How to reduce the harmful side effects of noise and vibration caused by hydraulic power units in industrial environments?

Nothing is more jarring than the high-frequency noise emanating from a hydraulic power unit. The noise primarily originates from the hydraulic pump, but secondary harmonics and reverberation also contribute, creating a sound distinct from other machine noises. The typical hydraulic hum is a nightmare that machine operators must mentally block out, sending chills down their spines.

Noise levels are a primary concern for engineers and designers of hydraulic power units. Therefore, manufacturers design components that are inherently smoother and quieter. Hydraulic designers may also choose components that are less prone to noise. Meanwhile, technicians can minimize the conversion of vibration into annoying and potentially dangerous noise.

No pump can perfectly transmit pressure without generating bumps, turns, or turbulence; every resistance introduces vibration and noise. Lens plates, gears, blades, inlets, and pistons often have sharp edges, sharp turns, or blocked flow paths. Fluid flow in a pump is far from laminar. Even if every metal pump component is chamfered, beveled, and honed, the resulting noise reduction is likely to be offset by a similar decrease in efficiency.

From a design and installation perspective, manufacturers have a responsibility to strive for innovation and to develop noise reduction solutions. Furthermore, manufacturers must balance the diverse requirements of the pump, necessitating efficient operation at varying pressure and flow outputs. For example, a pump designed to withstand pressure pulses of 1800 rpm and 3000 psi may perform poorly at 3450 rpm and 1000 psi.

Pump pulses are a major driver of noise, vibration, and harshness (NVH). Each burst of hydraulic energy released from pump-driven elements (such as pistons) or chambers (such as vanes and gears) produces a pulse, and the frequency of this pulse determines the timbre of the hydraulic noise. The frequency is the product of the main engine speed and the number of elements or chambers. A 9-piston pump operating at 1750 rpm generates 15750 pressure signals per minute. Acoustically, the pump’s fundamental frequency is 262.5 Hz (15,750 divided by 60 seconds per minute), while also emitting high-amplitude harmonics at 525 and 787.5 Hz.

If you’ve ever designed or manufactured a hydraulic power unit, you know we don’t just weld or bolt the pump/motor unit together and hope for the best. Damping of sound and vibration begins at the connection between the pump and motor. The drive coupling transfers power from the motor to the pump and uses rubber inserts clamped between the couplings to reduce vibration transmission from one joint to the other. The coupling and inserts also help to prevent shaft misalignment.

Every time you introduce elasticity into the interface, you lower the natural frequency of the component. The elastomer (or actual spring) should be chosen with a natural frequency at least half the pump’s rotational frequency. For example, your 1800 RPM motor with a 30 Hz frequency should ideally be paired with an isolator providing 10 Hz or lower. This principle applies not only to coupling inserts but also to vibration-damping mounting feet used to connect horizontal pumps/motors to reservoirs.

Of course, besides the pump/motor unit, other components transmit sound and vibration. You might be surprised at how much sound energy is transmitted through pipes, such as tubing and hoses. The steel structure of pipes or hoses can vibrate in sync with specific harmonics, although longer pipes are somewhat mitigated by their naturally lower frequencies.

However, in long-distance ducts, using only pipes or flexible hoses along the entire length can achieve highly efficient acoustic energy dissipation, especially when harmonics overlap with the fundamental frequency. The ideal configuration for noise reduction via ductwork is to run a single pipe along the longest section of the duct, terminating at both ends with short flexible hose assemblies. Pipe sandwich technology not only reduces cyclic radial expansion on long flexible hoses but also isolates the duct from direct vibrations from the machine.