Among other factors, motor and gearbox lubrication programs require understanding and a controlled lubrication approach.
By Ken Bannister, MEch Eng (UK)CMRP, MLE, Contributing Editor
When a driven component is required to operate at a speed different than that of the attached motor (driver), a designer can choose from two basic power-takeoff speed-reduction/increaser methods. The first uses pulleys or sprockets of different diameters mounted to the motor and driven shaft, with power transmitted by a connective belt or chain. The second design connects the motor to the driven component through a gearbox, with the motor connected to the gearbox input shaft and the driven device connected to its output shaft.
When viewed in a maintenance-management-system database for lubrication purposes, belt/chain-drive motors and motor/gearbox units are rarely handled with separate PM work orders. Rather, the lubrication requirements are integrated as line items on a much broader machine PM work order. This is fine for sub-fractional and smaller horsepower motors. Larger, more expensive (and re-buildable), motors—usually 20 hp and more (there is no set rule to this)—require treatment as a separate entity from the parent machine, with their own asset numbers and PM/lubrication regimes, so as to compile work-history files. Furthermore, in the case of motor/gearbox combinations there are two specific entities, one electro-mechanical (motor), the other purely mechanical (gearbox), that are best treated individually when assessing and managing lubrication needs.
Assuring motor and gearbox reliability is the result of good alignment practices and, more importantly, effective lubrication practices.
Bannister on Lubrication
Accompanying this article is the first of a new series of monthly lubrication podcasts with Ken Bannister. This month, he provides additional information about factors involved in lubricating motors and gearboxes.
Motors are electro-mechanical devices that turn electrical energy into mechanical energy. Motor magnets and windings are wound on and around a central shaft. This shaft is simply supported by two or more rolling-element bearings at each end of the motor frame and housing. These bearings are the only lubrication points on a motor, and are virtually always grease lubricated. With rare exception, fractional- and small-horsepower motors use sealed bearings and make no provision for external bearing lubrication. If the motor is balanced, aligned, and not overloaded, it should deliver a long life with no additional lubrication. This is not usually the case with larger motors, which are often subjected to heavier and often more variable loads, requiring larger bearings.
Depending on the motor design and manufacturer, external grease fittings usually are installed on motors rated at 5 hp and become much more prevalent on 20-hp units. When motors become more powerful and heavier, they place more load on the bearing points, therefore requiring 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) need only be filled to 30% to 50% capacity, at any time. Because the bearings are hidden behind end plates, they are lubricated “blind” and are often subject to overfilling—especially with manual greasing. When this happens, the grease has nowhere to go except through the bearing cavity into the winding! Grease-filled windings lead to premature failure and a rapid decrease in motor energy efficiency, evident by the rise in motor’s amperage draw.
To alleviate this condition, larger motors are designed with a drain-plug or screw in the end cases that, once opened, will allow excess grease to flow through the bearing and out of the motor end case. If this is kept closed during the greasing process, excess grease will channel directly into the motor windings. If your motor has a grease fitting but no drain plug, use extreme caution not to over-lubricate, as the excess will make its way into the winding.
Over-lubricated bearings will produce excess heat through internal fluid friction that can easily be detected with an infrared camera. This can also be achieved by adding contaminated grease with a dirty grease nozzle or through cross contamination with a non-compatible grease.
Grease-gun inconsistency can be ironed out through use of a single-point auto lube (SPL) setup to deliver a small amount of lube on a continuous basis for as long as a year, depending on the size of bearing and lube reservoir.
SPL manufacturers have setup guidelines based on bearing size and altitude (atmospheric pressure is relational to constant-pressure grease flow) for initial setup, which can then be fine-tuned by monitoring amperage draw and/or bearing temperature. These signatures will be unique to each motor and will differ based on size and load.
Gearboxes are 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-sized to room-sized. With few exceptions, all are oil lubricated.
Depending on the style and size, gearboxes employ a number of methods to move the lubricant over the gears and bearings, the most popular being:
• Splash lubrication. This is a common gearbox-lubrication method in which 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 lubrication. This method is frequently found on mid- to large-sized gearbox assemblies that use a gear-driven pump, typically located inside the gearbox, to work in conjunction with the “splash” method. Pressure-lubrication systems draw lubricant from the reservoir through a pickup-filter screen and pump oil at pressure through an internal piping system to bearings and gears that would be difficult to service with splash lubrication.
• Mist, or atomized, lubrication. This approach, reserved for the largest of gearboxes, uses a vane-style pump that picks up lubricant from the reservoir and “slings” it at a plate, causing it to atomize into a micro-drop mist. The mist saturates all of the mechanical components within the sealed gearbox.
In all three lubrication methods, choosing the correct oil viscosity and additive package is most important. Typical to all gearboxes is the need to ensure:
• No cross-contamination of lubricants occurs during oil top-ups or change-outs. Label your gearbox with the correct oil specification.
• No dirt or water contamination is allowed into the gearbox.
• The drain, fill, and breather caps are always tightly in place.
• The gearbox is regularly wiped clean of dirt and debris that will act as a thermal blanket and unnecessarily heat up the oil.
• The gearbox is not over-filled creating churning (foaming) of the oil that can rapidly deplete the anti-foam additive, causing the oil to oxidize. This requires attaching low- and high-level markers to the gearbox sight gage.
If you have all of the above practices in check, make enquiries regarding the use of synthetic gear oils. These not only last longer but can cut your energy consumption as much as 4%. MT
Ken Bannister is co-author, with Heinz Bloch, of the recently released book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). As managing partner and principal consultant for EngTech Industries Inc. (Innerkip, Ontario), he specializes in the implementation of lubrication-effectiveness reviews to ISO 55001 standards, asset-management systems, and training. Contact him directly at firstname.lastname@example.org, or telephone 519-469-9173.