Moving heavy, unguided loads — vertically

Feb. 1, 2000
You might not think of a rodless cylinder for vertical linear motion applications, especially if those applications involve heavy loads. Here’s one, however, that can handle the load.

Recent versions of rodless cylinders now move heavier unguided loads, even if they are off-center, while providing the smooth, fast linear motion these devices are known for.

One of the latest designs comes from Tol-O-Matic Inc., Hamel, Minn., with the BC3 pneumatic rodless band cylinder, Figure 1. It uses a recirculating ball-bearing system to support the load. This ball bearing system also offers several advantages:
• The cylinder needs less force to start its motion (low breakaway friction point.)
• The bearings can handle heavy loads, up to 1,091 lb, because load weight is distributed among the bearing balls.
• Reduced wear enables the cylinder to consistently track linear movement over the life of the cylinder. Internally, the cylinder

has a tube with a piston that’s free to slide back and forth. In a mid-section of the piston, a bracket rides through a narrow slot in the top of the tube, connecting the piston to the carrier. Inner sealing bands along the tube prevent air from escaping and contaminants from entering the cylinder.

The carrier portion of the carrier-piston arrangement rides on the recirculating ball-bearing system, which rides on bearing rails that hold this assembly in place. The bearings are preloaded to eliminate excess carrier deflection when subjected to loads.

At each end of the cylinder are ports for the compressed air, which can be to 100 psi. Speed depends on pneumatic pressure and the load.

Current bore sizes are 1, 1-1/2 , and 2 in. Bending moment capacities for the 1-1/2-in. bore range from 583 to 1,009 lb-in., Figure 2.

Going up

This type of rodless cylinder suits applications involving off-center loads, such as this rotating pickand- place elevator, Figure 3. The cylinder mounts to a rotating platform. A gripper arm, attached to the cylinder’s carrier, picks up cast blocks from a nearby conveyor. The cylinder raises the gripper arm, then the platform rotates to the other side where the arm releases the block onto a feeder conveyor.

The support arm attached to the cylinder, weighs 25 lb and is 12 in. long. The gripper on the end of the arm weighs 3.5 lb. The load the gripper picks up weighs 10 lb. Thus, the cylinder must support a moment of 462 lb-in.

The stroke is 24 in. At a velocity of 3 ips, it takes 24 sec for a complete cycle: 8 sec down, 8 sec up, with a rotation time of 4 sec (8 + 8 + (4 × 2) = 24 sec). Using a rodless cylinder in this application eliminates external guide shafts and bearings.

As shown in Figure 4, these cylinders also function in horizontal applications. In this pick-and-place, the cylinder mounts on its side to a stationary member. A rod cylinder slide attaches to the cylinder. The rod slide supports a vacuum grid, which picks up a 4 × 8 ft plastic sheet weighing 25 lb.

The cylinder moves the rod slide, vacuum structure, and plastic sheet over to a conveyor. The sheet is set onto the conveyor and proceeds down the production line. The rod cylinder slide weighs 7 lb, the vacuum structure 30 lb. The cylinder must move a total weight of 62 lb a distance of 96 in. The bending moment at the end of the stroke (Mv) is 495 lb-in. The Ms value is 55 lb-in. (1 in. × 55 lb).

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