Abstract
Being a critical factor affecting the motion accuracy of precision machine tools, structural thermal elongation of precision ball screw unit is generally caused by the comprehensive influence from heat generations of screw-nut pair/bearings and time-varying ambient temperature. To resist 2 thermal disturbances above to guarantee precisely the original length of screw shaft, an active coolant control strategy is proposed in this paper. This strategy is based on a premise hypothesis: For the slender and long tubular structure of screw shaft, the screw shaft temperature is approximately equal to its recirculating coolant temperature. The reason is that the intensive forced coolant convection is capable of eliminating screw shaft temperature rises caused by friction heat generations and ambient air convections. Based on this premise, screw coolant temperature can be consistently controlled by an active strategy, further to correct the thermal elongation of screw shaft. It can be experimentally verified that the thermal variations of machine positioning accuracy caused by the active coolant control strategy are not more than 10 μm, which are lower than traditional strategy. Besides, based on detected structural temperatures of precision ball screw unit, the theoretical model above is further proved to be reliable by FE simulation method.
Highlights
In recent decades, precision ball screw unit has been widely applied into linear feed systems of various precision machine tools, to convert rotational motion into linear motion [1]
Being the vital factor causing the thermal variations of machine accuracy, the shaft thermal elongation of precision ball screw unit is generally attributed to 2 disturbing factors from a machining activity, which is revealed in Fig. 1: 1 friction heat generations from screw-nut pair and bearing groups, and 2 air convective heat transfer with time-varying ambient temperature
In order to resist thermal disturbances from internal heat generations and external ambient temperature change of precision ball crew unit onto its structural thermal behaviors, an active coolant control strategy is proposed in this paper
Summary
Precision ball screw unit has been widely applied into linear feed systems of various precision machine tools, to convert rotational motion into linear motion [1]. Being the vital factor causing the thermal variations of machine accuracy, the shaft thermal elongation of precision ball screw unit is generally attributed to 2 disturbing factors from a machining activity, which is revealed in Fig. 1: 1 friction heat generations from screw-nut pair and bearing groups, and 2 air convective heat transfer with time-varying ambient temperature. The disadvantage of this strategy has been gradually exposed: Because coolant supply temperature is dynamically determined based on the ambient temperature detections, disturbing heat transfers onto the screw structure can hardly be dissipated accurately This restricts the effectiveness of reducing structural thermal elongations of precision ball screw unit [2]. It is urgent to develop an active strategy regulating coolant temperature to control thermal behaviors of precision ball screw unit, to guarantee precisely its original length for machine accuracy stability
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