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.
Designing to reduce noise
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 [email protected] or visit www.parker.com/parflex.