Noise has always been a problem in hydraulic systems. And both pump pressure and pump size have about equal effects on hydraulic noise levels. However, pump speed has about a 300% greater effect on pump noise than either pressure or pump size. Indeed, some pump manufacturers have been known to recommend slower electric motor speeds to solve the problem.
Noise problems are compounded by the use of hoses too. Laboratory tests show that pump noise levels are increased by 2 to 3 dB(A) just by adding 12 ft of outlet and return lines. The lines do not generate noise. Instead they radiate noise when they respond to pulsations or vibrations. The pulsations are usually generated by the pump and the vibrations are radiated by large flat machine surfaces.
Not only do hydraulic lines radiate noise, they quite frequently provide the primary path for propagating noise from the pump to components that, in their turn, react to the noise and radiate additional sound. This helps explain why a great many pump manufacturers have a very low dB(A) pump rating, but when the pump is installed on a power unit, the sound rating is much higher.
It is almost impossible to forecast how much additional sound the hydraulic lines and surrounding structure will radiate. For this reason, many power units are enclosed after they have been manufactured and installed.
For many years, the problem of noise was solved by the use of nitrogen charged hydraulic accumulators. Initially, these devices were placed on the hydraulic line and it was hoped that the pulsations would enter the accumulator. However, in practice, pulsations bypassed the line leading to the accumulator.
Various different designs then evolved to try to solve this problem; in the main, the full flow was diverted into the accumulator. But the sizing of the design was difficult and the design was expensive. The pressure drop through the accumulators was also very high and created a problem in some instances.
This device, on the other hand, mounts an in-line nitrogen charged noise suppressor at the outlet of the pump. The tuning of the device is achieved by slight adjustments to the nitrogen precharge of the unit, an operation that is somewhat easier than wrapping the piping in sound absorbing tape or enclosing the entire power unit.
Usually mounted directly at the pump outlet, the suppressor reduces the noise of any hydraulic power unit before it travels through the piping and radiates off other structural components. In operation, it performs the same function for a hydraulic line as a silencer does on a car: it makes the unit quieter by absorbing sound. The unit also reduces hydraulic pulsations and hydraulic shock - effects that can cause pump wear and cause leakage at tube or pipe connections.
When the device is actually installed in a system, noise enters the suppressor and goes through three different noise baffles or diffusers. These metal baffles are designed to convert 1/2 inch diameter holes to 1/32 inch diameter holes. The total radial distance through these baffles is only 1/4 inch.
After passing through the holes, the noise then strikes a nitrogen charged rubber tube or bladder. This tube is usually charged with nitrogen at 50% to 60% of the hydraulic operating pressure. The 1/32 inch diameter holes are so small that the bladder cannot extrude into them. The bladder deflects each time it is hit by a pulsation, and this slight deflection of the bladder reduces both shock and noise. The large bladder area and the short travel distance combine to absorb high frequency pulsations over 600 Hz.
The suppressor has already found many applications. In one, a large pump manufacturer was requested to build sixty double pump power units for a large automotive company. They were required to meet a noise level of 80 dB(A). However, when they were completed, the noise level registered 82 dB(A). The pump manufacturer installed the new in line noise suppressor directly at the outlet of the pumps with the result that the noise level was brought down to 78 dB(A).
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Figure 4. Construction assembly, (A) As fluid eaters the suppressor, it flows radially through the 1/2 at holes. (B) The second chamber is the compressed spring. Oil freely flows through the compressed spring. (C) The third radial flow chamber is assembled over the spring which is compressed over the 1/2 in holes of the first chamber. Each chamber is sized to give adequate flow area for the fluid. (D) The assembled flow chamber shows that the original 1/2 inflow holes have been coverted to 1132 in only 114 in radial distance. (E) The complete flow chamber fits into [lie bladder. |
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Figure 1. The suppressor in
operation. Sensors,
mounted immediately
before and after the
Suppressor provided the
tracings shown above.
Figure 3. This cross section
of the Suppressor
shows the outer steel shell,
the nitrogen charge, the
bladder, the oil and the
diffuser tube. |
