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Hydraulic Accumulators Tame Shock and Vibration

March 2, 2011
Accumulators help ensure smooth, quiet, and safe operations.
Authored by:
Bryan McGehee

Application Engineer
Parker Hannifin Corp.
Global Accumulator Div.

Machesney Park, Ill.

Edited by Kenneth J. Korane
[email protected]

Key points:
• Quick-acting valves and mechanical impact can generate hydraulic shock waves.
• Shock and pulsating flow cause noise and can lead to premature system failure.
• In-line shock suppressors smooth out flow variations and can make equipment quieter.

Parker Hannifin

Hydraulic systems are noted for being highly responsive even when moving heavy loads. But the dynamic behavior that can give unmatched motion control sometimes produces nasty consequences — namely shock, vibration, and noise.

Pumps, for instance, often deliver pulsating flow that makes lines vibrate, while starting and stopping actuators can send shock waves through an entire circuit. Both can hurt overall performance, cause objectionable noise, and even lead to system failure. Here’s how accumulators let equipment run smoother, quieter, and safer.

Shock suppression
Many industrial and mobile machines experience severe mechanical and hydraulic shocks when a moving part — such as the bucket on a front-end loader — stops suddenly. Designs that let a cylinder bottom out but rely on relief, vent, or compensator valves that do not respond quickly enough will also generate hydraulic shocks. And quick-closing valves and pump start/stop cycles can trigger water-hammer-type ripples that travel through a system. These can build to peak pressures well in excess of normal operating pressures.

Shock waves can create unwanted noise and, in severe cases, even harm upstream components. Shock waves also sometimes excite natural harmonics in piping that resonate throughout the system, again causing noise and damage.

Regardless of the source of shock, putting an accumulator into such systems lets the unit’s trapped gas absorb surges and reduce or eliminate their harmful effects. While piston accumulators can be used, quicker-acting bladder accumulators are more often the best choice.

The accompanying Accumulator circuits schematics show the three most-common techniques for plumbing an accumulator into a system. The first uses a T-union in the hydraulic line. Install the accumulator as close as reasonably possible on the perpendicular branch of the T. A large fluid port on the accumulator is recommended to best absorb shock.

Another method for absorbing shock routes oil flow through the accumulator. The second schematic represents Parker’s Greer Pulse-Tone shock suppressor that has a baffle in the hydraulic port. The baffle directs oil into the bladder accumulator’s shell, thereby protecting downstream components against shock. (See the sidebar on hydraulic shock suppressors for more details.)

The last schematic shows an economical alternative for plumbing an accumulator into the system. A general rule of thumb is this type of installation reduces shock levels by about 5%.

Sizing for shock
When sizing for shock suppression, key factors are the mass and velocity of fluid in the hydraulic line and pressure of the shock waves. Calculate the required accumulator volume, V1, using:

The discharge coefficient, n, is based on factors such as the type and size of accumulator, pressure, discharge rate, and temperature. Specs are typically found in manufacturers’ catalogs. (See, p. 134, for additional details.)

However, when insufficient data are available to properly size accumulators, the following are good guidelines to aid design engineers.

• Always consult an accumulator expert (even if only to verify your calculations) to help avoid the costs and consequences of an improperly sized accumulator.
• Use the largest port available.
• Match the port and line sizes.
• Start with a precharge pressure that is 60% of the maximum operating pressure.
• A good rule of thumb is to set allowable pressure 5% above system pressure. Calculate the shock pressure and plug it into the equation. Repeat with double the initial estimate. This aids in understanding how accumulator size increases with shock pressure. Varying the precharge pressure also affects the results of this calculation.
• Compression ratio (shock pressure: precharge pressure) should not exceed 4:1.

Pulsation dampening
Design engineers often prefer hydraulic piston pumps, due to their compact size and high-pressure capability. However, these positive-displacement pumps generate pulsations — similar to a continuous sine wave — as the pistons stroke. Like shock waves, these pressure waves can induce vibrations detrimental to system components.

As pistons in the pump barrel rotate from low to high pressure, a small pressure ripple or pulsation is created each time the piston crosses over to the high-pressure region. For instance, a pump with nine pistons in the barrel, rotating at 1,725 rpm, produces 15,525 pulsations/min (a pulsation rate of about 260 Hz).

Fortunately, a properly sized accumulator effectively cushions these pulsations. Sizing an accumulator for pulsation dampening is similar to that for shock suppression:

With shock, accumulator size depends on the mass and velocity of fluid in the line. Specifying an accumulator for dampening, however, depends more on the pump design and internal pressures.

When sizing for pulsation dampening, first determine the type of piston pump. The number of pistons establishes whether the pump is simplex (one cylinder), duplex (two cylinders), triplex (three cylinders), and so on. Each additional cylinder introduces another sine wave into the system as the pump strokes, resulting in complex vibration patterns.

Also determine if the pump is single or double acting. A single-acting pump fills the cylinder only as the piston strokes in one direction (called the suction stroke), then forces the liquid from the cylinder on the return (discharge) stroke. A double-acting pump fills one end of the cylinder as it discharges liquid from the other end. On the return stroke, the end of the cylinder just emptied fills and the opposite end empties.

With this information, select the pump output coefficient from the table. A double-acting triplex pump, for example, has an output coefficient of 0.06.

Next, multiply the bore and stroke of a single piston by the number of cylinders in the pump to determine piston area and stroke.

Finally, consider operating pressure and maximum allowable shock pressure. As previously stated, when sizing for shock suppression, a good rule of thumb sets allowable pressure 5% above system pressure.

When sizing for pulsation dampening, however, a good baseline for maximum allowable shock pressure is 100 psi greater than maximum system pressure. The accumulator’s location in the circuit is also important. For best results, this is typically at the pump outlet.

Noise attenuation
Noise reduction has become increasingly important in both mobile and industrial systems, thanks to heightened concerns for worker safety and tighter guidelines that have regulators clamping down on excessively loud machines.

Given the complex and unpredictable interactions between components, trying to estimate in advance a hydraulic power unit’s noise level is nearly impossible. Nonetheless, here are some tips on how fluid-power designers can minimize noise and meet specifications.

Both the amplitude and frequency of noise are important. Sound meters typically measure amplitude in decibels (dB) and pass it through an “A” filter that reduces sound readings for frequencies below 1,000 Hz, because our ears are less sensitive to lower frequencies. Thus, the familiar dB(A) scale is designed to mimic amplitudes and frequencies most sensitive to humans.

Any improvements that reduce noise levels even a few decibels or shift the frequency lower have a major impact on the apparent noise a machine makes. (Note that sound intensity doubles with an increase of 2.71 dB(A).) Consequently, noise-reduction strategies involve minimizing pressure pulsations, mechanical vibrations, and radiated vibrations by either dampening or decoupling a noise source from a noise transmitter. For example, rubber isolation rails decouple motor vibrations from the machine base.

Noise in hydraulic equipment can be fluid borne, structure borne, or radiated. Pump pressure pulsations are the primary source of fluid-borne noise. Because fluid starts, accelerates, and stops with each stroke in a positive-displacement pump, fewer pistons, vanes, or gear teeth mean more flow variation and, therefore, a greater pressure ripple in the system. These ripples induce vibrations in the hydraulic plumbing that may transmit to the machine structure and radiate to the air.

Other causes of fluid-borne noise involve sudden changes in the kinetic energy of the fluid — from water-hammer shock, decompression shock, and turbulent flow.

Products, like in-line shock suppressors and bladder accumulators reduce pressure-ripple noise by smoothing out flow variations at the pump outlet. Less pressure ripple induces smaller vibrations in the plumbing and, therefore, makes for quieter equipment.

Structure-borne noise comes primarily from an eccentric load radiated from or conducted through the system. Whether it is from a prime mover like an engine or electric motor, or from an actuator, these oscillations can move structures back and forth at a natural frequency that generates audible noise.

When conducted noise reaches large, flat surfaces — such as the top and sides of hydraulic reservoirs — noise is amplified and radiated to the surroundings, much like an audio speaker. Measured noise levels can often be 3 to 20 dB(A) louder than sound levels at the motor and pump itself.

Mobile systems pose a unique challenge. With wide-ranging pressure and flow demands, using standard accumulators with a fixed precharge — particularly on load-sense systems — often exceeds the product’s design parameters. The best solution is to use noise-attenuation devices without a precharge.

Gas bottles, also known as attenuators, are a good alternative. Typically, ports in gas bottles match the line size on both ends and simply mount in-line. There are no moving parts or precharge to worry about.

Gas bottles act like an expansion chamber that lets pressure waves propagate inside the larger diameter and cancel each other out when reflecting off the bottle’s walls. An attenuated, lower-amplitude pressure ripple leaves the exit port. The ripple can be tuned and minimized by either varying the gas-bottle diameter or length on a trial-and-error basis.

Attenuators work exponentially better at higher rpms and pressures, which is why they are well suited for demanding mobile-equipment operations. Engineering and sizing assistance from the gas-bottle manufacturer is generally recommended in these applications.

Noise-reduction strategies
First, lower the pump speed. Lowering pressure or pump displacement helps, too. However, decreasing pressure or pump displacement has nearly an equal effect on reducing noise, but reducing rotating speed has about a 300% greater effect.

Second, decouple oscillations from conductors or potential speakers using dampeners such as rubber pads, wire-rope isolators, and tube clamps.

Third, use hydraulic tubing for 90 and 180° ends, not hose. A straight length of hose is fine to help decouple pump vibrations from the plumbing — yet for bends, use tube. One hydraulics manufacturer found that bent hose raised noise levels by up to 5 dB(A).

Fourth, use in-line shock suppressors, accumulators, or attenuators to reduce pressure-ripple-induced noise. On mobile equipment, only use attenuators.

While noise-reduction strategies can be complicated, applying fundamental design practices often help meet noise specifications. And don’t hesitate to contact the accumulator manufacturer’s application engineers for expert guidance, to double-check inputs and calculations, or simply to ask questions.

Hydraulic shock suppressors
Shock suppressors like Parker’s Pulse-tone units reduce pressure pulsations and shock waves traveling through a hydraulic line. They consist of: an inner radial chamber with a series of 0.5-in.-diameter holes; a compressed coil spring surrounding the inner chamber; an outer radial chamber dotted with 0.03-in.-diameter holes; and an elastomeric bladder around the outer chamber. A 0.25-in. gap separates the chambers.

In operation, oil flows through the inner radial chamber, spring, and outer chamber. The 0.03-in. holes maximize flow but keep the bladder from extruding through the outer chamber. Pulsations pass through the holes and strike and deflect the nitrogen-charged bladder. This deflection reduces shock and noise. Typical noise reduction is more than 6 dB.

The suppressor mounts as close to the pump as possible, usually directly at the pump outlet, to stop pulsations and noise before they travel through the piping and radiate off other structural components.

Generally, the chamber surrounding the bladder is charged with nitrogen to 50 to 60% of the hydraulic operating pressure. The combination of a larger bladder that can oscillate at high frequencies, and the short distance the pulsations travel once they enter the unit, contribute to the device’s effectiveness.

The in-line Pulse-Tone holds an advantage in systems where hydraulic pressure drops below precharge pressure, such as where the pump is unloaded at low pressure. Standard accumulators are not acceptable in these applications.

© 2011 Penton Media, Inc.

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