On a single-axis tracker, the torque tube is the backbone. It carries the modules, turns them, and passes wind torque down the row to the drive. When it is too flexible, the row twists, the modules point the wrong way, and in the worst case the row starts to oscillate.
This guide covers the solar tracker torque tube as a structural member: torsion, twist, dynamic stability and its interface with the piles. For the pile loads themselves, read our solar tracker foundation design loads guide. This article explains where those loads come from.
Quick answer. A solar tracker torque tube must carry bending between piles and torsion along the row. Torsion builds from the free ends toward the drive or locks. Twist and peak torque depend on row length and the number of restraint points. Closed tube sections resist torsion far better than open sections. Flat stow is the angle most prone to torsional instability.
TL;DR
- The tube carries bending between bearings and torsion along the row.
- Twist grows with row length and falls with torsional stiffness, GJ.
- A closed square tube can be over 1,000 times stiffer in torsion than an open section of the same steel.
- Research links torsional galloping mainly to tilt angle, with flat stow most exposed.
- The drive pier and locking points take the largest torsion. Their piles are not typical piles.
What does a torque tube do on a solar tracker?
A single-axis tracker rotates a row of modules about one axis. The torque tube is the long member along that axis. Modules clamp to it, bearings on the piles support it, and a drive turns it.
The tube has three jobs:
- Carry bending from module weight and wind between bearing piles.
- Carry torsion from wind pressure that acts off the rotation axis.
- Transfer rotation from the drive to every module in the row.
Each job has its own failure mode. Bending failures show as sag. Torsion failures show as twist, then as instability.
How does torsion build up along the row?
Wind on a tilted module creates a twisting moment about the tube axis. Each module adds a little torque. That torque accumulates along the tube until a point that resists rotation: the drive, a damper or a lock.
With one central drive, the tube on each side acts like a cantilever in torsion. Torque is near zero at the row end and highest at the drive. Twist is the reverse: zero at the drive and largest at the free end.
| Row restraint layout | Peak torque location | Twist pattern | Effect on piles |
|---|---|---|---|
| One central drive | At the drive | Largest at row ends | Drive pier carries most torsion |
| Drive plus locks or dampers | At each restraint | Shorter twisting lengths | Torsion shared across several piers |
| Distributed drives | Spread along row | Small and even | Lower peak per pier |
This pattern is why the drive pier is designed separately. It carries the row’s accumulated torque as an overturning moment and twist on the pile.
How much does a torque tube twist?
For a uniform tube, angle of twist is θ = TL / GJ. T is torque, L is length, G is the shear modulus of steel and J is the torsional constant.
The section shape decides J. For a thin-walled closed tube, J = 4A²t / p. Here A is the area enclosed by the wall centerline, t is wall thickness and p is the perimeter. For an open section, J is roughly the sum of bt³/3 over its plates.
| Section (illustrative) | J (mm⁴) | Twist over 30 m at 2 kN·m |
|---|---|---|
| Closed square tube, 125 × 125 × 3 mm | about 5,450,000 | about 8.2° |
| Open section, same wall length and thickness | about 4,400 | Not usable |
Arithmetic with G = 77,000 MPa and constant torque along the length, to compare section shapes. It is not a design check: real torque varies along the row.
The closed tube is about 1,240 times stiffer in torsion than the open section. That is why trackers use closed tubes. It also shows why row length and restraint spacing matter: twist scales directly with length.
Yield link. Twist is also an energy loss. Modules at the far end of a twisted row point away from the intended angle. Ask the supplier for the twist limit used in their design and the angle error it allows.
What is torsional galloping on solar trackers?
Torsional galloping is a self-excited oscillation. Wind feeds energy into the row’s twisting motion faster than damping removes it, and the motion grows.
Rohr, Bourke and Banks (CPP Wind, 2015) found the instability hard to suppress in wind tunnel tests. They recommended that isolated trackers and trackers at array edges not be stowed flat in high winds (Rohr et al., 2015).
Later experiments found the critical reduced wind speed depends mainly on tilt angle. Torsional stiffness, inertia and chord length matter less (ResearchGate, experimental study). A US national laboratory team has also measured full-scale tracker loads to study these instabilities (OSTI, Aeroelastic modeling and full-scale loads).
For design, this means three things:
- Stow angle is a structural decision. It is not only a control setting.
- Edge rows are different. They see higher loads and earlier instability.
- Static code coefficients are not enough. Tracker suppliers rely on wind tunnel testing for dynamic effects.
Our article on wind tunnel vs code-based wind loads covers when tunnel data is needed. ASCE/SEI 49-21, the wind tunnel testing standard, added requirements for products that include ground-mounted solar trackers (ASCE, 2021).
How does the torque tube connect to the piles?
Between the drive and the row ends, the tube sits in bearings on top of each pile. These bearings let the tube rotate, so they pass little torsion. They do pass vertical load, lateral load and some friction torque.
| Pile position | Main loads from the tube | Design note |
|---|---|---|
| Standard bearing pile | Vertical, lateral, uplift | Most of the row; uniform design |
| Drive pier | Accumulated torque plus normal loads | Higher moment; often a heavier section |
| Damper or lock pier | Torque at restraint points | Check per supplier load table |
| End pile | Higher wind at row end | Edge loads govern |
Pile installation tolerance matters here. If piles are out of line or out of level, the tube binds in its bearings. That adds friction torque and stresses the tube. Specify tolerances on the drawings and check them before the tube goes in.
For pile sizing from soil data, see our pile foundation design guide. For modeling the pile in software, see STAAD Pro pile foundation modeling.
What should an EPC check in the tracker supplier’s design?
Most EPCs buy the tracker as a supplied system. The torque tube design belongs to the supplier, but the foundation design often belongs to the EPC. These are the checks that connect the two.
- Load tables by pile position. Separate loads for bearing piles, drive piers, damper piers and end piles.
- Wind basis. The code edition, wind speed, exposure and any wind tunnel report used.
- Stow strategy. Stow angle, trigger wind speed and what happens on power loss.
- Twist limit. The allowed twist and its effect on tracking accuracy.
- Pile tolerances. Line, level and plumb limits for tube installation.
- Row length and restraint spacing. Changes here change torque and twist.
If any item is missing, ask for it in writing before the pile design starts.
How Heaven Designs helps
Our structural team designs tracker foundations from supplier load tables and site soil data. We check each pile position separately, including drive and damper piers. This work comes through our solar ground mount design and civil and structural engineering services.
We do not redesign a supplier’s torque tube or certify tracker dynamics. Those stay with the tracker supplier and their wind tunnel consultant.
To see a sample structural package, download design samples. To discuss a tracker project, get a project quote.
FAQ
What size torque tube does a solar tracker use?
It depends on the supplier, row length, wind speed and restraint layout. Tube size comes from the supplier’s structural design for the site wind. Ask for the design basis rather than comparing tube sizes alone.
Why are torque tubes square or round?
Closed sections resist torsion far better than open sections of the same steel. In our illustrative example, a closed square tube was about 1,240 times stiffer in torsion than an open section with the same wall length and thickness.
What is the safest stow angle for a tracker?
There is no single answer. Research shows flat stow is most prone to torsional instability, so many suppliers stow at a tilt in high wind. The right angle comes from the supplier’s wind tunnel testing for that tracker.
Which pile carries the most load on a tracker row?
The drive pier usually carries the largest moment, because the row’s accumulated torque reaches the ground there. End piles also see higher wind loads. Design both separately from standard bearing piles.
Does torque tube twist reduce energy yield?
Yes. A twisted row has modules pointing away from the target angle, which lowers output. Ask the supplier for the twist limit and include the loss in the tracker yield study.