Typically, there are two options of maintenance concerning electric motor bearings. One is to relubricate them and the other is to change them. Monitoring electric motor bearing condition is paramount to insure reliability. Electric motors are an asset that must be managed, and prolonging their life will insure more contribution to the bottom-line profit. Preventing collateral damage caused by electric motor bearing failure is true cost avoidance. There are several ways to monitor and measure bearing condition – ultrasonics, temperature, vibration analysis and shock pulse, just to mention a few.
Because lubrication is the first line of defense for prolonging bearing life, it is imperative that the lubrication film thickness between the rolling elements and the raceways be measured and monitored.
Lubrication film thickness: What matters
Lubrication is the vital element between any moving parts and an absolute requirement in rolling element bearings.
There has always been much discussion and debate regarding the questions of how much, how often and what kinds of lubricants should be used in rolling element bearings; and to many, it still remains a mystery. The following chart (Figure 1) indicates the many parameters related to the lubrication film thickness and film quality in a rolling element bearing. Even “sealed for life” bearings will lose their lubrication film thickness. Knowing how much lubricant is left in a sealed bearing can make the difference between productive uptime and unscheduled downtime. Knowing if lubricant is getting into the bearing will allow maintenance to maximize bearing life by optimizing oil film thickness and unscheduled downtime can be prevented.
Differentiating between bearing surface damage and lubrication film thickness, as a bearing fault, is a measurable function. Knowing the difference is valuable knowledge.
Figure 1.
Case study No. 1: Maxim Corporation
- Scrubber for cleaning contaminated water before dispersing into the waste water system.
- Four small 3/4-horsepower motors with integral “C” face mounted impeller pumps plumbed with PVC piping.
- All four motors are running at the same time. (Figure 2.)
After demonstrating how to monitor bearing condition and measure lubrication film thickness at this printed circuit manufacturing plant, we were asked to look at a small water scrubber. The screaming sound of a bad bearing could be heard, but they could not pinpoint which bearing was making the noise. Was it the drive end or opposite drive end of which motor? After a quick check, the bearing was identified and its condition was determined. This was a 6204.2RS sealed bearing (a sealed-for-life bearing). The cost was about $9.
We left the plant with the promise to provide a quotation for a portable data logger instrument.
During the night, the bearing froze up and tripped the motor, so only three motors were now running. They lost one-fourth of the capacity of the system to clean water.
The next day (before the repair could be made), the Bay Area Environmental Protection Agency inspector showed up. Water was still being dumped into the wastewater system. Because of the lessened capacity, when the water was tested, it did not meet the cleaned requirements. The fine was $32,000.
The sad part is that they knew which bearing was bad. They also knew how bad it was. If predictive maintenance or condition monitoring was a part of their normal operating procedures, this downtime and fine would have never happen.
Figure 2.
Case No. 2: Electric motor shop at Laprino Foods
- A 700-horsepower ABB electric motor for driving a fan for dehydrating milk.
ABB motors have long recommended the use of shock pulse to measure their motor bearings. Many ABB motors are equipped at the factory with SPM adapters for quick-connect measurements.
A new ABB motor was installed at Laprino Foods and failed within the warranty period. The bearing froze up and twisted the shaft. There was no lubrication in the bearing.
After careful review, the records showed that the maintenance personnel in the plant had indeed greased the bearing according to the recommended schedule.
The seal cap was removed from the end bell of the drive end exposing the bearing for inspection. It was found that a machining error was made during manufacture and the grease pathway from the grease zerk to the bearing lubrication notch did not line up. Grease could not get into the bearing. Even though the lubrication schedule was being adhered to, the bearing was not being measured, so no one could tell if lubricant was actually getting into the bearing.
Production was interrupted so a new motor was taken from the electric motor shop’s inventory to replace the damaged motor. Before installing the new motor, a test run was performed. While the motor was running, a measurement was taken while lubricant (grease) was applied to the zerk. Without disassembling the motor or causing an intrusive action, it was determined that the grease pathway was in the correct place and lubricant was truly getting into the bearing. The bearing’s true condition was determined and the lubricant film thickness was measured and recorded.
ABB honored the warranty. The damaged motor was repaired and stored for a spare and the new motor is still running today. An online monitoring system (MG-4) continues to monitor these ABB electric motor bearings.
Figure 3.
Case No. 3: Evanite Fiber Corporation
- A 125-horsepower motor and belt drive on an exhaust fan for a spun glass process used to make high-density filters.
- This example was not necessarily on the motor, but on the fan shaft.
One of the most common bearing installation faults that affects lubrication film thickness is the installation and set-up of the tapered bore, double-row spherical roller bearing using a split tapered adapter in a mounted unit (i.e. pillow block bearing). While this is not an electric motor bearing,it is in very close proximity to every drive motor.
The tapered adapter is used as a wedge device to hold the inter race of the bearing on to the shaft. All bearing manufacturers have recommended clearances for applying this type of bearing. This type of bearing in a pillow block housing is very common on larger fan applications and is a very popular style for many other applications.
Figure 4.
If the tapered sleeve (adapter) is drawn up too far by over-tightening the spanner nut (a very common practice), the internal clearance of the bearing will be removed, thus reducing the lubrication film thickness.
Even though these bearings are running with no apparent fault or vibration alarm, the reliability of the machine has been put into jeopardy because the operator cannot see the film thickness problem. Lubrication film thickness can be measured to determine the true condition of the bearing during start-up without setting a baseline and treading. Differentiating bearing surface damage faults and lubrication film thickness faults is powerful knowledge.
Case No. 4: Superior Lumber Company
- 25-horsepower motor driving an edger in a sawmill.
Standard foot mounted motors that are mounted in the vertical position must have special consideration related to lubrication.
The normally mounted bearing in a horizontal motor is supporting load/overhung load in the radical direction. If the motor is mounted in the vertical position, the bearing will see more thrust loads and the lubrication will run out through the opening of an open or shielded bearing. A sealed bearing will hold the lubricant reservoir in the lower half of the bearing because the bearing is lying on its side in this application. This position will also force all of the balls of a ball bearing to one side of the raceway, thus placing them all in a more loaded situation in both radial and thrust directions. Load affects the lubrication film thickness. Normally, a motor mounted in this position will need lubricating more often than what is recommended.
Figure 5.
Summary
The ability to measure the lubrication film thickness is paramount. Better asset management and machine reliability can be obtained by measuring the film thickness as soon as the machine is turned on. This will prolong bearing and machine life.
Lubrication film thickness in rolling element bearings is critical. It can be measured to detect and troubleshoot for proper lubrication amounts (over/under), the right kind of lubricant for the right job, to detect installation faults and even to pinpoint the lack of film thickness if there is a compatibility and/or contamination problem.
Knowing the film thickness of lubrication in rolling element bearings, even a sealed bearing, is information worth measuring. Lubricant should only be applied to rolling element bearings because of demand requirements. Measuring the bearing film thickness before you lubricate will insure that there is truly a lubrication requirement. Automatic lubrication supply systems can provide the optimum amount of lubrication at just the right time when signaled by an online measuring system. Knowing the lubrication film thickness in rolling element bearings is instrumental in best asset management practices. Knowing what really matters in rolling element bearings to insure machine reliability is knowing the lubrication film thickness.
