Dale Hamil
Illinois Electric Works
In oil-lubricated machines, leaks are a frequent and difficult-to-diagnose and correct condition. Due to design issues, there may even be a few machines where oil leaks are simply part of the landscape and not correctable without major modifications.
Overfilling is probably the number one cause. This may result from adding oil while the machine is running, filling to the idle level, rather than the run level that is sometimes quite different. In forced-lubricated machines, incorrectly sized orifices or operation beyond recommended flows and pressures may result in too much oil being introduced into the oil sump and overcoming the ability of the drain piping. Likewise, if the drain is not sufficiently sloped or is obstructed in any way, the oil may not have free egress from the bearing sump. If the oil is too viscous, then it may not drain fast enough. In closed loop systems, back pressure from the oil reservoir may prevent the free flow of return oil and a P-trap in the drain line or atmospheric break may be needed.
If the oil sight glass is the external tube type, make sure the cap is vented. If it is not vented, then an air pocket may falsely indicate that more oil is needed.
Installations where the base and/or the sight glass pipe are not level may give a false indication of the oil level. Constant-level oilers should be avoided, if possible, as setting the correct level of oil is somewhat problematic. If the oil level drops when the machine operates, a constant-level oiler will add oil, thereby overfilling the reservoir.
Check/replace sight glasses
Sight glasses should be checked for tightness and replaced if any sign of leaking is noticed. Often the sight glasses can be stained and the oil level can be difficult to determine with certainty. Replace any stained or mismarked sight glasses. If any of the threaded pipes or joints have been jostled, baked or heated, the sealant may become hard and ineffective, so it’s best to reseal all threaded pipe joints with new sealant.
Venting is another issue. Labyrinth seals may only be designed to handle 2 or 3 inches of water column (500-750 pascals) of pressure differential, so any pressure inside the bearing sump that exceeds this value compared to either the inside of the motor or atmosphere will tend to cause oil to exit through the seals. Since increasing the velocity of air results in lower pressure (Bernoulli’s principle), the cooling air inside the motor may be sucking oil out of the reservoir with its nearly stationary air. Venting from the bearing to the fan shrouds provides a barrier in the form of a siphon break, but varying pressure can lead to an oil leak; venting to atmosphere is usually preferred.
In some open and vertical motors that are particularly susceptible to this phenomenon, the rodent screens can become partially clogged causing the internal air velocity to increase and exacerbate the pressure differential issue. Forced oil may tend to pressurize the bearing housing sufficiently to overcome the labyrinth seal capacity to retain oil. There is usually a vent passage, often cast into the housing, between the inner and outer labyrinth seals. Insects are prone to build nests obstructing the vent. Dirt or other debris may also clog the vent. The vent passages must remain clear and functional.
The horizontal joints must be flat, free from any burs or defects, and lightly coated with sealant during final assembly. Be careful to not obstruct the horizontal oil return groove, if there is one. The choice of sealant is seldom a make or break decision, but it should not be something that is oil soluble or exceptionally thick.
In vertical motors with enclosing tubes (standpipes), the joint should be tight and tube perfectly vertical with a bead of epoxy to seal the joint.
Proper sealing
Non-contact labyrinth seals should be the size, fit and finish according to the manufacturer’s instructions. If they’re not available, use the values in ANSI/EASA AR100 as a guide. A good rule of thumb is .003” (0.08 mm) over the bearing inside diameter. An experienced mechanic will position the seal during assembly to be slightly more open at the top than at the bottom. Bolted-on-seals may need a gasket with sealant to prevent oil from going around the seal. Generally speaking, seals with pointed lands tend to seal better than flat lands. Any drain holes in the seals should obviously be at the bottom and direct oil back to the sump, if possible.
Contact-type fiber seals such as those made by Renk or Zollern must be sealed with Curil-T sealant (avoid substitute sealants) in the manner (see Figure) described in the bearing instructions. The sealant is designed to be placed on the radial lands of the outside of the seal, not on the flat or axial surface. Follow the manufacturer’s instructions.
Oil sumps are usually of a cast iron construction. Occasional blow holes and voids are not to be discounted as a source of oil leaks, especially if the housing has ever been exposed to below-freezing temperatures with water in the sump, or has sustained major mechanical damage.
Oil should be 100% drained during transportation. Residual oil that might remain in the sump in sufficient quantity may leak during movement or transport, and may not be compatible with the oil used in service, affecting viscosity and oil lubricating characteristics.
Problems caused by oil foaming
Oil foaming can cause the oil to effervesce and go over the recommended oil level. Water in the oil can cause the oil to foam, so oil sumps and cavities should be baked and dried after cleaning. Avoid the use of soaps, detergents or caustics during the cleaning operation as very small amounts of cleaning agents can contaminate the oil. Hot water flush for cleaning is recommended. In vertical motors, any locknuts or lockwashers should have any burs or protrusions smoothed to prevent excessive aeration during operation. Also in vertical motors, oil is metered through a hole and then pumped through the bearing, and some bearing cage designs may restrict the free flow of oil and contribute to aeration in operation. Oil-lubricated thrust bearings should have a machined bronze retainer which is ball guided (not race guided).
Vibration should be kept within the levels recommended in the ANSI/EASA AR100 as shaft oscillations may cause a pumping action. Shaft seal surfaces should have a 60-micro-inch (1.5 micro-meter) finish. If the seals are machined during the repair operation, and the direction of rotation is known, have the machinist tool the shaft so that the cut spirals into the bearing and not out of the bearing.
If the motor is being forced to run off of an electrical center for any reason, the anti-migration grooves or internal oil slingers may be discharging oil into the labyrinth seals instead of directly into the bearing housing.
Hint: As final proof during no-load testing, place plastic coated Kraft paper (see Figure 2), plastic side down, under the motor on each end. Even a single drop of oil will be clearly visible on the paper that may be otherwise missed. Any oil drips should be investigated and fully resolved before shipping.
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