Select figure to enlarge.

Making plastic trash bags requires a fast, reliable drive to operate a sealer at the end of a blown-film line. Unfortunately, one bag manufacturer’s system lacked both speed and reliability. After a retrofit, the system now has both speed and reliability. The higher speed alone increased productivity by 33%.

Previous system

To seal and perforate plastic trash bags, a 3-hp dc drive, through a 15:1 gearbox and a clutch-brake unit, rotated cams that move a 600-lb platen up and down, Figure 1. The platen measures 3 ft × 1.5 ft × 4 in..

The dc motor ran continuously while the clutch-brake engaged and disengaged to start and stop the cams. When the clutch is engaged, the cams start rotating, driving the platen down to seal and perforate a plastic bag. The cams then allowed the platen, which is forced up by springs and air cylinders, to return to the start position. The clutch disengaged, and the brake engaged, stopping the cams.

A photo-sensor detected a point on the plastic material and triggered the clutch-brake to start the cams moving. The cams were stopped when a proximity sensor detected that a cam completed a full 360-deg rotation.

Different plastic materials require different line speeds, which were set by the operators. In turn, the line speed determined the dc motor speed.

The machine was designed to operate up to 60 cycles per minute. However, when operated at 54 cpm, the clutchbrake failed about every 30 days, incurring a $700 repair cost for each failure.

Replacement system

Criteria for the new system called for an adjustable-speed drive that can change speed as the line speed changes and can cycle on and off to eliminate the clutch-brake.

The dc drive, clutch-brake, and worm gearbox were replaced with an adjustable- frequency inverter equipped with dynamic and dc-injection braking, and an integral gear 5-hp ac brakemotor. The brake is used only for emergency stopping.

Because the high-cyclic duty produces motor heating, the brakemotor also contains a separately powered blower that continuously moves a constant velocity of air over the motor regardless of motor speed.

The dc injection braking applies dc to the ac motor during the last eighth of the deceleration ramp. (For more information on this braking technique, see the article, “Selecting nonfriction stopping methods,” in this issue.)

The new system uses the same sensors. The photo-sensor initiates the sealing cycle, and the proximity sensor determines cam position to initiate the stopping sequence.

System sizing

To size a drive package for this application, requires determining the required torque, which in this application is based on both load inertia and the torque needed to compress the springs and air cylinders. However, calculations involving cam systems are complex, because the load inertia and load torque change during the cam’s rotation. To conserve time for this retrofit application, it is often beneficial to approximate some of the values.

(For more information on establishing inertia, see the Power Transmission Design Handbook.)

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