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The choice of ball screw bearing and influenced by the way of support on the positioning accuracy of the knowledge - bearing

by:Waxing     2020-06-04
Ball screw is to convert rotary motion to linear motion, or convert linear motion into rotary motion of the ideal products, ball screw because of friction resistance is small, high efficiency, high accuracy, is widely used in industrial equipment and precision instruments. Because of the screw in the process of work mainly bear axial force, the size of the axial force and axial stiffness of ball screw, the repositioning precision of the leadscrew, there are many factors that can affect screw axial stiffness, mainly includes: the axial stiffness of screw, the screw axial stiffness of supporting bearing and axial stiffness of ball nut, screw and the axial stiffness of the bearing, etc. Here mainly discuss screw axial stiffness of supporting bearing and screw. 1> All kinds of bearing axial stiffness contrast size quite a thrust bearing axial stiffness is about angular contact ball bearings and tapered roller bearing axial stiffness of two times, maximum axial stiffness of thrust roller bearing, ball screw support with bearing axial stiffness between taper roller bearings, thrust cylindrical roller bearings. The axial stiffness big also means bigger starting torque. Can choose according to actual needs both economic and can satisfy the requirements of the bearing. Of course consider the bearing capacity and fatigue life of bearing, will no longer be discussed here. Bearing stiffness can look it up in the product manuals. 2> Screw support way and the effect on the axial stiffness of support way and the influence of the stiffness of a ball screw: usually choose fixed screw drive on the supporting structure, so that we can reduce or eliminate screw and drive components of the joint, Parallel to the axial wrong, wrong, wrong) The impact on the screw position precision. With specific examples below to see the same screw with different ways to support its influence on the axial stiffness. CNC milling machine with ball screw feed mechanism, the average load Fa = 3000 n, choose CM5006 - 5 ball screw, screw length L = 1. 0m. Try to calculate screw in different support condition the influence of the axial stiffness. Ks = screw shaft axial stiffness ( KN /μm) d1 = 45。 2 mm diameter at the end of the screw ( mm) L0 = 1000 mm screw length L1 = 68 mm nut installation dimension E = 2. 06 * 10 ^ 5 mpa ( Screw modulus of elasticity) As = screw effective cross-sectional area ( mm^2) This analysis from the fixed end 100 mm to 800 mm screw within the scope of work axial stiffness influence on positioning accuracy. a> Fixed - - - - - - - - - - - - - - - - - - - Free support this structure, is suitable for the positioning accuracy is not high, screw rotational speed is low and ball nut from or by a very small radial force condition. b> Fixed - - - - - - - - - - - - - - - - - - - Simply supported way support this structure, suitable for positioning accuracy requirement is too high, under the operation condition of the radial force is small, because the other end is simply supported, critical speed is higher than the fixed - - - - - - - Free support. Because of the simply supported support can slide along the axial direction in a small range, so this way of support for the influence of the axial stiffness and fixed - - - - - - - The same way free support. c> Fixed - - - - - - - - - - - - - - - - - - - Fixed support this way of supporting axial stiffness, high repeat positioning accuracy, can withstand the bigger radial load, normally this kind of structure but also exert a certain tension in advance on screw, the screw bear radial load capacity is greatly increased, and in the case of temperature, screw can also keep a certain tension in advance, so that even if has a temperature change, the change in temperature will not affect the length of the screw, just change the size of the screw inner stress. Screw axial stiffness calculation calculation under different conditions the axial displacement caused by the axial stiffness in this case, screw axial stiffness influence on positioning accuracy is as follows.
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