Between tracking updates, a solar tracker's drive sits unpowered — and the wind does not. The row presents hundreds of square meters to every gust, and the torque that gust generates must go somewhere. The answer that makes modern trackers possible is the self-locking slew drive: a worm gear whose geometry makes back-driving impossible, so the structure holds its position like a brake that never releases.
How self-locking works
In a worm drive, the motor drives the worm, the worm turns the wheel — and in the reverse direction, the mechanics invert: wind torque on the wheel tries to turn the worm, but the worm's shallow helix angle cannot convert that force into rotation. The load is locked by geometry, not by friction pads or motor current. This irreversibility is why trackers do not need motor brakes, why wind stow survives a power failure (the row simply holds where it stopped), and why backlash management — not holding torque — is the design challenge.
What it buys the tracker
Safety first: on grid or controller failure, the row freezes in place instead of windmilling — a passive fail-safe that wind tunnels and certification labs reward. Economics second: no brake motors, no holding current, smaller motors sized for tracking speed rather than wind holding. Accuracy third: the same irreversibility that blocks wind also blocks backlash drift between updates, so the row holds its angle within the drive's specification until the next angle-sensor correction.
The trade to respect
Self-locking means the row cannot be back-driven by hand either — maintenance stowing uses the motor or a handwheel provision, and emergency manual override must be designed in, not improvised. Efficiency is the worm's other tax: part of the tracking energy becomes heat in the mesh, which the gearbox's size already budgets for.
The decision rule
Any tracker exposed to real wind gets self-locking or an equivalent holding strategy — the question is never whether to hold, but how honestly the drive's irreversibility is rated against the site's gust loads. Pair it with true-angle feedback and the stow logic, and the row's worst day is just another update cycle.
Do self-locking drives need maintenance?
Grease discipline and backlash checks — the worm mesh wears like any gear, and its self-locking character is a property of geometry plus friction condition. A starved drive can lose its lock character, which is why lubrication intervals are a structural spec, not housekeeping.
Why not use a motor brake instead?
Brakes work, but they add a component, a failure mode and a holding-current decision — and they release when power dies unless engineered otherwise. Worm irreversibility is the brake that is already in the gearbox.
