RULES / OPERATIONS
Conveyor safety switches and silo flow aids: what they do
What each conveyor safety switch and silo flow aid actually does — pull-cord stops, belt drift, zero-speed, plugged-chute and rip detection, vibrators, air cannons, fluidising pads, bin activators — and what none of them can do.
Part of the guide: Why material stops flowing: bridging, ratholing and mass flow
Two families of device appear on almost every handling line and are specified with the least thought. Safety switches — pull-cords, drift switches, speed monitors, chute detectors — exist to stop a conveyor when something has already gone wrong. Flow aids — vibrators, air cannons, fluidising pads, bin activators — exist to restart a silo that has already stopped. Both are reactive, both are cheap relative to what they protect, and both get blamed for problems they were never designed to solve.
Why do two different families belong in one note?
Because they share a failure mode: being used in place of design. A drift switch on a belt that mistracks every shift is doing its job while the alignment problem continues. A vibrator that runs all day on a hopper is compacting the material it is meant to move. Each device below does one specific thing in one specific place, and the rules are about keeping it there.
What are the rules?
Pull-cord stops along every accessible length
Anywhere a person can reach the belt, a pull-cord must be reachable too — on both sides where there is a walkway on both sides. It stops the conveyor and everything feeding it, latches until it is reset at the switch, and is tested by pulling it, not by looking at it.
Drift switches at both ends and wherever the belt has history
Belt misalignment switches sit at head and tail and at any point where the belt has wandered before. They protect the belt from cutting itself on the structure; they do not cure the mistracking, whose causes are in the belt tracking note. A switch that trips regularly is a symptom, not a solution.
Zero-speed monitoring on every drive and elevator
A speed switch on a non-driven pulley catches a slipping belt, a snapped belt and a stalled elevator before the drive burns the lagging or the boot fills. Under-speed gives a warning; zero-speed stops the feed. Elevators need it most, because a stopped belt under a running feed buries the boot very quickly.
Plugged-chute detection at every transfer that can block
A tilt switch, a paddle or a level probe in the chute stops the incoming conveyor before the blockage climbs to the head pulley. Anything wet, sticky or lumpy can block, for the reasons in the transfer point note. Put the probe where the build-up starts, not where it is convenient to fit.
Bearing temperature and speed on elevators
Bucket elevators run enclosed and fast, with bearings at the top of a tall casing: a hot bearing, a slipping belt or a belt rubbing the casing is a fire risk as well as a breakdown. Temperature probes at head and boot, a speed monitor and a belt alignment sensor are the usual set; the elevator discharge note covers the machine itself.
Rip detection where a trapped object is likely
Where tramp metal, bar or a lump can pierce the belt at the loading point — under crushers, reclaim hoppers, scrap feeds — a rip detector under the loading zone stops the belt before a longitudinal tear runs its whole length. It protects the most expensive component on the conveyor.
Interlock the line in both directions
Downstream equipment starts first and upstream stops first; a stopped conveyor stops what feeds it, and a tripped device on one machine takes out the machines above it. Without the interlock, every switch protects one conveyor while the one behind it buries the transfer.
Guard the nip points; a switch is not a guard
Pull-cords and drift switches limit the consequences of contact. Guards prevent it, at every pulley, idler nip and drive a hand or clothing can reach. Fixed guards, interlocked access doors and isolation for maintenance come first; the switches are what remain after the guards.
Test on a schedule
A switch that has never tripped may never trip. Corrosion, dust build-up, a bridged contact after a nuisance trip, a probe painted over: none of it shows until the device is needed. Trip every device on a fixed schedule, record the result, and treat a bypass as a fault to be closed the same day.
Flow aids recover; they do not design
A vibrator, an air cannon or a bin activator breaks an arch that has already formed. None of them turns a funnel-flow hopper into mass flow or makes an undersized outlet adequate, and a vibrator run continuously on a cohesive material compacts it. The silo flow note sets out the geometry that flow aids can only supplement.
Which device does what?
Safety switches
| Device | What it senses or does | Where it goes | What it does not do |
|---|---|---|---|
| Pull-cord stop | Stops and latches the conveyor and its feed | Along every accessible length, both sides | Prevent contact; it limits the consequences |
| Belt drift switch | Detects the belt edge running off line | Head, tail, and points with tracking history | Correct the tracking |
| Zero-speed switch | Detects a slipping, broken or stalled belt | A non-driven pulley on every drive and elevator | Detect a belt that runs but is blocked |
| Plugged-chute detector | Detects material building up in a chute | Every transfer that can block | Clear the blockage |
| Bearing and speed monitor | Detects hot bearings, slip and misalignment | Elevator head and boot | Replace routine inspection |
| Belt rip detector | Detects a longitudinal tear as it starts | Under the loading zone where tramp objects arrive | Stop a tear that began upstream of it |
Flow aids
| Device | What it does | Where it goes | What it does not do |
|---|---|---|---|
| Vibrator | Shakes the hopper wall to break an arch | Lower cone, on a mounting plate | Help cohesive material if run continuously |
| Air cannon | Releases a burst of air to dislodge build-up | Cone and outlet area, aimed at the arch | Fix an outlet that is too small |
| Fluidising pads | Aerate a fine powder so that it flows to the outlet | Hopper cone, for fluidisable powders only | Work on wet, coarse or fibrous material |
| Bin activator | Vibrates a live-bottom section so the outlet keeps drawing | Under the hopper, above the feeder | Change the flow pattern in the silo above it |
| Level probe | Reports material level for control and alarms | Silo top, chute, or where a rathole hides | Say what is stuck below the level it sees |
Where do these devices fit in an upgrade?
They are usually the first items on the list, because they are among the cheapest changes to an old line and the ones that most reduce the consequences of a fault. The retrofit or replace note puts them in context, and our equipment and components range covers the switches, detectors and flow aids described here, selected for the line they go on.
QUESTIONS WE HEAR
Should safety switches be wired to the PLC or to a safety circuit?
Emergency stops and the devices that protect people belong in a dedicated safety circuit — a safety relay or safety controller — that stops the drive independently of the process controller, with the process system told what happened. Process protection such as chute detectors and speed monitors can act through the control system. The plant's safety assessment decides which is which.
Can an air cannon damage a silo?
Yes, if it is fitted without checking the shell. The burst loads the wall locally and the nozzle penetrates it, so the position, the reinforcement pad and the aim have to be engineered with the silo's structure in mind, and the cannon must never be aimed at a feeder or a gate. Fitted properly, it is a well-proven device for the right material.
How often should conveyor safety switches be tested?
On a schedule the plant sets and keeps, tied to its maintenance system, rather than at an interval taken from a catalogue. Pull-cords and drift switches are commonly tested at every planned stop and the less accessible devices at each shutdown. The interval matters less than the record: an untested switch is an assumption.
Is a vibrator or an air cannon better for a blocked hopper?
They suit different problems. A vibrator works on granular materials that arch mechanically and on thin steel cones; on cohesive, fine or wet material it tends to compact rather than free it. An air cannon breaks a cohesive arch or wall build-up in a burst, without continuous energy input. Neither replaces an outlet that is too small.