Chapter 2 Engineering Design of Servo Mechanism
18
When selecting a ball screw, it is not economical to make its fatigue life uselessly long because the ball
screw must be so much big. For reference, general target value of fatigue life is shown bellow.
Machine tool 20,000 hours
Industrial machine 10,000 hours
Automatic controller 15,000 hours
Measuring instrument 15,000 hours
Therefore, from equation (7), (8) (T clearance Ca = 7056 kgf)
L
Ca
Ff N
t
mw m
3
6
1
60
10
≒380000 25000 (hr)
[Selection of accuracy and clearance]
(a) Accuracy grade
Positioning accuracy ± 0.10/700 (mm)
From Table 2-1
Accuracy grade: C5
E = ± 0.040/1000 (mm)
e = 0.027 (mm)
Table 2-1. Allowance of accumulated main lead error (±E) and fluctuation (e)
Unit: m
Accuracy grade C0 C1 C2 C3 C5
more than or less ±E e ±E e ±E e ±E e ±E e
– 100 3 3 3.5 5 5 7 8 8 18 18
100 200 3.5 3 4.5 5 7 7 10 8 20 18
200 315 4 3.5 6 5 8 7 12 8 23 18
315 400 5 3.5 7 5 9 7 13 10 25 20
400 500 6 4 8 5 10 7 15 10 27 20
500 630 6 4 9 6 11 8 16 12 30 23
630 800 7 5 10 7 13 9 18 13 35 25
800 1000 8 6 11 8 15 10 21 15 40 27
1000 1250 9 6 13 9 18 11 24 16 46 30
1250 1600 11 7 15 10 21 13 29 18 54 35
1600 2000 – – 18 11 25 15 35 21 65 40
2000 2500 – – 22 13 30 18 41 24 77 46
2500 3150 – – 26 15 36 21 50 29 93 54
3150 4000 – – 30 18 44 25 60 35 115 65
4000 5000 – – – – 52 30 72 41 140 77
5000 6300 – – – – 65 36 90 50 170 93
6300 8000 – – – – – – 110 60 210 115
8000 10000 – – – – – – – – 260 140
Screw part
effective
length
(mm)
10000 12500 – – – – – – – – 320 170
Source : NSK catalog
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