The Pressure-Flow Conundrum And What It Means For Hydraulic Machine Reliability

In most hydraulic fluid power applications there are essentially two ways to get the required power output: high flow at low pressure; and low flow at high pressure. This is of course because power is a product of flow and pressure; increase pressure and you can reduce flow proportionately but still get the same power output. For example, 100 liters/minute at 200 bar equates to the same power output as 50 liters/minute at 400 bar.

Higher pressures mean higher force and torque is available from smaller components. And because the components are smaller in displacement, higher speeds are possible from smaller flows. Smaller flows mean pipework, valving and even the tank can be made smaller - remember, the old rule of thumb for tank size, which is largely ignored these days, is 3 to 5 times pump flow per minute.

So the power density of individual components and the system as a whole increases with pressure. But there are a number of disadvantages with higher operating pressures. These include:

• The inherent heat dissipation of the system is lower due to the reduced surface area of the tank (it's smaller in volume), pipework and components. In other words, a bigger heat exchanger is required for a high-pressure system of the same power and efficiency.
• Greater variation in fluid volume (compression) at higher pressures means reduced stiffness of the system and less favorable dynamic response.
• Higher noise levels as a result of higher peak pressures.
• The potential for greater friction and wear, resulting from heavier loads on bearings and sliding surfaces, and more severe damage resulting from aeration, cavitation and micro-dieseling due to higher compression ratios.
• Increased potential safety hazard from components and conductors containing fluids at higher pressures.

In the majority of applications though, the advantages presented by higher operating pressures outweigh the disadvantages. Which is why for several decades now, we've seen the average operating pressures of hydraulic equipment - particularly mobile hydraulic equipment, increase. And this is a trend which is expected to continue. But clearly, it comes with some design problems. Here's a few that come to mind:

• Heavier hose construction, i.e., less flexible lines, longer bend radii and possibly, special fittings and assembly methods.
• Heavier valve bodies, actuators, pumps, pipes -- or the use of more exotic, and expensive, materials in their construction.
• More advanced seal materials, new groove designs and closer tolerances - to ensure sealing integrity doesn't suffer.

But beyond these design and material-strength issues, also consider for a moment how higher operating pressures impacts machine reliability. We know that force in a hydraulic system is a product of pressure and area. So when operating pressure increases - so do loads on lubricated surfaces.

Oil viscosity and film strength are vital to maintain full-film lubrication between heavily loaded contacts. I already consider the oil to be THE most important component of any hydraulic system. But this will definitely be the case for machines operating at increased pressures. Oil selection AND maintenance will be critical for optimum reliability.

Similarly, contamination control will be more important than ever. Because the more heavily loaded the lubricated contacts - the more susceptible they are to wear and damage from water and particle contamination.

For machine designers, issues such as tank size, installed cooling capacity, filtration, contamination control, and oil recommendations will be even more important than they are now. Because the impact of mistakes or omissions in these areas at the design stage will have an even greater impact on machine reliability.

For hydraulic equipment users whose maintenance practices are unsophisticated or non-existent - their hydraulic equipment operating costs can only go up! Due to a likely higher incidence of premature failures resulting from temperature, lubrication, oil degradation and contamination issues.

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