Chapter 12 Guidance for ordering goods 12.1 Capacity selection Determination of the driver unit capacity requires general consideration of load inertia, load torque, required positioning accuracy and required maximum speed, following the recommended steps below: 12.1.1 Calculation of load inertia and torque 12.1.1.1 Torque calculation Load torque is generated by friction of the drive system and load torsion. 2πM is the work performed by the motor when it rotates by one revolution at the torque M; FL is the mechanical work performed when the motor moves by a mechanical displacement distance L at force F.
In a real machine tool, due to presence of transmission efficiency and friction coefficient, the torque required by a ball screw to conquer external load P and move at a constant speed is shown in the following figure: Z 2 Z 1 - Number of teeth Servo motor Example of a servo feed drive system It shall be calculated by using the formula below: Finally, the torque of the servo motor is preliminarily determined based on the condition of M 1 ≤the servo motor’s rated torque M S.
204 Technical Manual for EA190 Series Servo Driver V100 2πM = FL M Motor shaft torque F A force required to make a mechanical part move along a straight line L The mechanical displacement distance when the motor rotates by one revolution (2πrad) Ball screw M1 Driving torque in case of movement at a constant speed (N•mm) Pretightening torque of a double-nut ball screw (N•mm) Fa0 For pretightening force (N), usually 1/3 of the maximum axial workload Fmaz is taken, i.e. Fa0 = 1/3Fmaz; when it is difficult to calculate Fmaz, Fa0 = (0.1~0.12) Ca (N) can be used.
Ca The rated load of the ball screw pair can be found in the product sample hsp Screw lead (mm) K For the pretightening torque coefficient of a ball screw, take 0.1 - 0.2 P External load (N) applied axially to a screw, P = F+μW F Cutting force (N) applied axially to screw W Normal load (N), W = W1+P1 W1 Gravity of moving part (N), including the maximum gravity bearing capacity P1 Clamping force of clamping plates (such as the headstock) μ Guide rail friction coefficient, μ = 0.09 for sliding guide pair of plate attached with teflon, μ = 0.03-0.05 when there is lubrication, and μ = 0.003-0.004 for linear rolling guide η1 For ball screw efficiency, take 0.90-0.95 MB Friction torque of a supporting bearing, also known as starting torque (N•m), which can be found in the special bearing sample for ball screws Z1 Teeth of gear 1 Z2 Teeth of gear 2