Hydraulics, like any other engineering discipline, has design and maintenance pitfalls. Neglected or ignored, these issues ultimately lead to system failure. Predictive maintenance procedures analyze the machine as it is running and point out future weak links to be addressed before they become catastrophic. Preventive maintenance is a proactive maintenance approach that keeps failures from occurring.
Sources of hydraulic trouble often fall into one of two interrelated categories. The first is that the original equipment designer was unaware of system parameters. Responses I’ve heard from my clients include:
- “Did I forget to tell you this is going to Mexico and it will be operating outside?”
- “Oh yeah, this power unit also runs part of this other gargantuan machine over here behind this wall.”
- “Well now that it’s here, I guess we should have told you that the power unit is going into this hot, small closed room with no ventilation. It has to be really quiet in here as the company president’s office is next door.”
- “No, all together the load is 2,000 lbs., not 380 lbs. I don’t know where they got that number from!”
The second source of hydraulic system trouble is temperature. Hydraulic fluid loses many of its beneficial protective properties and characteristics when overheated. Root causes for high temperature include environment, improper system sizing and additional load not included in the original design. Inadequate research to determine the proper fluid for the design may also be a factor. Too often general-purpose fluid is used where a specific type would have been better suited. Hydraulic fluid is application-specific and selection requires the system designer or the original fluid supplier to be involved.
Collecting documentation before a system fails is critical to effective predictive maintenance. Pay attention to the standard recommendations on this topic. Listen to the machine operator and the floor mechanic. They live with the machine day in and day out and are usually attuned to incipient problems.
When the machine is functioning well, take the time to document the motor current and fluid pressure as well as temperature in the reservoir and at external machine parts. Make note of adjustable system settings, such as flow controls.
Observe the machine when it’s running. Examine the hoses. Listen to the sound the machine is making. If some component is too hot to be touched, record the temperature and query the machine manufacturer about the maximum allowable temperature. Ask if higher system temperatures in May, June and July are acceptable or whether external cooling is needed. Oil coolers come in a wide variety of styles and sizes, the choice being largely dependent upon how much heat you need to remove.
Check for leaks, but remember that an extra quarter turn on a fitting usually does more harm than good, depending on the type of thread. There’s probably a reason for the leak or a fitting working loose. Reasons could include too much hose flex and incorrect angle of approach from the hose to the port.
When a failure occurs, analysis is a critical step in resolving long-term issues. This is when careful documentation is a must. Machine symptoms, problem analysis, attempts at repair, even if unsuccessful, provide valuable history in distinguishing an isolated incident, such as infant and manufacturing flaw mortality, from a trend failure. Record the pressure, temperature, any odd noises and physical observations of the machine components at failure. If there’s a similar failure in the near future, compare the failure mode documentation to identify trends. Documentation also helps in troubleshooting failures that occur immediately after a system failure. Looking in system areas that were last disturbed often leads to a faulty component. This is where the documentation of attempted repairs proves invaluable.
Documenting things before problems occur is strongly encouraged for predictive reference and comparison when something does go wrong. Adopting this habit has often led me to uncover insidious machines problems where repeated failure trends might be ignored or overlooked.
System design oversights that lead to problems such as cylinder seals failing or system components breaking repeatedly are addressable and obvious. Unanticipated side loads and heavier-than-expected loads are less obvious and are usually the most troublesome. They’re interrelated and the documentation can help pinpoint and solve the design flaw. For example, a power unit undersized by 10% or 15% produces higher system temperatures that will, over time:
- Change system fluid viscosity and alter the design parameters for the least reliable mechanical component.
- Destroy the stability of the hydraulic fluid, making it more prone to overheating.
- Be a possible cause of cavitation and loss of lubrication, events that initiate a death spiral for the hydraulic pump.
The thermal problems might show up during the first hour of operation. Or, for a machine operating at 35% of its rated capacity during winter in the Upper Peninsula of Michigan, the problems may not become apparent until July when it operates at full load.
Dismantle and inspect failed components that haven’t reached the end of their useful life span. Metrics for expected life are readily available from the component OEM. Cylinders are usually rated in terms of total distance traveled; valves are rated in terms of number of actuations.
Automobile automatic transmissions have evolved over 70 to 80 years into reliable, self-contained hydraulic systems that use a reservoir, filter, hydraulic spools, clutches and hydraulic pump, to name a few components. There are many more transmission repair shops in hilly northern New York and Central Pennsylvania compared to Rochester, with its relatively flat terrain. Climbing grades at speed introduces a high hydraulic load that produces high temperatures. Heat dissipation is inefficient because recovery time going down the mountain isn’t long enough and summer pavement temperatures are high. Those who tow boats or trailers usually invest in transmission coolers as recommended by manufacturers of both the load and the automobile. Drivers who don’t invest in such cooling systems shorten the useful life of their transmissions. Nevertheless, with or without coolers, owners also tend to neglect the recommended frequency of fluid and filter changes. They don’t inspect the fluid levels or check to see if the fluid is beginning to darken or smell burned.
Pay close attention to the OEM’s instructions and recommendations. Send fluid samples to a lab for analysis to determine contaminant content and particle size. Examine the lab results and recommendations with a critical eye. Perhaps the test results have uncovered a potential problem that you and the OEM need to address. Heed the recommendations about cleaning fluid strainers and changing filter cartridges.
Keep it clean
|Chips accumulating around the breather cap of a power unit can lead to a contaminated system.|
Cleanliness is a simple—but often overlooked— common sense item. You’ll probably find dirt and chips around the hydraulic reservoir fill cap (Figure 1). Make sure that area is kept clean because removing the cap to inspect the fluid can dislodge undesirables that will fall into the tank.
When adding fluid, use a clean funnel and clean the top of the drum and bung before you pour (Figure 2). If you use a barrel pump, make sure it’s specifically designated for use on clean hydraulic fluid and never used for waste oil or solvents.
Inspect new hoses and clean them with a blast of compressed air. If possible, run clean oil through the hoses before installing them. Follow OEM recommendations for replacing fittings and using thread sealer. Small pieces of Teflon tape have clogged many a small hydraulic orifice.
Steven P. Thomsen, P.E., owns Rochester Industrial Design Inc. in Rochester, N.Y. Contact him at firstname.lastname@example.org and (585) 329-1425.