Many designers have long accepted leaks as inherent to hydraulic systems, even though advances in technology should have eliminated hydraulic leakage a long time ago. Hydraulics suffers a similar identity crisis when it comes to noise. Noise certainly cannot be eliminated, but a number of products and techniques exist to at least bring noise down to an acceptable level. The problem is that noise reduction is a complex subject, and investing a great deal of time, effort, and money may produce only modest improvements.
Sources of noise
A hydraulic system's greatest contributor to noise is the power unit, Figure 1. Noise not only emanates directly from the electric motor and pump, but also is caused by pressure fluctuations in the hydraulic fluid and by components — either resulting from these pressure fluctuations or from physical vibration. Transmitting vibration of the pump-motor assembly to the reservoir can transform this physical vibration into sound - in the same way a loudspeaker transforms electromagnetic vibrations into sound.
Electric motor noise comes from bearings, the rotor and stator assembly (the characteristic hum), and, especially, the fan. A standard electric motor contains a fan with blades designed to provide cooling whether the motor shaft rotates clockwise or counter-clockwise. A fan designed for rotation in only one direction will generate less noise, so the expense of this option may be warranted if the application demands quiet operation.
Pump noise stems from rolling and sliding of bearings and pumping elements (vanes, pistons, rotors, gears, etc.), plus pressure fluctuations that result from the cyclical nature of the pumping process. Metal housings, whether part of the hydraulic pump or an electric motor, do little to prevent noise from being transmitted to the surrounding environment. Moreover, because the pump generally is coupled to an electric motor (and the coupling itself is a source of noise), noise control often involves treating the pump-motor combination as a unit. This design technique has produced power units where the pump-motor combination is submersed in oil or where the entirepower unit is submerged in the reservoir. This technique uses liquid to dampen sound waves by acting as a buffer between the pump-motor housing and the surrounding atmosphere.
Valve noise has occurred in cabs of construction and other mobile equipment for years. Often, a high-frequency, random noise occurs when fluid, traveling at high velocity through the valve, undergoes a rapid and severe drop in pressure. This causes air dissolved in the fluid to form bubbles which, when they collapse, generate noise. Other types of noise — such as chattering, squealing, or buzzing — is generated when poppet-type valves do not seat properly.
Fortunately, most of these problems can be eliminated through better system design or by incorporating cushioning features into valves. A current trend replaces direct-operated valves with joystick-controlled remote electrohydraulic valves. This process of removing the hydraulics from the equipment cab offers other advantages beyond providing a quieter workplace environment.
Fluid conductors (tubing, hose, fittings), often are overlooked as noise sources. However, pressure pulsations in plumbing can distribute noise over a large area. Pressure pulsations can shake hose and tubing, causing rattling and eventual leakage.
Although reducing fluid-borne noise can be complicated, many manufacturers suggest rules of thumb to help reduce noise. For example, terminating a long run of metal tubing with a section of hose at each end helps isolate noise sources, Figure 2. One might be tempted to simplify the design by instead specifying a single section of hose. Hose, however, is very sensitive to pressure pulsations, so in long sections it can be a greater source of noise than metal tubing or pipe.
Securing tubing to framework with resilient clamps eliminates rattling and banging noise. However, care should be taken not to confine tubing too tightly, because lines may need to undergo thermal expansion. On the other hand, allowing a tube to fit too loosely could cause wear as the tube constantly rubs against a metal clamp surface. Likewise, resilient grommets should be used when a hose or tube passes through a hole in framework, covers, etc.
Actuators, especially hydraulic motors, also generate noise. Hydraulic motors sometimes are considered to generate noise equivalent to that of pumps. However, hydraulic motors often operate at relatively slow speeds, so motors generally operate much quieter than pumps do.