TechArchive

Original article date: June 2000

Noise in hydraulic systems has a nasty habit of being worse than expected during system installation. What’s on offer to combat the problem? DAVID WRIGHT investigates

Hydraulic sound isn’t pleasant, and is usually classified as noise. In excessive amounts, it can damage the health of workers in the area around a hydraulic system. Because the noise actually emitted by a hydraulic power unit often exceeds the sound rating stated in the manufacturer’s brochure, many units need sound-deadening enclosures. But this isn’t always the cleverest way to solve the problem.

According to Wilkes and McLean, pump pressure and size have about equal effects on hydraulic noise levels. Pump speed, on the other hand, has about 300% greater impact on pump noise than either pressure or size, which is why some pump manufacturers recommend slower electric motor speeds. Fixed pumps are usually quieter than variable displacement pumps.

The company’s lab tests show that pump noise levels are increased by 2 to 3dB(A) just by adding outlet and return lines. Not only do hydraulic lines radiate noise, but they often provide the primary path for propagating noise from the pump to components that, in turn, react to the noise and radiate additional sound. So many manufacturers have a very low dB(A) pump rating, but when the pump is installed on a power unit the sound rating is higher. Because it is almost impossible to forecast how much additional sound the hydraulic lines and surrounding structure will radiate, many power units are enclosed after they have been installed.

Wilkes and McLean has developed a suppressor, located as close to the pump as possible, usually directly at the pump outlet. It stops the noise before it increases as it travels through the piping and radiates off other structural components. The suppressor is tuned by slight adjustments to the nitrogen pre-charge. This is easier than wrapping the piping in sound-absorbing tape, or enclosing the entire power unit. The Wilkes and McLean in-line nitrogen-charged noise suppressor is installed at the outlet of the pump. Noise enters the suppressor and goes through three different noise baffles or diffusers. The noise hits a nitrogen-charged rubber tube, usually charged with nitrogen at 50% to 60% of the hydraulic operating pressure. The tube deflects each time it is hit by a pulsation, and this slight deflection of the bladder reduces both shock and noise. The large tube and short travel distance combine to absorb the high frequency pulsations over 600Hz.

Meanwhile, by analysing the pressure ripples within the new 02-series HPR hydraulic pump range, Linde Hydraulics engineers have been able to significantly reduce the amplitude of these pressure ripples over a wide range of pump input speeds and operating system pressures. One of the changes has been to alter the shapes of the noise reduction slots (in the valve plate), another is to add a silencer, very like the expansion chamber in a car silencer.

Linde now sells silenced versions of the HPR series -02 pumps (see graph) with sound reductions of between 2-4dB(A). This reduction helps the system designer control the overall airborne noise. All other elements which the pump supplies in the hydraulic circuit such as motors, control manifolds, valves and pipework will be subjected to a smaller exciting amplitude.

Both solutions are an improvement on older nitrogen-charged accumulators, where the compressible nitrogen in the relatively incompressible medium of hydraulic fluid reduces pulsations. In later designs, the full flow was diverted into the accumulator, but the pressure drop is often too high. Also, correctly sizing this type of accumulator is complicated and the circuit that directs flow into the accumulator is expensive.

Hagglunds Drives has developed low-speed hydraulic motors (pictured above) that generate very low noise levels. The company has a set of five noise control options that can be applied to hydraulic plant. Enclosure design, together with dampening technology allows Hagglunds to achieve typical noise levels of 60-65dB(A) in its quietest unit: the same level as `normal speech’ and about 10dB(A) less than the noise in an office.

• Wilkes and McLean
• Linde Hydraulics
• Hagglunds Drives

June 2000