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Linear Motion Control - 2011-09-29

Improved cooling for high-speed ball screws

Temperature control at the site of friction 

Whenever ball screws perform short movements at high cycle rates, nut cooling provides much better temperature control – and therefore more precise positioning – than the hollow-shaft spindle cooling system previously used. System solutions featuring this technology are now ready for series production; NSK has showcased them at the EMO trade fair.

Linear drives, which are positioned with a high level of precision, require minimal fluctuations in operating temperature. The movement of the nut on the spindle generates friction, which is converted into heat and causes dimensions to change. This factor should not be underestimated: a temperature increase of just 10oC causes a one-metre spindle to stretch by no less than 120 µm.

In practice, this can mean that machine tools do not achieve the necessary precision.

To comply with the demands made of a machine tool, ball screws can be used that have a hollow-shaft through which coolant is routed. This keeps the thermal strain on the ball screw constant to the greatest possible extent.

NSK has now developed an innovative alternative: a ball screw where the nut body is cooled.

The advantage of this new principle is that the drive’s temperature control system works at precisely the point where heat is introduced into the mechanical system, i.e. the area of contact between the ball and the raceway profile. The nut fitting is also effectively cooled and the guiding table likewise maintains its predefined temperature. To get the system ready for series production, it was crucial to ensure that the cooling did not affect the nut’s preload – and once again, the NSK development team rose to the challenge.

The nut housing cooling system can be incorporated into various series of high-precision and high-speed ball screws. The shaft diameters range from 40, 45 and 50 to 63 mm, with leads of 16, 20, 25 or 30 mm. Existing linear drives can also be retrofitted as the integrated cooling system does not alter the connection dimensions. This means the drives remain compact.

The performance delivered by the nut cooling system is at least as good as hollow-shaft cooling, and in many cases even better. The new systems are most effective when they are used in machines and handling systems that perform small movements at high speed with short cycle times. Many machine tools that process car vendor parts fit this description, for example, as do production systems for electronics and electromechanical plastic injection moulding machines.

In addition to the technical advantage, ball screws with nut cooling also boast financial benefits: they are cost-effective to manufacture and can easily be integrated into existing cooling systems.

NSK has given a practically oriented demonstration of the new cooling principle at the EMO trade fair. Two ball screws – one with nut cooling, the other uncooled –  moved in parallel while a thermographic camera shows changes in the two drive systems’ operating temperatures in real time.

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