Until a few years ago, lifetime-sealed rolling element bearings were not used on larger motors because the sealed-in grease was soon depleted or rendered unusable after relatively few operating hours. Open bearings and shielded bearings were therefore preferred in larger than 2-horsepower motors. They could be re-greased and generally made economic sense.
However, grease lubrication of electric motors can be maintenance-intensive. Grease must be applied in ways that ensure that only 30-40% of the free space between rolling elements contains grease. Care has to be taken to satisfy this need and the grease path through bearings has to be understood and pre-defined. Over-pressuring the bearing grease cavity or reservoir must be avoided for a number of reasons, and a drain plug must be removed when new grease is introduced to allow spent grease to be expelled.
From the early 2000s it has become possible to capitalize on another option: perfluoropolyether (PFPE)-based “Krytox®” greases. These PFPE-based lubricants are non-flammable and offer extremely high tolerance to heat and chemicals. Grease life comparisons with traditional greases are highly favorable (Figure 1): PFPE-based lubricants will not oxidize or gum up like other greases, nor can they be displaced by low-pressure steam and water.
In addition to these desirable features, PFPE greases contain no volatile organic compounds (VOCs) or chlorinated compounds. (It also should be noted that PFPE is the liquid base oil in the generic PFPE mineral oil-free family of products. PFPE is the actual lubricant whereas PTFE is the thickener in this family of mineral oil-free electric motor bearing greases.)
PFPE greases: A mature life extension option
In particular, Krytox® greases should be explored in view of a highly favorable long-term experience at a major pulp and paper mill located in Western Canada. Prior to using sealed bearings filled with PFPE grease of the right viscosity, the company faced a challenge regarding their electric motors. With over 3,000 motors on site, it was becoming increasingly more complex to properly and economically service each motor with customary lube planning and work procedures.
Figure 1. PFPE-based lubricants will not oxidize or gum up like other greases, nor can they be displaced by low-pressure steam and water.
The motor bearings and housings at this mill included cross-flow and same-side grease reservoirs, and single-shielded as well as double-shielded bearings. Some of the single-shielded bearings were oriented with the shield away from the grease reservoir while others–more appropriately–had the single shield positioned adjacent to the grease reservoir. Grease relief ports were needed for the various re-greasable motor bearing housings, but these ports were not always ideally situated, nor were drain plugs in these ports consistently removed before and re-installed after replacing or replenishing grease.
Without well-understood or uniformly specified bearing and housing configurations, re-lubrication routines were likely to include occasional over-greasing or under-greasing. Grease contamination was an additional risk factor and the Canadian paper mill, at one time, decided that leaving the bearings alone was the most prudent course of action. Therefore, the pulp and paper mill had chosen to run their electric motors without a re-lubrication process. They knew that electric motors in their harsh paper mill environment usually lasted no longer than 4-6 years, with over 60% of the breakdowns attributed to bearing lubrication failure. When motors would unexpectedly fail, the mill would often have to repair them in-house. This, usually, required overtime pay and costly downtime.
These important factors and considerations led the mill to move towards using only lifetime grease-lubricated sealed bearings whenever an electric motor had to be serviced. However, it was understood that lifetime lubrication with mineral oil-based greases would still require relatively frequent pre-planned bearing change-outs, and the anticipated bearing life without re-lubrication would stress the mill’s maintenance budget.
Results and savings
It was at that point that the PFPE alternative became very attractive, and the data in Figure 1 pointed to the likely superiority of Krytox® PFPE greases over the alternatives. Seven years later, the mill had successfully converted bearings to PFPE-based greases in over 100 motors. They also stocked more than 150 additional PFPE-based grease-filled sealed bearings, and planned to swap these into motors as the old bearings failed.
From the start of the PFPE greasing program, the mill has not had to replace any electric motors using PFPE grease, nor have they had a single bearing failure in an electric motor using PFPE lubricant. On top of the reliability successes, the use of PFPE grease has helped provide a simple solution with less manpower. For this mill, it clearly proved the most cost-effective option and seven years of tracking showed the mill to have nearly doubled the life of their electric motors. The labor component of periodic re-lubrication no longer exists, and each avoided motor replacement event saves the mill at least $4,000. To this Canadian paper mill, any maintenance program requiring periodic re-greasing would have meant more manpower and added overhead costs.
It can be demonstrated that PFPE greases are an optimum choice for long-life bearing lubrication in severe applications. Using these greases in sealed life-time lubricated bearings will virtually eliminate bearing failures attributable to lube application errors. Greased-for-life PFPE-filled bearings (Krytox®) will also eliminate the cost-of-labor component of periodically re-lubricating bearings.