Page 1594 - MISUMI Thailand Economy Series

[Technical Data]
Selection of Ball Screws 1

1. Ball Screw Selection Procedure 3. Axial Clearances of Ball Screws
Basic ball screw selection procedure and required evaluation items are shown below. Axial clearance does not affect positioning accuracy if the feed is unidirectional, but will generate backlash and negatively affect on
positioning accuracy if the direction or the axial load is reversed.
Determine the application parameters Moving mass, feed speed, motion pattern, screw shaft rotational speed, stroke, ■Types of Rolled Ball Screws
mounting orientation (horizontal or vertical), life, positioning accuracy
Accuracy Screw Lead Screw Shaft Length(mm) Axial Clearance
grade Type shaft dia. (mm) min. max. (mm)
Interim selection of ball screw specifications Temporary selection of ball screw lead accuracy grade (C5~C7),
shaft diameter, lead, and length. P.1589 , 1590 08 02 100 400
02
NG •Axial Load Capacity 10 600
Evaluation of various basic safety factors 04 0.05 or less
Confirm that the applied axial load is within the ball screw's axial load capacity rating.
P.1591 04
•Allowable Rotational Speed 12 05 800
Confirm that the intended shaft rotational speed is within the ball screw's allowable rotational speed 10 150
rating. P.1592
05
•Life 10 0.1 or less
Confirm that the ball screw satisfies the life requirement. P.1593 15 16 1200
C－TBSE
NG C7 C－TBSB 20
Evaluations based on required performances If higher positioning accuracy and/or improved responsiveness is needed, below parameters need to be evaluated.
•Screw shaft rigidity 05
•Effects of temperature variation on life P.1594 20 10 0.15 or less
20
2. Ball Screw Lead Accuracy 05 200
Ball screw lead accuracy is defined by JIS Standards property parameters (ep、vu、v300、v2Q).
Parameter definitions and allowable values are shown below. 25 10 2000
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. 25
■Table 1. Positioning Screw (C Class) Actual Mean Travel Error (±ep) and Variation (u) allowances Unit : μm 05 0.2 or less
Effective Thread Length ( lu ) Thread Effective Length (mm) Accuracy Grade 32 10 300
Actual Travel ( la ) Line Nominal Travel ( lo ) Line C5
+ Actual Mean Travel ( lm ) Line Specified Travel ( ls ) Line over or less Actual Mean Variation 32
Travel Error 0’ 315 315 23 18 Selection Example of Lead Accuracy
Travel Error
400
25
20
0
20
400
500
27
<Requirements>
35
800
25
630
· Stroke 720mm
0 0 t 500 630 30 23 · Ball screw diameter F15, lead 20.
− 3 V 800 1000 40 27
1000 1250 46 30 · Positioning accuracy Ú0.05mm/720mm
1250 1600 54 35 <Selection Details>
p Select an appropriate lead accuracy grade based on the application
π e ■Table 2. Positioning Screws (C Class) variation per 300mm (300) Variation per rotation (2π) standard values Unit : μm
2 V requirements.
2π u V Accuracy Grade C5 C7
300mm (1) Evaluating the screw thread length
Parameters V300 V2π V300 Stroke＋Nut Length＋Margin＝720＋62＋60＝842mm
Standard Values 18 8 52 * The Margin shown above is an overrun buffer, and normally
determined as 1.5～2 times the screw lead.
Lead 20×1.5×2 (both ends)=60
Terms Symbols Meaning
Actual Mean Travel Error ep A value that is Specified Travel subtracted from Actual Mean Travel. (2) Evaluating the lead accuracy
P.1589 Table 1. is referenced and an Actual Mean Travel Error
The maximum difference of the actual travel contained between two lines drawn ±ep for 842mm ball screw thread.
parallel to the actual mean travel, and is defined by three parameters below. C3 · · · Ú0.021mm/800～1000mm
Variation uu Variation for the effective thread length of screw shaft. C5 · · · Ú0.040mm/800～1000mm
u300 Variation for an arbitrarily taken length of 300mm within the effective thread length of screw shaft. (3) Determining the lead accuracy
u2Q Variation for an arbitrary one revolution (2πrad) taken within the effective thread length of screw shaft. It can be determined that a C5 grade (Ú0.040/800～1000mm)
Specified Axial travel compensated for temperature rise and loading ball screw can satisfy the required positioning accuracy of
Travel ls conditions, in relation to the Nominal Travel (Lead). Ú0.05/720mm.
Specified A value that is Nominal Travel subtracted from Specified Travel, over the effective thread length.
This value is set to compensate for possible screw shaft expansion
Travel Target t 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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