What creates noise in hydraulic systems and why is it important to eliminate it? Noise in hydraulic systems is generated primarily by the mechanical workings of the pump and fluid pulsations exiting the pump as it supplies the flow for the system. It can be also created by any element that causes turbulence or fluid velocity change. Noise is additive, so small amounts of noise from many components can be effectively amplified resulting in a significant noise problem. The transmission of the noise to the operators of the equipment can cause fatigue, nerve damage, and require operators to wear additional hearing protection.
For this discussion, we will use the term noise to refer to audible as well as inaudible waves in the fluid. Both can be troublesome for the fluid power system designer and end user so it will be our attempt to find ways to eliminate or at least minimize noise.
Noise is undesirable because it can cause additional load on hydraulic components leading to premature failure, additional system cost, operator fatigue, and potential hearing loss. The U.S. Department of Labor’s Occupational Safety and Health Administration (OSHA) states that exposure to 85 dB(A) of noise for more than eight hours per day can result in permanent noise induced hearing loss (NIHL)1.
Noise is known to cause many issues with components in hydraulic systems but in particular steel tube assemblies are known to be very susceptible to vibration failure.
Mechanical resonances occur within a system when it is able to store and easily transmit energy between two or more modes. When the frequency of oscillations in a system approach the system’s natural frequency, mechanical resonance in the form of vibrations occur. Each component in the system has a natural frequency and when combined will have a new set of natural frequencies depending on their damping and energy transmission properties.
Vibration can travel through the system via the fluid and/or metal components transmitting to all parts of the equipment. Noise travels easily though the metal components such as pumps, valves, cylinders, steel tubes and elbow fittings but can also travel through the steel wire reinforcement in hose.
Designing for noise elimination
Noise is commonly eliminated by adding attenuators which can be tuned to cancel the frequency out of the system. Attenuators are effective but relatively expensive and bulky units. One cost effective and simple solution for reducing noise and/or adding damping and ramping characteristics to hydraulic functions is the use of hoses with varying rates of volumetric expansion (VE). Like accumulators, hoses have a capacitance characteristic but to a much lesser extent, that is, the higher the volumetric expansion the greater the accumulator effect.
A quick and easy solution that some designers have discovered to eliminate noise in power steering systems, hydrostatic pumps, pump outlets, motor inlet/outlet and PTO’s is to utilize thermoplastic fiber reinforced hose. This hose is constructed using a variety of smooth bore polymer inner cores for a high degree of chemical compatibility, high strength fibers, and polymer jacket. Fiber braided thermoplastic hose is available in pressure ratings from 500 psi to 7500 psi.
Parker's Parflex Division is contacted at least once a month by companies looking to bring the noise down below audible noise level (vibration). Through years of studying noise and its effects on hydraulic systems, as well as, working closely with our customers to reduce application specific noise, Parflex has developed an extensive line of thermoplastic hoses with a high degree of dampening effects.
Parflex 515H, 510C, 540N, 520N and 53DM are most suited for reducing noise and have a working pressure range of 1500 to 5000 psi at a 4:1 design factor. Hose selection tools and application engineering expertise are available through Parflex for your equipment hydraulic design needs.
Download the full, updated article, "Reducing Noise in Hydraulic Systems," which includes new customer case studies.
1 Occupational Health and Safety Administration [OSHA], 2002
Article contributed by Greg Hayes, OEM Sales Manager at Parker Parflex Division
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