Park Ridge, IL--When it comes to hydraulic noise, engineers around the globe agree on one simple fact: Hydraulic noise comes from hydraulic pumps--right?
Wrong. Although thousands of engineers believe otherwise, the fact is that the loudest hydraulic noise is rarely generated by the pump. "Customers often think it's the pump," explains Paul Baumgartner, a Vickers applications engineer. "But when you give them a replacement pump, they usually find that the noise level hasn't changed. That's when they realize that the pump is not the problem."
Baumgartner says he recently fielded a call from a customer who was understandably frustrated over the noise emanating from his hydraulic system. The customer blamed his hydraulic pump, saying that it was five decibels (db(A)) too loud.
When Baumgartner and other Vickers engineers examined the pump, however, they found no problem. Instead, they traced the noise to a different locale: a bolt on the side of the hydraulic system's reservoir had been over-tightened, thus damaging the isolation material beneath it. Result: The reservoir resonated like a drum. "By tightening or loosening the bolt, we could produce almost any sound we wanted," he recalls.
For Baumgartner, such experiences are all too common. But their existence serves to point out one of the realities in the battle against hydraulic noise: Hydraulic noise sources are many and varied.
Quieter products. At one time, the popular image of the noisy pump held some truth. But in recent years, hydraulic pump manufacturers have taken pains to reduce noise levels. The reason: Industry now demands it.
Some automakers, for example, have taken the lead in noise reduction efforts by standardizing noise levels at certain plants. Maximum noise levels at those plants typically range from 75-80 db(A) at full flow and full pressure. Some plants also call for reduction in maximum motor operating speeds--from 1,800 to 1,200 rpm.
OSHA's maximum permissible exposure limit for noise is 90 decibels, measured as an 8-hour, time-weighted average. OSHA also requires a hearing conservation program if workers are subjected to a noise level that equals or exceeds 85 db(A) for more than 8 hours daily as a time-weighted average.
To meet the demand for reduced noise, pump manufacturers have made their new products downright stealthy. Parker Hannifin, for example, recently introduced the PFVI, a vane pump with a hydraulically balanced rotor and vane assembly, which cuts noise. Last fall, Parker also rolled out the PVP-100 axial piston pump, another product aimed at reducing noise for industrial customers. Similarly, Commercial Intertech, Youngstown, OH, has rolled out the Ultra PowerMax Stealth, a gear pump that operates on the principle that a low amplitude pressure ripple will significantly reduce system noise and vibration. The firm's engineers say that the pump has reduced noise levels from 3-11 db(A) in tests.
One of the most profound changes in noise reduction technology, however, is still the Integrated Motor-Pump from Vickers. The Integrated Motor-Pump dramatically reduces noise by eliminating the cooling fan from pump-motor assembly. Instead of air cooling, the Integrated Motor-Pump employs hydraulic oil cooling. As a result, Vickers engineers were able to surround the unit in a sound shroud without concern for reduced air flow.
Introduced in 1994, the Integrated Motor-Pump has made in-roads at theme parks and in factories around the world. Nuova Plastic Metal, Chiampo, Italy, recently incorporated the Integrated Motor-Pump in its Dynamika Series of plastic injection molding machines, resulting in noise reductions of 70%, the company says. The reason for the dramatic change, Nuova engineers say, is that the unit's electric motors used to be located outside the machine, whereas the motor-pump can now be placed internally. For workers, they say, the results are dramatic. "In the average plastics processing plant, normal conversations were almost impossible because of the noise," notes Merrick Adelstein of Nuova. "But when we run this machine at exhibits, we have no problem talking to customers."
Structure-borne noise common. Reduction in pump and motor noise, however, hasn't eliminated the problem of noise in general. The reason is that any part of a hydraulic system can start buzzing, whining, or banging, given the proper conditions. "The pump-motor is the source of energy, and everything else you hear is the result of it," explains Paul Smith, manager of industrial applications engineering for Vickers.
As a result, hydraulic noise levels of 85-90 db(A) are still commonplace. And that's significant, when you consider that noise levels double for each increase of 3 db(A).
Still, hydraulics industry engineers insist that high noise levels shouldn't be the norm. Much of today's noise is structure-borne and can be easily minimized, they say. To accomplish that, they offer a series of common sense design recommendations for any hydraulic system. Those include:
- Isolate manifolds.
- Use rubber grommets between the manifolds and frames.
- Isolate pump-motor assemblies from reservoirs by employing isolation mounts or shock mounts.
- Use cast iron motors.
- Operate motors at lower rpms and use larger pumps to compensate.
- Wherever possible, use rubber hose instead of steel tubing.
- Employ elastomeric or polymer-type clamps for hoses.
- Don't use metal on metal.
- Don't undersize pumps.
Engineers explain the rationale behind some of these recommendations. "When you rigidly mount a manifold to a frame, you make the whole frame vibrate," explains Jennifer Hoey, an engineer for Parker Hannifin's Hydraulic Pump/Motor Division, Ostego, MI. "If you use isolation, much of the noise disappears."
"The more rigid the plumbing, the more opportunity there is to transmit vibration and noise through your system," explains Scott Kane, technical services manager for Parker Hannifin's Fluid Connectors Group.
"If you undersize your pump, it will whine when it tries to supply the flow you need," Kane says.
Industry engineers say that it's important for users to remember that most noise is simply a byproduct of the resistance to flow. As a result, much of that noise is structural, rather than component-borne. "None of the components are inherently responsible for the noise that's produced," notes Brian Burgess, product manager for fixed displacement products for Parker Hannifin. "Most noise is the result of work being done in the hydraulic circuit."
Do you have a noise problem?
If the measured noise level exceeds your motor and pump's db(A) rating plus 3 decibels, there's a strong probability you have structure-borne noise.
Pump up the volume
TYPICAL SOUND LEVELS db(A)
Jet aircraft 140
Elevated train 120
Subway train 110
Noisy industrial plant 100
Noisy office 80
Average street noise 70
Typical office noise 60
School classroom 40
- Noise output levels for pumps may be higher or lower, depending on the pump's horsepower, operating environment, and amount of structure-born noise.