Hydraulics At Work

How Efficient Are YOUR Hydraulic Machines?

Not properly considering a hydraulic machine's efficiency can be a costly mistake.

In the current economic climate, the debate concerning peak oil (the end of cheap mineral oil) and global warming (the end of the planet as we know it) have been sidelined to some extent. But these two major issues have not gone away. And they will almost certainly influence the source and cost of our energy needs in the not-too-distant future.

And there's no escaping the fact that industry is a huge consumer of energy. If peak oil and global warming are real issues - and there's a growing body of evidence to suggest they are, then I predict energy management will become a major issue for industry over the next decade and beyond. And a large part of this burden will fall on machine designers.

Efficient By Design

Reliable machines will not be enough. Highly efficient, reliable machines will be required. This begs the question: how efficient are the fluid power machines you design, build, maintain or repair? Maybe this is an issue you don't think too much about. But according to a study by the ORNL/NFPA mentioned in the January 2014 issue of Hydraulics & Pneumatics, the average efficiency of fluid power systems is just 21%!

As a consultant advising clients in a diverse range of industries, power transmission efficiency is an issue I deal with a lot. Let me illustrate with a couple of examples:
 
One client, the designer of a three-wheeled vehicle, approached me to design a hydraulic drive. He wanted to power at least two-wheels, ideally three.

To keep cost to a minimum, the machine designer asked me to consider gear pumps and motors. A gear pump or motor in good condition is 85 percent efficient. So a gear pump driving a gear motor has a best-case efficiency of 0.85 x 0.85 = 0.72. That's 72 percent - not considering losses through valves and conductors.

But say a gear-type flow divider was included to achieve multiple wheel drive. The theoretical efficiency would now be 0.85 x 0.85 x 0.85 = 0.61. That's 61 percent, not including losses through valves and conductors. Compare this with a chain drive in good condition, which is 97 to 98 percent efficient. This explains why you don't see too many hydraulic bicycles around!

In this application where the available input power is limited by space and weight, the question I had to ask my client was: Can you afford to lose 40 to 50 percent of available input power to heat? In his case the answer was no. But in a similar industrial application we have the luxury of installing a bigger electric motor, without being too concerned about the energy losses - for the time being at least.

Another client is examining his options for a rotary drive in a remote location with no access to the electricity grid. He can generate his own electricity but likes the idea of using an air motor. He wants to know comparative costs. It's not a big drive - only 20 horsepower.

An air motor has an efficiency of around 15 percent. So I explained to my client that to drive his 20 horsepower air motor he'll need a 140 horsepower air compressor! That pretty much settles it in his application. He's not going with the pneumatic option. In an industrial situation though, where a large air compressor is already available, these energy losses may be tolerated - for now.

Contrast these two examples with this one: another client for whom I am advising on the design of a 6,000 ton press. Regardless of efficiency, hydraulic power transmission is really his only option. But this is also a relatively efficient use of hydraulics. One of the reasons for this is, in high force applications, the efficiency of a hydraulic cylinder approaches 100 percent.

And because it's a high-pressure application, piston pumps will be essential. The overall efficiency of an axial piston pump in good condition is 92 percent. So the theoretical efficiency of the press hydraulic circuit is 0.92 x 1 = 0.92 or 92 percent - not including losses through valves and conductors.

A significant, 'built-in' inefficiency in this application however, is the compressibility of the hydraulic fluid - particularly given the necessarily high working pressure and large volume of the cylinder. But clearly, this is not an application for gear or chain drives.

Consider Efficiency

This is not to say hydraulics, and even pneumatics, aren't appropriate solutions for rotary drives. Energy efficiency is just one of many issues that must be considered when selecting a power transmission option. But like the many factors that influence machine reliability, if efficiency is overlooked at the design stage and not considered during the equipment selection process, this can increase the lifetime ownership cost of the asset. And even more so in the years come.

In other words, not properly considering a hydraulic machine's efficiency can be a costly mistake. To discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.

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