I need clean, quiet hydraulics

QUESTION: Hydraulics is the primary power source for our metal-forming machines. It’s been dependable, does its job, and we understand it well. However, we’re losing some new business opportunities because of noise problems and environmental concerns. What’s out there to overcome these issues?  —From April ’07 Control Design

ANSWERS:

It’s Doable
Noise and environmental issues provide an opportunity for a positive, practical, and economical solution in many installations where hydraulic drives are used.

Hydraulic drives have become a very reliable and economical drive technology, but their extraordinary high power-to-weight ratio (power density) brings with it high noise levels. Environmental concerns from extensive use of petroleum oil as a medium also need to be addressed.

Noise from hydraulic drives comes from three sources: airborne, fluid-born and structure-born. Airborne noise can be attributed to vibrations in the drive reflected and amplified through mechanical structures, and from fluid-born noise transferring its energy or vibration into the adjacent air mass.

Fluid-borne noise is caused by the pumps used. Pulsations from pumps (gear, vane and piston type) transfer energy by way of vibrations and result in fluid-born noise that will cause mechanical elements to be excited.
 
In most cases the most cost-effective solution is to concentrate on the pump. Manufacturers of hydraulic pumps typically publish noise data (sound pressure dBA) for their pumps at a known drive speed, flow, and pressure. This sound data is typically collected and recorded in an anechoic chamber or room conforming to a DIN Norm 45 635, part 26.

Data from these tests in a controlled environment only measure the sound pressure from airborne noise emitted from the pump. While it is important to begin pump selection with relative noise data, differences of ±5 dBA can be seen based on the manufacturers’ tolerance, control settings, and the mounting of the pump. It would be equally important to note that one should gather similar data from the motor manufacturer.

OEMs can address noise reduction:

Inlet to pump: 

• Increase inlet diameter to pumps to a maximum oil velocity ≤ 0.5-1.0 m/s
• Design inlet lines straight and short, avoid elbows, use full flow inlet valves
• Terminate pump inlet line in oil tank ≥ 4 inlet line diameters below air/oil

Surface:

• De-couple pump inlet by submersing the pump in the tank. This could be a fluid immersed motor/pump, but most often just is a submersible pump. This solution reduces air-born noise as well.
• Use soft connections on pump inlet, hose, or expansion joint
• Provide adequate tank volume and design (baffles) to dissipate entrained air in

The Oil

• Avoid termination of any return lines near pump inlets
• Provide for oil inlet head pressure to pump manufacturers’ recommendations

Pump Outlet, Service Lines, Return Lines:

• Use hose on pump outlet, selected for fluid line velocities ≤ 4 m/s
• Don’t use sharp bends on outlet lines to avoid reduction and distortion of wall

Cross section on pipe/tube

• Secure outlet lines with clamps to ground noise to structure
• Consider fluid line silencers (mufflers)
• Limit return line oil velocities to ≤ 2 m/s

Structure-born noise reduction

• Add flexible motor/ pump shock mounts with three degrees of freedom in shear rather than compression
• Add a damping flange to replace the typical rigid bell housing or pump L-bracket with a kinematically matched pump motor coupling 
• Design and arrange fixing points of support where the reservoir and frame is most rigid and of substantial mass
• Add additional high mass elements or gusset support areas 
• Remove/design to remove other mechanical sound bridges 

These can include pressure, suction, and return lines that are rigid. Sometimes experimenting with adding a small section of hose can decrease noise transmission dramatically.

Environmental concerns prompted manufacturers to develop fluids that are both environmentally friendly and suitable for many modern hydraulic drives. Products categorized as HEES are prepared from rapeseed and soybean base chemicals. These are environmentally friendly and have measured success in hydraulic drives. Synthetic anti-wear hydraulic fluids, type HETG with better than petroleum properties, fire-resistance, and that are biodegradable have been employed with great success in many applications.  —Andrew Binversie, Manager, Presses, Series, and Light, Bosch Rexroth.

Hydraulics Getting Better All the Time