Why Bar Screens Fail: Engineering Insights into Torque Overload from Jammed Rollers
An evidence-based look at what goes wrong in typical multirake screen installations.
Introduction
Mechanical bar screens are essential in wastewater treatment. Yet across countless installations, these systems are abandoned, bypassed, or overloaded within months of commissioning.
This isn’t a fabrication flaw. It’s a design oversight - and it begins with torque.
This article presents technical insights based on our internal torque modeling studies. It is not a critique of any particular brand or model, but a data-grounded explanation of why so many multirake screens fail under routine conditions - and what engineers can do about it.
The Problem Isn’t Rolling Friction — It’s That Rollers Rarely Roll
Rollers in multirake screens are intended to glide along stainless steel guide tracks with minimal resistance. And yes - rolling friction is low. Our modeling shows it contributes less than 0.2% of total torque.
But in the real world, rollers rarely roll for long.
Debris, fibers, corrosion, and grit cause rollers to jam, converting rolling contact into sliding contact - drastically increasing resistance. And this happens not during extreme overloads, but under routine operation.
This isn’t an edge case. This is routine.
Torque Modeling of a Generic 9-Meter Multirake Screen
Using a 9-meter inclined screen with M112 ISO chains and 359 rollers, we modeled jamming scenarios based on common wastewater conditions.
Jammed Rollers | Jammed % | Added Force (kN) | Total Force (kN) | Running Torque (kNm) | Starting Torque (kNm @1.75×) |
|---|---|---|---|---|---|
0 | 0% | 0 | 14.5 | 1.93 | 3.38 |
36 | 10% | 22.3 | 36.8 | 4.89 | 8.56 |
72 | 20% | 44.5 | 59 | 7.85 | 13.74 |
108 | 30% | 66.8 | 81.3 | 10.81 | 18.91 |
Even 10% jamming more than doubles the torque requirement.
At 30%, torque demands increase by over 5×, often exceeding the installed system’s design limit.
How Torque Values Were Calculated
1. Total Number of Rollers
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Chain pitch: 101.6 mm
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Total loop length ≈ 2 × 9 m + 0.26 m = 18.26 m
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Number of rollers per chain = 18.26 / 0.1016 ≈ 180
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Two chains per screen → Total rollers = 180 × 2 = 359
2. Load per Roller
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Estimated load: 1051.4 N per roller
(Includes chain weight, guide pressure, and comb load divided over engaged rollers)
3. Friction Type When Jammed
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Jammed rollers slide instead of roll
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Friction pair: dry SS304 on SS304
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Coefficient of friction (μ): 0.6
Fslide = μ × roller load = 0.6 × 1051.4 ≈ 630.8 N per roller
4. Force From Jammed Rollers
Jammed % | Jammed Rollers | Additional Frictional Force |
|---|
Add to base system force (clean chain): 14,500 N
5. Torque Calculation
T = Ftotal × R
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Where:
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R = pitch radius of drive sprocket = 0.133 m
Total Force (N) | Running Torque (Nm) | Starting Torque (Nm @1.75×) |
|---|
And Then What Happens?
Many facilities respond by installing larger gearboxes and motors to push through the jams. This allows the screen to continue running — temporarily.
But the excessive torque doesn't disappear. It just shifts the failure point downstream to:
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Raking combs, which bend or shear under load
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Screen frames, which crack at welds or anchor points
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Foundations, which may loosen or distort over time
You can't out-muscle a jam. You have to design around it.
What Engineers Can Do
Size drives with 2× torque buffer — jamming is not exceptional, it’s expected
Allow for accessible roller inspection and cleaning
Integrate mechanical torque monitoring for early jam detection
Avoid equipment with permanently submerged rollers
Use torque modeling as standard practice — not just flow rate calculations
Coming Next: When More Torque Becomes the Problem
In our next Knowledge Center article, we’ll explore what happens when oversized gearboxes solve one problem but create another — leading to structural failures in screen frames, combs, and civil foundations.