Multiple drive concepts for machine tools

When looking at machine tools, details of the installed drive technology usually remain concealed. In principle, there are various possibilities for selecting main, feed and auxiliary drives for the movements that need to be carried out.

For main drives, closed-loop controlled electric synchronous and asynchronous motors are predominantly used. Their applications include kit or housed motors for use in turning, milling and grinding machines as well as in machining centers. The traditional spindle drives with housed motors are widely popular as main drives. Compared with motor spindles, they are less costly, considering the secondary costs of both systems.

On one hand, the interposition of gear boxes enables the rotational speed and torque to be tuned to the machining task. On the other hand, the gear boxes cause unwanted radial forces, noise and increased wear.

Meanwhile, the main drives using kit motors with an integrated spindle have become technically sophisticated. Since gear box and clutch can be eliminated, these drives make a centric rotational movement possible without suffering from shearing force. They stand out—thanks to their long-term running smoothness and minimal wear, they are used especially for high-performance machining.

The generation of higher torques is still quite costly, since either a planetary gear has to be integrated into the spindle or a higher motor power needs to be selected. To carry out preventive maintenance and servicing, integrating monitoring sensors into the spindle to allow the acquisition of measurement data will become the standard. Cooling with oil, air or glycol is still always necessary.

For the drive technology of feed drives, the choice is either electromechanical or hydraulic systems. To make this choice, the specific advantages and disadvantages of each system should be considered. Today, the electric servomotor with ball-screw assembly dominates worldwide. It converts the rotational movement into a linear movement. Here, synchronous housed motors are preferred, as they need to cope with higher requirements in terms of positioning, synchronized operation and dynamics, more so than the main drive.

Due to its high static rigidity, this drive system is suitable for various applications. It is also considered traditional, but prone to wear. Depending on installation conditions and strength of the torques that need to be generated, the servomotor is connected to the spindle either directly or, for example, via a synchronous belt. Although the principle of the electric linear motor was invented as early as the 19th century, the technology has only found its way into machine tool applications since the early 1990s.

Load regimes
The drive's load regime is an aspect that limits its use. Of course, this does not mean that when processing with large forces, ball screw assemblies and hydraulic drive solutions can be dispensed with. The other main criterion is the fact that supporting machine elements—such as the swarf cover with its maximum permissible sliding speed and the carriage guide with its damping behavior—can also limit the area of application. The benefits of linear motor drives are counteracted by the associated investment costs, and this has so far prevented a worldwide breakthrough of this drive technology.

Hydraulic feed drives are in demand when their benefits have a significant impact. In particular, this relates to installation in confined spaces, demand for high dynamics and generation of great feed forces. The hydraulic feed drive should be positioned accurately to the micrometer.

Practical applications demonstrate that the hydraulic linear drive is long-lasting, works free of play and tends to be more durable than a comparable drive with ball-screw assembly. With electric feed drives, each specific performance (torque and rotational speed) needs to be installed, while the hydraulic axis can draw energy according to demand from a hydraulic fluid accumulator. This means that the installed input power can be reduced by up to 80 percent.

Auxiliary drives reflect a wide variety of solutions. Across the whole spectrum of auxiliary-drive functions in machine tools, there is neither a significant trend nor certain tried-and-tested solutions that stand out. The choice depends on the solution that best fits the actual drive situation. And it is not unusual for one machine group with a closed sequence of functions to combine drive types of various technologies.

There are examples of this in applications where electromechanical drives for vertically or diagonally moved carriages are used with hydraulic or pneumatic weight compensation. Here, weight compensation can be understood as a passive auxiliary drive in the widest sense; its task is to compensate the weight force of the moved mass. It can be achieved in a number of ways, such as a hydraulic solution that uses a hydraulic fluid accumulator. If the weight force that needs compensating is small, a pneumatic gas spring can carry out the compensating function. The advantages of these solutions lie in their adaptable dynamic behavior and favorable energy balance.

Pneumatic drives
Pneumatic drives are suitable for installation in handling devices because of their low weight, simple control structure and movement speed. This applies to feed and load units for smaller masses, which are thus integrated into the workpiece flow of the production process. Clamping of the tools and workpieces within machine tools is important, as the quality of the processing result is significantly influenced by the accuracy and repeatability of the clamping process. Hydraulic clamps represent a special type of auxiliary drive. They are used particularly in machines with unattended workpiece load and unload, since they are easily automated. The clamping elements' high force density favors the construction of clamping devices in the smallest spaces.

There are many electric, hydraulic, electromechanical and pneumatic drive concepts available as solutions to drive tasks in machine tools. The design engineer and the whole team need to decide which type of drive concept is right for the task, taking into account the many constraints. A good automation supplier having expertise in these technology groups will consider and advise customers in their decisions.

- Rainer Drechsler
Head, Machine Tool Hydraulics Applications Center
Bosch Rexroth AG