Page 1594 - MISUMI SINGAPORE Economy Series

[Technical Data]
Selection of Ball Screws 1

1. Ball Screw Selection Procedure
Basic ball screw selection procedure and required evaluation items are shown below.
Determine the application parameters Moving mass, feed speed, motion pattern, screw shaft rotational speed, stroke,
mounting orientation (horizontal or vertical), life, positioning accuracy

Interim selection of ball screw specifications Temporary selection of ball screw lead accuracy grade (C5~C7),
shaft diameter, lead, and length. P.1589 , 1590
NG •Axial Load Capacity
Evaluation of various basic safety factors
Confirm that the applied axial load is within the ball screw's axial load capacity rating.
P.1591
•Allowable Rotational Speed
Confirm that the intended shaft rotational speed is within the ball screw's allowable rotational speed
rating. P.1592
•Life
Confirm that the ball screw satisfies the life requirement. P.1593
NG
Evaluations based on required performances If higher positioning accuracy and/or improved responsiveness is needed, below parameters need to be evaluated.
•Screw shaft rigidity
•Effects of temperature variation on life P.1594
2. Ball Screw Lead Accuracy
Ball screw lead accuracy is defined by JIS Standards property parameters (ep、νu、ν300、ν 2π).
Parameter definitions and allowable values are shown below.
In general, a ball screw lead accuracy grade is selected by evaluating if the Actual Mean Travel Error of a candidate is within the allowable positioning error.
■Table 1. Positioning Screw (C Class) Actual Mean Travel Error (±ep) and Variation (u) allowances Unit : μm
Effective Thread Length ( lu ) Accuracy Grade
Thread Effective Length (mm) C5
Actual Travel ( la ) Line Nominal Travel ( lo ) Line
+ Actual Mean Travel ( lm ) Line Specified Travel ( ls ) Line over or less Actual Mean Variation
Travel Error
Travel Error 0’ 0 315 315 23 18
25
400
20
500
20
400
27
35
25
800
630
t 500 630 30 23
0
− ν 0 3 800 1000 40 27
1000 1250 46 30
1250 1600 54 35
pe ■Table 2. Positioning Screws (C Class) variation per 300mm (300) Variation per rotation (2π) standard values Unit : μm
π
2
ν
2π ν u Accuracy Grade C5 C7
300mm
Parameters ν300 ν2π ν300
Standard Values 18 8 52
Terms Symbols Meaning
Actual Mean Travel Error ep A value that is Specified Travel subtracted from Actual Mean Travel.
The maximum difference of the actual travel contained between two lines drawn
parallel to the actual mean travel, and is defined by three parameters below.
Variation νu Variation for the effective thread length of screw shaft.
ν300 Variation for an arbitrarily taken length of 300mm within the effective thread length of screw shaft.
ν2Q Variation for an arbitrary one revolution (2πrad) taken within the effective thread length of screw shaft.
Specified ls Axial travel compensated for temperature rise and loading
Travel conditions, in relation to the Nominal Travel (Lead).
Specified A value that is Nominal Travel subtracted from Specified Travel, over the effective thread length.
Travel Target t This value is set to compensate for possible screw shaft expansion
and contraction due to temperature changes and applied loads. The
Value value is to be determined based on experiments or experiences.
Actual Travel la Actually measured travel distance
A straight line representing the actual travel trend.
Actual Mean lm A straight line obtained by the least-squares method
Travel or other approximation methods from the curve
representing the actual travel.
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