MRO Magazine


5 symptoms that warn of motor and gearbox failures

Motors are electro-mechanical devices that turn electrical energy into mechanical energy. The motor magnets and windings are wound onto a shaft that’s simply supported by two or more rolling-element bearings at each end of the motor frame. These bearings are the only lubrication points on a motor and are virtually always grease-lubricated.

Failure Symptoms
For motors…
#1—Motor runs hot
#2—Motor winding is full of grease

For gearboxes…
#1—Gearbox has sludge in the bottom of reservoir
#2—Gearbox always runs hot
#3—Gearbox uses too much energy

Most fractional and small hp motors don’t make provisions for bearing lubrication, which is designed to utilize the original grease lubricant fill within the sealed bearing to provide lubrication throughout the life of the motor. Loads are light and the lubricant rarely needs replenishment if the motor is balanced and aligned correctly.

Depending on the motor design and manufacturer, external grease fittings are introduced for motors rated in the 50-hp and above category. When motors become more powerful and heavier, they place more load on the bearing points. This requires grease replenishment on a more frequent basis.

If a motor is to operate at peak efficiency, its bearing cavities (the available space between the balls, raceways, cage and seals) should only be filled to 30 to 50 percent capacity, at any time. Motors that are designed to be greased will have a drain plug to allow excessive grease to flow through the bearing and out of the motor.

If this is kept closed during the greasing process, excessive grease will channel directly into the motor windings. Unfortunately, motors with grease points, suffer a disproportionate amount of premature failures caused by over-lubrication. The most common lubrication-related symptoms are:

Symptom #1—"Motor runs hot"
• Cause/remedy: Over-lubrication is the biggest sin with many maintainers believing a little lubrication is good and a lot of lubrication is a lot better! Ensure that maintenance professionals are fully trained to correctly lubricate a motor; all grease guns must have identical output when instructing to lubricate by "number of shots." Remove all grease nipples from fractional and lower hp motors.

Symptom #2—"Motor winding is full of grease"
• Cause/remedy:  (see symptom #1).

Gearbox lubrication
Essentially, gearboxes are simple self-contained mechanical devices that allow power to be transmitted from an input shaft to an output shaft at different speeds through the meshing of different-sized gear sets held on each shaft. The gears and shafts are supported on bearings contained within a sealed "box" that also serves as a reservoir for the lubricating oil. Gearbox dimensions can range from palm-size to room-size, and with few exceptions, are oil-lubricated.

Depending on the style and size, gearboxes use a number of methods to move the lubricant over the gears and bearings, with the most popular being:

• Splash-lubrication. This is the most common gearbox lubrication method in use. Here the reservoir is filled part way with lubricating oil to ensure partial coverage of all the lower mating gears. At speed, these gears use surface tension on their teeth to "pick up" lubricant and transfer to other gears and bearings through meshing and by "flinging and splashing" the lubricant in all directions within the sealed reservoir.

• Pressure-lubricated. This method is commonly found on mid to large-size gearbox assemblies. It uses a gear-driven pump (usually located inside the gearbox) to work in conjunction with the "splash" method, which picks up lubricant through a pick-up filter screen and pumps oil at pressure through an internal piped-delivery system to bearings and gears that would be difficult to service with traditional splash lubrication.

• Mist or atomized lubrication. This method is usually reserved for the largest of gearboxes, using a vane-style pump that picks up lubricant from the reservoir and "flings" the lubricant at a plate, causing it to atomize into a fog, which in turn, saturates all the mechanical components within the sealed gearbox.

Three of the most commonly found symptoms, or complaints, found with all gearbox types are:

Symptom #1—
"Gearbox has sludge in the bottom of reservoir"

• Cause/remedy: The mixing of different specification-lubricating oils during oil "top-ups", or oil changes. Attach a label to the reservoir clearly indicating the correct lubricant manufacturer, product name and viscosity to be used—without exception! Also, clearly indicate the same information on the preventive maintenance (PM) work order.

• Cause/remedy: Water reacting with the lubricant’s detergents can create soap sludge. If the reservoir is regularly cleaned with water, ensure the fill cap and breathers (if applicable) are waterproof and in place at all times, or position a water-deflection shield over the reservoir.

• Cause/remedy: The lubricant has oxidized that causes the viscosity to increase (oil gets thicker) through the mixing of products, excessive heat, introduced contamination, or extended use. Always use the correct lubricant for application-related ambient temperatures. When changing your lubricant, ensure the cap and breathers are always re-installed, and that the lubricant is transferred using dedicated clean equipment. Use oil analysis to determine when the oil is starting to "break down" its additive package and when a change is needed.

Symptom #2—
"Gearbox always runs hot"

• Cause/remedy: Excessive dirt covering a gearbox will create a thermal blanket, raising the lubricant’s temperature and rendering it less effective. Make gearbox cleanliness an essential part of your PM program.

• Cause/remedy: Incorrect lubricant viscosity through the mixing of lubricants or incorrect lubricant choice. Too light of an oil will result in boundary layer friction, and too heavy of a lubricant will result in fluid friction, both raising lubricant temperatures. Consult the manual specification or with a lubrication expert to ensure the correct lubricant viscosity choice. Also, consider the use of a synthetic lubricant, which will run cooler than a mineral oil.

• Cause/remedy: The reservoir isn’t filled to the correct level; too little lubricant will result in boundary layer friction, and too much lubricant will result in fluid friction. Ensure the reservoir employs a visible sight gauge that’s clearly marked with the upper and lower reservoir limit lines—much like a dipstick.

• Cause/remedy: Sludge in the gearbox (see symptom #1 cause/remedies).

Symptom #3—
"Gearbox uses too much energy"

• Cause/remedy: All the symptoms found above (symptoms #1 and #2) will result in an energy increase. The use of synthetic lubricants in gearboxes has shown to reduce energy consumption by as much as four percent.

Ken Bannister of Engtech Industries Inc. is a production and maintenance management specialist. This article was previous published in 2008.

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