A solar tracker gearbox is not selected on speed, because it barely turns: a single-axis row moves through 180 degrees in a day, which is a few thousandths of a revolution per minute at the output. It is selected on three other things - the torque needed to hold the array still in a storm, the backlash that decides whether the controller can settle, and a service life of twenty years in dust, UV and temperatures that swing fifty degrees. Get the holding case wrong and the row walks itself apart in the first gale.

What a Tracker Actually Asks of a Drive

A tracker has one job: keep the module plane perpendicular to the sun within a degree or so, all day, every day, for two decades. The drive does that job in short bursts - a second of movement every few minutes - and spends the rest of its life holding position.

That duty profile is unlike any other gearbox application in this catalogue. There is no continuous running to wear the teeth in, no thermal cycling from the load itself, and almost no oil-side heat. What there is, is an enormous number of starts, a static load that reverses with the wind, and a mechanism that must not creep.

Three motions exist, and each sets a different requirement.

• Horizontal single axis (HSAT) - a north-south torque tube rotating east to west, typically over ±60 degrees, with backtracking near sunrise and sunset to avoid row-to-row shading.
• Tilted or vertical axis - one axis inclined or vertical, used where the site geometry favours it.
• Dual axis - azimuth plus elevation, two drives per unit, each with its own holding case.

The Ratio Comes From the Sun, Not From the Motor

Tracking speed is a function of the earth, not of the load: fifteen degrees per hour is the solar rate, and a tracker that follows it exactly needs an output speed of roughly 0.0007 rpm. Practical drives run faster than that and move intermittently, which is why the ratio is chosen for torque rather than for speed.

The ratio band is wide because the same torque can be reached with a single worm stage at low ratio and a large ring, or with a two-stage reduction and a smaller ring. The choice is commercial as much as technical: a bigger ring costs more steel, a second stage costs more efficiency.

Self-Locking: The Feature That Stands In for a Brake

A worm drive locks when the lead angle of the worm is smaller than the friction angle of the bronze wheel. In practice that means ratios above roughly forty to one are self-locking in the holding direction: the load cannot drive the worm backwards, so the array stays where it was put without a brake, without power, and without a control loop.

For a tracker this is not a convenience, it is the safety case. A storm arrives at night when the controller is asleep. If the drive is not self-locking, the only thing holding a hundred square metres of module against a gust is a brake that has not been tested since commissioning.

Two consequences follow:

• Holding torque must be stated separately from running torque. A slew drive may run at 1,200 N·m and hold at 6,000 N·m, and the tracker needs the second number.
• Efficiency is low by design. A self-locking worm stage runs at 30–50 per cent efficiency, which is why the motor is bigger than the running torque alone would suggest, and why a two-stage self-locking unit is rarely more efficient than a single stage with a larger ring.

Wind Is the Load, and Stow Is the Design Case

The static load on a tracker is its own weight, and that is handled by the torque tube bearings and the pile. The load that reaches the drive is the torque about its own axis, and wind is what produces it.

Wind pressure acts on the module plane, and the centre of pressure sits above the axis of rotation. That offset turns a lateral force into a torque at the slew drive. At a design gust of 42 m/s the dynamic pressure is about 1.08 kN/m², and with a drag coefficient near 1.2 the module plane sees roughly 1.3 kN/m². A row of 120 m² therefore takes about 156 kN, and a 100 mm offset between centre of pressure and axis converts that into 15.6 kN·m before any safety factor - which is why trackers stow flat or at a defined angle when wind exceeds the operating limit, and why the drive is checked in that stow position rather than in tracking position.

The design check is straightforward: holding torque at the slew output must exceed the stow wind torque plus bearing friction, multiplied by the site's safety factor. A factor of 1.5 is common; a factor of 2 is cheap insurance on a site with hail and gust fronts.

Backlash Decides Whether the Controller Can Settle

Tracking accuracy of ±0.5 to ±1 degree costs very little energy - a one-degree error is under one per cent of annual yield - so backlash is not primarily an energy problem. It is a control problem.

A drive with 0.3 degrees of output backlash gives the controller a dead band it cannot close. The motor moves, the array does not, the controller commands more movement, and the row hunts. Hunting loads the mechanism, wastes power and can trip the drive on overcurrent.

What keeps it manageable: a single-stage slew with a preloaded worm, a control dead band slightly wider than the backlash, and a movement strategy that approaches position from one direction rather than reversing into it.

Twenty Years in Dust, UV and Frost

A tracker gearbox is usually a sealed, grease-filled unit with no oil change in its life, mounted at knee height in a field.

The conditions that decide whether it lasts:

• Dust. Fine soil dust migrates into any unsealed joint and mixes with grease to form a lapping paste. A sealed-for-life slew ring with a protected worm housing is the answer; a breather on a tracker is a liability, not a feature.
• UV and ozone. Seals and cable glands degrade before the steel does. EPDM rather than nitrile, and a housing that shades its own seal, both help.
• Low temperature. Grease that stiffens at minus twenty turns the running torque into a start-up problem. The grease grade has to be stated for the site's minimum, not for a factory default.
• Corrosion. Coastal and agricultural sites are the two worst. A coating system chosen for C4 or C5 environment, and fasteners to match, cost little at order stage.

Motor, Control and the Start That Matters

The motor on a tracker is small - typically 0.37 to 1.1 kW - and it starts thousands of times a year. That start count, not the running hours, is the duty that sizes the thermal capacity of the motor and the wear allowance of the worm.

Direct-on-line starting is normal on small drives, and the inrush is what the site's supply has to carry. Where a variable frequency drive is used, the soft start removes the shock but adds a low-speed running condition in which the worm gets no oil film from rotation; a grease-filled unit handles that better than an oil-filled one.

There is also a cold-start case worth checking: grease at minus twenty, a gear train at 40 per cent efficiency, and a controller that expects movement within ten seconds. If the motor cannot break the array loose, the controller will keep trying and the drive will sit in stall.

The mechanism inside a tracker slew drive is a worm set driving a bronze wheel bolted to the slewing ring. The same principle, built for continuous industrial duty rather than a twenty-year field life, is what a helical worm gearbox supplies where a plant needs a slow, self-locking output in a compact right-angle layout - same arithmetic, different duty profile.

The Numbers to Send With an Enquiry

A tracker drive quotation needs five figures, and most enquiries arrive with two.

1. Holding torque required in stow - from the row geometry, the design gust and the site safety factor.
2. Running torque - bearing friction plus module unbalance at the worst tilt angle.
3. Rotation range and speed - degrees of travel, and the time allowed for a full sweep.
4. Environment - minimum and maximum ambient, dust class, coastal or inland, snow load.
5. Interface - mounting flange and bolt pattern on the torque tube, and the controller's output.

The online selection system takes the torque and speed pair and returns a frame; the holding case still has to be argued by hand, because no catalogue table knows the wind speed at the site.

FAQ

Why is a solar tracker gearbox self-locking?

Because the worm lead angle is smaller than the friction angle between worm and wheel, so the load cannot drive the mechanism backwards. That is what holds the array in position overnight and in a storm without a brake, and it is the single most important property after holding torque.

Can a standard industrial gearbox be used on a tracker?

Mechanically it can carry the torque, but the duty does not suit it: an industrial unit is built for continuous running with an oil sump, a breather and periodic oil changes. A tracker needs a sealed, grease-filled unit that survives twenty years of intermittent movement, dust and frost without service.

How much backlash is acceptable in a tracker?

Around 0.1 to 0.3 degrees at the output is normal, and it costs almost nothing in yield. What it costs is control stability, so the controller dead band should be set wider than the backlash and the approach should be made from one direction.

Does the gearbox need a brake?

On a self-locking single-axis drive, no - the worm holds. On a dual-axis elevation drive where the module mass can overhang the axis, a brake or a counterbalance is usual, because the load can be reversible at certain angles.

A tracker drive is chosen on its holding torque, its self-locking behaviour and its survival in a field - then confirmed on running torque and backlash, in that order.