What is the real difference between internal and external circulation?

Internal circulation hides the return path inside the screw: shorter nuts, nothing protruding, better shielding — but the screw needs a bore, which sets a minimum diameter. External circulation needs an external guide pipe, allowing smaller screws, but adds axial length and slightly reduces stiffness.

Can ball turns be changed later?

No. Ball turns are fixed when the nut is made. At the same diameter and lead, 2.8 turns and 4 turns are different models with different dimensions — you replace the whole nut.

Why do ball turns use decimals (2.8, 4.8)?

The decimal is the average number of ball wraps, used to fit as many balls as possible inside one envelope without ball-to-ball interference. It is an industry-standard encoding, not a rounded figure.

How do I read ball turns on SFK, which has no trailing number?

SFK/XSK are K-type miniature nuts whose size is set by diameter + lead only; makers do not encode ball turns for them because the construction is fixed and turns vary with envelope. Size these on Ca and envelope directly.

Ball Screw Circulation & Ball Turns: Internal, External and Cap Compared

How to choose ball-screw circulation: internal, external and cap types compared, plus how ball turns (1.8 to 7.6) affect load capacity and stiffness, with the real distribution across 23 series and 264 models.

For the same screw, circulation decides structure and heat dissipation while ball turns decide load capacity and stiffness. The tables below use the real distribution across this site's 23 series and 264 models.

1. The three circulation families

Balls travel forward along the screw raceway and return at the far end; that return path is the circulation type. It decides nut length, sealing method and heat path.

Circulation	Typical series	Structure and use
Internal circulation	SFU / SFNU / SFNI / SFI / OFI / OFU / SCI / SCNI	The return path runs inside the screw, so nothing protrudes — compact and well shielded; the mainstream general-purpose choice.
External circulation	SFV / OFV / BSHR（d≤12）	The return path is external and needs a guide pipe; better heat dissipation and contamination resistance, but longer axially and slightly less stiff.
Cap / return-pipe circulation	SFA / SFB / SFJ / SFY / SFYA	End caps catch the balls at both ends, giving easy mounting and optional sealing; this is how high-speed feed and high Dm-N are achieved.
Flangeless / special	SFK / XSK / BSHR（d≥14）/ XSYR / XSUR	Nuts without a flange or in special sizes, for very tight spaces, miniature ends and thin linear-stage mounts.

Model codes under each series are decoded field by field in the model code guide.

2. All 23 series and their real model counts

The table groups this site's 264 ball-screw models by circulation. Model counts show how broad each series is.

Series	Circulation / construction	Models	Ball turns
SFV	External circulation · heavy duty (flange)	32	2.7 / 2.8 / 3.8 / 4.8 / 1.8 / 7.6
SFA	Cap circulation · high-speed feed	31	2.8 / 3.8 / 1.8
SFY	High Dm-N value · cap circulation (single/double lead)	24	3.6 / 5.6 / 1.6
SFYA	High Dm-N value · cap circulation	24	3.6 / 5.6 / 1.6
SFB	Cap circulation · upgraded SFA (DIN 69051 FORM B)	20	2.8 / 3.8 / 1.8
SFNU	Internal circulation · reinforced sealing (flange); left-hand thread available	15	4 / 3
SFK	K-type miniature nut	14	—
SFU	Internal circulation · reinforced sealing (flange)	12	4
SFNI	Internal circulation · reinforced sealing (flange); left-hand thread available	12	4 / 3
BSHR	No flange · miniature (d≤12 external / d≥14 internal)	12	2 / 2.5 / 3 / 3.5 / 4
OFI	Internal circulation · double nut (flange)	11	4
OFU	Internal circulation · double nut (flange)	11	4
SCNI	Standard internal circulation (no flange)	11	4
OFV	External circulation · heavy-duty double nut (flange)	10	4.8
SCNH	Linear-stage specific (no flange)	9	1.8 / 2.8 / 3.8 / 4.8 / 5.8
SFI	Internal circulation · reinforced sealing (flange)	4	4
SCI	Standard internal circulation (no flange)	4	4
SFJ	Cap circulation · quiet, reinforced sealing	2	1.8
XSK	K-type miniature (special)	2	—
XCNH	Linear-stage specific (no flange)	1	1.8
SFM	Milling-machine specific; left-hand thread available	1	4
XSYR	XSY miniature	1	—
XSUR	Internal circulation (special)	1	—

3. Ball turns decide load capacity and stiffness

Ball turns (the trailing number, e.g. the 4 in SFU1605-4) is how many times the balls wrap around the screw. More turns fit more balls in the same envelope, raising load capacity and stiffness at the cost of a longer, costlier nut.

Ball turns	Ca (relative)	Stiffness (relative)	Nut length	Typical use
1.8 / 2 / 2.5	~1.0×	~1.0×	shortest	Miniature shafts, light load, tight spaces
2.7 / 2.8 / 3	~1.3×	~1.4×	short	General drive, fast feed
3.5 / 3.6 / 3.8	~1.6×	~1.8×	medium	Medium load, higher stiffness
4 / 4.8	~2.0×	~2.4×	long	Heavy load, long life (most common)
5.6 / 7.6	~2.8×	~3.4×	longest	Extra-heavy load, milling / heavy cutting

Ball turns alone are not the whole story: at 4 turns, Ca for Ø16 and Ø25 differs several-fold. Size on diameter, lead and turns together — see the sizing guide.

4. Filter all 264 models by circulation and ball turns

Matching models 264

Model	Ø (mm)	Lead	Turns	Circulation	Accuracy

4. Choosing circulation by duty cycle

General automation lines with tight space → internal circulation (SFU/SFNU/SCI): the most common and best shielded.
High-speed machining needing heat dissipation → cap circulation (SFA/SFB) with replaceable seals; the top pick for fast feed.
Heavy load, large lead, high Dm-N → SFY/SFYA or SFV (external, heavy duty).
Linear stages and thin mounts → flangeless SCNH/XCNH.
Shafts ≤12 mm → BSHR or SFK/XSK miniature nuts.
To remove axial clearance, any circulation type can be preloaded — see the preload guide.

FAQ

What is the real difference between internal and external circulation?: Internal circulation hides the return path inside the screw: shorter nuts, nothing protruding, better shielding — but the screw needs a bore, which sets a minimum diameter. External circulation needs an external guide pipe, allowing smaller screws, but adds axial length and slightly reduces stiffness.
Can ball turns be changed later?: No. Ball turns are fixed when the nut is made. At the same diameter and lead, 2.8 turns and 4 turns are different models with different dimensions — you replace the whole nut.
Why do ball turns use decimals (2.8, 4.8)?: The decimal is the average number of ball wraps, used to fit as many balls as possible inside one envelope without ball-to-ball interference. It is an industry-standard encoding, not a rounded figure.
How do I read ball turns on SFK, which has no trailing number?: SFK/XSK are K-type miniature nuts whose size is set by diameter + lead only; makers do not encode ball turns for them because the construction is fixed and turns vary with envelope. Size these on Ca and envelope directly.

Ball Screw Selection Model Code Sizing L10/Nc/Pc Lead Accuracy Preload P0–P4 Full Model List

Series and circulation data come from this site's 264 ball-screw model pages; ball turns come from the model-code suffix. Relative Ca and stiffness figures are engineering multiples at the same diameter.

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