Hydraulic Accumulators Tame Shock and Vibration

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 tinyurl.com/47e3grh, 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.