FREEIntroduction to Uptime Elements Reliability Framework and Asset Management System

The fact you're reading this tells me you are involved in industry, directly or indirectly. 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. At least some of this burden will fall on the maintenance department.

Reliable Machines Not Enough

Having reliable machines will not be enough. Highly efficient, reliable machines will be required. Some who achieve it will gain a real advantage. For others, it will be mandatory just to remain competitive.

This begs the question: How efficient are your machines? If your current focus is on keeping your machines running, then it's an issue you probably don't think much about.

Efficient By Design

For me as a fluid power consultant advising clients in a diverse range of industries, machine efficiency is an issue I deal with a lot. Let me illustrate with a couple of examples.

Three-wheel Drive

One client, the designer of a three-wheeled vehicle, approached me to design a hydraulic drive. He wanted to power at least two of the 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 any hydraulic bicycles around!

In this application, where the available input power is limited by space and weight, the question I asked 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 option of installing a bigger electric motor, at least for now.

Rotary Drive

Another client examined 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 the debate in his application. He's not going with the pneumatic option. In a plant situation, however, we might just order a bigger compressor without thinking much about it.

Building a Press

Contrast the first two examples with this one: 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 that the efficiency of a hydraulic cylinder approaches 100 percent.

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 1.00 = 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 (the subject of my last column), 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, are not 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 ownership cost and, therefore, unit cost of production, and perhaps even more so in the years to come.


So what does this have to do with the maintenance department? When asked to advise management on the acquisition of new equipment, be aware of and consider the efficiency of the various power transmission options. And in the case of hydraulic power transmission, proper maintenance is essential to ensure the equipment operates reliably and efficiently.

About the Author

Brendan Casey has more than 19 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his Web site:

Brendan Casey

Brendan Casey

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