Ahead of the bell-shaped curve

DN Staff

March 26, 2001

3 Min Read
Ahead of the bell-shaped curve

Eden Prairie, MN-Hydraulic systems are the source of excessive airborne noise in many plants and manufacturing facilities. Traditional approaches to minimizing noise levels include shrouding the pump, which involves placing the pump assembly in a box-like structure lined with noise dampening materials on the box sides.

Fluid silencers are another approach to noise reduction. They attenuate the pump pressure ripple by adding expansion volume at the pump outlet. However, silencers also discharge small increments of pressure pulsation that change the peak-to-peak energy output and result in pressure fluctuations coming from the pump. Both approaches mask symptoms and do not attack the real problem-vibration originating in the pump's structural components.

Engineers at Eaton Hydraulics discovered that excessive noise originated from the bell-shaped geometry of their previous pump, which produced structural vibrations that were converted to noise. Their approach to noise reduction addressed the problem at the source.

The designers determined that fluctuations in fluid pressure changed the force exerted against the pump's housing walls. Force fluctuations produce vibration in the pump's structural components and in the entire hydraulic circuit. The structure-borne vibration emits the audible air-borne noise.

Bell shape of the old pump geometry creates structural vibrations that are converted to audible noise. Below, the teardrop shape of the new M-series pump design makes the pump stiffer and helps reduce audible noice by 9dB.

Eaton engineers knew that reducing the internal pulsation would reduce air-borne noise. They found that by changing the shape of internal components from a bell to a teardrop and making the pump stiffer, they could reduce noise levels within operating levels of 1,800 rpm and up to 280 bar.

"In conventional pumps, the housing extends from the mounting flange in a hollow rectangular shape," says Brent Coppock, the industrial piston product manager at Eaton Hydraulics (see old design, previous page). "When excited by the pumping energy, the flat sides of the envelope flex and balloon with the pump's modal frequencies." Modal analysis, which includes the study of mode shapes and natural frequencies that amplify noise, was used for designing the pump's structure.

Eaton's approach to the design of its new M-series piston pumps is a teardrop structure (see new design, previous page). This design moves natural structural frequencies outside the spectrum of the pump's pressure ripple. "As more mass is shifted to the flange end of the pump, the structure becomes stiffer and the motion of the sides of the housing is reduced," says Coppock.

Coupled with improvements in port timing, modeling, and groove design, Eaton reduced noise levels in its M-Series piston pumps by 9 dB. Since sound levels are dependent on the environment, the actual noise reduction may vary from application to application.

At Tower Automotive's truck frame assembly plant in Milwaukee, hydraulic designer Don Colvin uses the new pumps for opening and closing tooling on assembly fixtures for manufacturing DaimlerChrysler's Dodge Ram trucks. "On assembly lines where noise levels were a strong consideration, the noise reduction is very noticeable," he says.

Additional Details

Contact Brent Coppock, Eaton Hydraulics; 14615 Lone Oak Rd., Eden Prairie, MN 55344; Tel: (952) 294-7982; E-mail: brentcoppock@

eaton.com; or Enter 501

Typical Applications

Construction equipment

Industrial machinery

Material handling equipment

M-Series ripple data

The lower the ripple, the lower the noise inducing vibration. Engineers reduced the ripple by about 25% by gearing the timing of the pressure rise rates into and out of the chamber.

Pressure (bar)

Peak-to-peak ripple


7 bar


8 bar


11 bar

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