Chapter 2 Engineering Design of Servo Mechanism
21
3) Motor rated torque (T
R
)
Refer to motor catalog.
4) Evaluation
() .TT
N
N
T
PD R
1
2
02≦
T
P
: Friction torque by external load (Nm)
T
D
: Friction torque by pre-load (Nm)
T
R
: Motor rated torque (Nm)
N
1
: The number of gear A teeth
N
2
: The number of gear B teeth
5) Others
It is recommended to use the grinding ball screw. Rolling ball screw has so large backlash that it
may not move properly.
[ 2 ] Gear sliding resistance
Table 2-3 shows gear transmission efficiency
Use gear of which transmission efficiency is 98% or more except for skew gears. In this case,
sliding resistance of gear (friction torque) can be disregarded.
Table 2-3. Classification and type of gear
Classification of gear Type of gear Efficiency (%)
Spur gear
Rack
Parallel axis Internal gear 98.0 - 99.5
Helical gear
Helical rack
Double helical gear
Straight bevel gear
Intersecting axis Spiral bevel type gear 98.0 - 99.0
Zerol bevel gear
Skew axis Cylindrical worm gear 30.0 - 90.0
Crossed helical gear 70.0 - 95.0
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