C O U N T E R P O I N T
Machinery & Equipment MRO
What makes a hydraulic system reliable? BY MARK BARNES AND JEFF SMITH
Fluid power is an effective way of producing, transferring and controlling energy to produce lateral or rotary motion. This could be subcategorized into hydraulics and pneumatics. In some organizations hydraulic systems are cursed as unreliable and troublesome, in others they are a non-issue. Some systems are clean, dry and trouble-free, some are sprinklers and high-volume parts consumers. In this instalment of Counterpoint, Smith and Barnes share their perspectives on what is required to create a trouble-free system, whether mobile or stationary.
P oorly designed fluid power systems lead to poor maintenance practices. There are three key factors that enable reliability in fluid power systems: design, system tuning and corrective maintenance. Having Jeff Smith investigated multiple failures and conductAcuren ed failure analysis on many hydraulic issues, three common causes seem to arise. Poorly designed systems often lead to excessive heat, shock-loaded functions, lack of ingress control and unmanaged ingress (contamination) and egress (leakage). Poor design is also compounded by improper setup and poor post-failure maintenance practices. System design is often centered on basic functionality and developed by matching the requirements to Pascal’s law. Pressure = force/area Area = pressure/force Force = pressure x area Flow rate requirements are calculated (Cyl. Area x Stroke/time). Power requirements are determined KW = (LPMxPressure)/600). Motor speed requirements are aligned Motor Speed = (LPM/ cc per Rev. of pump). So, although most hydraulic systems are mathematically correct, the application fails to deliver the required reliability.
Why? System design: Most hydraulic systems engineered fail to evaluate the consequences of component failure and the system degradation that will occur. One example of this is on haul trucks where I have seen one manufacturer use the hydraulic system for braking. When a wheel bearing fails, a massive amount of metal contamination dumps into the system. This has resulted in the aftermarket coming up with rare earth magnets and other
solutions for an issue that could have been resolved in the design stage. To emphasize the number of design issues just consider the aftermarket products sold: desiccant filters, advanced filtration, contamination control fill ports, sample ports, coolers, cylinder rod protection, etc. If any of the items listed are installed and add value, why were they not in the initial design? I believe that equipment manufacturers should utilize RCM design. If you follow the aviation world, you realize that plane maintenance centres do not use RCM (MSG-3); that is in the hands of the equipment manufacturers. They apply manufacturer-developed programs to assets that have had the failure modes considered at the design phase. Optimal design of hydraulic systems considers operational, shock and environmental loading and protects against cascading damage. System tuning: Have you ever listened to a band and heard a random note or an out-of-tune instrument. Regardless of the song played, the result is painful. Hydraulic systems should be tuned like a guitar; everything must interact correctly to protect the system or function. Optimal design and contamination control will do nothing if a relief valve is set below system pressure. The heat generated will bake the oil and cascade the damage throughout the system. Another example is a cross line relief system. Its purpose is to cushion against shock loading, and this can be the difference between smooth operation and an operator bouncing around in an excavator cab every time he stops a swing. If your current practices are to only address setup when there is a failure, you are ignoring the natural degradation of the system components. If you are profiling your hydraulic system (trending pressure over time) you can easily find and resolve slight tuning issues. Post-failure maintenance: Having watched organizations replace components only to have the next downstream component fail far too many times, I struggle to understand why we wouldn’t clean the system as part of the repair. The saying, “We don’t have time to do it right, but we have time to do it over,” comes to mind. I have also seen the other extremes where even unrelated systems are taken apart and inspected. Assuming there is a pre-planned work order for the task of replacing a failed component, it should