How to Select Ball Screws: Page 1

1. Ball Screw Selection Process

The following is an outline of the basic process for selecting ball screws, and relevant factors to consider.

2. Ball Screw Lead Accuracy

Ball screw lead accuracy is a special characteristic stipulated under JIS standards (ep, V_u, V₃₀₀, V_2π).
Definitions and tolerance values for each are as described below.
In general, confirmation is made to determine whether the ball screw's average travel length deviation falls within tolerance values for positional accuracy when selecting a ball screw precision class.

Term	Symbol	Meaning
Average travel length deviation	ep	Value obtained by subtracting standard travel length from average travel length.
Deviation range	Vu V300 V2π	The following three items are determined based on the maximum amount of deviation of the actual travel length line positioned between the two lines drawn parallel to and on either side of the average travel length line: Maximum deviation relative to the maximum length of the screw thread Maximum deviation over a chosen 300-mm section relative to screw thread maximum length Maximum deviation over a chosen section comprising one rotation (2π rad) relative to screw thread maximum length
Standard travel length	ls	Axial travel length corrected for displacement caused by temperature rises, loads and/or similar, relative to nominal travel length (lo).
Standard travel length target value	t	Standard travel length relative to screw thread effective length, with nominal travel length subtracted. Used to determine displacement correction in cases where temperature changes, external loads and/or other such factors are expected to cause thread length elongation/contraction. Said values are determined experimentally and/or through practical experience.
Actual travel length	la	Actual, measured travel length.
Average travel length	lm	A straight line representing the general, average direction of actual travel. This line is calculated based on the curved line of actual travel length, using the least squares method or simple approximation method.

Table 1: Positioning-use (C series) tolerance for average travel length deviation (±ep) and deviation range (vu) Unit: μm

Screw Thread Effective Length (mm)		Precision Class
		C3		C5
Exceeds	Max.	Average travel length deviation	Deviation range	Average travel length deviation	Deviation range
	315	12	8	23	18
315	400	13	10	25	20
400	500	15	10	27	20
500	630	16	12	30	23
630	800	18	13	35	25
800	1000	21	15	40	27
1000	1250	24	16	46	30
1250	1600	29	18	54	35

Table 2: Positioning-use (C series) deviation range (V300) wobble (V2π) standard values for a 300-mm section Unit: μm

Precision Class	C3		C5
Item	v300	v2π	v300	v2π
Standard Value	8	6	18	8

Table 3: Conveyor-use (Ct series) deviation range (V300) standard values for 300-mm section Unit: μm

Precision Class	Ct7	Ct10
v300	52	210

Conveyor-use (Ct series) average travel length (ep) is calculated as ep = 2 × l_u ÷ 300 × V₃₀₀.

3. Ball Screw Axial Clearance

Axial clearance is not an essential element of positional accuracy when travel is only in one direction. However, it may be a causal factor in backlash that affects positional accuracy when travel occurs in the reverse direction as well, when axial loads are applied, etc.
Select ball screw axial clearance specifications based on positional accuracy requirements.

Rolled ball screw

Type	Common type(s)	Screw shaft diameter	Lead	Axial clearance	Screw shaft length (mm)
					MIN	MAX
Compact nut, precision class C10	BSSC	8	2	Max. 0.05	100	400
		10	4		150	600
		12	4		150	800
		15	5	Max. 1.10	150	1,200
			10		200	1,200
		20	5		200	2,000
			10	Max. 0.15	250	2,000
		25	5	Max. 1.10	200	2,000
Standard nut, precision class C10 C-VALUE Standard nut precision class C10	BSSZ BSSR C-BSSC	8	2	Max. 0.05	100	400
			4*1		100	380
		10	2*2		150	585
			4		150	600
			10*1		150	585
		12	4		150	800
			5*3		150	800
			10	Max. 1.10	150	800
		14	5＊		150	800
		15	5		150	1,200
			10		200	1,200
			16*4		200	1,200
			20		200	1,200
		20	5		200	2,000
			10	Max. 0.15	250	2,000
			20	Max. 1.10	250	2,000
		25	5		200	2,000
			10	Max. 0.20	300	2,000
			25	Max. 0.12	300	2,000
		28	6*3	Max. 1.10	250	2,000
		32	5*4	Max. 0.20	300	2,000
			10		300	2,000
			32	Max. 0.15	300	2,000
Block nut precision class C10	BSBR	15	5	Max. 1.10	150	1,200
		20			200	1,200
		25			200	1,500
		15	10		150	1,200
		20		Max. 0.15	200	1,200
		25		Max. 0.20	200	1,500
Standard nut precision class C7	BSST	8	2	Max. 0.03	100	380
		10	4		150	585
		12	4		150	795
		15	5		150	1,200
			10		200	1,200
			20		200	1,200
		20	5		200	1,200
			10	Max. 0.05	250	2,000
			20	Max. 0.03	250	2,000
		25	5		200	2,000
			10	Max. 0.07	300	2,000
C-VALUE Standard nut precision class C7	C-BSST	8	2	Max. 0.05	100	400
		10	2		150	585
			4		150	600
		12	4		150	800
			5		150	800
			10	Max. 1.10	150	800
		15	5		150	1,200
			10		200	1,200
			16		200	1,200
			20		200	1,200
		20	5		200	2,000
			10		250	2,000
			20		250	2,000
		25	5		250	2,000
			10		200	2,000
			25		300	2,000
		32	5		300	2,000
			10	Max. 0.14	300	2,000
			32		300	2,000

Precision ball screw

Type	Common type(s)	Screw shaft diameter	Lead	Axial clearance	Screw shaft length (mm)
					MIN	MAX
Standard nut, precision class C5	BSS	8	2	Max. 0.005	100	210
		10	2		100	315
			4		150	380
			10		150	450
		12	2		150	445
			4		150	400
			5		150	450
			10		200	600
		15	5		150	1,095
			10		200	1,095
			20		230	1,095
		20	5		200	1,000
			10		250	1,500
			20		250	1,500
		25	5		300	995
			10		300	1,500
			20		300	1,500
C-VALUE Standard nut, precision class C5	C-BSS	8	2	Max. 0.008	100	210
		10	2		100	315
			4		150	380
		12	2		150	445
			5		150	450
			10		200	600
		15	5	Max. 0.015	150	1,095
			10		200	1,095
			20		230	1,095
		20	5		200	1,000
			10		250	1,500
			20		250	1,500
		25	5		300	995
			10		300	1,500
			25		300	1,500
Standard nut precision class C3	BSX	6	1	0 (preloaded part)	80	205
		8	1		80	255
			2		100	240
		10	2		100	310
		12	2		150	390
			5		150	440
		15	5		150	590
Standard nut precision class C7	BSSE	8	2	Max. 0.030	100	210
		10	2		100	315
			4		150	380
		12	2		150	445
			5		150	450
			10		200	600
		15	5		150	1,095
			10		200	1,095
			20		230	1,095
		20	5		200	1,000
			10		250	1,500
			20		250	1,500
		25	10		300	1,500
			20		300	1,500

Example of Selecting Lead Accuracy

Usage conditions:

- Screw thread diameter is φ15, lead is 20
- Stroke is 720 mm
- Positional accuracy is ±0.05 mm / 720 mm

Selection:

Determine ball screw lead accuracy based on usage conditions to determine the appropriate precision class.

(1) Screw thread length
Stroke + nut length + excess: 720 + 62 + 60 = 842 mm
Note: "Excess" refers to an extra amount equivalent to 1.5 to 2 times the lead length, and is used as a measure against overrun.
Calculation based on a lead of 20 (with excess at both ends): 20 × 1.5 × 2 = 60.

(2) Lead accuracy
Using Table 1, calculate the tolerance for average travel length deviation ±ep based on a screw thread length of 842 mm.
C3・・・±0.021mm/800～1000mm
C5・・・±0.040mm/800～1000mm

(3) Lead accuracy selection
A C5 class (±0.040 / 800–1000 mm) screw is suitable for meeting positional accuracy usage conditions of ±0.05 mm / 720 mm.

Example of Selecting Axial Clearance

Usage conditions:

- Screw thread diameter is φ15, lead is 5
- Permissible backlash is ±0.01 mm

Selection:

According to Table 5, when using a screw with a thread diameter of φ15, a C5 class with max. 0.005 axial clearance fulfills the requirement of ±0.01-mm permissible backlash.

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