Special tuning chambers in pumps and hydraulic attenuators can reduce noise in hydraulic systems at or near its source but can add to the cost of the pump. Moreover, many other sources of turbulence in the system plumbing exist — such as elbow fittings and transitions from full bore hose to steel tubing. These can render quieter pump designs and noise attenuators less effective.

You can replace steel tubing with wire braided hose to reduce some noise generation from the full inside diameter and the additional volumetric expansion of the hose. However, the steel wire reinforcement still provides a conduit of energy transfer in the form of vibrations through the system.

An easy solution that some designers have discovered eliminates noise is to use 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 a polymer jacket. Also, fiber reinforced thermoplastic hose has an inherently higher VE rate than that of an equivalent bore wire reinforced rubber hose, resulting in a total lower Q factor.

Furthermore, because hoses with a higher VE have a greater damping effect, they create a natural ramp in the function they are supplying, which can benefit certain functions, such as swing and steering. This same concept applies to load sense lines, where some systems are designed with an oversized signal line to create a higher capacitance and, therefore, a slight delay in pump response.

Fiber braided thermoplastic hose is available in pressure ratings from 500 to 7500 psi. Products from Parker Parflex with a high degree of damping effect are 515H, 510C, 540N, 520N and 53DM. These hoses have a working pressure range of 1500 to 5000 psi at a 4:1 design factor. A wide offering of Parflex thermoplastic hose uses high strength fiber reinforcement in place of steel. Therefore, they do not as readily transmit hydraulic vibrations from one component to another. This can eliminate the additive effects of noise and potential of resonant frequencies propagating through a system by isolating the components. This, combined with a higher volumetric expansion, can act as an attenuator, driving fluid born noise from the system.

Cases in point

A manufacturer of rear-engine mounted motor homes needed to solve a noise problem in its vehicle’s power steering system. The power steering pump was 35 ft away from the steering actuator, and the wire braided rubber hose routed between the pump and motor picked up transmitted noise from hydraulic and other mechanical components to the steering column.

Technicians at the manufacturer contacted Parker, who suggested they use 53DM hose instead of the wire braided hose. This simple change resulted in a dramatic reduction in noise experienced by the operator and complaints about operator fatigue. In this example the fluid and system born noise was extensive. However, the long thermoplastic hose assemblies routed in the system enabled sufficient noise damping.

In another example, turf maintenance equipment was generating an annoying mechanical noise from its steering valve. Operators complained of a pulse in the steering column, resulting in a “popping” noise.

The equipment manufacturer’s engineering team experimented with different types of hoses and settled on Parflex 510C hose. The high volumetric expansion rate naturally damped the pressure surge, eliminated the pulse in the steering column, and eliminated the objectionable noise.


Many applications cannot benefit from using hose with high volumetric expansion, especially if they need quick response. But when damping from using hose with high volumetric expansion clearly will benefit the application, a simplified hose selection tool has been developed based on the flow diameter of the hose and system pressure rating. The tool displays available hoses in the selected size and pressure rating that impart the least and greatest volumetric expansion. This allows choosing minimal or maximum damping to match the application.

This information was submitted by Greg Hayes, Mobile OEM Product Manager, and Jeff Lemonds, Applications Engineer, of Parker Hannifin’s Parflex Div. For more information, e-mail jlemonds@parker.com or visit www.parker.com/parflex.