A lead screw is a rotation-to-translation device; a ball spline is a rotation-plus-thrust device that can carry moment and radial load while still sliding. Specifying one where the other belongs is a common 2026 mis-buy in cobot joints, Z-axis gantries, and indexing tables [S2][S3].
The 2026 component market sells both as catalog items from overlapping vendors: VXO and LMT list lead screws, ball screws, and linear motion products on the same price page, while RS Groups Taiwan catalogue lists lead screws, nuts, ball screws, and splines under one mechanical power-transmission tree at TWD 1,580-2,400 per piece for common sizes [S1][S2][S4].
Function and Mechanism
A lead screw pairs a helical thread on a shaft with a matching nut; the nut is typically polymer (acetal, PTFE-blend) or bronze, and sliding friction between thread flanks generates the axial force lead screw. Standardized Acme and Hi-Lead profiles are sold by Roton in inch sizes from 3/16"-20 Acme up through 1-1/2" and metric trapezoidal equivalents elsewhere [S6].
A ball spline replaces the sliding nut with a splined, grooved shaft and a ball-retained cage/nut that runs along the grooves. The balls carry torque through the angular-contact groove geometry while the same member slides axially on a secondary track, so one nut transmits rotation, axial force, and moment loads simultaneously [S3].
Efficiency, Speed and Backlash Behaviour
Lead-screw efficiency is governed by thread geometry and lead angle, with typical values of 30-50% for Acme single-start polymer-nut pairs at low rpm, climbing to 80-95% only on high-lead multi-start geometries with low-friction nuts and small diameters [S5][S6]. Self-locking behaviour is a function of lead angle below the friction angle and is generally maintained on Acme profiles but lost when lead exceeds roughly 1× diameter on multi-start screws [S5].
Ball splines are recirculating-element devices with efficiencies in the 90%+ band when preloaded, because rolling contact replaces sliding. Backlash on a single-nut ball spline is typically reduced to near zero with an angular-contact preloaded nut design, and Thomson's 2023 eNews notes that ball-spline automation is re-emerging in collaborative-robot joints where zero-backlash torque transmission is mandatory [S3].
Load Capacity and Combined-Load Handling
Lead screws are rated for axial thrust only; any radial or moment load must be reacted by a separate linear guide running parallel to the screw. Practical continuous thrust limits for inch Acme lead screws from Roton sit in the low-thousands-of-newtons range for 3/8"-1/2" diameters and climb into tens of kN for 2"+ industrial screw pairs [S6]. Buckling limits, not thread shear, often set the ceiling for long-stroke vertical axes.
Ball splines are explicitly designed for combined loading: the angular-contact ball path carries torque and moment loads while a separate ball track or the same track handles axial force and radial load. This is why the same shaft can replace both a linear guide and a torque-transmitting coupling in a robot shoulder or rotary actuator, and why Thomson positions the ball spline as a combined-load member rather than a screw [S3].
Decision Criteria: Which One Fits the Axis
Use a lead screw when the job is pure linear thrust at low-to-medium speed, when self-locking is required (vertical Z without brake), when the duty cycle is light, and when budget per axis is tight. Polymer-nut Acme pairs are the cheapest linear actuator in any catalog, and 2026 sourcing at VXO, LMT, and Kaiwo keeps lead times short on standard diameters [S1][S4][S5].
Use a ball spline when the shaft must transmit torque while sliding, when the axis sees radial or moment loads with no parallel linear guide, when zero backlash under reversal is required, or when the design consolidates a guide + coupling into one part. The combined-load envelope is the killer feature: a ball spline is rarely specified purely for linear travel [S3]. Cross-reference on cost and grade for the linear side lives in this ball screw 2026 buying guide, and the ball screw price 2026 breakdown shows why adding preloaded ends or end-machining often costs more than a standard nut.
Standards, Materials and Sourcing Signals
Lead screws and ball splines are typically specified to ISO 3408 (ball-screw terminology and acceptance), DIN 103 (metric trapezoidal threads), and ANSI B1.1/B1.3 (inch Acme/UN threads) depending on region.
2026 sourcing signals: RS Taiwan's mechanical-power-transmission category lists lead screws and ball screws under a single tree at TWD 1,580 for a DST-JFRM-282835DS14x25 lead-screw-and-nut pair and TWD 2,400 for a comparable ball-screw assembly, a useful ballpark for budget planning [S2]. Thomson's 2023 eNews (the most recent dedicated ball-spline article in the source set) flagged the ball spline as a re-emerging combined-load product for cobots and automation [S3], while VXO, LMT, and Kaiwo continue to push lead screws as their volume general-purpose linear actuator [S1][S4][S5].
Selection gates that push the call one way or the other in 2026: required efficiency above 80% on a reversing axis removes polymer-nut lead screws; required self-lock on a vertical axis removes ball splines unless a fail-safe brake is added; required torque transmission with linear travel removes lead screws entirely. Confirming these three gates first saves a re-spin of the BOM. For the deeper selection math on the linear-thrust side, the ball screw selection criteria 2026 article walks through load, life, buckling, and lead gates on the rolling-element screw family.