Traditionally, increasing the speed and, consequently, the productivity of a piece of production equipment meant increasing the size of the power unit. But larger power units mean more energy usage and with today’s soaring energy prices, purchasers are paying much more attention to how a piece of equipment will impact energy costs.
With a simple piece of hydraulic equipment -- an accumulator -- it is possible to boost speed and productivity without increasing the size of the power unit as long as there is dwell time in the hydraulic circuit.
For Example...
Consider this example of an injection molding machine: Total machine cycle time is 16 seconds — 8 seconds to cycle the cylinder and 8 seconds of dwell time. An increase in productivity is desired, but dwell time cannot be changed due to machine loading and unloading requirements. The machine has a 15-gallon power unit working at 1,000 psi, but it is capable of 2,000 psi. Traditionally, the productivity increase would be achieved by increasing the size of the power unit to deliver a higher flow rate to the cylinder and speed up the cycle times. Instead, a 5-gallon accumulator was incorporated to supplement the flow rate of the pump and provide fluid to the cylinder. The pressure can be increased to 1,650 psi. This reduces the cylinder cycle time from 8 seconds to 4 seconds while maintaining the necessary 8 seconds of dwell time. At the original 16 seconds, 3.75 parts were produced per minute. With the accumulator addition, 5 parts per minute can be produced, a 33% increase in productivity.
The size of accumulator needed to reduce cycle time is determined by the maximum pressure the system can withstand, the minimum pressure required to do the work, and the cubic inches of fluid required to supplement pump flow. The necessary dwell time is determined by pump size. If 311 cubic inches are discharged from the accumulator during the machine cycle, the pump must be sized large enough to put the 311 cubic inches back into the accumulator during dwell time.
Hydropneumatic accumulators are the type most widely used in industrial applications. In operation, an inert gas -- normally nitrogen -- is compressed to apply force to hydraulic fluid. Long used for supplementing pump flow, auxiliary power supply, leakage compensation, and holding applications, accumulators today are being used in many new and different applications thanks to their ability to improve productivity, reduce initial and operating costs, handle increased payloads, deliver energy savings, and prolong equipment life.
