Why Bar Screens Fail ? - Part 2: Why Oversizing Drives Leads to Structural Failure in Multirake Bar Screens

An evidence-based look at what goes wrong in typical multirake screen installations.

Recap: What We Learned in Part 1

In Part 1, we showed that even a modest 10% jamming of rollers in multirake bar screens more than doubles the required torque. At 30%, torque demand increases fivefold, pushing many systems well beyond their design limits.

But instead of redesigning, many operators attempt a “quick fix” — by simply increasing the torque. That brings us here.

The Quick Fix That Backfires

When screens jam too often, the typical response is to:

• Install a larger gearbox

• Add a more powerful motor

• Reduce output speed to increase torque

This allows the screen to power through temporary resistance. But it doesn't solve the problem — it just moves it somewhere else.

The torque now flows downstream through the machine. And when resistance builds up again, that energy gets unleashed into the weakest component.

A Cascade of Failures

1. The chain usually fails first — stretching, snapping, or jamming deeper into its guide as it absorbs the full tensile load from a blocked rake.

2. If the chain survives, the raking arm becomes the next fuse. Rigid and cantilevered, it often bends, twists, or tears at the mounting point.

3. If the arm holds, torque travels into the drive shaft and main bearings. Bearings heat up, shafts flex, misalignments appear. Failures start to cascade.

4. If everything else is oversized — then the screen body gives way. Deck plates deform. Rear plates twist. Brackets shear. Anchors pull out. Some screens become structurally unusable — and cannot be repaired.

Why Overload Switches Fail to Protect

Many screens are fitted with spring-loaded mechanical limiters or torque switches. They are meant to trip when torque spikes.

But in real plants:

• Jamming is instantaneous — limiters don’t trigger in time

• Springs fatigue

• Brackets bend or crack

• Trip points drift or seize

After a few overloads, these switches are often no longer effective — and the next jam becomes a catastrophic event.

The Paradox of Torque: When Power Becomes Destructive

Some installations now run screens with torque outputs as high as 25,000 N·m. That’s enough to rip through rags — and tear apart the machine holding the rake.

It’s like giving a bodybuilder enough strength to break their own bones.
A screen that can power through every jam is strong enough to destroy itself.

This is not durability. It’s delayed failure.

What’s the Real Solution?

The real solution isn’t more torque. It’s less — but smarter.

Cap the Drive Torque at or below 1,000 N·m, where the screen has just enough energy to do its job, but not enough to damage itself.

• Build in predictable failure points

• Make it mandatory for manufacturers to provide FEA-based design verification of all critical parts, showing they can withstand peak instantaneous torque spikes

• Design systems to deflect, yield, or shut down before structural limits are reached

But most importantly:
Stop responding to jamming by overpowering it.

​Coming in Part 3: Designing Screens That Don’t Break Themselves

In Part 3, we’ll explore design principles that reduce friction, eliminate roller jamming, and create systems that absorb or avoid overloads entirely.

From dry-side drives to cam-based rakes and transmission chain systems, we'll show how smarter mechanical design results in safer, longer-lasting screens — without brute force.