Mobile Hydraulic Equipment: Increasing the Pressure

According to an article published in the September 2007 issue of Hydraulics and Pneumatics magazine, the average operating pressure of mobile hydraulic equipment will increase to 450 bar by 2010.

This is an interesting development because while the operating pressures of mobile hydraulic equipment have been on the increase for more than 30 years, they have been relatively stable since the current average operating pressure of 350 bar was reached in the early 1990s (Figure 1).

Figure 1. Operating Pressure Trend of Hydraulic Excavators1

Mobile hydraulic equipment already has a harder life than its industrial counterpart. There are four key reasons for this:

Operating Environment - There is plenty of industrial hydraulic equipment working in dirty, dusty conditions and temperature extremes, and for mobile equipment, this is the norm rather than the exception.

Tank Size - Due to space and weight restrictions, the reservoir capacity of mobile hydraulic equipment is typically less than ideal. This means that less oil is circulating in the system and there is a shorter dwell time in the tank, which gives up contaminants such as particles, water and air.

Cooling Capacity - The reservoir is not the only component on mobile hydraulic equipment that goes to the chopping block. All too often, the cooling system suffers the same fate. As a result, installed cooling capacity is marginal at best or simply inadequate.

Of course, ambient temperature at the machine's location influences the heat balance of the hydraulic system and, therefore, its operating temperature. To provide some anecdotal evidence of the possibilities in terms of reliability gains, I had a conversation recently with the designer of an efficient refrigerated oil cooler now fitted to more than 90 mobile machines working in hot climates. By reducing hydraulic oil temperature by up to 15 degrees Celsius as a result of installing these chillers, the users of these machines have reported a drastic reduction in hose, cylinder and pump failures.

Operating Pressure - Again, while there are many high-pressure industrial hydraulic systems, it is the mobile systems that always push the envelope.

Figure 2. Losses in a Hydraulic Drive (Bosch Rexroth)

Stressing the Reliability
In a hydraulic system, power is a product of flow and pressure. Similarly, force is a product of area and pressure. Therefore if pressure is increased, flow and area can decrease for the same power and force, respectively. When space and weight are a machine issue, a more compact hydraulic system with higher power-to-weight ratio is desirable.

So while this forecast increase in average operating pressure is not surprising, it will increase the stress on mobile hydraulic machines.

Beyond the obvious material-strength issues (seals, hoses and other components must be designed to withstand the increased operating pressure), consider for a moment how this will impact reliability.

Higher operating pressures increase power lost through compressibility of the oil and internal leakage (Figure 2). While everything else is equal, this increases the heat load on the hydraulic system, which is not desirable on any machine where installed cooling capacity is already marginal.

As previously stated, force in a hydraulic system is a product of pressure and area. Therefore, when operating pressure increases, so do loads on lubricated surfaces.

Important Properties
Oil viscosity and film strength are vital to maintain full-film lubrication between heavily loaded contacts. I consider oil to be the most important component of any hydraulic system, and this will likely be the case for mobile machines operating at 450 bar. Additionally, oil selection and maintenance will be critical for optimum reliability.

Similarly, contamination control will be more important than ever. Why? Because the more heavily loaded the machine, the more susceptible it is to wear and damage from water and particle contamination.

Moving Forward
My prediction is this next generation of mobile hydraulic equipment will present challenges for both machines designers and equipment owners. Issues such as tank size, installed cooling capacity, filtration, contamination control and oil recommendations will be crucial. And the impact of mistakes or omissions at the design stage on equipment reliability will be even more significant.

For mobile equipment users whose maintenance practices are either unsophisticated or nonexistent, hydraulic equipment operating costs can only go up. This is due to a likely higher incidence of premature failures resulting from temperature, oil degradation, lubrication and contamination issues.

The good news is this new generation of mobile hydraulic equipment is still a couple of years away. So, regardless of which side you're on - original equipment manufacturer or end user - there's still time to get the house in order.

Reference

1. S. Ohkawa, A. Konishi, H. Hatano and D. Voss. "Piston Pump Failures In Various Type Hydraulic Fluids." Hydraulic Failure Analysis: Fluids, Components, and System Effects ASTM STP 1339. G. Totten, D. Wills and D. Feldmann, Editors. American Society for Testing and Materials, West Conshohocken, PA, 2001.