Linear Motion Systems: Rails, Ball Screws & Linear Motors — The Ultimate Guide

Almost every motor you bolt into a machine spins, and almost every job you actually want done is straight-line. A gantry slides a tool over a part; a Cartesian pick-and-place drops a chip onto a board; a CNC table feeds stock past a spindle; a humanoid's linear ankle pushes the foot. Somewhere between the rotor and the work, something has to turn rotation into translation — or skip rotation entirely. That something is the linear motion system, and it is where a surprising amount of a machine's real-world accuracy, speed, and stiffness gets decided.

This is the long version. We'll separate the problem into its three honest subsystems — the guide that constrains the motion to one axis, the drive that supplies the force, and the carriage that carries the load — because mixing them up is the single most common sizing error. Then we go through each technology family with real numbers and real parts: profile rails from THK, HIWIN, Bosch Rexroth, and NSK; ball, lead, and roller screws; GT2 and HTD belts; rack-and-pinion; and ironcore versus ironless linear motors from Aerotech, Beckhoff, and the like. Numbers with units. Opinions with reasons.

The take: For most machines in 2026, the default linear axis is a pair of profile rails plus a ground ball screw — it's stiff, ~90% efficient, accurate, and the supply chain is deep. Reach for a belt when the stroke is long and you care about speed more than micron accuracy; reach for rack-and-pinion when the stroke is measured in meters; and reach for a linear motor only when you genuinely need the bandwidth, acceleration, and zero-backlash directness that a screw can never give you — and you can pay for the magnets, the encoder, and the heat. Pick by the guide-drive-carriage trio as a system, never by the screw alone.

Companion reading: robot actuators, servo motors, gearboxes (harmonic & cycloidal), and industrial robot arms.

Table of contents

1. Key takeaways
2. Why linear motion is its own problem
3. The three subsystems: guide, drive, carriage
4. Profile rails and recirculating-ball linear guides
5. Ball screws vs lead screws vs roller screws
6. Belt and rack-and-pinion drives
7. Linear motors: ironcore vs ironless
8. The precision / speed / force / stroke tradeoff
9. Architectures: Cartesian, gantry, H-bot, CoreXY
10. Sizing: load, moment, life, critical speed, buckling
11. Accuracy, repeatability and straightness
12. Lubrication, sealing and contamination
13. A selection workflow
14. Frequently asked questions

Why linear motion is its own problem

Start from the prime mover. A rotary servo or BLDC motor makes torque and wants to spin; we cover sizing those in the servo motors guide. But a huge fraction of machine work is translation along a straight line, and there are exactly two ways to get there:

1. Convert rotary motion to linear with a screw, belt, rack, or cam. The motor still spins; a mechanism does the geometry.
2. Generate linear force directly with a linear motor — an "unrolled" rotary motor whose stator is laid flat and whose rotor becomes a moving forcer.

Both have to solve the same three problems that a rotary joint mostly gets for free from its bearing:

• Constrain the motion to one degree of freedom and reject the other five (two translations, three rotations). A spinning shaft in a bearing does this naturally; a sliding carriage does not, and the quality of that constraint is the linear guide.
• Carry the moment loads. A payload is almost never on the line of thrust. Offset mass creates roll, pitch, and yaw moments that try to cock the carriage — and moment capacity, not just direct load, is what sizes a real axis.
• Supply thrust efficiently enough that the motor and its gearbox don't have to be absurd.

Rule of thumb: a linear axis is only as good as its weakest of {guide, drive, carriage}. Engineers over-spec the screw and under-spec the rails constantly, then wonder why the tool point shakes.

The reason linear motion gets its own discipline — and its own catalogs from THK and Rexroth that are thicker than most engineering textbooks — is that all three problems interact. The guide spacing changes the moment capacity. The screw's end fixity changes its critical speed and its buckling load. The carriage's overhang changes the rail loading. You cannot size them independently and bolt them together; you size them as a system.

The three subsystems: guide, drive, carriage

Decompose every linear axis you ever build into these three parts and most of the confusion evaporates.

The guide carries the load and constrains the motion. It is the bearing of the linear world. Options, roughly in order of stiffness and cost:

• Profile rail (recirculating ball or roller) — the default. A hardened, ground rail and a block full of recirculating balls. Carries load and all three moments in one component.
• Round shaft + linear ball bushing (Thomson, Igus) — cheaper, more forgiving of misalignment, but lower moment capacity and stiffness; the shaft sags over span.
• Crossed-roller / box ways — old-school machine-tool ways and crossed-roller slides; very stiff and damped, but heavy and friction-y.
• Plain bearing / polymer slides (Igus drylin) — dry-running, light, quiet, corrosion-proof, low cost; lower load and precision, some stick-slip.
• Wheel/cam-roller systems (Hepco GV3, Bishop-Wisecarver DualVee) — V-guide wheels on a track; fast, debris-tolerant, long travel, lower precision.
• Air bearings — frictionless, sub-micron straightness, used in metrology and wafer stages; expensive and need clean dry air.

The drive turns the input (torque or current) into thrust along the axis:

• Ball screw / lead screw / roller screw — rotary input, threaded conversion, high force, moderate speed.
• Belt (GT2, HTD, AT) / rack-and-pinion — rotary input, long stroke, high speed, lower stiffness/accuracy.
• Linear motor — electrical input straight to thrust, no conversion mechanism, highest bandwidth.

The carriage is the moving structure that bolts to the guide blocks and holds the payload. Its job is stiffness and a sane center of gravity. A carriage that puts the payload far above or ahead of the guide blocks loads them in moment, and moment is what kills L10 life.

The cleanest way to think about a machine is: for each axis, choose a guide, choose a drive, choose how the carriage hangs the load, then size all three against the same duty cycle. The rest of this guide is the menu for each slot plus the math to size it.

Profile rails and recirculating-ball linear guides

The profile rail linear guide — sometimes called a "linear guideway" or "LM guide" (THK's trademark that became generic) — is the component most machines are built around, so it earns the most ink.

How it works

A profile rail is a hardened steel beam with precision-ground raceways (usually two pairs, in a "Gothic arch" or circular-arc groove). A block (the carriage, runner block, or "bearing") rides on it with two or four rows of balls that recirculate: balls roll along the loaded raceway, get scooped at the end, return through a channel in the block, and re-enter. That recirculation is what gives unlimited travel — unlike a crossed-roller slide whose rollers only roll the length of the cage.

The four-row "Gothic arch" geometry is the important bit: each ball contacts the groove at two points, and the four rows are oriented so the block carries load equally in all four radial directions (down, up, and both sides) plus all three moments — roll (Mr, about the travel axis), pitch (Mp), and yaw (My). That omnidirectional capacity is exactly what round shafting lacks.

Sizes and ratings

Profile rails come in standard widths, named by rail width in mm: 15, 20, 25, 30, 35, 45, 55, 65. Rough capacity ladder:

Rail size	Typical dynamic load C per block	Where it lives
15 mm	~8–14 kN	Small Cartesian, lab automation, 3D printers (linear-rail builds)
20–25 mm	~17–35 kN	Pick-and-place, light gantries, semiconductor handling
30–35 mm	~35–70 kN	Machine-tool sub-axes, robot 7th-axis tracks, mid gantries
45 mm	~70–110 kN	CNC axes, heavy gantries
55–65 mm	~110–250+ kN	Large machine tools, press feeders, heavy structures

Two load numbers matter and they are not the same:

• Dynamic load rating C — the load at which 90% of a population survives a nominal travel distance (THK and most metric makers use 50 km of travel as the reference; some legacy/US specs use 100 km, so always read the basis). C drives the L10 life calculation.
• Static load rating C0 — the load that causes a defined permanent indentation (~0.0001× ball diameter total). C0 protects against standstill shock, e-stops, and clamping loads, and it sets the static safety factor fs = C0 / applied load.

Sizing rule: for a smooth machine use a static safety factor fs of about 1.5–3; for machines with vibration, impacts, or e-stops, 3–5. The dynamic rating sets life; the static rating sets survival.

Preload classes

A block can be assembled with oversized balls so the rows are loaded against each other even with no external force. This preload removes internal clearance and increases stiffness, at the cost of rolling friction and accelerated wear. Manufacturers sell discrete classes; THK's nomenclature is typical:

THK class	Preload (≈ % of C)	Use
C0	~0 (clearance to slight)	Low friction, light load, axes where smoothness beats rigidity
C1	~2–5% of C	General precision machines; the common default
C2	~8–13% of C	High rigidity, heavy cutting, vibration, single-rail/single-block layouts

HIWIN (ZF/Z0/ZA), Rexroth, and NSK have equivalent ladders. The tradeoffs:

• More preload → more stiffness and less deflection under load, which matters for cutting accuracy and to keep a tool point from drooping under cantilevered mass.
• More preload → more friction and heat, which matters for low-thrust drives (belts, small linear motors) and for back-driven or hand-loaded axes.
• More preload → shorter life if combined with high external load, because the effective load on the balls is preload + external; the L10 calculation must use the combined value.

Most general machines run C1/medium. Go to C2 only when you've justified the stiffness need; go to C0 when friction or smoothness dominates (e.g., a delicate ironless-linear-motor scanning stage).

Accuracy and precision grades

Separate from preload, profile rails ship in accuracy grades that bound the running parallelism, height tolerance, and height variation between blocks. THK's ladder, roughly: Normal (no symbol), High (H), Precision (P), Super-precision (SP), Ultra-precision (UP). As you climb:

• Height tolerance of the block tightens (e.g., from ±0.04 mm Normal toward ±0.005 mm UP).
• Running parallelism of the raceway against the mounting face tightens — this is the wave you feel as the carriage travels, the source of vertical/horizontal "waviness."
• Block-to-block height variation tightens, which lets you run two parallel rails without one fighting the other.

Grade rule: buy accuracy grade to match the machine's required straightness, and buy it on both rails of a parallel pair. A precision block on a normal rail, or mismatched blocks across a gantry, throws away the money you spent.

Roller versions (THK SRG, Rexroth roller rail, HIWIN RG) swap balls for crossed cylindrical rollers: line contact instead of point contact gives substantially higher stiffness and load capacity for the same size, at higher cost and slightly more sensitivity to mounting flatness. Use roller rails for heavy cutting and maximum rigidity; balls for everything else.

Products and where they show up

• THK — invented the LM guide; SR/SHS/SSR (ball), SRG/SRS (roller/caged). The reference everyone is benchmarked against.
• HIWIN — HG/EG/MGN series; MGN9/MGN12 are ubiquitous in hobby and small-machine builds; strong price/performance.
• Bosch Rexroth — ball and roller rail systems, deep in machine-tool and factory automation; integrates with their actuator modules.
• NSK — NH/NS series; strong in semiconductor and precision.
• Misumi — sells THK-compatible and house-brand rails configured online by length; the fast path for one-off machines.
• Igus drylin — polymer plain-bearing rails (W/T/Q series); dry, light, corrosion-proof, for washdown and low-load axes.

Ball screws vs lead screws vs roller screws

The screw is the most common rotary-to-linear drive, and the three families differ enormously. The headline numbers:

Drive	Efficiency	Backlash	Self-locking	Load capacity	Speed	Relative cost
Ball screw	~90–95%	Near-zero (preloadable)	No	High	High	Medium
Lead (ACME/trapezoidal) screw	~20–50%	Yes (unless anti-backlash nut)	Often yes	Medium	Low–medium	Low
Planetary roller screw	~80–90%	Near-zero (preloadable)	No	Very high	Very high	High–very high

Lead screws

A lead screw is a threaded rod and a nut in sliding contact — typically an ACME/trapezoidal thread or a polymer nut on a steel screw. The sliding friction is the whole story:

• Low efficiency (~20–50%) means a big fraction of motor torque becomes heat, and you size the motor up accordingly.
• Self-locking is the upside of that friction. If the lead angle is shallow enough (efficiency below ~50% in the back-drive direction), the screw holds position with the motor off — no brake needed. This is why 3D-printer Z axes, jacks, and many vertical hold-position axes use lead screws.
• Backlash is inherent in a plain nut. Anti-backlash nuts (spring-loaded split nuts, or polymer nuts like Igus drylin) take it out at the cost of wear life and added drag.
• Cheap and quiet. A stainless lead screw with a Delrin/Igus nut is a few dollars and needs no lubrication. For low-duty, low-load, cost-sensitive axes it's the right answer.

Thomson, Nook, Misumi, and Igus all sell lead screws and anti-backlash nuts off the shelf.

Ball screws

A ball screw replaces sliding with rolling: hardened balls run in a matched helical groove between screw and nut, recirculating through a return tube or internal deflector, exactly like a profile-rail block wrapped around a screw. Consequences:

• ~90–95% efficiency — rolling friction is tiny, so most motor torque becomes thrust and very little becomes heat. This is the single biggest reason to choose a ball screw.
• Not self-locking — a vertical ball-screw axis will back-drive under gravity if you cut power. Add a motor brake.
• Backlash is preloadable to near-zero. Use an oversized-ball preload, a double-nut preload, or a lead-offset preload to remove axial play. Preload buys stiffness and zero backlash at the cost of friction and life — the same tradeoff as rail preload.
• Accuracy grades (per JIS/DIN/ISO): from C10/C7 (rolled, transport-grade, ±0.05 mm/300 mm class) up through C5, C3, C1, C0 (ground, precision, down to a few µm/300 mm). Rolled screws are cheap and fine for general motion; ground screws are for positioning accuracy.

THK, HIWIN, NSK, Bosch Rexroth, KSS, and Misumi cover the market. Leads (axial travel per revolution) run from ~1 mm (fine, high force, slow) to 25–50 mm (coarse, fast, lower force).

Roller screws (planetary roller screws)

A planetary roller screw replaces the balls with a set of threaded rollers that planet around the screw inside the nut. Many lines of contact instead of point contacts at discrete balls give:

• Much higher load capacity for a given diameter (often 2–3×+ a ball screw), because contact is distributed across many roller threads.
• Much higher speed and acceleration — no balls to recirculate and slam, so DN-type limits are higher; some run leads down to 1 mm at high rpm.
• Long life — distributed contact and no recirculation impacts.
• Fine leads available that ball screws struggle to make (e.g., 1–2 mm at high diameter).
• Efficiency ~80–90% — a bit below ball screws because of more contact, but far above lead screws.

The cost is several times a ball screw. Use roller screws where ball screws run out of headroom: electric press actuators replacing hydraulics, high-cycle servo presses, heavy fast pick-and-place, and aerospace/defense actuation. Makers: Rollvis, Ewellix (formerly SKF), GSA, Creative Motion. This is the technology quietly enabling the all-electric heavy actuators discussed in the robot actuators guide.

Speed from rpm and lead

The core conversion is trivial and you should have it memorized:

Linear speed v (mm/s)  = (motor rpm / 60) × lead (mm/rev)
Linear travel per rev   = lead (mm)
Thrust F (N)            = (2π × η × T_motor (N·m) × 1000) / lead_mm
   where η = screw efficiency (≈0.9 ball, ≈0.3–0.5 lead)

Example: NSK ground ball screw, lead = 10 mm, motor at 3000 rpm
  v = (3000 / 60) × 10 = 500 mm/s = 0.5 m/s
  With a 1.0 N·m servo and η = 0.9:
  F = (2π × 0.9 × 1.0 × 1000) / 10 ≈ 565 N thrust

Notice the lead trades speed for force directly: halve the lead and you double the thrust and halve the speed at the same rpm. That single choice, plus the motor's torque-speed curve, sets the axis envelope. We size the rotary side of this in the servo motors guide; the gear-ratio analog of "lead" is covered in the gearboxes guide.

Belt and rack-and-pinion drives

Screws are great until the stroke gets long. Screw cost, mass, critical speed, and buckling all scale badly with length, so past roughly 1.5–3 m you switch to a belt or a rack.

Belt drives

A toothed belt over a driven pulley converts rotation to translation with the carriage clamped to the belt (or the belt fixed and the motor riding the carriage). Belt tooth profiles you'll meet:

• GT2 / GT3 (2 mm, 3 mm pitch) — curvilinear tooth, low backlash, the standard for small/medium motion and 3D printers. GT2 is everywhere in light automation.
• HTD (3M, 5M, 8M) — deeper curvilinear teeth, more power, used for larger axes.
• AT (AT5, AT10, AT20) — trapezoidal, steel- or aramid-corded, very high stiffness and force for industrial linear units; the choice when a belt axis must be reasonably rigid.

Why belts:

• Speed. Belt axes routinely run 3–10 m/s and accelerate hard, because there's no screw whip or DN limit — only pulley rpm and belt dynamics.
• Long stroke, low cost. Travel is limited only by belt length; a 5 m belt axis is cheap next to a 5 m ground ball screw.
• Low moving mass if the motor is stationary and only the carriage and a length of belt move.

Why not belts:

• Compliance. A belt is a spring. Belt stretch under load gives the axis a finite stiffness, which shows up as positioning error, settling time, and a resonance you must keep below your control bandwidth.
• Accuracy. Practical positioning error of a motor-side-encoded belt axis is ~50–200 µm depending on tension, length, and load. To do better, put a linear encoder on the load and close the loop there.
• Tension maintenance. Belts stretch and need re-tensioning; over-tension shortens bearing life, under-tension causes tooth skip and backlash.

Bosch Rexroth, Festo, Igus (drylin ZLW), Misumi, and Bishop-Wisecarver sell complete belt-driven linear units. The handheld rule: belt for speed and reach, screw for force and accuracy.

Rack-and-pinion

A rack is a straight gear; a pinion on the motor output rolls along it. Racks bolt end-to-end, so the axis can be arbitrarily long — tens of meters on a machine-tool gantry or a robot 7th-axis track.

• Unlimited stroke by tiling rack segments (ground racks have matched ends so the tooth pitch is continuous across joints).
• High thrust and high speed simultaneously, limited mainly by the pinion and gearbox.
• Stiffness far better than a belt of equal length — it's a gear mesh, not a spring.
• Backlash is the catch. Single-pinion rack-and-pinion has gear backlash. Fixes: a preloaded pinion against the rack, or a dual-pinion / electronic-preload drive where two motors (or one split path) push against each other to take up lash, or a master/slave torque-biased pair on a servo axis.

Helical racks are quieter and stronger than straight; ground racks (Güdel, Atlanta, Wittenstein, Apex) hit DIN quality grades that matter for positioning. Rack-and-pinion is the default for the long overhead axis of large gantries and for the linear track that carries an industrial robot arm along a production line.

Linear motors: ironcore vs ironless

A linear motor is a rotary servo/BLDC motor cut open and laid flat. The stator becomes a track of permanent magnets; the rotor becomes a forcer (the moving coil assembly) that produces thrust directly when driven with field-oriented control. There is no screw, belt, or gear — the electromagnetic force is the thrust.

Consequences, good and bad:

• Zero backlash, zero mechanical wear path. Nothing meshes or threads. The only wear is the guide.
• Huge acceleration and bandwidth. Direct drive means no reflected screw inertia and no compliant transmission between motor and load. Accelerations of 5–10 g are routine, and some short-stroke stages exceed 20 g. Settling times and bandwidth crush any screw axis.
• Smoothness limited by cogging and force ripple, not by a nut or belt.
• No reduction. A rotary motor + screw gives you a built-in mechanical advantage (the lead acts like a gear ratio); a linear motor has none, so it makes peak force purely from current — and from heat.
• Not self-locking. Cut power and there's nothing holding position; vertical axes need a counterbalance or a brake.
• Feedback must be a full-stroke linear encoder (optical or magnetic scale). There's no rotary encoder to count screw turns; commutation and position both come from the linear scale, so encoder quality directly sets your resolution and smoothness.
• Heat goes into the machine. The forcer dissipates I²R losses right at the work zone; high-duty linear-motor stages often need liquid cooling to keep thermal growth from eating accuracy.

Ironcore vs ironless

The big architectural fork:

	Ironcore (iron-core)	Ironless (air-core / U-channel)
Coil structure	Coils wound on a laminated iron core	Coils in epoxy, no iron, between two magnet rows
Force density	High (iron concentrates flux)	Lower for same size
Cogging / force ripple	Present (iron teeth attract magnets)	Essentially zero
Magnetic attraction to track	Large normal force (often > thrust) loads the guide	Zero net attraction
Stiffness / thrust	Best	Moderate
Best for	High-thrust, stiff, machine-tool and press axes	Smooth constant-velocity scanning, metrology, light stages

Ironcore linear motors make the most force per size because the iron concentrates magnetic flux — but that same iron is strongly attracted to the magnet track (a normal force that can exceed the thrust), which preloads the guide bearings and can cause cogging. Use ironcore when you need thrust and stiffness and can carry the attraction load.

Ironless motors put the coils in epoxy with no iron, sandwiched in a U-channel of magnets. No iron means no cogging, no force ripple, and zero net attraction to the track — the smoothest possible motion and no extra bearing load. The price is lower force density. Use ironless for constant-velocity scanning (wafer inspection, laser machining, metrology) where smoothness beats raw force.

Players: Aerotech (precision stages, ironless and ironcore), Beckhoff (AX5000/linear, and the XTS/XPlanar transport systems), ETEL (high-end direct drive), Kollmorgen (IC/ICD ironcore), Tecnotion, LinMot (tubular linear motors — a moving magnet rod through a stator, a clean form factor for press/insertion). Tubular linear motors deserve a note: the coil wraps fully around the magnet rod, so flux is used efficiently and there's no net side load — a nice middle ground for short-stroke, high-force insertion and pressing.

When to actually choose a linear motor: when you need acceleration or bandwidth a screw can't give, and the stroke is short-to-medium, and you can pay for the magnets, the linear encoder, the controller, and the thermal management. Otherwise a ball screw is cheaper, self-contained, and has a built-in reduction.

The precision / speed / force / stroke tradeoff

Every drive technology is a different bet on four conflicting axes: precision, speed, force, and stroke length. No technology wins all four, and the honest comparison is the most useful table in this guide:

Drive	Precision (positioning)	Top speed	Force/thrust	Practical stroke	Backlash	Efficiency	Self-locking
Lead screw	Low–medium (10–50 µm)	Low (≤0.3 m/s)	Medium	≤1 m	Yes (or anti-backlash)	20–50%	Often yes
Ball screw	High (1–20 µm)	Medium (0.5–2 m/s)	High	≤~3 m	Near-zero (preload)	90–95%	No
Roller screw	High (1–10 µm)	High (to ~2+ m/s)	Very high	≤~3 m	Near-zero (preload)	80–90%	No
Belt	Low–medium (50–200 µm)	Very high (3–10 m/s)	Medium	10+ m	Low (toothed)	~90%	No
Rack-and-pinion	Medium (20–100 µm)	High (to ~5 m/s)	Very high	Unlimited	Yes (dual-pinion preload)	~90%	No
Linear motor	Very high (<1 µm possible)	Very high (3–10 m/s)	Medium–high	Short–medium (encoder-limited)	Zero	N/A (direct)	No

Read it as a decision aid, not gospel — every cell depends on size, grade, and how you close the loop. But the shape is real:

• Want micron accuracy and high force in a compact axis? Ground ball screw on profile rails. The default.
• Want speed and long reach? Belt (medium reach) or rack-and-pinion (any reach).
• Want the highest dynamics and zero backlash and you'll pay for it? Linear motor.
• Want it cheap, low-duty, and self-holding? Lead screw.
• Want to push very hard, very fast, for millions of cycles? Roller screw.

Architectures: Cartesian, gantry, H-bot, CoreXY

How you stack axes matters as much as which drive you pick. The common multi-axis arrangements:

Stacked Cartesian (serial XY/XYZ). Each axis carries the next: X rides on Y rides on Z (or some order). Simple, intuitive, and every axis is independent — but the lower axes carry the mass of all the axes above them, including their motors. Moving mass grows fast, so dynamics suffer for the proximal axes. Standard for machine tools, dispensing, and most pick-and-place where the payload is modest.

Gantry (bridge). A bridge spans the work and moves over it, often driven by two parallel motors (one per side) on the long axis. Stiff, large work envelope, and the long axis is usually rack-and-pinion or dual ball screws. The catch is gantry skew — the two sides must stay synchronized or the bridge racks (twists); this needs either a mechanical cross-shaft or a tuned dual-drive gantry control with encoders on both sides and a controller that fights yaw. Standard for large routers, laser cutters, and gantry robots.

H-bot. A single belt routed in an "H" so that two stationary motors drive both X and Y; the tool head carries no motor mass. Moving X = both motors same direction; moving Y = both motors opposite. Brilliant low-moving-mass idea, but the H routing applies a racking moment to the gantry that the frame must resist, which limits stiffness and accuracy at speed.

CoreXY. A refinement of H-bot with two belts crossed symmetrically so the racking moment cancels. Same benefit (two stationary motors, light head) without the H-bot's twisting load. Dominant in fast 3D printers and light gantries. The cost is belt routing complexity and the compliance of long belt loops.

Architecture	Moving mass	Stiffness	Drive typically	Best for
Stacked Cartesian	High (axes stack)	High	Ball screw / belt	Machine tools, dispensing, general
Gantry (dual-drive)	Medium	Very high	Rack-and-pinion / dual screw	Large routers, laser, gantry robots
H-bot	Low (head only)	Low–medium	Single belt	Fast light heads (budget)
CoreXY	Low (head only)	Medium	Two belts	Fast 3D printers, light gantries

Architecture rule: minimize moving mass on the fast axes and put stiffness where the tool point is. A light CoreXY head accelerates beautifully but flexes under cutting load; a stacked ball-screw machine is rigid but slow to move its proximal axes. Match the architecture to whether your job is fast-and-light or slow-and-stiff.

The kinematic mapping from motor coordinates to tool coordinates (especially for H-bot/CoreXY, where motion is a linear combination of both motors) is exactly the kind of transform handled in the motion planning & kinematics guide.

Sizing: load, moment, life, critical speed, buckling

This is where axes are won or lost. Five checks, each a separate limit, and the smallest resulting size is rarely the right one.

1. Load and moment on the guide

Resolve the payload (including its offset from the carriage center, and dynamic forces from acceleration) into a load on each guide block: a vertical/horizontal force plus the three moments — roll (Mr), pitch (Mp), yaw (My). An offset or cantilevered payload dumps its weight into moment, and moment loads divide unevenly across the blocks (a two-block carriage sees one block loaded more under a pitching moment). Check the combined load factor the catalog specifies:

Load factor = P/C + Mr/Mr_rated + Mp/Mp_rated + My/My_rated  ≤ 1
   (where P is equivalent direct load, C the dynamic rating;
    must be ≤ 1, with margin, for the chosen block)

Then apply the static safety factor fs = C0 / P_max (1.5–3 smooth, 3–5 with shock).

2. L10 bearing life

The fatigue life of a ball guide or ball screw follows the standard rolling-bearing power law. For ball elements the exponent is 3 (cube); for roller elements it's 10/3:

Linear guide L10 (km) = (C / P_equiv)^3 × 50   [THK basis, 50 km reference]
Ball screw  L10 (rev) = (Ca / Fa_equiv)^3 × 1e6
   C / Ca   = dynamic load rating
   P_equiv  = cube-mean equivalent load over the duty cycle
   Fa_equiv = cube-mean equivalent axial screw load

Example: rail block C = 30 kN, equivalent load P = 6 kN
  L10 = (30/6)^3 × 50 = 125 × 50 = 6250 km of travel
  At 0.5 m/s and a 50% duty over 16 h/day:
    daily travel ≈ 0.5 × 3600 × 16 × 0.5 / 1000 ≈ 14.4 km/day
    L10 ≈ 6250 / 14.4 ≈ 434 days → ~1.2 years before 10% fail

Use the cube-mean load over the real duty cycle, not the peak and not the simple average — the cube weighting means high-load segments dominate. A factor-of-two load error becomes an 8× life error.

3. Critical speed (screw whip)

A rotating screw is a shaft that whirls when its rotational speed approaches its first bending natural frequency — "whip." It depends on diameter, unsupported length, and end fixity (the support condition multiplier):

n_critical (rpm) ≈ K × f × (d_root_mm / L_mm²) × 1e7
   d_root = screw root diameter (mm)
   L      = unsupported length between bearings (mm)
   f      = end-fixity factor (fixed-free ~0.36, fixed-supported ~1.0,
            fixed-fixed ~1.47, supported-supported ~1.0; values per maker)
   K      = material constant for steel (~10 in this normalized form)
Operate at ≤ 0.8 × n_critical.

Example: d_root = 18 mm, L = 1500 mm, fixed-supported (f ≈ 1.0)
  n_crit ∝ 18 / 1500²  → critical speed drops with the SQUARE of length.
  Doubling the length quarters the safe rpm.

The square-of-length dependence is the reason long ball screws hit a wall: a 3 m screw may be limited to a few hundred rpm before whip, capping your speed far below the motor's capability. The fixes are larger diameter (root diameter goes up linearly, but mass and DN go up too), better end fixity, or — the usual answer past ~2–3 m — switch to a belt or rack.

4. DN value (ball recirculation limit)

Independent of whip, the balls themselves have a speed limit set by recirculation dynamics:

DN = screw_nominal_diameter_mm × rpm
Standard ball screws: keep DN ≤ ~70,000 (internal return) to ~100,000+ (end-cap, high-speed nuts)

Exceed DN and the balls jam or wear at the return path even if you're below critical speed. High-lead and high-speed nut designs raise the limit; roller screws sidestep it entirely.

5. Column buckling

A screw in compression (pushing a load away from the fixed bearing) can buckle like a column. The critical buckling load follows Euler, again with end fixity:

F_buckling (N) ≈ m × (d_root_mm)^4 / (L_mm)² × constant
   ∝ d_root^4 / L²   (Euler column, end-fixity dependent)
Operate at ≤ 0.5 × F_buckling (safety factor ~2).

Buckling matters at full extension on a long, slender, vertically-loaded or heavily-thrusting screw. Like critical speed, it punishes length (1/L²) and rewards diameter (here d⁴, even more strongly). If the screw must push hard at full reach, size for buckling first.

Sizing summary: run all five checks against the worst point in the duty cycle. The binding constraint moves with the job — short heavy axes are limited by load/life and buckling; long fast axes by critical speed and DN; cantilevered payloads by moment capacity. Never stop at "the thrust is enough."

Accuracy, repeatability and straightness

Three different numbers that get conflated constantly, and a controller fixes only some of them.

• Repeatability — return to the same commanded position from the same direction, measured as the scatter band. Usually the best number on the spec sheet (1–10 µm for a good screw axis, sub-µm for a linear-motor stage). It's what matters for pick-and-place: hit the same spot every time.
• Accuracy (positioning accuracy) — how close the actual position is to the commanded absolute position over the full stroke. Worse than repeatability, because it includes screw lead error, thermal growth, and Abbe error. Improved by error mapping/compensation in the controller and by closing the loop on a linear encoder.
• Bidirectional repeatability — repeatability including both approach directions. This exposes backlash and reversal error (the lost motion when you reverse direction). A unidirectional spec hides backlash; always read whether a number is uni- or bi-directional.
• Straightness and flatness — how much the carriage deviates from a perfect line vertically and horizontally as it travels. This comes from the rail set and its mounting, not the drive, and no amount of axis control fixes it unless you have multi-axis compensation. It's set by rail accuracy grade, mounting surface flatness, and how carefully you align the parallel rails.

Two error sources worth naming:

• Abbe error — angular error of the carriage multiplied by the offset between the measurement scale and the actual tool point. A 10 µrad pitch with a 100 mm tool offset is 1 µm of position error. Keep the feedback scale close to the work, and keep the carriage angularly stiff.
• Thermal growth — a steel screw grows ~11 µm per meter per °C. A 1 m screw warming 5 °C from its own friction grows ~55 µm — larger than the screw's grade error. Ground-screw machines that need µm accuracy either control temperature, use a cooled hollow screw, or compensate, and many high-end machines move the feedback off the screw and onto a glass/steel linear scale precisely to dodge thermal screw growth.

Spec-reading rule: demand bidirectional repeatability and full-travel accuracy, ask what reference temperature they're at, and treat straightness as a separate line item set by the rails. A screw's "C3 grade" tells you about lead error, not about whether your gantry tracks straight.

Lubrication, sealing and contamination

The fastest way to turn a 10-year L10 axis into a one-year axis is to starve or contaminate it. This section is where field reliability actually lives.

Lubrication. Recirculating-ball guides and ball screws need a lubricant film between ball and raceway — grease (NLGI 0–2, lithium or urea base) for most, oil for high-speed or high-temperature. Consequences of getting it wrong:

• Starvation breaks the elastohydrodynamic film; metal-to-metal contact spalls the raceway and L10 collapses. Catalog L10 assumes adequate lubrication.
• Relube intervals are specified in travel distance or hours; many blocks have grease nipples or accept an auto-luber. Honor the schedule — "lubed for life" blocks have a finite life and that life is shorter than the metal's.
• Speed/temperature push you from grease to oil. High-speed linear-motor stages and fast ball screws often use oil-air or circulating oil.

Sealing and wipers. Every block ships with end seals and often side/under seals; you can add double seals, scrapers, and metal scrapers for hard chips. Seals add friction (relevant for low-thrust belt/ironless axes) but multiply life in dirty environments. A ball screw exposed to swarf without a wiper or bellows is a wear experiment with a known bad ending.

Contamination control, by environment:

• Machine-tool / cutting — bellows or telescoping covers over the rails and screw; metal scrapers; positive coolant management. Chips and grit are the enemy.
• Washdown / food — stainless or coated rails, stainless screws, food-grade grease, or go to Igus drylin polymer guides that run dry and tolerate water.
• Cleanroom / semiconductor — low-particulate grease, special seals, sometimes positive-pressure purge (clean dry air into the carriage) to keep particles out, and ironless linear motors to avoid debris-attracting fields.
• Vacuum — special low-outgassing lubricants and materials; this is a specialist sub-field.

Field rule: the catalog L10 is a clean-and-lubricated number. In a real dirty machine, the actual life is the catalog L10 multiplied by how seriously you took sealing and relube. Most "premature bearing failures" are lubrication or contamination failures wearing a fatigue costume.

A selection workflow

Put it together into a repeatable procedure. Work top-down; don't start by picking a screw.

1. Define the duty cycle. Stroke, payload (and its center-of-gravity offset), required move time / speed / acceleration, cycles per day, environment (clean, chips, washdown, vacuum), and required accuracy/repeatability. Everything downstream is sized against the worst point of this, not the average.

2. Pick the architecture. Stacked Cartesian for general work, gantry for large stiff envelopes, CoreXY/H-bot for fast light heads. This sets which axes carry which masses and where you need stiffness vs. dynamics.

3. Choose the drive per axis from the tradeoff table:

   • Short, accurate, forceful → ball screw (or roller screw if very high force/cycles).
   • Long stroke, speed matters more than microns → belt (to ~10 m) or rack-and-pinion (any length).
   • Highest dynamics, zero backlash, budget allows → linear motor (ironcore for force, ironless for smoothness).
   • Low-duty, cheap, self-holding vertical → lead screw.

4. Choose the guide. Profile rail (ball) is the default; roller rail for maximum stiffness/cutting; round shaft (Thomson) for misalignment tolerance; Igus drylin for dry/washdown/light; cam-roller (Hepco/Bishop-Wisecarver) for fast debris-tolerant long travel; air bearing for metrology.

5. Size the guide: combined load factor ≤ 1 with margin, static safety factor fs per environment, then L10 in km against the cube-mean load. Pick rail size and preload class (light-to-medium default; C2 only if stiffness justified) and accuracy grade to match required straightness — on both rails of a pair.

6. Size the screw (if used): lead from the speed/force tradeoff (v = rpm/60 × lead), accuracy grade from required positioning, then verify critical speed (≤0.8× n_crit), DN (≤~70k–100k), buckling (≤0.5× F_buckling), and screw L10 in revolutions. If any fails on a long axis, go bigger diameter, better end fixity, or switch to belt/rack.

7. Size the motor and reduction against the reflected inertia and the torque-speed curve — see the servo motors guide and, if you're adding a gearhead, the gearboxes guide. Check the inertia ratio (load reflected to motor / rotor inertia) lands in a controllable range.

8. Decide feedback. Motor-side encoder is cheapest and fine when the transmission is stiff (ball screw); put a linear encoder on the load when the transmission is compliant (belt) or when you need accuracy beyond the screw's lead error and thermal growth.

9. Specify sealing, lubrication, and covers for the environment, and write the relube schedule into the maintenance plan. This is the difference between catalog L10 and field L10.

10. Prototype and measure bidirectional repeatability, full-travel accuracy, and straightness on the real machine. The spec sheet is a starting point; the assembled, mounted, loaded axis is the truth.

Follow that order and you'll avoid the classic failures: the over-spec'd screw on under-spec'd rails, the belt axis that can't hold position, the long ball screw that whips at half its target speed, and the beautiful linear-motor stage that cooks itself because nobody planned the cooling.

Frequently asked questions

When should I use a ball screw versus a linear motor? Default to a ball screw: it's cheaper, self-contained, has a built-in mechanical reduction (the lead), and a single rotary encoder closes the loop. Reach for a linear motor only when you need acceleration or bandwidth the screw can't deliver, the stroke is short-to-medium, backlash must be truly zero, and you can pay for the magnet track, the full-stroke linear encoder, the drive, and the thermal management. Most machines never cross that threshold.

Why are ball screws so much more efficient than lead screws? Rolling versus sliding. A ball screw's load rides on recirculating balls (rolling friction, ~90–95% efficient); a lead screw's nut slides directly on the thread (sliding friction, ~20–50%). The flip side is that lead-screw friction makes the screw self-locking, so it holds a vertical load with power off — which a ball screw won't do without a brake.

What is preload and why does it matter on both rails and screws? Preload is built-in internal load (oversized balls, or a double nut loaded against itself) that removes clearance so the element is stiff and backlash-free even at zero external load. The cost is friction, heat, and shorter life, because the balls see preload plus external load. Use light-to-medium preload by default; go heavy only when you've justified the stiffness, and use light/zero preload for low-friction or smooth-scanning axes.

What does the DN value limit, and how is it different from critical speed? DN (diameter_mm × rpm) limits the balls' recirculation dynamics — exceed it and balls jam or wear at the return path. Critical speed is a shaft phenomenon — the screw whirls when its rpm nears its bending natural frequency, which scales with 1/length². They're independent: a short fat screw can be DN-limited while a long thin one is critical-speed-limited. Check both, plus buckling.

How long should a profile rail or ball screw last? It's an L10 fatigue number: 90% of a population survive the calculated travel (rails, in km against a 50 km basis) or revolutions (screws). Computed from the cube-mean load over your real duty cycle, good axes reach years of operation. But the catalog L10 assumes clean and lubricated — starvation or contamination can cut field life to a fraction, so most "early failures" are really lube/seal failures.

Belt or ball screw for a long horizontal axis? If the stroke is past roughly 1.5–3 m and you care more about speed than microns, use a belt — it avoids the screw's critical-speed and buckling penalties (both ~1/length²) and runs 3–10 m/s cheaply. If you need micron positioning and high stiffness over that length, a belt won't give it; either accept a large ball screw with good end fixity or close the loop on a load-side linear encoder. Past a few meters, rack-and-pinion beats both.

What's the difference between ironcore and ironless linear motors? Ironcore coils are wound on iron, giving high force density but cogging and a strong magnetic attraction to the track that preloads the guide. Ironless coils sit in epoxy with no iron — no cogging, no force ripple, zero net attraction, the smoothest motion possible — but lower force density. Choose ironcore for thrust and stiffness, ironless for smooth constant-velocity scanning and metrology.

Why does my machine hit the right position repeatably but the wrong absolute coordinate? That's the difference between repeatability and accuracy. Repeatability (returning to the same spot) is set by the mechanics' consistency; absolute accuracy adds screw lead error, thermal growth (~11 µm/m/°C for steel), and Abbe error. Fix accuracy with controller error mapping or by moving feedback to a load-side linear scale. Repeatability you mostly buy in the hardware.

Do I need a linear encoder, or is the motor encoder enough? A motor-side encoder is fine when the transmission between motor and load is stiff and low-backlash — a ground ball screw qualifies. Put a linear encoder on the load when the transmission is compliant (belts stretch, long screws wind up) or when you need accuracy beyond the screw's lead error and thermal growth. The encoder also dodges thermal screw growth by measuring the actual carriage, not the screw turns.

What causes gantry skew and how do I prevent it? On a dual-driven gantry, the two sides driving the long axis can get out of sync and twist the bridge (racking it about the vertical axis). Prevent it with a mechanical cross-shaft tying both sides, or — more common now — a dual-drive servo control with an encoder on each side and a controller term that actively cancels yaw. Without one of those, the bridge binds and the position error grows with how far the sides drift.

When is rack-and-pinion the right call over a screw or belt? When the stroke is measured in meters and you need both speed and high thrust with more stiffness than a belt — large gantries, machine-tool long axes, and the linear track that carries a robot arm down a line. Racks tile end-to-end for unlimited length. Fight the gear backlash with a preloaded pinion or a dual-pinion (electronic-preload) drive.

Can polymer plain bearings (Igus drylin) replace ball rails? For the right job, yes. Drylin runs dry (no lube), is light, quiet, corrosion-proof, and cheap, and it shrugs off washdown and dust that destroy ball guides. The tradeoffs are lower load capacity and stiffness, some stick-slip, and a wear allowance instead of a fatigue life. Use it for light, low-precision, dirty, or wet axes; keep ball rails for load, stiffness, and µm accuracy.