Power-and-Free Conveyor

Overview

A power-and-free conveyor is an overhead conveyor that separates the driving chain from the carriers it moves, so that carriers can be stopped, queued, switched onto branch lines, and released without halting the chain. A plain overhead chain conveyor moves every load at the same speed all the time, but a power-and-free system lets one carrier sit at a workstation while others keep moving past it. That ability to accumulate and route makes it the backbone of paint lines, assembly lines, and overhead storage buffers, where parts have to wait at processes that take different amounts of time.

The system runs on two tracks, one above the other. The upper Power Track carries a continuous driving chain, the lower Free Track carries the load trolleys, a Caterpillar Drive pulls the chain, the Load and Free Trolleys hang the load, Switches and Stops route and stop carriers, Chain Take-Up keeps the chain taut, and a Control System system tracks every carrier.

Two tracks, one idea

The whole design rests on the split between power and free. The Power Track is an enclosed Power Rail Section that guides the Drive Chain on small Chain Trolley trolleys. Hanging down from the chain at intervals are spring-loaded Pusher Dog fingers. The Free Track below carries the load trolleys, which have no drive of their own. When a pusher dog catches the Engaging Dog on a carrier, it tows the carrier along; when a stop drops that engaging dog, the pusher rides over the top and the carrier coasts to a halt while the chain keeps running. Both tracks hang from the building on Track Hanger hangers, and an Anti-Backup Dog pawl on inclines stops a stopped carrier from rolling back.

The carriers

Each carrier is a short train of trolleys on the free track. A Front Trolley leads with the retractable engaging dog, a Load Trolley in the middle takes the weight, and a Rear Trolley trails to keep the carrier square. They are joined by a Load Bar from which the part hangs on a Carrier Hook or a custom fixture. Every trolley rolls on hardened Trolley Wheel wheels riding on Ball Bearing bearings, held in a stamped Trolley Bracket. The carrier length and hook design are matched to the product, whether that is a car body, an engine, or a rack of small parts.

Drive, chain, and take-up

The chain is pulled by a Caterpillar Drive, usually a caterpillar drive that grips the chain between two toothed Caterpillar Wheel wheels and pushes it forward with Drive Dog teeth. The Drive Gearmotor feeds through a reducer, and an Overload Clutch slips if the chain jams so nothing breaks. The Drive Chain itself is built from forged Chain Link links joined by hardened Chain Pin pins, with Attachment Link links carrying the pusher dogs. Over a long circuit the chain stretches as the pins wear, so the Chain Take-Up unit slides a Take-Up Sprocket on a spring-loaded carriage to remove the slack and hold tension constant.

Routing and accumulation

What sets power-and-free apart is what the Switches and Stops section does. A Track Switch is a movable rail segment that diverts a carrier onto a branch, a spur, or a parallel line, thrown by an Air Cylinder fed through a Solenoid Valve. A Stop Mechanism drops the engaging dog so the carrier parks, and a line of parked carriers forms an accumulation bank. Because each carrier stops on its own, a slow station never starves the line behind it, and parts can be banked between operations as live storage.

Control and use

The Control System system runs a PLC that knows where every carrier is, fires the switches and stops in sequence, and ramps the chain through an Variable-Frequency Drive. Carrier position is confirmed by Proximity Switch switches, and on systems that route mixed product an Carrier ID Reader reads a tag on each carrier so the control sends it to the right station. The controller does more than route: it sequences the stops so a bank of carriers releases one at a time at machine pace, it holds carriers back to balance work across parallel stations, and it logs how long each carrier waited so the line can be tuned. Power flows to the carts through the chain and the track, while signals run on a separate bus, which keeps the wiring simple on a circuit that may stretch for hundreds of metres through a plant.

Power-and-free conveyors come in two broad weights. Light overhead units handle garment, parcel, and small-part work, where carriers weigh a few kilograms and the track hangs from the roof on Track Hanger hangers. Heavy floor-supported inverted systems put the track below the load on stanchions, so a car body rides above the chain through weld, paint, and assembly; this keeps the dirty chain away from the painted surface and lets workers reach the underside. A common layout combines a main loop with several spurs, each spur a short branch where carriers park for a slow operation while the main loop keeps moving. Both forms share the same principle, a constantly running chain that picks up and drops carriers on demand, which is what turns a conveyor into a flexible routing and buffering machine that absorbs the uneven pace of real production.