The lubrication schedule and the type of lubricant are determined by the type of load and the ambient conditions. A belted load needs lubrication more often than a direct-coupled load. Motors that operate under extreme conditions (hot, cold, wet, frequent starts, vibration, etc.) need to be greased more often.
Counter to many grease manufacturers’ claims, there is no one grease that fits all conditions. Nor is there a specific lubrication schedule. However, a motor can’t be lubricated too often (provided the lubrication is done correctly). Grease scheduling should be frequent enough that the old grease doesn’t become caked or hardened. (Forcing hardened grease through the bearing while the motor is running can damage a bearing.)
Motor manufacturers estimate bearing life at up to 100,000 hours if the motor is direct-coupled and around 50,000 hours if it has a belted load. This estimate assumes that a good grease schedule is followed (with the right grease) and that the motor operates in ideal conditions. The speed of the motor, balance (of both motor and load components), and ambient conditions must be considered for individual bearing life expectancy. (Ideal operating conditions are rare.)
Grease Types
Grease is made of various materials that will hold oil. Oil is released from the grease over a period of time. When all the oil has been released, the consistency of the remaining material can range from firm to hard.
Grease types are designed for various conditions. It’s important to select a grease type that fits the condition. A selection of grease types follows:
• Petroleum oil grease is the most common (with a temperature range of 30 0 to 300 O F).
• Diester oil grease is used for cold conditions as low as —100 O F.
• Silicon grease is not recommended for heavy loading but has a wide temperature range (—100 to 300 F). Silicon grease of any type shouldn’t be used in a DC motor. Fumes from silicon break down the brush material next to the commutator, resulting in excessive brush dusting.
• Fluorosilicone grease works well where it might become diluted with solvents or chemical contaminants. (It also shouldn’t be used in a DC motor.)
• Perfluorinated polyether greases stand up under temperatures as high as 550 0 F and have good load-carrying capability.
Mixing different types of grease can cause bearing failure. If they’re not
Caution compatible, the greases will liquefy or will combine and become thick. In either case, the bearing lubrication won’t be effective. Figure 7.19 shows compatible and noncompatible greases.
Changing Grease Types
The right way to change incompatible grease types is to remove the bearing and wash it thoroughly with solvent, before filling it with new grease.
However, in most cases, this procedure takes too much time.
The method that follows, under “Lubrication Procedure, ” is recommended, with the following modifications. Flush the old grease for a longer time than described (to remove as much old grease as possible). Grease the motor using this procedure at least three times (at no more than one-week intervals).
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Aluminum complex x c c |
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Barium x c |
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Calcium x c c c B c |
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Calcium 12-hydroxy c c c x B c c c c |
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Calcium complex B x c c |
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Clay c c |
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Lithium c c x c c |
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Lithium 12-hydroxy B c c x c |
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Lithium complex c C c C c c x |
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Polyurea c x |
FIGURE 7.19 Compatibility grease chart.
Lubrication Procedure
High-pressure greasing equipment should not be used on motors. Nor should grease be forced into the bearing at a fast rate with a hand-operated grease gun. That would force grease through the bearing seal and onto the winding of the motor.
Grease won’t break down the motor’s insulation. However, it does impede the dissipation of heat from the winding. If grease gets into the air gap between the rotor and stator, the motor becomes excessively loaded.
If the motor has grease fittings, called zirks, a purge plug should be at the bottom of the bearing enclosure. Remove the purge plug and clean the grease zirk thoroughly. Run the motor, and pump the grease through the bearing slowly, until about half an ounce of new grease comes out of the purge hole.
The motor should run at least 2 hours before the purge plug is replaced.
(This allows the bearing to expel excess grease.)
A bearing should be filled when hand-packing it with grease. But the motor’s bearing housing should be filled to only about one-third. This gives the bearing room to expel excess grease. If it can’t expel excess grease, the grease will churn, causing it to release its oil too soon. (All grease types are designed to release oil over a long period of time.)
Shielded (or sealed) bearings should be used if there is no purge plug. Replace the grease zirk with a plug, and tag the motor to prevent installation of another grease zirk.
Alternate Lubrication Methods
If the motor is in a remote location or needs lubricant at frequent intervals, self-actuated grease cups can be installed. They’re available in batteryoperated and gas-operated types and are designed to apply lubricant daily, weekly, bimonthly, or monthly. Extreme temperatures may affect the timing of the gas-operated types.
Oil mist is a method of lubrication that works well on high-speed machines. The oil is atomized, and is delivered with a mixture of air to the bearing under pressure. With the bearing enclosure pressurized, moisture and other contaminants are kept out. Normally the oil mist is applied for a short time before starting (to make sure the bearings aren’t dry).
Oil injection is another effective way of lubricating bearings. A measured amount of oil under pressure is squirted directly into the bearing.
Gearbox Lubrication
Gearboxes have an oil level that should be checked at regular intervals. Oil change should be done once a year under normal conditions. If the operating temperature goes above 200 0 F, oil change every 3 months is recommended. An infrared gun can determine the running temperature of the gearbox.
High humidity and wide temperature swings cause condensation inside the gearbox. The oil will become a whitish color when extreme condensation is a problem.
Particle Analysis
A regular program of particle analysis is recommended for gearboxes, which are vital to the operation of an industry. Wear particles from each component can be identified as well as any outside contaminants. Once a record of wear
has been established, any abnormal increase is easily detected. Repair can then be done before a catastrophic failure occurs.
Particle analysis is also done with grease. This service can be used to solve frequent bearing failure problems.
Particle analysis is described in detail in Chapter 8.
Shielded Bearings
Single-row bearings with shield(s) or seal(s) are used in most standard-duty electric motors. (Shields are used more often than seals.) A shield retains grease well, but does not keep out all contaminants. Inactive grease (next to the shield) keeps out some (but not all) types of contaminants.
Some motors have bearings with only one shield (facing the rotor) for retaining the grease. If grease is forced into this bearing too fast, it will go through the shield and onto the motor’s winding.
Sealed Bearings
Sealed bearings have flexible seals that rub the inner race. The seal has a small amount of drag that decreases the motor’s efficiency slightly. If a motor has to operate where sealed bearings are needed, efficiency can’t be a factor.
Seals keep out fine dust and contaminants of that nature. They are available in single, double, and triple seals.
If it isn’t possible to keep a timely lubrication schedule, the use of shielded or sealed bearings is recommended.
Labyrinth Seal
The labyrinth seal is a noncontact seal that keeps contaminants out of bearings very effectively (Fig. 7.20). High-speed applications need a noncontact type of seal.
Contaminants do several direction changes after entering the seal. When they get to a cavity between the rotor and stator, they are expelled through a purge hole. Any contaminant that isn’t expelled is held by centrifugal force until the shaft stops. At this point, an O-ring, held by centrifugal force, pulls together and seals off the path where the contaminant could have entered the bearing. (The movable O-ring also seals in any bearing lubricant that would flow out.) At rest, the motor is hermetically sealed.
FIGURE 7.20 A noncontact seal that keeps most contaminants out while keeping the grease in. Inpro/Seal Co.
Labyrinth Seal Construction
The labyrinth seal has two major parts: a rotor (Fig. 7.21 a) and a stator (Fig.
7.21b). Both parts have O-rings that isolate the motor bearing from outside contaminants.
The motor housing must be machined to accept the stator. The stator has an O-ring and a press fit. The O-ring and the tight pressed fit combine to seal the stator.
The rotor has two O-rings as well as a pressed fit. One O-ring seals the shaft from outside contaminants and is stationary. The other O-ring is movable and contacts both the stator and the rotor when the motor is at rest, hermetically sealing it. When the motor runs, this O-ring is pulled away from the stator by centrifugal force. It is held away from the stator by centrifugal force in a space machined into the rotor. There is no contact between the stator and the rotor shaft while the motor runs. Consequently, there’s no wear or efficiency loss.