This article discusses the value of proactive maintenance and the use of off-line filtration to reduce wear particles and improve reliability. The author has applied these principles to closed systems to protect the lubricant and machinery from lubricant-induced wear. This article discusses applying filtration in the 5 to 10- micron range to stamping presses that range from 100 to 400 tons each. These presses have an open lube system as well as a high rate of contamination from exposure to the clutch and brake mechanism. The performance of different manufacturers of depth filters or various filter systems is not compared in this article.
The Limitations of Oil Analysis
Of all the predictive methods, oil sampling is possibly the most misunderstood and ineffectively used. Because of the cost (typically $40 to $50 per sample), the tendency is to sample on a quarterly or semiannual basis. The credibility of oil sampling also suffers until one realizes there is experimental error in the analysis as well as human error in procuring the sample. Many failures are also missed in large-volume reservoirs due to poorly considered alarm levels. The absolute value of ferrous content in parts per million is a poor indicator of long-term issues.
One or two samples do not make a trend. In fact, the sampling should initially be taken at a high frequency, at least once per month, to establish the standard deviation of the component being monitored. Alarms should not go off until two standard deviations of the normal data stream have been exceeded. Furthermore, the analyst must realize that spectroscopic testing is not accurate when the particles exceed five microns, leading some technicians to believe that only ferrographic analysis has any value.
Ferrographic analysis microscopically examines the particles’ magnetic attraction, shape and color to determine if severe wear is occurring. The cost of the sample is typically double that of other techniques because of the lab time and technical skill required.
The most effective lubricant maintenance is accomplished when sampling is performed proactively or with an eye to remove the causes for wear.
Causes of Wear
Abrasive contamination by particles as small as five microns.
Water entrainment as low as 200 ppm or 0.02 percent.
Incorrect viscosity or additive package.
The above conditions can be evaluated through basic spectrographic analysis when specified in lab instructions. If one test must be chosen, it should be a particle count with an ISO cleanliness target suitable for the equipment. Initially setting a target of ISO 17/14 or better is an achievable goal for lubrication of plain bearings using 150 viscosity lube oil, and an ISO 15/12 or better is recommended for hydraulic systems with lighter 46 viscosity hydraulic oil and close-fitting valve spools. As hydraulic system pressures and valve sensitivity increase, this target must be tightened (a lower ISO target selected). A target of ISO 15/12 is also important for high-speed ball bearing units with lighter viscosity (such as 46 to 68) antiwear oils and thin hydrodynamic oil films.
Table 1. Conversion of Particle
Count to ISO Cleanliness Numbers
Reviewing Actual Sample Data
The sample results in Tables 2 and 3 illustrate examples of contamination and the effect of off-line filtration on heavier lubricating oil in a stamping press. The samples are from two different locations using the same 150 viscosity lubricating oil.
The first case has a sample location ID# V1-003-8, which is a 150 viscosity oil from a lubrication system feeding a large stamping press. The reservoir contains about 120 liters and has a low rate of contamination from external sources. When a single bypass filter installed and left in place for six weeks from May 6 to June 18, 2002, the cleanliness improved from 21/14 to 17/13, which fell within the target cleanliness range of 17/14. Note that the iron PPM has decreased from 13 to 1.5 and copper from 7.0 to 2.0, indicating a reduction in bearing and journal wear. Oil was changed in January 2002 which explains the apparent lack of change between May 25, 2001 and May 6, 2002.
|Figure 1. Clean return flow from dual filter shortly after installation. Compare to pre-sample in Figure 2. Flow rate is about one gallon per minute. Note clarity of oil compared to sample.||Figure 3. Use of an ARO diaphragm air pump, with ½-inch diameter port size, rated 13 gpm to circulate lube oil through the bypass filter at 45 psi.|
|Figure 2. Oil sample taken June 12, 2002 before filtration (very black). Oil type is ESSO Spartan 150 lubricating oil.||Figure 4. Dual filter installation on sample point V1-002-3. Note unique manifold block and sample point take off beside gauge on inlet to filter.|
The second case shows sample location ID# V1-002-3. The reservoir contains approximately 200 liters of 150 viscosity lube oil at about 100ºF. A dual filter supplied by a different manufacturer (Figures 1, 2 and 4) was left in place for about two months with samples taken June 12, June 19, June 28 and June 30 of 2002. After two days, there was dramatic improvement in the color of the oil. This oil was subject to severe contamination due to dust from the brake and clutch of the press. Although the particles were not highly abrasive, they had recently contributed to sticking of the solenoid valves and lubrication distributor blocks. This filter has saved regular bimonthly oil changes of 200-plus liters of oil and used oil disposal costs. The iron PPM shows a repeatable decrease indicating decreased wear.
It is significant to note that even though the 1-micron bypass filter has been installed for almost two months, the ISO cleanliness still remains at range 22 for 5-micron particles on sample point V1-002-3. The conclusion is that either the filter efficiency is poor at the 5-micron particle range, or the filter has become plugged and the differential pressure is forcing the contaminants through, or has caused channeling of the filter media. Both are possible.
One of the more unique features about this installation is the use of a pneumatic diaphragm pump (Figure 3) to circulate the oil. This is an advantage over the electric-driven gear pump because the filter unit requires no pressure control or relief valves which reduces the initial cost of the filter units by about 50 percent. The pump is supplied with air at aproximately 75 psi and the gauge on the inlet to the filter indicated 45 psi at a flow of around one liter per minute. There is no concern regarding filter plugging, overheating and pump damage because the air-driven pump will simply stall. Therefore, there is no danger of leaving the circulation system running unattended for several days.
Note that the iron component follows a bathtub curve correlating with the fall and rise of sub-15 micron particles (Figure 5), which implies that wear is directly related to the presence of particles of this size. Based on research, the normal oil thickness in journal bearings (as used in stamping presses) is about 10 microns and the most damaging particle size is a particle which bridges this gap, or 10 microns. Larger and smaller particles produce a lower rate of wear as they fail to pass or pass without causing abrasive wear.
Using Statistics to Set Alarm Levels
See Figures 5 and 6 for spreadsheets to trend any value change and attribute a statistical alarm status based on sample standard deviation. This includes spreadsheets to plot contamination by volume from particle count. The values plotted were taken from sample data sheets (Tables 2 and 3).
The use of bypass filtration is useful in reducing journal wear and sticking of hydraulic or lubrication system valves in the old lower press. The oil has been in use more than four months, where it typically would have been changed at least once, and no sticking or plugging problems have been observed. After the initial capital cost of the pump, hoses and filter canisters, the cost of two filter elements (approximately $100) is far less than the cost of 200 liters of oil, the cost of disposal and environmental savings.
The use of a pneumatic diaphragm pump rated 13 gallons per minute with ½-inch diameter ports was successful where there is insufficient pressure or capacity from the existing oil circulating system to feed the filter in a passive mode. Tests with a smaller diaphragm pump with ¼-inch ports rated five gallons per minute were not successful.
The ¼-inch port pump had been used successfully with lighter 32 and 46 viscosity hydraulic oil. The failure appeared to be caused by small suction port size and the higher 150 viscosity lubricating oil at 100ºF. The port and required fitting bore size prevented the pump from achieving more than 25 psi discharge to the filter before suction head became too low and cavitation occurred. Minimum suction hose size appeared to be 3/8-inch ID nominal with ½-inch ID optimal. Suction hose length should also be limited to no more than six feet.
The optimum inlet air pressure required for the diaphragm pump was found to be 75 psi. This produced a filter inlet pressure of 45 psi at the filter when 150 viscosity lubricating oil at 100ºF oil temperature was pumped through a 3/8-inch ID hose at about one gallon per minute. Filters used were the larger dual parallel system mounted on an aluminum manifold provided by FiltaKleen (Figure 4).
Further testing is required to establish optimum filter change intervals or if more parallel filters and reducing the filter supply pressure would improve filter efficiency with longer intervals between filter changes.
Duchowski, J. Collins, K. and Dmochowski, W. “Experimental Evaluation of Filtration Requirements for Journal Bearings Operating Under Different Contaminant Levels.” Lubrication Engineering, June 2002.
Duchowski, J. and Collins, K.“Cleanliness Requirements for Journal Bearing Lubrication.” Practicing Oil Analysis magazine, July-August 2000.
“Particle Counting.” Practicing Oil Analysis magazine, July-August 2002.
Fitch, J. “Proactive and Predictive Strategies for Setting Oil Analysis Alarms and Limits.” Presented at Enteract 98.
Davis , A. “Setting Viscosity Alarms and Limits.” Practicing Oil Analysis magazine, January-February 2003.
Garvey, R. “Is Your Particle Counter Giving You PPM and Size Distribution.” Practicing Oil Analysis magazine, January-February 2003.
Maintenance Technology International Inc. The Practical Handbook of Lubrication. “Suggested ISO Cleanliness Codes.” p. 192-195.
Friction Lubrication an Wear Technology., ASTM Vol. 18. “Abrasive Wear,” p. 184-190 and “Lubricant Analysis,” p. 299-312.
Oil Analysis provided by Wear Check Testing Labs, Mississauga Ontario .
Parker filtration equipment provided by Darren Beudreau, Pneutech.
Triple R filtration equipment provided by David Murray, Triple R
KleenOil filtration equipment provided by Paul Dumont, Proactive Maintenance Solutions.
FiltaKleen filtration equipment provided by Garth Owens and Robert Andrews, FiltaKleen Industrial Inc. www.filtakleen.ca