Solid and hollow spline shafts up to roughly 2 m are catalog-standard across major suppliers, with the SNR/NTN BSP family covering diameters from 4 mm to 100 mm in solid form and offering hollow variants for cable routing or weight reduction [S5]. Thomson markets ball splines in multiple shaft sizes and lengths for precision linear and rotary motion [S1], and THK publishes a complete ball spline product line spanning high-torque, medium-torque, and rotary-with-bearing nut configurations [S2].
For designers sizing long-stroke systems, the practical length ceiling is set by manufacturer stock programs and the ability to ship, support, and align a single rigid shaft, with 2 m a common upper bound rather than a hard physical limit. NB Corporation notes that the basic dynamic load rating of a ball spline runs 5-12 times that of a comparable slide bush, an advantage that compounds as stroke length grows [S4].
Shaft Length Programs: Solid, Hollow, and Cut-to-Length
SNR's BSP catalog exposes a length field directly in the type code: position 7 accepts any value in millimetres, and the series is documented with shaft diameters from 4 mm to 100 mm, plus a hollow-shaft option designated by shaft type code "K" against the solid "L" [S5]. Hollow shafts matter in long-stroke Z-theta axes because cables and airlines can be passed through the bore, removing the drag and snag risk of an external umbilical, a configuration used in SCARA Z-axis cable routing [S3].
IKO's ball spline rolling guides cover a smaller diameter window of 4 mm to 12 mm and ship in both standard and high-rigidity long cylinder versions, with cylinder length selected to match the application stroke [S6]. PBC Linear, selling 60 Case linear shafting separately, offers inch and metric diameters cut to length with a controlled surface finish that shortens break-in time on long shafts [S7]. A separate engineering discussion notes that commercial 8 mm linear shafting typically holds to about 0.02 mm diameter tolerance, useful context when deciding whether to substitute a generic linear shaft for a true grooved spline [S8].
Groove Count and Raceway Geometry: Why 4 Beats 6 for Most Strokes
Ball spline shafts use 2, 4, or 6 longitudinal grooves, and the choice has direct consequences for length, compactness, and load capacity. NB Corporation's whitepaper states that the basic dynamic load ratings of a ball spline are typically 5-12 times that of a slide bush of similar size, and the most popular configuration is the 4-groove shaft, because in 6-groove shafts the active and recirculating ball paths can collide, forcing the nut to protrude beyond the shaft end and lengthening the overall envelope [S4].
Gothic-arch 4-point-contact groove geometry is the precision choice for larger spline sizes, because the arch removes internal clearance and lifts moment-load capacity. A 4-groove gothic-arch nut also lets active and return paths sit side by side, holding axial length down, an important consideration when total stroke is already eating into a 2 m budget [S4].
Comparison of Common Long-Shaft Configurations
Four ball spline configurations cover the bulk of long-shaft applications, and the differences are easiest to read side by side: [S4]
<b>Solid shaft, cylindrical nut (e.g. SNR BSP 25 CN, 100 CN):</b> the baseline. High torque capacity per unit length, simplest sealing, lowest cost per metre. Use when stroke is linear only and weight is acceptable [S5].
<b>Solid shaft, flange nut:</b> easier face mounting to a machine wall or carriage, slightly more axial envelope, similar torque. Use when the carriage has a perpendicular mounting face [S5].
<b>Hollow shaft (BSP K series):</b> trades bending stiffness for through-bore access, suiting SCARA Z-theta axes where cables, air lines, or a through-rod for a tool changer must pass through the shaft [S5][S3].
<b>Rotary ball spline (with angular contact bearing, crossed rollers, or gear on the nut OD):</b> adds a theta axis to the Z stroke, turning the assembly into a combined linear-and-rotary actuator. Heavier, longer, and more expensive, but it eliminates a second axis stage [S3][S2].
Selection criteria: stroke length, moment load, need for through-bore, need for theta rotation, and envelope. For pure linear anti-rotation on a stroke approaching 2 m, a solid 4-groove gothic-arch shaft with a cylindrical nut is the default. For combined Z-theta, move to a rotary ball spline and accept the additional mass.
Standards, Precision Classes, and Preload
SNR BSP shafts are offered in two precision classes, N (normal) and P (precision), and three preload bands, Z0 (no preload), Z1 (light), and Z2 (medium), encoded in positions 8 and 9 of the type code [S5]. This granularity matters on long shafts because preload sets the running friction, the breakaway torque at reversal, and the susceptibility to stick-slip at low speeds. Light preload (Z1) is the usual long-stroke default, with Z2 reserved for high-stiffness machine-tool axes where the extra drag is acceptable.
Sealing is selected separately: code UU ships with end seals fitted, AA ships without, and the seal choice is the biggest variable in service interval on a long exposed shaft [S5]. THK's ball spline line lists matching variants, including high-torque caged ball, high-torque, medium-torque, and rotary-with-support-bearing nut types, giving designers a torque-capacity ladder rather than a single product [S2]. Thomson notes ISO and AS certifications in its compliance documentation, relevant for medical, food processing, and aerospace builds where supplier quality system status is gated [S1].
Length Limits: Where the 2 m Ceiling Comes From
Few manufacturers publish an absolute maximum spline length, but the practical ceiling around 2 m is set by three coupled limits: shipping and handling of a long, ground, precision shaft without inducing a permanent bow; critical speed and column buckling under compressive load; and the cumulative angular error over the full stroke, which scales with length and tolerance class [S5][S4]. NB Corporation's whitepaper flags that 4-groove shafts let the nut carry moment loads that would deflect a 6-groove equivalent of the same diameter, a real benefit on long shafts where mid-span sag is a concern [S4].
For strokes above the single-shaft practical limit, the alternatives are a telescoping or multi-stage ball spline, a linear guide on a separate parallel rail with the ball spline used only for anti-rotation, or a re-circulating long-stroke ball spline offered as a special by most vendors [S3][S4]. A plain linear shaft substituted for a spline shaft loses torque transmission entirely, so it only works when the load is purely linear and anti-rotation is provided elsewhere, as discussed in the ball spline encyclopedia entry and the shaft fastening reference.
Failure Modes and Sizing Checks on Long Shafts
Four failure modes dominate on ball spline shafts approaching 2 m. First, mid-span deflection under combined cantilever moment and axial load, mitigated by selecting the smallest diameter that meets load and stiffness, not the largest. Second, critical-speed whirling in high-rectilinear-speed applications, which can be checked against the same column-buckling formulas used for long ball screws, and a spline shaft is no exception. Third, seal wear on long strokes, where total seal travel per cycle is stroke times cycles, and a UU-sealed BSP on a 2 m stroke at 10 cycles per minute accumulates 1,200 m of seal travel per hour [S5].
Fourth, ball-circuit brinelling during shipping or installation when a long shaft is supported at only two points; NB Corporation specifically warns that on some 6-groove designs the balls can fall out of the nut if the two are separated, so handling protocol matters more as length grows [S4]. For installation, the spline shaft typically requires shaft collars or end blocks to locate it axially against the housing, and any misalignment with the driven carriage is multiplied by shaft length, so the alignment budget on a 2 m shaft is roughly an order of magnitude tighter than on a 200 mm shaft.
Typical Use Cases at Long Stroke
Long ball spline shafts appear in three application clusters. Paper and converting machinery uses single-shaft ball splines to let spool carriers travel horizontally while staying rotationally locked, replacing a two-shaft linear bushing arrangement with one shaft and saving alignment work [S3]. SCARA robots use hollow rotary ball splines to combine the Z stroke and the theta wrist axis into a single actuator, with cables routed through the hollow bore [S3].
CNC automatic tool changers use rotary ball splines to deliver both the linear drawbar motion and the rotational orientation in a single compact unit, cutting the moving mass relative to a stacked linear-axis-plus-rotary-axis design [S3]. For a deeper dive into the SCARA Z-theta use case, the related article on ball screw spline combined shafts covers the integrated variant where a ball screw and ball spline share a single shaft. Misumi's older ball spline overview similarly catalogues these use cases across packaging, inspection, and assembly [S9].
Sourcing Long Ball Splines: Catalog vs Special
Standard catalog ball spline shafts in popular diameters (typically 16, 20, 25, 32, 40, 50 mm) ship in lengths up to roughly 2 m from stock at SNR/NTN, THK, NSK, NB, and Thomson, with hollow variants and special end machining treated as options rather than customs [S5][S2][S1]. Specials, including non-standard lengths above the catalog max, special end machining, custom grease ports, or non-standard nut orientations, are usually quoted against a drawing, with lead times in weeks rather than days [S5].
For procurement, the most useful datasheet fields are shaft diameter, length, precision class (N vs P), preload (Z0/Z1/Z2), seal code (UU/AA), and shaft type (solid L vs hollow K), all of which appear in the SNR type code at positions 2 and 6-9 [S5]. THK's lineup adds a torque-class dimension (high vs medium torque) and rotary-with-bearing options that have no direct SNR equivalent in the BSP naming scheme [S2]. Buyers should also confirm ISO/AS certification status with the supplier for regulated-industry builds, where Thomson and THK both publish compliance documentation [S1][S2].
Track the following signals when monitoring this category: supplier catalog updates on maximum length per diameter, additions of hollow-shaft variants to mid-range sizes (currently strong at 4-100 mm solid, more limited in hollow), and any move to integrate measurement or electronic test and measurement feedback into the nut body for closed-loop long-stroke positioning. For adjacent measurement and test needs on long machines, integrated encoders and inline load cells are the most common add-ons.