Proximity Sensors for Cranes, Hoists and Material Handling

On an EOT crane the sensor is doing safety related work: stopping long travel before the end stop, preventing two cranes colliding on a shared runway, limiting hoist height, detecting slack rope. When one fails the crane either stops producing or hits something.

The environment is the hard part. Continuous vibration, impact when the end stop is reached, temperature swings under a plant roof, and on a gantry, weather.

The sensing jobs

End of travel on long and cross travel. Usually a slowdown limit and a stop limit in series, sensing a steel flag or dog on the runway. This is the classic M30 application, because the gap between the trolley and the runway flag cannot be held tightly and 15 mm of sensing distance absorbs it.

Anti collision between cranes. Two cranes on one runway need to see each other before they meet. Inductive sensing is too short range for this; use photoelectric in retro reflective mode, which reaches 20 m in the M30 body, with the reflector on the opposing crane.

Hoist upper and lower limits. Rotary geared limit switches handle the counting, but a proximity switch on the drum or a flag on the rope guide gives an independent backup, which is worth having on a hoist that lifts over people.

Slack rope and rope drum rotation. A speed and rotation sensor on the drum detects a drum that has stopped turning while the motor is still commanded, which is what a snagged or slack rope looks like electrically.

Specifying for vibration

Vibration kills crane sensors in two ways and neither is the sensing element.

The mounting works loose. Locknuts on a threaded barrel will back off under continuous vibration unless they are properly torqued and locked. Use both nuts, torque them, and use a thread locking compound or a tab washer. Then check them at every scheduled inspection, because they will still move.

The cable fails at the gland. Continuous low amplitude vibration work hardens copper where the cable leaves the body. Support the cable within 100 mm of the sensor so the vibration is taken by the clamp rather than the gland, and use a connector version so a failed cordset can be swapped in minutes instead of rewiring back to the panel. See connector or cable.

Impact protection

A sensor sensing an end of travel dog will eventually be hit by it, usually because the dog has shifted or the trolley has overrun. Two defences:

• Mount the sensor so the dog passes across the face rather than towards it. A sensor that sees a flag sweeping past is never struck head on.
• Recess the face slightly behind a sacrificial non metallic plate and add the plate thickness to your gap calculation.

Choosing a larger body helps here too. The extra sensing distance of an M36 or M50 lets you set the sensor further back from the path of travel.

Weather and washdown

Indoor plant cranes are straightforward. Gantries in a yard, cranes in a steel plant near the water sprays, and anything in a coastal location need the ingress rating specified deliberately. IP67 covers temporary immersion, which is what driving rain amounts to. Where there is high pressure washing, IP69K. The difference is explained in IP67, IP68 and IP69K.

In a steel plant, add heat to the list. Near the caster or the rolling line, see high temperature sensors and our notes on steel plants and rolling mills.

Fail safe wiring

On a limit that stops motion, wire it normally closed. A broken cable, a disconnected plug or a failed sensor then reads the same as the limit being reached, and the crane stops. Wired normally open, the same failures read as the limit never being reached, and the crane runs into the end stop.

This is the single most important decision on the list and it costs nothing. Specify the NC variant at the order stage, not the NO.

Spares policy

A crane down for a sensor is a production line down. Hold spares for the parts fitted, not a generic selection, and label the spares with the crane and the position. Our plant spares list guide sets out how to build a list that is actually usable at two in the morning.

If the existing sensors are imported and hard to source, send us the part numbers. Most can be cross referenced to an equivalent we build in Mumbai and hold, see cross referencing imported switches.