Why Rectangular Washers Bend in Solar Projects
(And What It Reveals About Your Fastener Quality)
On paper, everything often looks correct.
The drawing specifies M10 HDG bolts.
Torque values are followed.
Rails are aligned properly.
Installation proceeds as planned.
Yet during tightening, something unexpected happens.
The rectangular washer bends.
This issue is far more common in solar installations than many teams admit—and it is never harmless. A bent washer is not a cosmetic defect; it is a structural warning sign that the joint is not behaving as designed.
The Overlooked Reality: Washers Are Load-Bearing Components
In many projects, washers are treated as minor accessories—often described casually as “cut pieces” or “spacers.” In reality, within solar mounting structures, washers are critical load-distribution elements
A washer’s role is to:
1. Distribute clamping force evenly
2. Prevent localized crushing of rails
3. Maintain long-term joint tension
4. Resist vibration and cyclic wind loads
When a washer bends during tightening, the joint immediately begins to lose its integrity.
If a washer bends:
1. Bolt tension drops
2. Load shifts to one edge of the joint
3. Contact stress increases sharply
4. Fatigue begins earlier than expected
5. Wind and vibration gradually loosen the assembly
In short, a bent washer turns a “tight” joint into an unstable joint.
When a washer bends during tightening, the joint immediately begins to lose its integrity.
If a washer bends:
1. Bolt tension drops
2. Load shifts to one edge of the joint
3. Contact stress increases sharply
4. Fatigue begins earlier than expected
5. Wind and vibration gradually loosen the assembly
In short, a bent washer turns a “tight” joint into an unstable joint.
Washer bending is rarely caused by over-tightening alone. It is usually the result of multiple specification and manufacturing shortcuts working together.
1. Insufficient Thickness
The most common cause is washer thickness below requirement.
Rectangular washers thinner than 2 mm simply do not have the stiffness needed to span rail slots under bolt preload. When torque is applied, the washer deflects instead of distributing load
2. Incorrect Base Material
Washers made from soft mild steel deform easily under compressive load.
Solar applications require:
1. Hot Rolled (HR) steel for strength
2. Stainless steel where corrosion and stiffness are both critical
Soft material bends long before the bolt reaches its design clamping force.
3. Hand-Fabricated or Torch-Cut Washers
Hand-cut washers lack:
1. Flatness control
2. Parallel faces
3. Uniform thickness
Uneven surfaces concentrate load at corners, accelerating bending during tightening.
4. Galvanizing Before Cutting
When plates are galvanized first and cut later, the exposed edges lose coating protection. This often comes with inconsistent thickness and reduced surface hardness, weakening the washer structurally.
5. Slot and Washer Mismatch
Rectangular washers must bridge rail slots properly.
If the washer is too small for the slot width:
1. Load transfers to edges
2. Washer behaves like a lever
3. Bending begins immediately under torque
This is a design-stage error, not an installation mistake.
Common Washer Sizes Affected in Solar Projects
Washer bending is frequently observed in standard solar fastener combinations such as:
1. M8, M10, and M12 HDG bolt assemblies
2. Rectangular washer sizes like:
A. 28 × 18 mm
B. 40 × 20 mm
C. 50 × 25 mm
For these applications:
1. Minimum recommended thickness: 2 mm to 3 mm
2. Washer width must fully cover the rail slot with margin
3. Flatness tolerance must be controlled
Ignoring these parameters makes washer deformation almost inevitable.
What Professional EPC Teams Specify Clearly
Well-executed solar projects do not leave washer selection to chance. Their BOQs and drawings explicitly mention:
1. Exact washer dimensions (length × width × thickness)
2. Minimum thickness requirement
3. Coating type (HDG, mechanical galvanizing, SS)
4. Material hardness or grade
5. Certification and test documentation
When these details are missing, fastener performance becomes unpredictable—no matter how good the bolt itself may be.
Why This Matters Long Term
Solar plants rarely fail because of modules or inverters alone.
They fail slowly—at connections.
And connections fail at joints.
A bent washer is often the first visible symptom of a joint that will loosen, vibrate, or fatigue over time. Left uncorrected, it can lead to:
1. Re-torquing cycles
2. Rail movement
3. Structural noise and vibration
4. Premature maintenance costs
Final Truth
In solar structures, small components carry big responsibility.
Washers are not secondary items—they are structural elements that protect the entire joint. Fixing washer specifications early prevents much larger failures later.
Before the biggest problem shows up, fix the smallest component.