Electronic shear pins: protecting conveyors and mixers without the downtime

A mechanical shear pin is a deliberately weak link. When a conveyor, auger or mixer jams, the pin breaks before the gearbox or drive shaft does. It is simple and cheap, but every break stops the line until someone finds the failed pin, fits a new one and restarts the machine.

Where mechanical shear pins fall short

• Downtime on every trip. The machine stays down until a technician replaces the pin, often in an awkward place.
• An imprecise breaking point. Pins weaken with fatigue and corrosion, so they can break under normal load, or fail too late when a real jam occurs.
• No adjustment. Changing the protection level means changing the pin.
• Only one kind of fault. A pin reacts to overload. It cannot tell you that a belt has snapped, a chain has come off or a hopper has run empty.

How an electronic shear pin works

An electronic shear pin replaces the weak link with a measurement. A load control monitors the true power drawn by the motor, which rises and falls with the mechanical work the machine is doing. When power crosses a set limit, a relay stops the motor before the drive train is damaged. Nothing breaks, so the machine can be restarted as soon as the cause is cleared.

Because the trip point is a setting rather than a piece of metal, it can be placed close to normal running load and adjusted when the process changes. The same control can also set a low limit, catching loss of load such as a broken belt or an empty feed.

Why power rather than current

Motor current changes little at light and moderate loads, because much of it magnetises the motor rather than doing work. Power follows the load in a straight line, so a power-based control sees a developing jam earlier and can be set tighter without nuisance trips. Our technology page explains the difference in more detail.

Getting the settings right

• Start-up delay. Motors draw a surge when they start. An adjustable start-up timer ignores it so the control does not trip on every start.
• Trip delay. A short delay filters out brief, harmless peaks while still stopping the motor quickly on a sustained jam.
• Rate of change. On lightly loaded equipment a jam may barely move the absolute power reading. A rate-of-change set point trips on how quickly the load rises instead.

Which control to use

The Load Controls fast response family is designed for this job. The PFR-1550 has a single set point, the PFR-1750 has two so it can alarm before it stops the machine, and the ROC-50 adds rate-of-change detection. All three respond in 25 milliseconds. On variable frequency drives, their “V” versions work with a UPC-FR fast-response power cell on the drive output.

Typical applications include belt and screw conveyors, mixers and agitators, augers, crushers and material handling equipment.

Tell us about the machine and its motor and we will recommend a control and send a quotation.

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Related: Why fault detection speed matters, Conveyors: jam detection