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by Heinz P. Bloch, PE, Process Machinery Consulting

Bearing lubrication is an essential component of proper motor maintenance. Here, Heinz P. Bloch explains the differences between re-greasable and lifetime bearings, and introduces recently advanced PFPE greases and the financial benefits they may have for business owners.

Bearing lubrication guidelines have existed ever since bearings were first designed and manufactured. The vehicle owners of antiquity had leather cups secured to the bearing housings of their supply wagons and war chariots. These cups were filled with animal tallow and topped by a smooth stone. This stone applied the pressure needed to force the tallow or grease into the wheel bearing clearance gap. Similar single-point lubricant application devices exist today, except that metal or plastic have taken the place of leather cups. Also, instead of tallow, users now have access to a multitude of different grease formulations. And yes, the pressure-applying stone has been out of favour for a while. It was replaced by a spring or by pressurisation via an inert gas.

Open and shielded bearings can be re-greased

While the above could rightfully be called ancient history, even modern grease application practices follow historical precedents and are influenced by long-established perceptions. One of these precedents is depicted in Figure 1, representing a reasonable and widely used standard re-lubrication guideline published decades ago (Reference 1 and 2). This guideline is still valid within the context of its intended application; it relates to premium mineral oil-based greases at ‘average’ temperatures but severe loads.

Common re-lubrication intervals (Figure 1) are often recommended as a function of shaft speed ‘n’ (r/min) and bearing bore ‘d’ (mm).  As illustrated here at 1,000r/min, 80mm bore diameter ball bearings should be re-greased after approximately 3,000 operating hours. In high temperature applications, bearing manufacturers generally recommend suitable adjustment factors (Reference 1 and 2). Because adjustment factors are percentage multipliers (0.7, 0.8, or other numbers below 1.0), it can be shown that bearings at higher than ‘typical’ operating temperatures will require more frequent grease replenishment. Not to be overlooked, effective motor re-greasing is greatly facilitated by knowing the grease path through the bearing. For that reason, best-of-class user-purchasers specify bearing configuration just as they, of course, specify motor voltage, power, speed, service factor, and wire insulation grade.

Figure 1. Greased bearing re-lube intervals as a function of size and speed.

Figure 1. Greased bearing re-lube intervals as a function of size and speed.

Anticipated life of bearings which cannot be re-greased

But what if timely and precautionary grease replenishment was not possible because a bearing had been provided with seals? Sealed bearings (Figure 2, Item 5) are often called lifetime lubricated bearings. For sealed bearings with mineral oil-based greases, Figure 1 can still be consulted to make rough estimates of probable operating life to failure. Without grease replenishing, it was usually assumed that a particular bearing would live until double the recommended time-between-replenishments would be reached.

The following example will illustrate this approximation:

This time, we opt to use Figure 1 and assume 60mm (shaft) diameter ball bearings operating at 3,600r/min. A vertical line would be plotted from 3,600r/min (on the x-axis) to intersect the 60mm curve. From there, a horizontal line would be drawn to the ‘a’ column (on the y-scale). It would show grease replenishment recommended after 2,000 operating hours, or about four times a year. However, because sealed bearings cannot be re-lubricated, cautious industrial users often invoke a somewhat conservative rule of thumb. This rule of thumb would be to multiply the recommended interval by a factor of two. In this instance, one would replace such bearings after one year of operation – a rather costly practice. Therefore, a more sophisticated user would probably purchase re-greasable 60mm bearings for the aforementioned 3,600r/min motor running conditions and periodically re-lubricate these ‘open’ or ‘shielded’ motor bearings with premium-grade (mineral oil-based) grease twice yearly.

PFPE greases: a mature life extension option

However, another option – perfluoropolyether (PFPE)-based greases – has recently been advanced. PFPE-based lubricants are non-flammable and offer extremely high tolerance to heat and chemicals. They will not oxidise 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.

In particular, these greases should be explored in view of highly favourable 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 its 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.

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 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.

Figure 2. Bearing nomenclature with terminology item 5 (seals versus shield) of interest.

Figure 2. Bearing nomenclature with terminology item 5 (seals versus shield) of interest.

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 is 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 four to years, with over 60 per cent 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 life-time grease-lubricated sealed bearings whenever an electric motor had to be serviced.  But lifetime lubrication with mineral oil-based greases would still require relatively frequent pre-planned bearing change-outs; the anticipated bearing life without re-lubrication would stress the mill’s maintenance budget. It was at that point that the PFPE alternative became very attractive and Figure 3 provided analytical back-up for the anticipated superiority of PFPE greases.  It 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 (Reference 3).

The Weibull plot in Figure 3 shows the failure probability of an optimally formulated PFPE-based motor bearing grease to be 20 times less than that of the traditional premium shear-stable polyurea grease. The PFPE grease provider’s plot was rather compelling and prompted the paper mill to request an accelerated bearing life test. The test results closely matched expectations and convinced the mill’s maintenance manager to move ahead on a proposal to use only PFPE pre-filled ‘lifetime lubricated’ bearings. From then on, these would be installed whenever motors had to be serviced in any way or required replacement for any reason.

Results and savings

Seven years later, the mill had successfully converted bearings to PFPE-based greases in over 100 motors. They also stocked over 150 additional PFPE-based grease-filled sealed bearings and planned to swap these into motors as the old bearings failed. From the time PFPE lubricants were applied, the mill has not had a single unplanned bearing failure, nor have they had to replace any electric motors which use this grease. 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 labour component of periodic re-lubrication no longer exists and each avoided motor replacement event saves the mill at least $4,000. For this Canadian paper mill, any maintenance program requiring periodic re-greasing would have meant more manpower and added overhead costs.

Figure 3. Bearing life plots with three different greases.

Figure 3. Bearing life plots with three different greases.

The paper mill was probably correct in disregarding the (academically justified) minor incremental frictional losses.  All bearings consume energy and the – conceivably – higher loss attributable to PFPE greases can be explained. If they could even be measured, higher losses would simply be due to PFPE grease having a somewhat higher density than that typically found in ‘traditional’ electric motor greases. That said, and as demonstrated by this case history, PTFE-based greases can be used to move productivity and failure avoidance at pulp and paper mills to new levels. On top of being strongly endorsed by end users, PFPE greases are recommended by many major bearing manufacturers for long-life lubrication of electric motors. At an average replacement cost of $4,000 per motor, many users have joined the Canadian paper mill in seeing favourable results add up rather quickly.

It can be demonstrated that PFPE greases are an optimum choice for long-life bearing lubrication in severe applications. Using these greases in sealed lifetime lubricated bearings will virtually eliminate bearing failures attributable to lube application errors.  Greased-for-life PFPE-filled bearings will also eliminate the cost-of-labour component of periodically re-lubricating bearings.

References

  1. SKF Bearing Maintenance Handbook, (1996) Publication 4100/I E.
  2. SKF General Catalogue, (2003) Publication 5000 E.
  3. Bench Test Data, DuPont ™ Krytox® Lubricants, Data Sheet K-23698-4.

 

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