Through the use of leakage-free seated valves, two-waypressure reducing valves and a higher efficiency radial piston pump,Schwaebische Werkzeugmaschinen GmbH has been able to achieve energy savings of73 percent on its BA 400 machining centers.

"One of the main ways to save energy with a machine tool isto control the creation of heat, so you don't need to cool the tool downagain," says Ullrich Breuner, sales manager with Hawe Hydraulics, which workedwith Schwaebische Werkzeugmaschinen to increase the efficiency of its BA 400machining centers. "Working with Schwaebische Werkzeugmaschinen, we starteddeveloping seated valves that are leakage free, which is a very differentfunction compared to spool valves."

Breuner says the compact hydraulic powerpacks used to revamp the BA 400 help save energy in three ways. The first, andmost obvious way, is by using the leak-free seated valves to eliminatepermanent leakage. A second way is the use of two-way pressure-reducing valvesto control the different pressures for different functions required by themachine tool while also limiting the leakage rate. The third way involves useof a radial piston pump, which is more efficient than a gear pump.

"A major benefit in developing these machines has been therealization that if you reduce the leakage rate, the application can use asmaller pump," says Breuner. "If you don't have the permanent leakage rate andthe need to re-fill, it's possible to use a 10-liter rather than a 15-literpump. You are still able to achieve the same clamping time but use a smallerpower unit because you have eliminated the leakage. And by using a submersiblemotor, it helps create a compact system that limits the amount of oil needed."

Radial Piston Pump Efficiency

The BA 400 series machining center isused for small to medium-sized workpieces, as well as ferrous metal machining.The machining center is available with two or four spindles and provides torqueup to 200 Nm. It is designed for clamping blanks while workpieces are machinedon the opposite worktable; it can also be used for multiple workpiece clampingon a double swivel table with three linear axes and two rotary axes. Theworkpiece handling is done via robots or linear gantries.

Before implementation of the Hawe solution, the machine wasfitted with standard hydraulic controls, but Schwaebische Werkzeugmaschinen waslooking for a solution that would yield substantially more energy savings.

The Hawe system eliminates use of a gear pump to generatehydraulic energy and replaces it with a radial piston pump, which offers ahigher efficiency than gear pumps or vane pumps. The choice of the pump's power pack (HK series) also comes with asubmerged motor for the drive.

One key advantage is that thesignificantly smaller motor keeps the overall dimensions of the power packcompact; 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 outputwhere a standard motor would break down due to thermal reasons. In a submergedmotor, after approximately 30 min, the oil reaches a steady-state temperature,which is considered an ideal condition for operating machining tools. Andbecause the HK power pack is built into an extruded aluminum casing withcooling gills, it allows unhindered heat transfer, as the air flowing by caneasily 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 thestrain on the environment. For the machining center, the power pack's selectedsize holds 4 gal of oil. Using its radial piston pump, the power packsupplies pressurized oil for operating pressures of up to 10,000 psi (700 bar).

Breuner says those pressures are unusual for machining toolsbecause the majority of applications operate at pressures between 2,200-4,400psi (150-300 bar). But because Hawe manufactures all of its pressurizedcomponents from steel, such high operating pressures are not problematicbecause the components can withstand these loads over long periods. Seated valves made from steel wear less than slidevalves in cast housings.

A cooling wheel mounted at the top of the power pack coolsboth the oil and motor, which means that an additional external cooling unit isavoided by using Hawe's modular construction kit system. Due to the size of themotor, low oil volume and compact dimensions, the power pack can be built intothe machine and integrated into the machining center.

Machine Operation with the New Pump

The machining center's working modevaries depending on the workpiece. Many applications work with a mixture ofdifferent workpieces, so that both the machine and hydraulics supply must beable to handle everything from continuous operation to machine set-up time. Atstandstill, the Hawe system operates in storage mode and saves additionalenergy which can be used in the machining center.

A smooth switching accumulator charge valve recharges thestorage whenever the duty switch point is reached. Because storage is chargedwhen the machine is started, the machine control monitors this operationconstantly. Once the predefined pressure has been attained, the valve willswitch itself into depressor circulation. At the slightest pressure, oil isre-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 offilters and valves can be combined with the BA block to achieve highlycustomized specifications.

The BA 400 machining center comes with a combination ofsub-plates and a seated valve block (NBVP 16), which again increases themachine's efficiency. Because the seated valves are pre-controlled, they workwith minimum switching capacity. The directional seated valve provides a maximum volume flow of 5.3 gal/min andmaximum operating pressure of 5,800 psi (400 bar).

The use of directional seated valvessaves considerable electric power during operation of the machining center. Amachine with 15 spool valves and/or 3-way pressure reducers accumulates aleakage of approximately 0.4 gal/min at 2,200 psi (150 bar), andthe configuration requires an additional drive input of 375W compared to asolution with seated valves.

In a two-shift operation, each with eight hours per day and200 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 hasbeen available for more than 10 years," says Breuner. "But, until recently, fewcompanies were thinking about energy savings. Now companies are thinking moreabout the total lifecycle, not only the initial investment but also ongoingoperational costs. If you need 1,200 kilowatts less per year to run the machineand there are 10 machines, the savings are significant."