Operating principles of rotating unions

4th Quarter 2011 Electrical switching & drive systems & components

Most modern machine tools and machining centres are equipped with coolant because high-speed cutting tools require both cooling and lubrication to reduce the rate of tool wear and prevent overheating, which degrades the tool’s strength.

Coolant has to be transferred through the spindle and tool directly to the cutting edge. To get there, a rotating union is needed as a junction between the spindle and coolant supply. Compared to external coolant systems, a through-spindle coolant (TSC) system, as supplied by Deublin, can pay for itself in terms of lower operating costs for both tools and coolant. Better control of tool overheating also allows faster feed rates, higher productivity and higher surface quality.

A rotating union is a precision mechanical device that allows transfer of pressurised fluid from a stationary source such as a pump into rotating machinery for heating, cooling or transfer of fluid power. A typical rotating device would be a spindle with cutting tool. Rotating unions also can transfer an air/oil mist, cutting oils, and even dry air. In certain machine tool applications, rotating unions are also used to transfer hydraulic fluid or air for clamping or sensing.

A rotating union consists of a rotor that spins at the same speed as the machine tool spindle; a non-rotating element that closes precisely against the rotor; a housing that connects the supply hoses to the non-rotating element; and seals that contain the coolant fluid. Bearing-supported unions connect the rotor to the housing with one or more bearings. Bearingless unions do not have their own bearings but use the bearings of the spindle. Each has their relative advantages and disadvantages.

Seals are the heart of the rotating union. They must contain very high pressures while rotating at very high speeds. At 20 000 rpm, for example, the seals are moving at a relative speed of nearly 5 m/s, while containing 140 bar of fluid pressure.

Suitable products for all cases are available from Deublin. Rotating unions such as Pop-Off, AutoSense or All-Media are ideal. They separate the seal faces if no medium pressure is applied and adjust the distance between the seal faces in conformity with the medium.

High quality rotating unions are designed to resist the impurities in coolants occurring in most production plants. However there are some guidelines for obtaining a long service life and maximum productivity. To read the full story visit http://motioncontrol.co.za/+rotatingunions

For more information contact Mike Vernes, PECS, +27 (0)31 303 3701, [email protected], www.pecspower.co.za

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