Through the use of leakage-free seated valves, two-way
pressure reducing valves and a higher efficiency radial piston pump,
Schwaebische Werkzeugmaschinen GmbH has been able to achieve energy savings of
73 percent on its BA 400 machining centers.
"One of the main ways to save energy with a machine tool is to control the creation of heat, so you don't need to cool the tool down again," says Ullrich Breuner, sales manager with Hawe Hydraulics, which worked with Schwaebische Werkzeugmaschinen to increase the efficiency of its BA 400 machining centers. "Working with Schwaebische Werkzeugmaschinen, we started developing seated valves that are leakage free, which is a very different function compared to spool valves."
Breuner says the compact hydraulic power packs used to revamp the BA 400 help save energy in three ways. The first, and most obvious way, is by using the leak-free seated valves to eliminate permanent leakage. A second way is the use of two-way pressure-reducing valves to control the different pressures for different functions required by the machine tool while also limiting the leakage rate. The third way involves use of a radial piston pump, which is more efficient than a gear pump.
"A major benefit in developing these machines has been the realization that if you reduce the leakage rate, the application can use a smaller pump," says Breuner. "If you don't have the permanent leakage rate and the need to re-fill, it's possible to use a 10-liter rather than a 15-liter pump. You are still able to achieve the same clamping time but use a smaller power unit because you have eliminated the leakage. And by using a submersible motor, it helps create a compact system that limits the amount of oil needed."
Radial Piston Pump Efficiency
The BA 400 series machining center is
used for small to medium-sized workpieces, as well as ferrous metal machining.
The machining center is available with two or four spindles and provides torque
up to 200 Nm. It is designed for clamping blanks while workpieces are machined
on the opposite worktable; it can also be used for multiple workpiece clamping
on a double swivel table with three linear axes and two rotary axes. The
workpiece handling is done via robots or linear gantries.
Before implementation of the Hawe solution, the machine was fitted with standard hydraulic controls, but Schwaebische Werkzeugmaschinen was looking for a solution that would yield substantially more energy savings.
The Hawe system eliminates use of a gear pump to generate hydraulic energy and replaces it with a radial piston pump, which offers a higher efficiency than gear pumps or vane pumps. The choice of the pump's power pack (HK series) also comes with a submerged motor for the drive.
One key advantage is that the significantly smaller motor keeps the overall dimensions of the power pack compact; and it loses less power in idle mode than a standard air-cooled motor. Plus, at peak demand, submerged motors generate up to twice the rated output where a standard motor would break down due to thermal reasons. In a submerged motor, after approximately 30 min, the oil reaches a steady-state temperature, which is considered an ideal condition for operating machining tools. And because the HK power pack is built into an extruded aluminum casing with cooling gills, it allows unhindered heat transfer, as the air flowing by can easily dissipate the low thermal energy.
A key advantage of this design is its low oil volume which, depending on the size of the power pack, can be kept between 0.8 to 4 gal. Lower oil demand brings down the costs of exchanging oil as well as the strain on the environment. For the machining center, the power pack's selected size holds 4 gal of oil. Using its radial piston pump, the power pack supplies pressurized oil for operating pressures of up to 10,000 psi (700 bar).
Breuner says those pressures are unusual for machining tools because the majority of applications operate at pressures between 2,200-4,400 psi (150-300 bar). But because Hawe manufactures all of its pressurized components from steel, such high operating pressures are not problematic because the components can withstand these loads over long periods. Seated valves made from steel wear less than slide valves in cast housings.
A cooling wheel mounted at the top of the power pack cools both the oil and motor, which means that an additional external cooling unit is avoided by using Hawe's modular construction kit system. Due to the size of the motor, low oil volume and compact dimensions, the power pack can be built into the machine and integrated into the machining center.
Machine Operation with the New Pump
The machining center's working mode
varies depending on the workpiece. Many applications work with a mixture of
different workpieces, so that both the machine and hydraulics supply must be
able to handle everything from continuous operation to machine set-up time. At
standstill, the Hawe system operates in storage mode and saves additional
energy which can be used in the machining center.
A smooth switching accumulator charge valve recharges the storage whenever the duty switch point is reached. Because storage is charged when the machine is started, the machine control monitors this operation constantly. Once the predefined pressure has been attained, the valve will switch itself into depressor circulation. At the slightest pressure, oil is re-fed into the tank.
Hawe's modular system provides options for the input block, such as pressure and return filters or TUV-certified valves. A variety of filters and valves can be combined with the BA block to achieve highly customized specifications.
The BA 400 machining center comes with a combination of sub-plates and a seated valve block (NBVP 16), which again increases the machine's efficiency. Because the seated valves are pre-controlled, they work with minimum switching capacity. The directional seated valve provides a maximum volume flow of 5.3 gal/min and maximum operating pressure of 5,800 psi (400 bar).
The use of directional seated valves saves considerable electric power during operation of the machining center. A machine with 15 spool valves and/or 3-way pressure reducers accumulates a leakage of approximately 0.4 gal/min at 2,200 psi (150 bar), and the configuration requires an additional drive input of 375W compared to a solution with seated valves.
In a two-shift operation, each with eight hours per day and 200 working days per year, this adds up to 1,200 kWh for each machine. Plus, the heat generated by the additional energy input also needs to be lowered, which means that the power saved doubles once more.
"The hydraulic power pack system has been available for more than 10 years," says Breuner. "But, until recently, few companies were thinking about energy savings. Now companies are thinking more about the total lifecycle, not only the initial investment but also ongoing operational costs. If you need 1,200 kilowatts less per year to run the machine and there are 10 machines, the savings are significant."