Fasteners for Solar Tracker Structures: A Complete Selection Guide

1. A tracker that collapsed

In 2022 a solar plant in a windy part of northern China lost several trackers after two years in service. Multiple column bolts had fractured and the structures came down.

The root cause was fatigue: wind-induced vibration worked on bolts that had been assembled with insufficient preload. Once the joint opened, the bolt saw the full alternating wind load instead of a small fraction of it, and the fracture followed.

The remedy was to recalculate the preload to VDI 2230, move to class 10.9 bolts with zinc flake, and add locking nuts.

2. Fixed tilt versus tracker

3. Corrosion protection options

Note that hot-dip galvanising adds 50–80 μm of zinc and changes the thread fit unless the thread is cut after dipping or the nut is tapped oversize. For structural bolting, specify galvanising to ISO 10684 and match the nut accordingly — this is a common source of assembly problems on site.

4. Wind-induced fatigue

Trackers oscillate under wind excitation, so the bolts see alternating load. The countermeasures are:

• Raise the preload towards 90 % of yield, so the external load only modulates a small fraction of the bolt force.
• Use rolled threads rather than cut threads, for the 20–30 % fatigue gain at the root.
• Remove stress concentrations: radii at shank transitions, no undercut at the thread run-out.
• Keep the joint stiff. A thick flange and a short grip length reduce the load factor Φ and therefore the stress amplitude.

5. Grounding and conductivity

The structure must be grounded for lightning protection, which has a direct consequence for coating choice:

• Zinc flake coatings are not conductive, so they cannot be used at a grounding point.
• Use galvanised or stainless bolts at grounding locations.
• Grind the contact faces to bare metal before assembly, and re-protect the joint with a conductive paste rather than paint.

6. Decision tree and FAQ

Step 1: fixed or tracker?

Fixed → class 4.6, hot-dip galvanised. Tracker → class 10.9, zinc flake over galvanising.

Step 2: coastal?

Yes → 316 stainless, and plan for periodic inspection rather than assuming 25 years of no maintenance.

FAQ

Q: Do solar structures use self-drilling screws? On profiled steel roof sheeting, yes — self-drilling screws with EPDM washers. Primary steel beams and tracker joints use bolts, because a self-drilling screw cannot develop the clamp load a structural joint needs.

Q: Is 25 years with no maintenance realistic? For hot-dip galvanising in a dry inland environment, yes. On the coast, no — plan for inspection and touch-up, and specify the coating on that assumption.

Q: Why not stainless everywhere? Cost, and galling. Stainless bolts in stainless structure seize during assembly without a lubricated coating, and the price premium over galvanised steel is hard to justify inland.