The S1-273 lathe studied in this paper is an all-functioned
CNC lathe fabricated by a Chinese machinebuilding
mill. A schematic diagram of the main transmission
is shown in Fig. 1. The lathe is driven by a
continuous speed regulation alternating current (AC)
motor by means of a timing belt and two pairs of shifting
slide gears. The timing belt transmits the power and
motion from the motor to shaft I in the headstock. The
diameters of the driving and the driven pulley are 125
and 230 mm respectively. The motion through the gear
pairs with teeth 28/70 and 49/49 is transmitted from
shaft I to shaft II, and through the gear pairs with teeth
59/47 and 30/76 from shaft II to shaft III, i.e. the spindle.
The rated speed of the motor is 1500 rev/min, its
maximum speed is 3000 rev/min and rated power is 11
kW. The rated speed is defined as the speed at which
the motor may output the maximum torque or power, in
general, it is the lowest speed that can deliver the rated
power [13,14]. Conventionally, the speed range from the
rated speed to the maximum speed is called the constant
power region; the speed range below the rated speed is
called constant torque region. The spindle speeds of this
lathe range from 20 to 2000 rev/min. It is obvious that
the use of shifting slide gears is not to enlarge the speed
range but to enlarge sufficiently the constant power
region within the whole speed range.
The speed diagram of the lathe is shown in Fig. 2.
The vertical coordinate is the speed in logarithmic scale.
The transmission shafts are shown as vertical parallel
lines at equal distances from each other. The Roman
numerals at the bottom of the diagram correspond to the
numbers in Fig. 1. The points on each vertical line indicate
the margins of speed ranges for each speed level.