Chuck Yung 
EASA Technical Support Specialist 

Have you ever repaired a sleeve bearing motor, only to have the customer complain that it leaks oil? Perhaps the motor had a history of oil leaks, and the windings were oil-saturated when you dismantled it. Two-pole machines are especially notorious as chronic oil leakers. The first step toward correct­ing an oil leak is to identify the cause. 

A good place to start is to determine whether the motor has a forced-oil system. If so, check for a metering plate in the oil supply line. The typical metering plate (see figure) has about a 3/32 diameter orifice to meter the volume of oil. Often installed in a pipe union, the metering plate is easily lost when the motor is removed from service. The repairer rarely gets the forced-oil system with the motor.  The customer does not recognize that little piece of metal that looks like a conduit knockout, and that tiny hole can t possi­bly be for oil flow. So it gets thrown away. 

Oil Flow 
In a closed system, oil flow is determined by two things: pressure, and the size of the smallest opening through which the oil passes. Comparing the 3/32 orifice to the 3/4 pipe, it is easy to see why the motor develops a leak. Here is the usual scenario: 

To remove the motor from service, the plumbing is disconnected at a convenient place the pipe union. The metering plate is lost. When the motor is re­installed, maintenance personnel notice the low oil pressure, and increase the pressure to the correct level. But now that same amount of pressure is pushing oil through a 3/4 pipe instead of through the 3/32 orifice. The higher volume of oil has to go somewhere, usually into the motor enclosure. 

If the motor interior is pressurized or under a vacuum, air is drawn through the labyrinth seal. (A result of the Venturi effect that causes an airplane wing to create lift, or a siphon gun to work.) Since the labyrinth seal is attached to the bearing chamber, the vacuum in the labyrinth seal in turn siphons air through the bearing chamber, carrying with it oil droplets. That Venturi effect is responsible for most of the 2-pole oil leaks our industry sees. The 2­pole machine moves so much air that this has been an on-going problem. Manufacturers have used numerous designs to mitigate the problem, but our industry repairs a lot of older motors. 

Many 2-pole machines have the inside labyrinth seal vented to the air baffle. The intent is to equalize the pressure between the oil chamber and the inside of the motor, thereby preventing airflow through the oil chamber.  The position of the fan relative to the air baffle is critical to this design, but a 2-pole motor has weak magnetic centering forces. (You may have noticed this when test running a 2-pole: The shaft can be displaced axially, much more easily than a low speed motor.) 

When the fan position relative to the air baffle is changed, there are corresponding changes in the differential pressure between the labyrinth seal and the interior of the motor.  As the differential pressure changes, the Venturi effect draws air through the labyrinth seal. The solution is to bypass the bearing chamber. 

Blocked Openings 
How is this accomplished? The inside labyrinth seal should be vented to the exterior of the motor (see figure) instead of to the air baffle.  Many designs incorporate a vent opening cast into the end bracket, usually in the bottom half. Mud-daubing insects may block these openings, or a well-meaning customer may plug the opening thinking it is the source of an oil leak. After all, he sees evidence of oil running out of that opening. 

By venting the inner labyrinth seal to atmo­sphere, air drawn through the labyrinth seal passes through the vent opening, rather than the bearing chamber.  The bottom line is that if the air doesnt pass through the bearing chamber it won t carry oil with it. That means no more oil leak. 

Note: A few repairers install lip seals on the inner labyrinth seal, trying to stop the nuisance oil leak. Tighter clearances did not fix the problem, so the lip seal is a desperate measure. A lip seal creates friction, and a 2-pole shaft has a high peripheral speed. The result is heat, and a brittle lip seal. Heat also transfers along the shaft to the babbitt bearing. Occasionally we see felt or similar material used to fill one of the labyrinth grooves, to form a better seal with the shaft to prevent leakage. These materials inevitably wear or harden, and lose their effectiveness. 

Categories: Bearings, Lubrication, Oil lubrication

Tags: Sleeve bearings Oil leak(s)

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