Prevention and cure
The power unit generally holds potential for the greatest reduction in noise for a given amount of time, effort, and expense exerted. As mentioned, an optional cooling fan may reduce noise from the motor. Also, using a motor that operates at 1,200 instead of the usual 1,800 rpm may reduce noise. However, expect a 1,200-rpm motor to be larger, heavier, and more expensive.
Pump noise may be reduced by running a large pump at a lower than normal speed (which can also increase pump life) or specifying four or five small pumps for a power unit instead of the usual one or two large pumps. Size and the type of pump (piston, vane, gear, etc.), number of pumping cycles per rotation, system pressure, and, especially, pump speed all influence noise. Check with the manufacturer for assistance in determining what parameters will best suit your application.
In addition to specifying quiet pumps and motors, you can also reduce noise by:
- using vibration-damping mounts to mount the pump and the motor to a subframe
- mounting the subframe to the power unit frame using vibration-damping mounts
- installing a flexible coupling between the motor and pump (and aligning it properly before startup)
- using hose sections between tubing and components that are mounted to framework, and
- as a last resort, treating noise as a symptom rather than at its cause may be the only recourse for some applications. Installing sound-damping materials around the motor-pump or power unit not only adds expense and complexity to the system, but complicates maintenance and may hinder air circulation for cooling. Acoustic filters, which use internal reflections and resonant frequiencies to cancel out noise, may also be effective. However, they must be tailored to the application and tend to be expensive.
Not allowing air to dissolve in hydraulic fluid goes a long way toward preventing cavitation, both in the pump and in downstream components. Cavitation usually causes noise when air bullbe suddenly collapse as fluid becomes pressurized in the pump. Air can be removed most effectively when fluid is in the reservoir. Given enough time, air will separate from the fluid, so the path from the return line to the pump inlet should be as long and with as little turbulence as possible. In addition, incorporating a fine-mesh screen promotes removal of air. Furthermore, tests have shown that positioning a 60-mesh screen 30° from horizontal may remove as much as 90% of entrained air, Figure 2.
Another method of quieting the power unit is to reduce pressure pulsations. Accumulators often are specified for this purpose, but their effectiveness is limited because they dampen pressure pulsations within a range of frequencies for a given size and precharge pressure. Moreover, accumulator calculations are complicated, and several accumulators may be required to dampen the full range of pulsation frequencies experienced by a system.
An alternative is to mount an in-line surge suppressor to dampen pulsations over a wide range of frequencies, Figure 3. One such suppressor consists of a housing containing an annular area that holds a pressurized charge of nitrogen, a cylindrical membrane, and a perforated tube, Figure 4. Under normal operation, fluid simply passes through the suppressor by entering one end of the tube and exiting the other. However, if pressure increases - from pump pulsation, for example - the fluid passes radially outward through the tube perforations, overcomes the nitrogen charge pressure, and expands the diaphragm outward. Allowing pressure fluctuations to act against the pressurized nitrogen cushions the vibration, so output pressure is much smoother - and, therefore, pump operation is quieter. Moreover, sizing is simple, because the suppressor is selected according to the size of the pump discharge line.
Finding out more
However you decide to make the hydraulic systems you design run quieter, component manufacturers prove an invaluable resource. Not only can they provide specifications on components, but they may also have useful literature containing more information on noise control of hydraulic systems. Engineering service laboratories who specialize in design and testing of hydraulic systems may also provide solutions. Whether affiliated with major component manufacturers or engineering laboratories, application engineers possess a wealth of knowledge that may include solutions to noise problems very similar to those experienced by your applications.
But resources don't end there. Dozens of books, technical reports, and papers exist to help you learn more about controlling noise in hydraulic systems. Calling on these resources may not make you an expert on the subject, but you'll certainly be more able to decide which solutions are most practical for your applications.