Life of ball screws
Even the ball screw is used under correct conditions, it would still fail after a period of time due to deterioration. The elapse of time Until it is out of service is called the service life of the screw, which is generally classified into the fatigue life when delamination phenomenon occurs and the accuracy deterioration life caused by wear-out, etc.
Basic static load rating Coa
The basic load rating is an axial static load which will produce a permanent combines deformation at contact points of the balls to ball grooves equal to 0.01% of the ball diameter.
Basic dynamic load rating Ca
The basic dynamic load rating is an axial load which allows 90% of a group of identical ball screws (rotated under the same condition) to rotate without flaking for 1 million revolutions. This basic dynamic load rating is shown in the table of dimensions
Relation between load and service life
L (1/P)3
L: Service life
P: Load
Fatigue Life
Table 3.8
Usage |
Life in hours (h) |
Working machines |
20000 |
General industrial machines |
10000 |
Automatic control machines |
15000 |
Measurement machines |
15000 |
Average load Pe
(1)When axial load keeps changing from time tu time, please calculate in order to find out the average load for the equivalent fatigue life under different load condition changes (as Diagram 3.30)
| Axial Load (kgf) |
Rotating Speed (min-1) |
Time(%) |
| P1 |
n1 |
t1 |
| P2 |
n2 |
t2 |
| - |
- |
- |
| - |
- |
- |
| Pn |
nn |
tn |
But,t1+t2+t3+¡K¡K+tn=100
Pmax:Maximal axial load (kgf)
Pmix:Minimal axial load (kgf)
When load changes according to sine curve (Fig.3.31)
Pe = 0.65Pmax (Fig.A)
Pe = 0.75Pmin (Fig.B)
Fig.3.31
Calclation of Life
The fatigue life is generally expressed by the total number of revolutions.The total rotation hours or total travel distance may also be used to express life.The fatigue life is calculated as follows:
Where,
L: Rated fatigue life (rev) Lt: Life in hours (h)
Ls: Life in travel distance (Km) Ca: Basic dynamic load rating (kgf)
Pa: Axia (kgf) n: Rotating speed (rpm)
fw:Load factor(Factor depending on operation conditions) l:Lead(mm)
Smooth operation without impact............1.0 ~ 12
Normal operation.....................................1.2 ~ 1.5
Operation with impact and vibration........1.5 ~ 3.0
Basic Dynamic Load Rating Ca
Ca=Pe¡Pfs
Basic Static Load Rating Coa
Coa=Pmax¡Pfs
Table 3.9 Factor of safety(fs)
| Usage |
Operation |
fs |
| Industral machines |
Normal operation |
1.0¡ã1.3 |
| Operation with impact and vibration |
2.0¡ã3.0 |
| Working machines |
Normal operation |
1.0¡ã1.5 |
| Operation with impact and vibration |
2.5¡ã7.0 |
| 
