Spindle Knowledge

Rotary feedthrough

A rotary feedthrough provides gases and liquids with a sealed transition from a fixed object to a rotating object or between two rotating objects.
In the machine tool spindle, the rotary feedthrough is attached above the floating bearing and feeds the coolant for the tool into the shaft. In wet processing, this is oil or water-based coolant and in dry processing it is air.  With minimum quantity lubrication, where the coolant is precisely metered to suit the machining and the workpiece, an oil-air mix is used as coolant.
As a result, the rotary feedthrough is exposed to high rotational speeds and variable coolant quantities because it transfers media from the stationary housing to the rotating shaft. Single-flute rotary feedthroughs are most often used in the field of machine tool spindles. These feed the coolant in one direction in the shaft. Multi-flute rotary feedthroughs feed gases or liquids into a rotating element and then back out again.  Some manufacturers offer a special type of rotary feedthrough, which enables the spindle shaft to be cooled. This type of cooling can reduce the disadvantageous effects of spindle warming on the accuracy and bearing service life.
Rotary feedthroughs are manufactured to suit the respective media being channelled. Particular care must be taken to ensure that the rotary feedthrough is capable of dry running so that it is not damaged in the event of dry running due to incorrect media being used. For this reason it is necessary to ensure that the rotary feedthrough is suitable for the respective process before a change to either wet or dry processing is implemented.
Rotary feedthroughs are available for rotational speeds of up to 100,000 rpm
and pressures of up to 500 – 600 bar, depending on the medium [GAT].
Due to the media incorporation and the resultant higher relative movements between stationary and rotating elements, there is a high potential risk for the entire spindle system with rotary feedthroughs. As such, in the event of a defective seal water can enter the spindle and destroy electrical components or the roller bearings.