A trend in industrial hydraulic power unit design has been to use a variable-speed electric drive to power a fixed-displacement hydraulic pump. Although the concept has been toyed with for decades, it wasn’t until today’s high-speed controls, quick-response motors, and sophisticated software came together to produce motor-pump combinations that have the power and responsiveness of their electrohydraulic valve controlled counterparts.
Moog Industrial Group, East Aurora, N.Y., has developed its speed controlled pump system, which machine builders can use to lower energy consumption in hydraulic machines by 30% or more when compared to conventional approaches. The system includes Moog’s RKP fixed displacement radial piston pump, a brushless servomotor, and a modular multi-axis programmable MSD motion control servodrive. The system changes the speed of the motor driving the pump, thereby controlling fluid flow. The resulting power unit is not only more efficient, but is quieter and provides easier setup for operators as well.
Lower cost of ownership
Most industrial machines undergo partial, medium, and full loading as part of normal operating cycles. In tests conducted by Moog, the speed controlled pump under medium load exhibits 20-30% higher efficiency compared to a conventional system. Moreover, the speed controlled pump can reduce energy consumption by 90% when that same machine is running at no load or in a standby mode. And performance compared to a traditional system under full load is nearly identical. This means users can conserve energy that would otherwise be wasted when machines operate at less than full load, and with no loss in performance.
Energy is certainly a major cost of operating a machine over its lifetime. But maintenance costs and downtime can also make hefty contributions. Moog’s tests show that the total cost of ownership will be lower than with conventional hydraulic technologies. This is because the speed controlled pump system has fewer valves, conductors, and other components that would otherwise require replacement. Furthermore, the payback period for the initial investment is also shorter for operators due to the energy savings.
The speed controlled pump system not only offers lower maintenance costs than with conventional hydraulic systems, but also drastically reduces maintenance costs when compared to all-electric motion systems. Electromechanical drives on all–electric injection molding machines are generally built into the framework of the structure — and the main component, the ballscrew, has a finite, rated life. Consequently, when the ballscrew needs to be replaced as part of routine maintenance, the electromechanical infrastructure needs to be totally disassembled and re-assembled.
The cost of rebuilding a hydraulic machine with Moog’s Speed Control Pump system is much lower because it is an integrated modular unit with all components easily accessible for maintenance and upgrades. And with Moog’s global support network, OEMs and end users have access to service experts anywhere in the world.
Not just any pump
Dave Geiger, of Moog, says that the design of the radial piston pump lends itself to this application. “The radial piston pump, by nature, has high volumetric and mechanical efficiencies. This makes it better suited to this type of application than other pump designs.” The speed controlled pump system is also quieter than conventional hydraulics, with acoustic emissions up to 9 dBA lower under partial load conditions.
Moreover, when required, the pump can feature dual-displacement operation, capable of intelligently switching from one to the other displacement. For example, during an injection molding machine’s pressure-holding phase — when high pressure at low flow is needed — the pump can save up to 90% of the energy otherwise needed.
Geiger explains, “In its high-displacement mode, the pump runs at maximum displacement, which produces high flow at a relatively low pressure. Once higher pressure is required, control algorithms cause a high-speed logic element to shift the pump’s displacement control ring into its low-displacement position. This shift occurs in 15 to 20 msec and decreases displacement so the pump can maintain high pressure at low flow. The sophisticated algorithms produce a bumpless transition between high-and low-displacement modes.”
Moog has also developed a system with the pump and motor submerged inside the hydraulic reservoir, which produces even quieter operation. This technique also reduces the size of the motor because the hydraulic oil dissipates heat more effectively than air does. Air cooled motors require large surface areas to dissipate heat, so cooling the motor with oil reduces the surface area (size) of the motor.
Of course, the system would be incomplete without Moog’s MSD servodrive controlling its brushless electric motor. Depending on pressure and flow demand values, the MSD controls the motor’s speed and torque. When pressure needs to be held while holding position, the MSD controls motor torque, rather than speed. Pump and servomotor characteristics are stored in the servodrive, creating a system that can communicate with external systems over a field bus.
For more information on Moog’s speed controlled pumping system, or other components, call (800) 272- 6664, or visit www.moog.com/about/industrial-group.