Linear Motion Essentials
Lead Screw Versus Ball Screw
Compare thread lead, pitch, friction and motion trade-offs.
You must choose between real alternatives covered by “Lead Screw Versus Ball Screw” for a defined machine, circuit or installation.
Compare sliding lead screws with recirculating ball screws, calculate travel from pitch and lead, and separate backlash, accuracy, resolution and repeatability before choosing a drive.
- Select or check first
- 1. Define the motion. Record usable stroke, move time, velocity, acceleration, settling time, duty cycle and the number of expected moves. Include stopping distance and end clearances.
- Controlling assumption
- Load, motion profile, geometry, alignment, duty, lubrication and environment match the selected mechanism and ratings.
- Top risk
- A vertical load needs an independent safe-holding strategy. Do not rely on assumed thread friction to protect people or equipment. Assess gravity-driven descent and stored energy, then provide an appropriately engineered brake, counterbalance, restraint or other protective measure as the machine risk assessment requires.
- Proof
- 7. Test the assembled axis. Measure position in both directions and across the stroke under representative load and temperature. Record motor current, temperature, noise, reversal behavior and protective-function results.
Start with the basics Optional foundation before “Lead Screw Versus Ball Screw”
Mechanical-design basics
Mechanical parts must carry loads, guide motion and remain aligned; similar-looking components can behave very differently under real force and duty.
- Force and torque
- Force pushes or pulls along a line. Torque is a turning effect equal to force multiplied by its perpendicular lever arm.
- Speed and acceleration
- Speed describes how fast position changes. Acceleration describes how fast speed changes and often creates the highest inertial load.
- Strength and stiffness
- Strength concerns permanent damage or failure. Stiffness concerns elastic deflection under load; a part can be strong enough yet too flexible for accurate motion.
- Load path
- The load path is the complete route by which force travels through shafts, bearings, fasteners, frames and foundations. Every interface on that route must be checked.
- Tolerance, clearance and alignment
- Tolerance permits controlled variation, clearance provides intentional space, and alignment keeps interacting axes or surfaces in the required relationship.
How to use this article: Sketch the load path and motion direction first, then attach units, worst-case loads, duty and acceptance limits to every calculation.
On this page
- Rolling contact and sliding contact spend energy differently
- Pitch and lead are equal only on a single-start thread
- Lead and efficiency connect torque to axial force
- Backlash is only one part of positioning performance
- Never turn self-locking into a safety assumption
- Select the complete drive in a traceable order
A lead screw and a ball screw can perform the same headline task: relative rotation between a screw and nut produces linear travel. Their contact mechanisms are different. A conventional lead screw relies on sliding contact between the nut and thread flanks, while a ball screw circulates rolling elements through matching grooves. That difference changes friction, heat, required drive torque, back-driving tendency, maintenance, cost and the ways clearance can be controlled. It does not make either mechanism the automatic winner.
Rolling contact and sliding contact spend energy differently
Inside a ball screw, balls roll between the helical raceways of the screw and nut and then pass through a return path. Rolling contact can provide high mechanical efficiency and predictable life when the assembly is correctly selected, aligned, lubricated and protected. The return system, precision manufacture and additional components also add cost and packaging constraints. Contamination or unsuitable lubrication can damage the raceways or interfere with recirculation.
A lead-screw nut slides along the thread. Sliding friction generally converts more input energy into heat, so more motor torque may be needed for the same thrust and lead. Yet the simpler interface can be economical, quiet and well suited to moderate-speed adjustment, positioning and light automation. Nut materials and thread forms vary widely: a polymer nut designed for dry running behaves differently from a lubricated bronze nut. Product-specific data, not a category stereotype, decides the usable load, speed, temperature and life.
Diagram description
The left half shows a metallic ball screw and nut with visible balls circulating around the screw groove. Nearby notes associate it with rolling contact, typically higher efficiency, preload options, lubrication and cleanliness needs, and applications with higher precision, load or duty. The right half shows a threaded lead screw inside a sliding nut. Its notes describe sliding contact, typically lower efficiency, anti-backlash options, comparatively simple construction and general-purpose or cost-sensitive use. A footer states that performance depends on the selected design.
| Question | Ball screw tendency | Lead screw tendency |
|---|---|---|
| How is force transmitted? | Balls roll in matched helical grooves and recirculate through the nut | The nut material slides along the screw thread flanks |
| Where does input energy go? | Lower friction can leave more input power available for useful motion | More sliding friction is commonly converted to heat |
| Can the load drive the input backward? | High efficiency often makes back-driving more likely | Some low-lead sliding combinations resist back-driving, but not every lead screw is self-locking |
| How is reversal clearance addressed? | Selected ball size, nut geometry or preload can reduce axial clearance | Anti-backlash nuts can take up clearance, often using spring force or split-nut geometry |
| What service questions matter? | Lubricant, seals or wipers, recirculation path, cleanliness and alignment | Nut material, approved lubricant or dry-running rule, wear, temperature and replacement access |
| Where can it fit well? | High duty, speed, thrust or positioning performance when justified by selection data | Cost-sensitive, moderate-duty, adjustment or clean/simple mechanisms when ratings are sufficient |
Pitch and lead are equal only on a single-start thread
Pitch is the axial spacing between corresponding points on adjacent thread forms. Lead is the nominal axial advance produced by one relative revolution of the screw and nut. A single-start thread has one continuous helical path, so its lead equals its pitch. A multiple-start thread has two or more interleaved helices. Its lead equals the pitch multiplied by the number of starts.
Diagram description
The upper drawing labels a screw shaft end, nominal diameter d, nut length and usable travel. A central formula states lead equals pitch multiplied by number of starts. The lower diagrams compare a single-start thread, for which lead and pitch are equal, with a two-start thread whose lead is twice the pitch. Text distinguishes adjacent thread spacing from axial travel per revolution.
Lead from pitch and thread starts
L = p × Nₛ
Use this geometric relationship only when pitch and the number of starts describe the same thread. Lead and pitch use units of length; the number of starts is dimensionless.
- L
- lead, or nominal axial travel per relative revolution
- p
- pitch, the axial spacing between corresponding adjacent thread forms
- Nₛ
- number of interleaved thread starts
A single-start screw with 5 mm pitch has a 5 mm lead. A four-start screw with 2 mm pitch has an 8 mm lead, so one revolution nominally advances the nut 8 mm.
Nominal speed from screw lead
v = n × L
Keep the time units aligned: revolutions per minute multiplied by millimetres per revolution gives millimetres per minute. This kinematic result does not approve the screw speed; critical speed, nut speed, lubrication, heat, supports and motor limits still apply.
- v
- nominal linear speed
- n
- relative rotational speed
- L
- screw lead
At 400 rpm, a 5 mm lead gives 2,000 mm/min nominal travel. An 8 mm lead at the same speed gives 3,200 mm/min. The higher lead moves farther per turn but may require more input torque for the same axial force.
Lead and efficiency connect torque to axial force
An ideal screw trades rotation for translation. In a simplified steady lifting case, greater axial force or greater lead requires more drive torque, while higher mechanical efficiency reduces the required input torque. The useful equation below is a first estimate, not a motor-sizing procedure: acceleration, breakaway friction, seals, support bearings, transmission losses, preload, gravity direction, duty and a suitable design margin must also be included.
Simplified driving torque estimate
T ≈ F × L ÷ (2π × η)
The relationship follows from equating rotational work with useful linear work for one revolution. It assumes steady motion and a representative efficiency for the actual screw, nut, direction, lubrication, load and condition. Do not use an assumed category value for final selection.
- T
- approximate input torque at the screw
- F
- required axial force
- L
- lead expressed as length per revolution
- η
- mechanical efficiency as a decimal between zero and one
For the same 1,000 N axial force and 5 mm lead, a hypothetical 0.90 efficiency gives about 0.88 N·m, while 0.35 gives about 2.27 N·m. These numbers only demonstrate the influence of efficiency; they are not ratings for all ball screws or lead screws.
Why the simplest torque estimate can still mislead
Efficiency may differ between driving and overhauling directions and may change with load, speed, temperature, lubrication, wear and thread geometry. Breakaway torque can exceed steady running torque. A preloaded nut adds internal force even when the external load is small. Support bearings, seals, coupling and gearing also consume torque.
A complete motion profile adds moving mass, acceleration, process force and orientation. A vertical axis also needs a controlled response to power loss. Use the selected manufacturer's calculation method and rated data, then verify current, temperature, speed and stopping behavior on the integrated machine.
Backlash is only one part of positioning performance
Axial backlash is free relative movement that can appear when the direction of force reverses and clearance between screw and nut changes from one flank to the other. It can create a dead zone during reversal. Lost motion measured at the carriage may be larger because couplings, bearings, mounts, guides and the structure also deform or move. Always state where and under what load the movement is measured.
| Term | Practical meaning | What it does not guarantee |
|---|---|---|
| Backlash | Clearance-related free movement observed around a reversal | Total position error across the stroke |
| Lost motion | Reversal difference that can include backlash, elastic deformation and other system effects | A single component's axial clearance alone |
| Repeatability | How consistently the system returns to a target under stated conditions | Closeness of that target to the commanded absolute position |
| Accuracy | Closeness of measured position to the intended position over a defined range | Small command increments or good repeatability by themselves |
| Resolution | Smallest input or measurement increment the system can represent | Motion of the load by that increment under every condition |
| Stiffness | Resistance to displacement under load | Zero backlash, zero lead error or infinite load capacity |
Ball-screw preload can remove specified axial clearance and increase nut rigidity, but it also increases internal loading, required torque and heat; excessive preload can reduce service life. A lead screw may use an anti-backlash nut that pushes opposing thread surfaces into contact. Its compensation range, spring force, wear allowance, torque and temperature limits remain product-specific. Neither approach corrects screw lead error, guide error, thermal growth or a flexible machine frame.
Never turn self-locking into a safety assumption
A load can back-drive a screw when axial force produces rotation at the input. The low friction that makes a ball screw efficient also commonly makes it easier to back-drive. Some single-start trapezoidal lead-screw combinations can resist back-driving under stated static conditions, while high-helix or multi-start designs may not. Actual behavior depends on lead angle, friction, nut and screw materials, lubrication, wear, vibration, load and direction.
Select the complete drive in a traceable order
From motion requirement to verified mechanism
- 1. Define the motion.Record usable stroke, move time, velocity, acceleration, settling time, duty cycle and the number of expected moves. Include stopping distance and end clearances.
- 2. Define every load case.Include payload, fixtures, process force, friction, gravity, acceleration, impact and abnormal but credible conditions. State whether the screw is in tension or compression.
- 3. Separate performance terms.Specify accuracy, repeatability, permitted reversal error, stiffness and measurement location individually. Decide whether direct load feedback is needed.
- 4. Choose a mechanism and lead.Compare ball and lead screw products whose force, speed, life, environment and service data cover the task. Use lead to connect motor speed to travel speed and to estimate torque.
- 5. Check physical limits.Verify critical speed, compression buckling, nut speed, permissible load, life, support arrangement, useful travel, lubrication, thermal behavior and mounting alignment using the selected product's data.
- 6. Engineer safety and service.Address back-driving, gravity, trapping and rotating-part hazards. Provide guards, limits, safe isolation, condition checks and access for lubrication or nut replacement.
- 7. Test the assembled axis.Measure position in both directions and across the stroke under representative load and temperature. Record motor current, temperature, noise, reversal behavior and protective-function results.
The comparison belongs inside the full linear-motion system described in From a Motor to a Moving Carriage. The screw creates thrust and travel, but the motor, coupling, supports, guides, base, controls and safeguarding determine whether the machine delivers that motion usefully. A ball screw is compelling when its rolling efficiency, duty capability and controllable clearance solve measured requirements. A lead screw is compelling when its sliding interface, simplicity and product-specific performance solve them more appropriately.
E2 · Educational scene
Explore the decision in three states
Select a state to compare the starting condition, the important change and the evidence needed before acting. The same meaning remains in the static list.
Start
Compare sliding lead screws with recirculating ball screws, calculate travel from pitch and lead, and separate backlash, accuracy, resolution and repeatability before choosing a drive.
What changes
A vertical load needs an independent safe-holding strategy. Do not rely on assumed thread friction to protect people or equipment. Assess gravity-driven descent and stored energy, then provide an appropriately engineered brake, counterbalance, restraint or other protective measure as the machine risk assessment requires.
Verify
7. Test the assembled axis. Measure position in both directions and across the stroke under representative load and temperature. Record motor current, temperature, noise, reversal behavior and protective-function results.
- Compare sliding lead screws with recirculating ball screws, calculate travel from pitch and lead, and separate backlash, accuracy, resolution and repeatability before choosing a drive.
- A vertical load needs an independent safe-holding strategy. Do not rely on assumed thread friction to protect people or equipment. Assess gravity-driven descent and stored energy, then provide an appropriately engineered brake, counterbalance, restraint or other protective measure as the machine risk assessment requires.
- 7. Test the assembled axis. Measure position in both directions and across the stroke under representative load and temperature. Record motor current, temperature, noise, reversal behavior and protective-function results.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Lead Screw Versus Ball Screw?
1. Define the motion. Record usable stroke, move time, velocity, acceleration, settling time, duty cycle and the number of expected moves. Include stopping distance and end clearances.
Which assumption can change the conclusion?
Load, motion profile, geometry, alignment, duty, lubrication and environment match the selected mechanism and ratings.
What can go wrong if I copy the example directly?
A vertical load needs an independent safe-holding strategy. Do not rely on assumed thread friction to protect people or equipment. Assess gravity-driven descent and stored energy, then provide an appropriately engineered brake, counterbalance, restraint or other protective measure as the machine risk assessment requires.
How do I verify the result in the real system?
7. Test the assembled axis. Measure position in both directions and across the stroke under representative load and temperature. Record motor current, temperature, noise, reversal behavior and protective-function results.
Sources and further reading
- ISO 3408-1:2006 — Ball screws — Part 1: Vocabulary and designationInternational Organization for Standardization · 2026-10-12Authoritative scope and vocabulary reference for ball-screw terminology; only public metadata and the public abstract were used.
- ISO 3408-2:2021 — Ball screws — Part 2: Nominal diameters, leads, nut dimensions and mounting bolts — Metric seriesInternational Organization for Standardization · 2026-10-12Public scope reviewed for nominal ball-screw diameters, leads, nut mounting dimensions and preferred combinations; no paywalled clauses were inferred.
- Specifying, Selecting and Applying Linear Ball Screw DrivesThomson Industries, Inc. · 2026-10-12Manufacturer technical paper reviewed for rolling-contact construction, efficiency context, lead accuracy, backlash, preload, selection inputs, critical speed and contamination cautions. Product examples were not generalized as universal ratings.
- Conditions of the Ball Screw — Selection CriteriaTHK Co., Ltd. · 2026-10-12Manufacturer engineering guidance reviewed for axial clearance, preload, rigidity, torque, heat and service-life trade-offs.
- DryLin trapezoidal lead screw nut — Technical dataigus GmbH · 2026-10-12Product-family technical data reviewed for sliding screw efficiency ranges, high-helix behavior, dry-running materials, anti-backlash options and conditional self-locking claims; values were kept product-specific.
- Machine Guarding eTool — IntroductionU.S. Occupational Safety and Health Administration · 2026-10-12Primary safety source reviewed for hazards associated with machine motion and power-transmission components; cited as general safety context, not a complete machine-specific or worldwide compliance plan.