Direct drive improves spindle system

In machining centers employed on production lines, it is important to obtain maximum speed from the machine without sacrificing accuracy. One of the limiting components for both of these parameters is the spindle used for axis movement. Conventional machines employ a rotating threaded spindle to move the tool carriage back and forth as required. An alternative method, developed by Bosch Rexroth AG, uses a non-rotating spindle. A nut in the carriage, rotated by an alternating current servomotor, produces the relative tool carriage movement along the spindle.

The MHS 40 includes an ac servomotor that directly drives a nut supported in a bearing. This bearing is resistant to axial forces that occur during high acceleration along the spindle.

A stationary spindle and tool carriage driven by the motor/nut can give a faster drive with more accurate positioning.

Particularly attractive for applications where spindle lengths as long as 7.5m are involved, the MHS 40 direct-drive system offers several advantages:

• The system keeps rotating mass to a minimum. In addition, the dynamic effects of the spindle, including the resonances, are minimized. This means that linear speeds as high as 120 m/min can be achieved.

• Since the spindle does not rotate, no expensive thrust bearings are needed to support the spindle under high axial forces. These forces can arise when the system is subjected to extreme acceleration conditions.

• With the spindle stationary, it can also be optionally cooled by coolant fluid fed through a central axial hole.

• Where one tool head is not enough, a number of tool heads can be employed on a single spindle. In another configuration, it is possible to mount the servo-motor/nut unit in a stationary position and allow the spindle to be driven up and down.

In this machining center, a stationary MHS 40 motor/nut unit powers the vertical axis. The spindle moves up and down to provide Z-axis movement. On the X-axis, the spindle is completely stationary and the tool carriage moves along it, directly dirven by the MHS 40.

Although the servomotor/nut configuration gives a faster drive, Bosch Rexroth has achieved this without sacrificing positioning accuracy. This is at least in part due to the direct drive which needs no couplings or drive belts. When the servomotor/nut is used in combination with a glass scale, or when it is used with a measurement system integrated into the guide rails, the system has a specified accuracy for linear positioning of more than one micrometer.