A cost effective solution for reducing noise and adding damping and ramping characteristics to hydraulic functions is to use 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 VE, the greater the damping effect.
Some examples are manufacturers of mobile equipment using hoses with a high VE to reduce hydraulic shock in steering systems while subsequently reducing hydraulic induced noise and vibration. High VE hoses can also be used in applications where sudden movement is detrimental, such as swing functions and even ramping on loadsense lines. On the other hand, hoses with low VE should be used when rapid function response is critical. Noise generation in hydraulics Noise in hydraulic systems is generated primarily by the pump and fluid pulsations exiting its outlet. Noise can be also generated 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 significant noise transmission. Transmission of noise to human operators of the equipment can cause fatigue, nerve damage, and require them to wear hearing protection.
Noise can be 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. Other effective methods of noise reduction are less intrusive and inexpensive ∇ specifically, using thermoplastic hydraulic hoses in place of conventional wire reinforced rubber hoses.
Noise is caused by pressure waves and flow surges occurring in the hydraulic fluid that cause vibration of mechanical components. Audible and inaudible noise can be transmitted by components of the hydraulic system to other elements of the machine. Audible noise emanates from multiple areas of the machine, but inaudible noise can travel throughout a system and become audible far from its original source. Both can be troublesome, so reducing them provides many benefits. Audible noise can be hazardous to human operators, but inaudible noise can cause additional load and wear on hydraulic components. Combined, they can lead to premature failure, additional system cost, operator fatigue, and potential hearing loss.
The U.S. Department of Labor∏s Occupational Safety and Health Administration states that exposure to 85 dBA of noise for more than eight hours a day can result in permanent hearing loss. About 30 million workers in the U.S. are exposed to hazardous noise levels, making occupational hearing loss one of the most common occupational afflictions.
Mechanical noise can cause damage to many different components in hydraulic systems, but steel tube assemblies, in particular, can be susceptible to vibration failure. Mechanical resonances occur within a system when it can store and easily transmit energy between two or more components. When the frequency of oscillations in a system approaches its natural frequency, vibrations occur. Each component in the system has a natural frequency. When combined, and depending on their damping and energy transmission properties, they will have a new set of natural frequencies.
Vibration can be transmitted to all parts of the hydraulic system via the fluid and the metal components. Noise readily travels though metal components, such as pumps, valves, cylinders, steel tubes, and fittings, but can also travel through the steel wire reinforcement in hose. For example, assume a gear pump is used in the swing drive of a typical excavator. If the gear pump operates at 2000 rpm and has 8 teeth per gear, sinusoidal waves at a frequency of 267 Hz will result. This means all components or assemblies (from a single tube installation, implement, to the entire machine) with natural frequencies of or near 267 Hz, 534 Hz, 801 Hz, etc. will resonate due to harmonics.
Lowering resonant frequency by damping can reduce vibration of assemblies. Quality factor, Q, is a dimensionless parameter that describes how damped a resonator is. A low value of Q indicates a high rate of energy loss relative to the stored energy of the resonator. So a low Q makes oscillations decay more quickly. Therefore, components and designs that reduce Q in a system are beneficial for reducing noise transmission.