
B–65142E/04
4. SELECTING A MOTORDESCRIPTIONS FOR THE α series
29
The load torque applied to the motor shaft is generally given by the
following equation:
Tm +
F L
2ph
) Tf
Tm : Load torque applied to the motor shaft
F : Force required to move a movable part (table or tool post) along
the axis
L : Traveling distance of the machine tool per revolution of the mo-
tor = P (Z1/Z2)
Tf : Friction torque of the nut of the ball screw or bearing applied to
the motor shaft (input if necessary)
F depends on the weight of the table, friction coefficient, whether cutting
is in progress, and whether the axis is horizontal or vertical. If the axis
is vertical, F also depends on the presence of a counterbalance. For a table
with a horizontal axis, F is calculated as follows:
When Tf = 2 (kgf·cm) = 0.2 (Nm)
When cutting is not executed:
F = µ (W + fg)
Example)
F = 0.05 (1000 + 50) = 52.5 (kgf) = 514.5 (N)
Tm = (52.5 0.8)/(2 π 0.9) + 2 = 9.4 (kgf⋅cm) = 0.92 (Nm)
When cutting is in progress:
F = Fc + µ (W + fg + Fcf)
Example)
F = 100 + 0.05 (1000 + 50 + 30) = 154 (kgf·cm) = 1509 (N)
Tmc = (154 0.8)/(2 π 0.9) + 2 = 23.8 (kgf⋅cm) = 2.3 (Nm)
To satisfy condition 1, check the data sheet and select a motor whose load
torque (rated torque at stall) when cutting is not executed is 0.92 (Nm) or
higher and the maximum speed is 3000 (min
–1
) or higher. Considering
the acceleration/deceleration conditions, provisionally select α2/3000
(rated torque at stall is 2.0 (Nm)).
When calculating the torque, take the following precautions:
D Allow for the friction torque caused by the gib fastening force (fg).
The torque calculated only from the weight of a movable part and the
friction coefficient is generally quite small. The gib fastening force
and precision of the sliding surface may have a great effect on the
torque.
D The pre–load of the bearing or nut of the ball screw, pre–tension of the
screw, and other factors may make Tc of the rolling contact
considerable. In a small, lightweight machine tool, the friction torque
will greatly affect the entire torque.
4.1.1
Calculating the Load
Torque and Load
Inertia
Calculating the load
torque
D Cautions