Ball screws convert rotary to linear motion through recirculating ball bearings rolling between matched helical grooves, reaching mechanical efficiencies of 90% or higher [S5][S1]. Lead screws do the same job through direct sliding contact between a polymer or metal nut and a steel or bronze thread, with typical efficiencies in the 20% to 40% band [S9][S5]. The 50 to 70 percentage-point efficiency gap is the single largest performance divider between the two technologies and the first number a process engineer should put on the spec sheet.
Backlash behaves inversely to efficiency in this comparison. Ball screws can be preloaded against the raceways to remove measurable lost motion, with options like oversize balls, offset preload, or constant-pressure preload reducing axial play to near zero [S6][S4]. Lead screws carry larger inherent clearance because the sliding nut must physically clear the screw thread, although anti-backlash nut designs using a second spring-loaded half-nut cut the lost motion substantially [S4][S2]. For positioning accuracy below roughly 0.05 mm with high duty cycle, the ball screw is the default; for coarse motion at low cost, the lead screw stays competitive.
Efficiency numbers: where the watts actually go
A 50 W input example shows the gap: a lead screw at roughly 35% efficiency delivers about 17.5 W of useful output, with the remaining 32.5 W turning into friction and heat at the sliding interface [S9]. A ball screw on the same load typically moves 45 W or more to the load, cutting waste heat by an order of magnitude and letting a smaller motor, smaller drive, and smaller power supply do the same work [S5][S2]. Sliding contact in the lead screw is the root cause: kinetic friction coefficients between steel and polymer sit in the 0.05 to 0.15 range, while rolling contact in a ball screw drops the effective coefficient to roughly 0.001 to 0.005 [S1][S8].
The efficiency drop of a lead screw also rises as lead angle increases. Standard ACME and trapezoidal profiles, with leads in the 2 mm to 20 mm range on 10 mm to 25 mm screws, are the sweet spot for self-locking, and the same geometry is what caps efficiency near 35% [S9][S2]. High-helix leads and multi-start screws push efficiency above 50% but sacrifice the self-locking property that is often the reason a lead screw was chosen in the first place [S4][S5].
Backlash, repeatability, and preload options
Backlash on a single-nut lead screw is typically 0.10 mm to 0.50 mm depending on size and lead, while a preloaded ball screw can be specified at 0.01 mm or below for precision classes C0, C1, C3, C5, C7, and C10 under ISO 3408-style accuracy grades [S4][S6]. Ball nut preload methods include offset-lobe preload, where two nuts are offset axially to remove internal clearance, and constant-pressure preload using a spring or shim stack to keep the nut loaded against the raceway at all times [S6][S2].
For lead screws, the practical anti-backlash construction is a two-piece nut with a spring or wave washer pushing the halves apart, or a single nut running against an axial spring on the screw end; both reduce but do not eliminate the gap, and wear widens the gap over time as the polymer nut deforms [S4][S2]. Where lead screws win on backlash is repeatable, smooth hand-cranked positioning: ACME and trapezoidal threads with high threads-per-inch give fine hand-feel resolution that a coarse-pitch ball screw cannot match [S5].
Speed, duty cycle, and lifetime comparison
Ball screws support continuous-duty dynamic loads at DN values (nominal diameter in mm times rpm) commonly reaching 100,000 to 150,000 for rolled screws and up to 200,000 for ground screws, with surface speeds above 1 m/s routine in machine-tool service [S2][S5]. Lead screws, because of frictional heating at the sliding interface, are usually limited to surface speeds of 0.5 m/s or below and are most often run intermittently or in hand-actuated service [S5][S7]. Rated life for a ball screw is commonly stated at 1 million to 10 million revolutions at full dynamic load, with L10 life calculation following the standard cube-rule load-life relationship [S2][S6].
Lead screw life is dominated by nut wear rather than fatigue, with polymer nuts typically rated for thousands of hours of operation under rated load and bronze nuts offering longer life at the cost of higher friction [S4][S7]. The wear pattern is the deciding factor in cost-of-ownership: a 400-parts-per-day production machine running a ball screw will keep position accuracy over years, while the same machine on a lead screw will see the nut clearance grow and parts drift out of tolerance [S3].
Self-locking, vertical loads, and brake requirements
Lead screws with thread leads under roughly one-third the screw diameter are self-locking, meaning the nut cannot back-drive the screw under load and the load stays put when the motor is de-energized [S2][S5]. This is a structural feature of low lead-angle sliding contact, and it removes the holding brake from vertical-axis designs, cutting cost and failure modes in lifts, Z-axis stages, and any suspended load [S4][S5].
Ball screws are generally not self-locking at common leads, and a vertical or suspended load will back-drive the screw when power is removed, which can drop the load or crash the stage [S2][S4]. The mitigation is a failsafe brake on the motor or on the screw itself, sized to hold the worst-case load with a 1.5 to 2.0 safety margin; cost and complexity rise accordingly [S4][S2]. For new designers this is the single most common reason a lead screw stays in the bill of materials even when a ball screw would be more efficient.
Cost, maintenance, and environmental fit
Lead screws are cheaper at the part level and require less maintenance: polymer nuts can run dry or with minimal grease, which suits cleanrooms, medical devices, food production, and packaging lines where lubricant migration is unacceptable [S5][S2]. Ball screws require oil or grease for the recirculating ball paths, and contamination from dust, wood chips, weld spatter, or washdown ingress will damage the raceways and accelerate fatigue [S5][S2].
For machine tools, automation gantries, semiconductor stages, and any high-cycle positioning axis, the ball screw is the default [S1][S5]. For 3D printers, low-end CNC conversions, manually adjusted fixtures, and low-speed vertical lifts, the lead screw keeps the design simple and cheap. Where the two are paired with linear guides, designers should remember that the support rail carries side loads while the screw carries only axial load, and a worn ball bearing in the support carriage will mimic screw backlash in the system.
Application matrix: pick the screw that fits the duty
High-speed, high-precision, continuous-duty positioning: ball screw, with preload, ground or rolled to C5 or better, paired with a servo drive and a holding brake if the axis is vertical [S5][S2]. Light-duty, low-cost, infrequent positioning with a vertical load: lead screw, ACME or trapezoidal thread, polymer nut for self-lubrication, no brake needed [S2][S4]. Hand-cranked adjustment with fine resolution: lead screw with high threads-per-inch, which gives finer hand-feel than a coarse-pitch ball screw [S5]. Cleanroom or food-grade service: lead screw with polymer nut and dry running, since the ball screw grease is the disqualifier [S5].
Frame the decision in four axes and the answer usually falls out cleanly: efficiency above 80% or high duty cycle means ball screw; self-locking without a brake means lead screw; backlash below 0.05 mm means preloaded ball screw; clean or food-grade service means polymer-nut lead screw [S1][S4][S5]. For a wider look at how recirculating element design choices affect a related component, the Ball Retainer vs Full Complement linear guide comparison walks through a parallel preload versus load-capacity trade-off, and the Ball Bearing Bore and OD size chart gives the metric reference series used inside most ball nuts. For a more general look at rolling-element geometry and load rating math across linear motion, the ball screw encyclopedia page collects the same formulas in one place.