In asynchronous assembly systems, modular pallet-transfer conveyors get the right parts to the right spot, at the right time.

The most common pallet-transfer conveyor carries pallets on twin strands of belt or chain. Each strand rides in a track within an extruded aluminum profile. The profiles are separated by space, and only the edges of the pallet are supported. The strands provide just enough friction to move the pallet. But, if the pallet encounters a gate or another pallet, it stops and the strands slide freely beneath it.

The strands can be belts, roller chain or flat-top chain. Belts are made from static-dissipative rubber, and generate the most friction for moving pallets. They are used when pallets must be positioned precisely. With thick belts made from low-durometer rubber, the product may be able to ride directly on the conveyor, without a pallet. One drawback of belts is that they are more difficult to repair or resize than roller chain or flat-top chain.

As its name implies, roller chain consists of steel and plastic rollers linked together in a chain. Roller chain can carry heavy loads with minimal chain tension and power consumption. "With roller chain, starts and stops have a low impact," explains Kevin D. Gingerich, marketing services manager for Bosch Rexroth Corp. Linear Motion and Assembly Technologies (Buchanan, MI). "The chain moves forward, but the rollers can spin. So, pallets accelerate slowly after stopping, which is good if you have a sensitive product or a heavy product, and you don't want to jerk it too much on the conveyor."

Flat-top chain is a roller chain topped with flat caps of low-friction plastic or steel. Because it can flex from side to side, flat-top chain is used for carousel and serpentine layouts. An advantage of both roller chain and flat-top chain is that individual links can be removed for easy repair or resizing.

Pallet-transfer conveyors are available for handling products as small as cosmetics cases or as large as kitchen appliances. The conveyors can be arranged in many ways: side-by-side, rectangular, carousel, serpentine or even one atop the other.

"Because the conveyors are built with extruded aluminum framing, they are like tracks for a model train," says Gingerich. "You can easily add new sections."

Many options are available for moving pallets from one conveyor to another, says Joe McCollum, senior technical specialist with Siemens Dematic Material Handling Automation Div. (Grand Rapids, MI). Transfer modules lift and shift pallets from one conveyor directly across to another. They can be used at the end of a line or in the middle of one. Elevator modules are used in over-and-under conveyor setups, or to transfer pallets to another level of the factory. Curved modules or turntables transfer pallets between parallel conveyor sections.

There are also many options for stopping pallets. Cushioned stops bring pallets to a gentle halt. Lift-and-tilt and lift-and-rotate modules facilitate manual assembly operations. Lift-and-locate units raise pallets off the conveyor for automated processes. Dowel pins in the stops engage holes in the pallet for precise positioning. If the process exerts downward force on the pallet, such as a pressing operation, cams can be used to lift the pallets instead of pneumatic cylinders.

Hold-down rails can be attached to the sides of the tracks to prevent pallets from tipping, says Richard A. Shore, president of Automation and Modular Components Inc. (Auburn Hills, MI). These rails are used at stations where screwdriving or other operations might tilt the pallet. Another way to provide additional locating accuracy is with guide rails. Located below the conveyor track, the rails match corresponding slots underneath the pallet. The rails are tall enough to slide into the slots and stabilize the pallet, but not so tall that they lift the pallet off the strands.