Why doesn’t your robot arm land exactly where you told it to?
Five numbers on a joint module spec sheet quietly decide how precise a robot really is. Here’s what each one means, in plain English.
Every robot joint has a motor, a gearbox, an encoder and bearings working together — and none of them are perfect. The gap between “where you told it to go” and “where it actually stops” gets measured five different ways, and each one tells you something different about how the joint will behave.
Backlash
In plain terms: the tiny bit of “play” in the gears. Spin a joint one way, then reverse — there’s a brief moment where nothing happens yet, because the gears are still taking up slack before the output actually starts moving back.
Lost Motion
In plain terms: send the joint to the exact same spot from two different directions — once approaching from the left, once from the right — and it usually won’t land in exactly the same place. That difference is lost motion.
Transmission Error
In plain terms: even while spinning smoothly with no reversing involved, the output almost never rotates in perfectly even steps. Tiny manufacturing imperfections make it speed up and slow down slightly, over and over, through each rotation.
Positioning Accuracy
In plain terms: how close the joint actually gets to the exact spot you commanded, measured against a fixed, known reference point.
Repeatability
In plain terms: how tightly the joint returns to the same spot, over and over, under the same conditions — regardless of whether that spot is the “correct” one.
All five, side by side
| Spec | What it’s really asking | Watch this if you’re doing… |
|---|---|---|
| Backlash | How much slack before direction reversal takes effect | Frequent direction changes |
| Lost Motion | Difference in landing spot from opposite approach directions | Bidirectional precision tasks |
| Transmission Error | Ripple in motion during continuous rotation | Smooth, continuous trajectories |
| Positioning Accuracy | Closeness to the exact commanded position | Working to fixed external coordinates |
| Repeatability | Consistency returning to the same taught position | Repetitive pick-and-place, dispensing |
Frequent direction changes
Prioritize low backlash and low lost motion — they control how the joint behaves the instant it changes course.
Continuous trajectories
Transmission error, encoder quality and calibration matter most for keeping motion even throughout.
Absolute positioning
Look at positioning accuracy — and evaluate it at the whole-system level, not just the gearbox.
Repetitive automation
Repeatability is usually the number that matters most for pick-and-place and assembly lines.
None of these five specs tells the whole story by itself. A joint can be excellent on paper for one and mediocre for another — which is exactly why it’s worth understanding what each one actually measures before comparing spec sheets.
The right joint module is the one matched to your motion pattern — not just the one with the smallest number on a single line.
Not sure which spec matters for your application?
Tell us your torque, speed, payload and positioning requirements — we’ll help you match the right joint platform.
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