What is in this article?:
- BOOK 2, CHAPTER 1: Hydraulic Accumulators (part 2)
- Using an accumulator as an emergency power supply
- Using accumulators for leakage makeup
- Using accumulators as shock absorbers
Using accumulators as shock absorbers
Accumulators can reduce damage from shock in some circuits if correctly applied. In other applications, an accumulator may add shock by releasing stored energy too quickly.
The top half of Figure 1-28 illustrates one way shock is produced. Flow velocity in a hydraulic circuit may be 25 to 30 fps and not cause any problems. However, if oil flow stops abruptly, as seen in Figure 1-28’s middle example, damaging shock can rip out tubing, blow seals, and split pump housings with ease. A column of moving fluid has a lot of energy that can get out of control.
The third example in Figure 1-28 shows a small accumulator teed into the line at the shut off that stops flow suddenly. An accumulator spreads the shock energy over a short period of time and eliminates the potential for damage.
To absorb flow shock, the accumulator is usually pre-charged at about 70 to 80% of system pressure. At this pre-charge pressure, only a small amount of fluid enters the accumulator subsequent to a shock situation. There is also little fluid transfer to take away from or add to the normal pump flow.
When it is necessary to stop a heavy load, such as shown in Figure 1-29, try using an accumulator and a hydraulic cylinder. The accumulator’s pre-charge pressure holds the cylinder extended, thus making it ready for the advancing mass. When the load contacts the cylinder, it mechanically forces it to retract. As the cylinder retracts, fluid flows into the accumulator and gas pressure increases. As pressure increases, the higher resistance slows the mass more. After the load decelerates, the cylinder might try to push the part away. Add valves between the accumulator and the cylinder to control the shock absorber after it finishes decelerating the load.
Some large, slow-turning piston pumps send a shock wave into the circuit every time a piston discharges oil. On the left of Figure 1-30, the piston pump does not have an accumulator at the discharge port. Pressure at the gauge will fluctuate from less than system pressure to well above it without an accumulator.
On the right side of Figure 1-30, adding a small accumulator reduces discharge flow and shock damage. A portion of the sudden discharge flow from an advancing piston goes into the accumulator and discharges smoothly while waiting for the next stroke. The pre-charge pressure for this type of accumulator circuit is about 60 to 75% of maximum system pressure.
Accumulator manufacturers have formulas in their brochures to calculate any situation mentioned here. Some suppliers have computer programs that do all the math after asking for circuit parameters.