Conveyor belts handling fragile products must, above all, be gentle. And for stepping conveyors with high stop-and-start cycling, the braking must be especially so. Such was the case for a maker of ornamental sheet glass cutting machines, which are used in window manufacturing plants, glass distributorships, and custom glass fabricators.
Glass sheets, ranging in size from 2 in. to 2 ft across, lie flat on a wide rubber conveyor belt while a programmable X-Y cutting head creates the required pattern. The brake must keep the belt absolutely stationary during cutting or the piece will be ruined. When the piece is completed, the brake releases so the belt can index forward to feed the next sheet into place for cutting. Then the brake resets. The belt’s indexing cycle typically ranges from 20 sec to 1 min, so a lot of stopping and starting is involved in the continuous glass-cutting operation.
Early mechanical designs jerked the belt to a stop and were a leading source of service calls for the machine as a whole, not only the brakes. Jerky stops also caused the glass sheet on the conveyor to shift, creating scrap and wasting material. To achieve gentle motion, the company, assisted by Pawling Corp., Engineered Products Div. in Pawling, N.Y., switched from a mechanical brake to a pneumatic brake.
The pneumatic brake was chosen based on success in other conveying applications, as well as its dual pressurizing connections that deliver fast action. With a predicted life of 40,000 cycles, pneumatic stops typically outlast the machines on which they are installed. And because they need no servicing, they can be put anywhere without regard to maintenance access.
The brake operates on a drive belt that runs underneath the main conveyor belt and moves it by friction. It consists of a long hollow sealed tube of reinforced EPDM, with a mounting foot molded in, which runs axially underneath the drive belt. Called Presray Pneuma-Seal, the tube is inflated and deflated on command through two pressurizing connections. When inflated, the tube expands to engage the underside of the drive belt, gently but quickly stopping and holding the entire system in position. When deflated, the tube’s top surface recedes 3/8 in. — enough clearance to free the drive belt to index the conveyor belt forward. An override clutch disengages the motor during the braked condition. Other than expansion and contraction of the cross section, there are no moving parts.
In addition to the original goal of gentler motion, the switch in braking systems improved reliability, saved space in the clutter underneath the conveyor, and cut cost by about $500 per machine.
For more information: Pawling Corp., Engineered Products Div. (845) 855-1000 www.pawling.com