Ball splines can and do accept radial loads perpendicular to the shaft axis; torque transmission through axial ground grooves remains their primary design intent [S1].
Catalog ratings therefore list torque as the headline figure, with radial capacity derived as a secondary check on groove geometry, number of circuits, and preload [S1]. Standard ball splines carry two, three, or four grooves, with some designs reaching six; life is calculated with the standard bearing-life equation using both radial and torque terms [S3].
Gothic-Arch vs Circular-Arc Grooves: the Geometry That Sets Radial Capacity
Ball spline grooves come in two contact profiles, and the choice swings radial stiffness more than torque capacity [S3]. A Gothic-arch groove gives four-point contact per ball, raising rigidity and moment-load support at the cost of higher friction; a circular-arc groove gives two-point contact, with smoother running and lower drag [S3]. For a four-groove Gothic-arch design, the assembly has 16 contact points; a two-groove Gothic-arch unit has 8, and the higher contact count scales torque capacity directly with the number of load-carrying balls in the circuit [S3].
Preload works the same way it does on profiled rail guides: increasing preload tightens radial and moment stiffness but raises heat generation and reduces travel life, so the radial-load check should always be run against the manufacturer's published dynamic load rating rather than a generic catalogue value [S3].
How a Ball Spline Compares to a Linear Bushing on Radial Load
Where a linear bushing can only support pure radial loads, a ball spline can withstand overhung loads and moment loads because the axial shaft grooves prevent the nut from rotating relative to the shaft [S3]. THK's Medium Torque Type ball spline explicitly markets large load capacity in both the radial and torque directions, enabled by two to three rows of load-carrying balls along the shaft [S5]. Ball splines distribute the radial reaction along the engaged length of the shaft rather than concentrating it at the loaded zone, so they handle off-center loads common in tool-change and pick-and-place cells better than a comparable bushing on a plain shaft [S2].
For context on the load-direction definitions: radial load acts at right angles to the rotation axis, while axial (thrust) load acts parallel to it, and most bearing choices collapse to which direction dominates the duty cycle [S4]. A shaft collar or shaft key cannot substitute here; a ball spline's radial rating is set by the rolling-element contact, not by a mechanical stop.
What Spline Type Fits Which Load Mix
Selection reduces to three load-direction questions: is the duty mostly torque, mostly radial/moment, or a true mix? Thomson's catalogue answer is short: "Yes. Ball splines can accept radial loads but their primary purpose is to resist torque" [S1]. THK splits the offering into High Torque Caged, High Torque, Medium Torque, and Rotary with Gear or Support Bearing, with the Medium Torque line explicitly designed for radial-and-torque combined duty [S5]. Misumi's catalog notes the same in one line: the spline "can bear radial loads and rotational torque simultaneously" [S7].
A standard (non-rotary) ball spline is the right pick when torque is the dominant load and radial/moment is a secondary reaction; a rotary ball spline, which adds an angular-contact bearing, crossed-roller bearing, or gear on the outer diameter of the nut, is the right pick when the shaft must also free-spin relative to the nut [S3]. Rotary ball splines add a second bearing race, so radial stiffness of the assembly depends on the rotary element as well as the spline grooves, and the catalogue rating has to be read as a system value, not just a spline value.
Sizing, Accuracy, and the Limits of the Radial Rating
Ball spline sizing borrows from both linear-bearing and ball-screw practice: life comes from the standard bearing-life equation applied to combined radial and torque loads, critical speed depends on shaft root diameter, unsupported length, and end fixity (fixed-fixed vs fixed-free), and accuracy is graded on radial run-out of the shaft and nut body plus perpendicularity of the flange [S3]. Accuracy classes are not harmonised to DIN or JIS, so a "precision" grade from one maker can equal a "high" grade from another [S3].
For shafts longer than about 2 m, deflection under radial load starts to dominate the design, and the unsupported length between end bearings becomes the first thing to check; ball spline shaft deflection under radial load walks through the equation set and boundary-condition choices that drive that check. Shaft length options on commercial ball spline lines have been pushed out to 2 m in recent catalogue releases, which is useful context when the application is a long-reach gantry or a telescopic axis [S2][news: ball-spline-shaft-length-options].
Integration Notes: Couplings, Bearings, and the Shaft Seat
End fixity is set the same way it is on a ball screw, with a support-bearing block on the drive end and a floating support on the opposite end, so a shaft coupling or shaft fastening choice on the driven end should be picked to avoid inducing a bending moment into the spline shaft. Because the spline grooves are the load path, a coupling that imposes even a small angular misalignment will feed a steady moment load into the spline, and the radial life drops faster than the torque life on the same misalignment. [S3]
Two practical signals to watch in service: (1) rising drag torque at zero radial load usually points to preload or lubrication drift, not groove wear, and (2) a step change in radial deflection under a constant moment load is the early indicator that the Gothic-arch contact pattern is brinelling, which means the radial rating on paper is no longer the radial rating on the machine.