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Machine Tool Hydraulics: Using Oil Analysis as an Essential PM Practice

I am often surprised that so many metalworking facilities do not use oil analysis to monitor their machine tool hydraulic units.  In discussing the issue with maintenance managers, I receive a number of explanations: the reservoirs are too small, pressures are low, pump replacements are fairly inexpensive, etc.  A closer examination would reveal a number of very good reasons to implement periodic oil analysis as an integral component of the machine tool preventive maintenance program.

Yes, the reservoirs tend to be quite small.  Most machine tools operate with hydraulic oil volumes of only 10 – 30 gallons.  It would seem to make sense to simply replace the fluid annually and forget about it otherwise.  After all, the cost of 20 gallons of premium hydraulic oil would only be about $200.  So, why spend $25 periodically to check the oil? There are some very good reasons.
  1. Hydraulic Fluid Costs:  Premium hydraulic fluids are fully capable of functioning satisfactorily for 3-5 years, provided you keep them clean and dry.  Periodic oil analysis will confirm that the oil is in good condition and may reliably remain in service.  Why spend $200 each and every year if you don’t have to?  When you consider that many facilities have 50 – 100 hydraulic units, the savings begin to pile up.  It is also worthwhile to consider some of the other costs associated with oil change-outs:  time and labor to drain the oil, transfer to temporary storage, disposal, environmental costs, and inefficient use of a non-renewable resource.  A modest oil analysis program provides a more sustainable alternative.  It is not unusual for hydraulic fluid consumption to fall to 30% of its original volume with effective use of oil analysis technologies.
  2. Machine Tool Repair Costs:  Keep in mind that oil analysis not only monitors the condition of the oil, but also gives us valuable information about the condition of the hydraulic components.  Taking samples prior to scheduled unit PM services will help to determine exactly what will be needed during the service.  For example, oil that is simply “dirty” could be filtered with a portable filtration unit while the PM service is being conducted.  Where the presence of wear metals is detected, preparations could be made in advance to inspect or replace the suspected component.  Indications of abnormally high temperature would lead to a check of the cooling unit.  Best of all, if the sample results are normal, the oil and hydraulic system components will require little or no maintenance effort during the service.  Identifying minor problems, and fixing them on a planned basis during routine services, is ideal for avoiding unexpected failures.  Such failures result in higher repair parts costs, production disruptions, excessive costs for emergency parts shipments, higher inventory costs, and a lot of high-stress activity. When you consider all the costs involved, it is hard to imagine that an unexpected failure would not produce at least $2,000 in avoidable expense.
Here are some guidelines for implementing a reasonable program:
  1. Choose the right oil analysis program: You will need an oil analysis service that is inexpensive, fast, and reliable.  In addition to the standard analysis that provides viscosity, wear metals, additive metals, oxidation, and water, you should also be sure to include particle count (ISO Cleanliness Code) in your requirements.  The cost of such services should be less than $25 with a turnaround time of about 48 hours at the lab.
  2. Select the sampling schedule: Your sampling schedule should coincide with your preventive maintenance services.  Ideally, you would sample the hydraulic fluid a few weeks before a scheduled service.  In this way, a great deal of important information would be available in scheduling the time and resources needed for the service.  Many companies sample at the beginning of each Quarter, taking samples from all units scheduled for PM services during the Quarter. Two samples per year are generally enough to effectively monitor and protect machine tool hydraulics.
  3. Get your oil supplier involved: Your oil supplier can recommend, and usually supply, an appropriate oil analysis program.  Good suppliers will assist in training your maintenance staff to sample properly, process the oil samples, and interpret the results.  Beyond this, an experienced lubrication engineer can manage and analyze the test data over long periods of time to pinpoint common deficiencies and address plant-wide issues. We call this “engineered” oil analysis and it will produce the best results. 
  4. Choose your cleanliness target: Hydraulics OEMs will tell you that 80% of premature component failures are due to undetected and unresolved fluid contamination.  Where pressures are high and servo or proportioning valves are involved, the cleanliness requirements can be quite demanding.  A good starting point for most machine tool hydraulic systems would be about 18/16/14, with future refinements where needed.
  5. Invest in the program:  Purchase a good quality portable filtration unit.  A 5 - 10 gpm unit with 12 and 6 micron (Beta 1000) filters will do a great job in efficiently cleaning contaminated oil.  Over time, install quick connects on the reservoirs to make filtration quick, safe, and easy.
  6. Periodically review program results: Take some time each year to look at the results and savings achieved through your oil analysis efforts.  It is not unusual, for a facility with 50 – 100 units, to conservatively avoid $30,000 in unnecessary costs.  Obviously, avoiding one unexpected failure on a critical unit could save that much alone.  Nothing garners management support like documented savings and consistent payback on investments.  Make your engineered oil analysis program an important part of ongoing sustainability efforts.
Oil analysis has become a key technology for monitoring and maintaining critical hydraulic systems.  Put it to use in your metalworking facility and see the impact for yourself.