More speed and accuracy, less scrap, fewer jams — these are just some of the factors leading design and plant engineers to consider servo drives for indexing applications, such as case packers, press, shear, and welder feeders, flying cutoffs, auger fillers, and high-speed diverters.

Where index distances and times change at almost every cycle — for example, where piece lengths change for every “pick” — a servo is worth its cost and programming effort. But a servo may not be the best choice to upgrade a drive that merely requires repeatable indexing to a setpoint.

Many indexing applications have been served well by open-loop drives using a clutch-brake, perhaps with different mechanical devices (arms or cams) to make periodic setup changes. But many of today’s production and quick-changeover demands exceed the capabilities of these drives.

Usually, the first step up in performance for these open-loop drives is closed-loop control of the clutch-brake with a PLC. But when PLC scan time becomes too slow or too variable for high repeatability at high cycle rates, most engineers think “servo.” However, servos offers features that may be unnecessary, and therefore too costly, for repeatable high-speed indexing.

A relatively new combination of an oil-shear clutch-brake equipped with an encoder and a closed-loop controller, Figure 1, solves the need for economical precision positioning system. Applications typically solved with servos can now be controlled with this new control system.

Applications

The combination of controller and oilshear clutch-brake has been applied to several high-speed, high-cycle indexing machines, including those that stack shingles and others that cut and seal plastic bags. Here are two examples.

Shingle catcher. In the roofing shingle industry, devices known as “autocatchers” catch and stack finished shingles into bundles for wrapping. The 36-in.- long shingles virtually fly into the catcher at conveyor speeds of 500 to 900 fpm, with an 18-in. gap between them. After catching a group of seven to eight shingles on two counter-rotating blades, called “star wheels,” the wheels must index 90 deg (±2 deg) in 60 to 90 msec to drop the shingles into a bundle-forming chamber, Figure 2.

PLC-based controls were unable to hold positioning accuracy of the blades at the high cycle rates needed. A new closed-loop, position-controlled oil-shear drive, developed in conjunction with Reichel & Drews Company Inc., the leading shingle industry OEM, has met this need in dozens of installations. The new drive indexes the blades at rates from 30 cpm for three-tab shingles to 180 cpm for laminated shingles. Accuracy must be maintained while the machinery warms up to operating temperature from ambient, which may be subfreezing because the equipment is usually located in sheltered, but unheated, areas.

The typical shingle catcher is timingbelt driven. The clutch output shaft usually runs at 933 rpm and is connected to a common shaft for the 4:1 ratio counter-rotating gearboxes that turn the two star wheels in opposite directions. Accel/decel times range from 0.02 to 0.04 sec. The kinetic energy per engagement is about 50 ft-lb, and the clutchbrake dissipates an average of about 0.50 thermal horsepower.

Feeding plastic bags in cut-and-seal machine. Cutting and sealing plastic bags from a web of material requires high-speed feeding with precision registration. Typical production rates are 120 to 180 bags per minute, with required indexing accuracy of ¹/₈ to ¹/₄ in.

Cut-and-seal machines use a variety of adjustable cranks, rack-and-pinion drives, and clutch-brakes to index the pinch rollers that feed material. These complex mechanisms are inaccurate and difficult to change for different bag lengths.

An oil-shear clutch-brake and closedloop position control retrofit package has proven to be a simpler, more reliable approach. With a timing belt connecting the clutch-brake to the pinch rollers, the controller gets its home signal from a photoelectric sensor that reads a registration mark on the web.

Drives that were retrofitted with this system typically use a 3/4-hp motor at 1,750 rpm, with a 1.175:1 timing belt ratio. The pinch rolls, operating at 1,489 rpm, index a 21-in. bag in 0.112 sec. Roll inertia is approximately 0.0306 lb-ft2, requiring 35 lb-in. to accelerate the feed mechanism in 0.050 sec. Bag length can be changed from the front panel of the controller.

Continue on page 2