Linear Motion Essentials
Selecting a Complete Linear Axis
Turn travel, load and accuracy requirements into a defensible component shortlist.
You must choose between real alternatives covered by “Selecting a Complete Linear Axis” for a defined machine, circuit or installation.
Build a linear-axis specification from function, travel, loads, moments, motion profile, accuracy, environment and service life; shortlist compatible guides, screws and drives; then verify the complete system with manufacturer evidence and commissioning tests.
- Select or check first
- 1. Freeze requirements and assumptions. Give every requirement an owner, unit, tolerance and verification method; close or bound unknown inputs.
- Controlling assumption
- Load, motion profile, geometry, alignment, duty, lubrication and environment match the selected mechanism and ratings.
- Top risk
- Do not divide total load equally by the number of blocks. Moments, spacing, mounting error, structure flexibility and preload can make block loads unequal. Use the candidate manufacturer's load-distribution and equivalent-load method, including contact or application factors where specified.
- Proof
- 7. Release a controlled record. Store calculations, catalogue revisions, drawings, settings, test evidence, maintenance instructions, residual risks and approved substitutions.
Start with the basics Optional foundation before “Selecting a Complete Linear Axis”
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
- Start with what the axis must do
- Turn the payload and process into forces and moments
- Specify the whole motion profile, not only maximum speed
- Translate “precise” into measurable errors
- Environment and maintenance belong in the initial selection
- Shortlist drive and guide combinations as a system
- Selection finishes with integration and evidence
A complete linear axis is a system, not a catalogue part chosen from payload mass alone. The guide must carry forces and moments with acceptable deflection and life. The screw, belt or other drive must create the required motion without buckling, overspeed, excessive heat or wear. The motor, coupling, bearings, structure, sensors, cable management and controller must work across the same travel, duty cycle and environment. Selection becomes defensible when every catalogue check traces back to a written requirement.
Diagram description
Seven numbered columns show a screw-driven carriage progressing from requirements, stroke and orientation, forces and moments, speed and accuracy, screw-guide-motor choices, catalogue review, and final integration with cable carrier and supports.
Start with what the axis must do
Describe the useful task in observable terms: move a payload between positions, scan continuously, press against a process, follow a profile, or hold against gravity. State the coordinate direction and mounting orientation. A horizontal transfer, a vertical lift and an inclined tracker support can apply very different loads to the same-looking components. Define normal operation, setup, homing, fault recovery, maintenance and safe state because the worst case may occur outside the normal cycle.
| Field | What to record | Why it changes selection |
|---|---|---|
| Task and orientation | Positioning, transport, pressing or following; horizontal, vertical or inclined | Sets gravity, holding, back-driving and failure-response conditions |
| Required working stroke | Distance between useful end positions | Defines only the useful motion, not yet rail or screw length |
| Travel allowances | Homing, braking, overtravel, end clearance, sensor and stop regions | Prevents the working cycle from consuming the mechanical safety envelope |
| Payload geometry | Mass, dimensions and center of gravity in every configuration | Converts weight and process forces into guide loads and overturning moments |
| Available envelope | Length, width, height, carriage space and service-removal paths | Eliminates candidates that cannot be mounted, aligned or replaced |
| Interfaces and datums | Mounting faces, hole pattern, shaft/coupling, motor, sensor and cable interfaces | Controls compatibility and the accuracy the machine structure can preserve |
Turn the payload and process into forces and moments
List forces along three defined axes and moments about them for every important phase: rest, acceleration, constant speed, deceleration, reversal, process contact, emergency stop and credible jam. Include payload, tooling, cables and moving brackets. The center of gravity creates a moment whenever its force acts at an offset from the guide system. Process forces can reverse direction, and a short shock may govern the static or structural check even when it contributes little to average fatigue load.
First estimate of an offset-load moment
M = F × d⊥
Use the perpendicular distance from the relevant reference point or guide arrangement to the force's line of action. A complete guide calculation distributes forces and moments among actual blocks using the manufacturer's method.
- M
- moment about the chosen axis, for example N·m
- F
- applied force, for example N
- d⊥
- perpendicular lever arm, for example m
A 200 N downward force whose line of action is 0.15 m from the guide reference creates a 30 N·m moment about the corresponding axis. Do not add unlike moment directions without a defined coordinate system.
Diagram description
A central motor, screw and rail assembly is connected to six labeled panels. Diagrams show working stroke, three directional forces, three moments, a speed profile, an accuracy datum and temperature, dust and lubrication symbols.
| Case | Quantities to calculate | Typical verification |
|---|---|---|
| Maximum stationary/process load | Forces and moments at the most unfavorable position | Static safety, permanent deformation, holding and structural deflection |
| Acceleration and deceleration | Inertial force from moving mass plus gravity and process load | Motor force/torque, coupling, screw thrust, guide distribution and frame reaction |
| Variable duty cycle | Load magnitude, direction, distance and time for each segment | Equivalent load, fatigue life, RMS/thermal motor demand and lubrication |
| Shock, jam or emergency stop | Bounded peak force/torque and stop distance | Static capacity, stop/bumper, shaft/coupling and machine-structure safety |
| Vertical power loss | Gravity load, back-driving tendency and stored potential energy | Brake, counterbalance, self-locking claim, controlled stop and safe maintenance |
Specify the whole motion profile, not only maximum speed
Record move distance, velocity, acceleration, deceleration, jerk or profile shape, dwell, settling time, moves per hour and operating hours. The axis may reach a catalogue's maximum speed yet fail to complete the required cycle because acceleration distance is too long. Reversals and short strokes can also create demanding lubrication and wear conditions. For a screw-driven axis, candidate lead changes linear distance per revolution, required motor speed, torque, resolution and back-driving behavior together.
Inertial force along the motion direction
Fi = m × a
Add or subtract gravity, friction and external process forces with consistent signs. The moving mass includes every translated component, not only the payload.
- Fi
- inertial force, N
- m
- total translated mass, kg
- a
- linear acceleration, m/s²
A 12 kg moving assembly accelerated at 2 m/s² requires 24 N for inertia before friction, gravity, screw efficiency and process forces are included.
Motion fields to put on the worksheet
- Move distance and direction for each segment, including return and recovery moves
- Maximum and typical velocity, acceleration, deceleration and allowed jerk
- Dwell and settling-time requirement at each working position
- Cycle time, cycles per hour, shifts per day and days per year
- Positioning accuracy, repeatability, resolution and allowable lost motion—kept as separate terms
- Applied force while moving and while stationary
- Emergency-stop or jam assumptions and the resulting stopping distance
- Noise, vibration and smoothness limits where the process needs them
Translate “precise” into measurable errors
Accuracy describes closeness to the commanded or reference position; repeatability describes how consistently the axis returns under stated conditions. Resolution is the smallest command or feedback increment, not proof that the mechanism reaches that increment. Backlash or lost motion, elastic deflection, thermal growth, pitch error, alignment, controller tuning and measurement location all contribute. State the load, direction, temperature, approach method and datum for any tolerance.
| Process statement | Useful engineering form | Common trap |
|---|---|---|
| Return to the same pickup point | Bidirectional or unidirectional repeatability at stated load and temperature | Calling encoder resolution repeatability |
| Follow a straight path | Straightness, pitch/yaw/roll and mounting-datum requirements over travel | Specifying only screw lead accuracy |
| Do not deflect during processing | Maximum displacement at tool point for defined forces and moments | Checking guide load rating but not stiffness or frame compliance |
| Reach the absolute target | Positioning error budget including mechanics, feedback, homing and thermal effects | Assuming high repeatability guarantees absolute accuracy |
| Settle quickly | Allowed error band and time after the move | Ignoring structural modes, controller bandwidth and preload/friction |
Environment and maintenance belong in the initial selection
Temperature changes lubricant behavior, dimensions, seals, motor capability and electronics. Dust, chips, water, washdown chemicals, salt, coolant or cleanroom limits change materials and protection. A bellows or cover needs space and can add drag. Mounting attitude can affect lubrication distribution and contaminant entry. Record the environment at the component, not only the room, and specify who will inspect, relubricate, clean, adjust and replace each wear item.
| Field | Record | Verification output |
|---|---|---|
| Temperature | Ambient, nearby heat sources, duty heating and gradients | All component ratings, lubrication, clearances and error budget remain valid |
| Contamination | Particle type/size, liquids, corrosion agents and cleaning method | Materials, seals, scrapers, covers, pressure or purge strategy where applicable |
| Required life | Travel distance, cycles, hours, calendar period and reliability objective | Guide/drive/bearing life plus planned replacement and downtime |
| Lubrication | Approved lubricant, quantity/method, access and interval basis | Maintainable ports and documented interval adjusted to real conditions |
| Safety | People, pinch/crush zones, vertical load, stops, guarding and isolation | Risk-reduction functions and verification plan integrated into the machine |
| Serviceability | Access, alignment method, replaceable lengths, lifting and spare strategy | Axis can be inspected and restored without uncontrolled risk or lost datum |
Shortlist drive and guide combinations as a system
Choose an architecture only after the requirement envelope is visible. A lead screw may suit moderate precision and load with different efficiency and wear behavior from a ball screw. A ball screw can provide efficient, accurate motion but needs checks for thrust, life, critical speed, buckling, support arrangement, lubrication and back-driving. Belt axes can suit long, fast travel while bringing belt stiffness and tension considerations. Profile rails and round shafts package and react loads differently. The shortlist should compare complete assemblies rather than isolated headline ratings.
Diagram description
Five panels show requirements, forces and moments on a carriage, candidate guides, ball screws, gearboxes and motors, verification gauges for load, speed, accuracy, temperature and life, and a final assembled axis with an integration checklist.
| Category | Candidate A / B / C evidence | Pass condition |
|---|---|---|
| Guide loads and moments | Block load distribution, permissible moments, static factor, equivalent load and nominal life | Manufacturer method passes every load case and required life |
| Drive mechanics | Thrust/torque, lead or ratio, efficiency, life, critical speed, buckling and support arrangement | Operating profile stays within applicable limits with stated margins |
| Motor and drive | Speed–torque curve, peak/continuous or RMS demand, reflected inertia, brake and supply | Thermal and transient requirements pass across the full cycle |
| Accuracy and stiffness | Catalogue grades plus calculated deflection and assembled error budget | Tool-point requirement passes under stated load and temperature |
| Environment and life | Temperature, seals/covers, corrosion, lubricant, interval and component life basis | Protection is compatible and maintainable for the actual exposure |
| Integration | Envelope, mass, mounting, datums, coupling, cables, sensors, controls, guarding and delivery | Interfaces fit and the machine can be built, commissioned and serviced safely |
Selection finishes with integration and evidence
From shortlist to released axis
- 1. Freeze requirements and assumptions.Give every requirement an owner, unit, tolerance and verification method; close or bound unknown inputs.
- 2. Calculate all governing cases.Trace maximum and varying forces, moments, speeds, acceleration, life and fault loads through guides, drive, motor, supports and machine structure.
- 3. Build a comparable shortlist.Use exact catalogue configurations and record evidence, assumptions, margins, exclusions and interface drawings for each candidate.
- 4. Review integration before purchase.Check tolerances, mounting preparation, alignment, coupling, lubrication access, cable motion, sensors, guarding, transport and replacement.
- 5. Inspect and commission the assembly.Verify identity, damage, mounting, alignment and lubrication, then test homing, limits, direction, motion profile and faults at controlled energy before full load.
- 6. Validate against the process.Measure travel, load, speed, positioning behavior, deflection, temperature, vibration and cycle performance using calibrated methods where required.
- 7. Release a controlled record.Store calculations, catalogue revisions, drawings, settings, test evidence, maintenance instructions, residual risks and approved substitutions.
Use Lead Screw Versus Ball Screw to compare drive principles, Linear Guides and Their Load Ratings to interpret guide forces and moments, and How Assembly Errors Damage Motion Systems before treating nominal component performance as assembled-machine performance. A good shortlist is not the candidate with the largest rating; it is the smallest set of documented systems that passes every requirement and can be safely integrated, verified and maintained.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Selecting a Complete Linear Axis?
1. Freeze requirements and assumptions. Give every requirement an owner, unit, tolerance and verification method; close or bound unknown inputs.
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?
Do not divide total load equally by the number of blocks. Moments, spacing, mounting error, structure flexibility and preload can make block loads unequal. Use the candidate manufacturer's load-distribution and equivalent-load method, including contact or application factors where specified.
How do I verify the result in the real system?
7. Release a controlled record. Store calculations, catalogue revisions, drawings, settings, test evidence, maintenance instructions, residual risks and approved substitutions.
Sources and further reading
- LM Guide General Catalog — Selection CriteriaTHK Co., Ltd. · 2026-10-12Primary manufacturer catalog reviewed for the conditions-to-type-to-load-to-safety-to-life-to-environment selection sequence and rating definitions.
- LM Guide Selection CriteriaTHK Co., Ltd. · 2026-10-12Current manufacturer guidance reviewed for applied/equivalent loads, static safety, average load, nominal life, accuracy, rigidity and environmental selection.
- Nominal Life — LM Guide Selection CriteriaTHK Co., Ltd. · 2026-10-12Manufacturer technical guidance reviewed for statistical nominal-life meaning, 50 km versus 100 km reference bases, variable-load and environmental factors.
- NSK Linear Guides Long-Life Series DH/DS ModelsNSK Ltd. · 2026-10-12Primary manufacturer data reviewed for preload/rigidity, static and dynamic ratings, impact/vibration factors, equivalent load and fatigue-life interpretation.
- Catalogs and CAD Drawings — Precision Machine ComponentsNSK Ltd. · 2026-10-12Current manufacturer catalog index reviewed to confirm exact guide, ball-screw, actuator and installation data must be selected by ordered configuration and revision.
- General Product Information — Load-Bearing Capacity and Service LifeBosch Rexroth AG · 2026-10-12Primary manufacturer technical material reviewed for resultant loads/moments, average speed and service-life calculation context.
- NSK Engineering Tool User ManualNSK Ltd. · 2026-10-12Manufacturer documentation reviewed for required operating inputs, life/load calculations, catalog data and retaining calculation results.