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Single Axis Stages:What Actually Matters When You Specify One

Beyond travel length and load capacity — the parameters that determine whether a stage performs or just moves.

A single axis stage is, in concept, simple: a carriage constrained to one degree of freedom, driven along a linear path. In practice, the gap between “it moves” and “it moves correctly” is where most integration problems live. This article focuses on the parameters that are routinely underspecified — and why they matter more than the headline numbers in a datasheet.

Bearing Type Sets the Performance Ceiling

The bearing system is the single largest determinant of stage behavior. Crossed-roller bearings offer high rigidity and low runout — typically <1 µm straightness error. However, they are preloaded at the factory and not field-adjustable. Recirculating ball bearings handle longer travel at lower cost, but introduce periodic error (ball-pass frequency). This error shows up in high-resolution positioning as repeatable noise. Air bearings eliminate friction entirely and achieve sub-nanometer runout. On the other hand, they demand clean, dry supply air. They also require a granite or lapped-surface reference, and reject any moment loading that exceeds the film stiffness.

The wrong choice here doesn’t just limit performance — it can make a stage unusable for a specific task. This is true regardless of how capable the drive and controller are.

CROSSED ROLLER

High rigidity, sub-µm runout, fixed preload

BALL BEARING

Long travel, cost-effective, periodic error

AIR BEARING

Near-zero friction, nm runout, environment-sensitive

Drive Mechanism and Error Mapping in Single Axis Stages

Lead screws are the default for cost and self-locking behavior. However, the nut-screw interface introduces backlash unless anti-backlash preload is applied. This increases drag and thermal load at speed. Ball screws eliminate most backlash but are not self-locking; vertical axes require a brake or counterbalance. For the highest accuracy, ground ball screws with C3 or better classification are necessary. Even then, thermal expansion of the screw itself becomes a limiting error source above a few hundred millimeters of travel.

Linear motors bypass the screw entirely: no mechanical contact between thrust source and carriage means no backlash, no periodic lead error, and no mechanical resonance from the drive train. However, the tradeoff is that they require a linear encoder with resolution commensurate with the application. In addition, the system is only as stiff as the servo loop allows — there’s no passive mechanical compliance to fall back on.

Encoder Resolution vs. System Resolution

Encoder resolution is frequently confused with positioning accuracy. A 1 nm resolution encoder on a stage with 500 nm of friction-induced stick-slip delivers 1 nm of digital feedback and 500 nm of actual repeatability. Here the encoder is resolving motion that isn’t happening in a controllable way. What determines useful positioning resolution is the combination of encoder resolution, bearing friction characteristic (especially at low velocity), drive mechanism stiffness, and controller update rate. These interact — improving one without addressing the others yields diminishing returns.

Load, Moment, and Off-Axis Forces in Single Axis Stages

Datasheets list axial load capacity, but real payloads generate moments. A 2 kg load mounted 50 mm ahead of the carriage center creates a pitch moment of roughly 1 N·m. On a compact stage with a 60 mm bearing span, this translates directly into increased preload on the leading edge of the rail. Additionally, there is reduced preload — potentially negative — on the trailing edge. The result is non-repeatable runout that varies with carriage position and worsens with velocity due to dynamic effects.

When specifying a stage, moment loading should be calculated explicitly. It should also be checked against the bearing system’s rated moment capacity, not assumed to be covered by a safety factor on linear load.

What to Specify, in Order

A practical specification sequence: start with required straightness and flatness over travel (this selects bearing type). Then define the useful positional accuracy and repeatability (this selects drive type and encoder), and compute the actual load and moment envelope (this validates the mechanical sizing). Velocity and acceleration come last — they are usually achievable once the above are correctly matched. In fact, most failed stage integrations trace back to starting from travel and load. Eventually users discover that accuracy requirements weren’t satisfiable with the chosen architecture.

Single axis stages are mature technology. The components exist across every performance tier. The engineering work is in matching architecture to application — which requires being specific about what the stage actually needs to accomplish.

Motion Engineering · Single Axis Stages · May 2026

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