced against the increased cost for larger bearings and their inherent inefficiency losses.

The minimum life expectancy of bearings is statistically predictable based on the operating conditions.

A B-10 rating for vertical motors means that under continuous operation for 1 year (8760 hours) at rated speed, only 10% of the bearings are expected to fail. For in-line published thrusts, this rating is good for 2 years.

When rated average life is given, 50% of bearings will provide satisfactory service under the conditions specified. This value is usually about five times the minimum life of the bearing.

Ball bearing failures vary inversely with the third power of the imposed load. If the average life of a bearing at a given load is 3 years, then under half load, it will last eight times longer, or 24 years. It is not reasonable to assume this long a life for any bearing with maintenance procedures being what they are in most cases. A 15- to 20-year life expectancy would probably be closer to the truth provided routine maintenance is performed.

There is no guarantee that any particular bearing will achieve the life expectancy predicted. The manufacturer's warranty, usually 1 year, is the only assurance available.

Sleeve bearings, which are often used on fractional horsepower motors, are made from babbitt, brass, or bronze. Graphite may be impregnated into the metal to enhance the lubrication. Because the metal in these bearings is much softer than the steel shaft of the motor, the bearing will wear rather than the motor shaft.

Starting of Flooded Screw Compressors

Similar to oil-free compressors, oil-flooded compressors with sleeve bearings must start unloaded. In most cases, unloaded start can be accomplished by moving the slide valve to the minimum flow position. Opening the recycle valve can be helpful but is not always necessary. As with any hydrodynamic bearings, prelubrication is required to prevent bearing damage. This is accomplished with a separate oil pump driven normally by an electric motor and sometimes by a steam turbine. The flow of oil required for prelubrication is normally 15%–25% of the fully flow required. One option is to use a small prelube pump and have the main oil pump be shaft driven. Another option is to have a separate 100% oil pump that provides the oil for prelubrication as well as normal operation. To prevent filling the compressor body completely with oil, automated shut-off valves block the flow of oil to the main injection ports in the rotor chamber, and the oil only flows to the bearings and driveshaft seal. Since the pressure requirement for the oil is the same during prelube as during operation (normally 250–320 kPa higher than gas discharge pressure) but the required flow is much lower, the oil system must be designed to handle both prelube and normal operation.

Once the compressor has reached minimum speed, the prelube pump (if used) is turned off. The recycle valve can be closed, and the slide valve can move to increase the flow capacity of the machine. The suction and discharge check valves will open and gas is moved downstream.

Bearing Installation