The choice of ball screw bearing and support their effects on the positioning accuracy

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 working process of the main axial force, the size of the axial force and axial stiffness of ball screw, the screw of the influence on repeated positioning accuracy, and influences the axial stiffness of a ball screw element has a lot of, 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 according to 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 be selected according to the practice demand both economic and can satisfy the request of bearing. Also can be thought of bearing load and fatigue life, will no longer be discussed here. Bearing stiffness can check it on the product manual. 2> Screw support way and the effect on the axial stiffness of support way and the influence of the stiffness of a ball screw: ordinary choice under the condition of screw drive on the fixed support structure, it can reduce or eliminate screw and driving element connecting place (in the wrong Parallel to the axial wrong, wrong, wrong) The impact on the screw position precision. With detailed examples below to see the same screw with different support way influence on the axial stiffness. CNC milling machine with ball screw feed mechanism, the uniform 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 E size = 2. 06 * 10 ^ 5 mpa ( Screw modulus of elasticity) As = screw effective cross-sectional area ( mm^2） This article from the fixed end 100 mm to 800 mm screw within the scope of work axial stiffness influence on positioning accuracy. a> Fixed - - - - Comfortable way support this kind of structure, suitable for precision request is not high, screw rotational speed is low and ball nut from or by a very small radial force condition. b> Fixed - - - - Simply supported this construct support way, suitable for precision request is too high to accept the operation condition of the radial force is small, because the other end is simply supported, critical speed is higher than the fixed - - Comfortable support. Because of the simply supported support can slide along the axial direction in a small scope, and therefore this way of support for the influence of the axial stiffness and fixed - - Comfortable support. c> Fixed - - - - Fixed support structure this way of supporting axial stiffness big, repeated positioning accuracy is high, can accept the big radial direction load, usually under the condition of the structure but also exert a certain tension in advance on screw, the screw radial load could progress greatly, and under the conditions of temperature, screw can still insist on 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 is calculated under different conditions the axial displacement caused by the axial stiffness in this case, screw axial stiffness influence on positioning accuracy is as follows.