Tips for Filtering High Viscosity Gear Oils

Tags: gear lubrication, viscosity
  Keeping gear oil clean isn't hard and may actually save you some money.

We have large circulating systems that service industrial gear sets with heavy gear oils (ISO 460-680). We tried using 25 micron filtration but this was as fine as we could go due to the high fluid viscosities. We have since abandoned the filters because of the high pressure drop. What can we do to remedy this?

It is true that high viscosity oils are not as easily filtered. However, all of this is influenced by several variables that need to be considered:

Filterability. Some oils are unfilterable for many reasons that have nothing to do with viscosity. The filterability of your gear oil may need to be evaluated. There are different procedures for doing this such as the draft procedure ISO/DIS 13357-2 and ASTM F52-69. Besides the effects of viscosity and the presence of solid particles, an oil’s filterability is influenced by additives (e.g., VI improvers and precipitants), oxide insolubles, carbon insolubles, wax insolubles, and poorly blended oils.

Oil Temperature. Make sure the oil is at operating temperature when the filter pressure drop is evaluated. Gear oils, at operating temperature, may have considerably lower viscosity. It depends on the exact temperature and the viscosity index of the oil. And, most filters have viscosity sensitive (i.e., pressure drop sensitive) bypass valves. Therefore, during cold start or other cold operating temperature conditions, all or a considerable portion of the flow bypasses the filter. It is true you won’t be filtering at this time but as the oil heats up, a higher percentage of the flow (eventually all) will pass through the filter. During bypass, a well designed filter housing will not permit particles from being pulled off the front side of the filter.

Flow Rate. Many offline filters for gear lubricants run at very low flow rates. Depending on ingression rate and system size, flows as low as one gpm can often maintain the required target cleanliness level. At low flow rates the pressure drop through the filter is reduced considerably.

Filter Size. High surface-area pleated elements such as those used to filter bearing oils on paper machines can handle relatively high viscosity and high flow rates. These are rather expensive at initial installation but in the long run can be a big cost saver.

Media Type. High pressure-drop depth media filters are not suitable for high viscosity gear oils. The best filter media has high pore density and hyper-fine fiber diameters. Hydraulic filters sold by leading manufacturers are generally of this type. Filter manufacturer’s published P-Q curves on their elements showing the relationship between flow/viscosity to pressure differential. Another major consideration is ingression. Every effort needs to be made to minimize particle ingression into the gear case. Once ingression is under control, the job of filtering the oil is simplified. Clean oil is vital to machine reliability. It is rarely a good financial or business decision to accept dirty oil in critical lubricating oils.

Just Getting Started?
We’ve decided to re-engineer our oil analysis program. We plan to install sample ports on our equipment, improve contamination control, bring a small oil analysis lab on site, and re-examine the lab we are using. We are now trying to select which machines at our power station to include in the initial program. What do you suggest?

It is usually advisable not to attempt to include all your equipment as this can dilute the available time and budget which will adversely affect the quality and success of the program. It is best to begin with fewer machines but insure that those machines that are included have the required elements. You might consider using the following criteria in selecting the machines to include in your program.

a. Machine Criticality. Machines that are vital to a process or operation should be given higher priority. If the downtime of any given machine can affect the productivity of the overall operation or process, resulting in idle labor and loss of production, these machines should be the first to be included.

b. Safety Risk. Machines that present safety risk upon failure should also be given priority.

c. Fluid Environment Severity. Machine lubricants and hydraulic fluids operating in harsh operating environments are also good candidates. Such machines are those that experience high loads, pressures, temperatures, duty cycle, contamination, and shock loads.

d. Failure-Prone Machines. These are machines that have had a history of poor reliability.

e. High Cost of Repair. Some machines are extremely expensive to repair. Many use components that are special ordered and therefore not in spare parts inventory. The overall cost of repair, including labor, material, and downtime needs to be considered.

Crank Up the Heat.
I have learned that high temperature encourages additive depletion and base stock oxidation in bearing oils. We are a paper mill and have been told by the paper machine supplier that our main lube oil reservoir should be maintained at 180 degrees F. Why are these high temperatures recommended, considering the harm it can cause?

You are correct, high operating temperatures encourage additive and base oil failure. The widely used rule of thumb is that for every 18 degrees F increase in temperature the life of the oil is cut in half. Still, it is common for high temperatures to be recommended in paper mill applications. Reasons include better water shedding ability, lower foaming tendency, better air release, improved particle settling rate, better drain back flows, and more rapid vaporization of water. However, most of the benefits from raised temperatures can be sufficiently achieved at, say, 140 degrees F. At this lower temperature viscosity will be higher and therefore, you might want to revisit the viscosity grade selection. Lower viscosity oils, in general, are more resistant to oxidation and thermal failure. It is always good advice to discuss a proposed temperature and/or viscosity change with your machine and oil suppliers.