In a previous blog post, I talked about the importance of reliability and how it affects life-of-machine ownership cost (LOMOC). But to minimize LOMOC, having a reliable machine is not enough. A highly efficient, reliable machine is required. So this begs the question: how efficient are your hydraulic machines?
As a hydraulics consultant advising clients in a diverse range of industries, the issue of efficiency is one I deal with a lot. Let me illustrate this 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 many hydraulic bicycles around!
In this particular 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 option of installing a bigger electric motor - and then paying for it over the life of the machine!
Another client, 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 a plant situation though, we might just order a bigger compressor without thinking too much about it - and then keep paying for it for years to come!
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 that the efficiency of a hydraulic cylinder in this type of application 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 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.
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 ownership cost and therefore unit cost of production. And perhaps even more so in the years come, if energy costs continue to increase.
So when you're acquiring new equipment, be aware of and consider the efficiency of the various power transmission options. And remember that in the case of hydraulic power transmission, proper maintenance is essential - not only to ensure the equipment operates efficiently, but also reliably. For more on improving the efficiency AND reliability of your hydraulic machines, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.