Automated Bakery Bets Its Dough on Oil-Shear
Clutch/Brakes, Wins Quarter Million Bagels per Month
30x Longer Life Reduces Maintenance Costs, Turns
Clutch/Brake Replacement Time Into Productive Uptime
APRIL, 1995 - Nine oil-shear clutch/brakes increase Sara Lee's bagel production by at least a quarter million bagels per month - probably more. So estimates Mike Baxter, maintenance superintendent at Sara Lee's Greenville (SC) automated bakery, where high-cycling, continuous "stop and start" indexing typically wore out each dry friction clutch/brake after only four weeks of service.
Replacing a clutch/brake takes at least 45 minutes, though the more difficult-to-reach units can take as long as 90 minutes. However, because the line follows a step-by step process dependent on the synchronization of all of its clutch/brakes for precise indexing, replacing any of the units means shutting down the entire line. For nine replacements per month, Sara Lee's monthly loss totaled 243,000 bagels.
To turn this downtime into productive time, Sara Lee has done away with nearly all dry friction units, replacing them with Posidyne* oil-shear clutch/brakes from Force Control Industries. Through a patented process, the oil-shear units use a film of oil as the "friction" surface, significantly reducing the heat and mechanical wear of engagement. The oil-shear clutch brakes survive Sara Lee's bagel line not for four weeks, but for as long as 2-1/2 years, cycling 30 million times or more without error. In addition, because oil-shear operation allows the two mating discs in typical clutch/brakes to be replaced with an integrated "stack" of several smaller-diameter, low-inertia discs, the clutch/brakes reduce the load on Sara Lee's drive motors.
"Even if productivity stayed the same, the oil-shear clutch/brakes would have improved our profitability," Mike Baxter notes. "They deliver 30 times the life, for a cost that's about four times what we used to pay for comparable dry friction units. By eliminating dry friction, we've saved money just on clutch/brakes alone."
Indexing is Key
As in any fully automated food processing line, repeatable indexing is key to consistent, efficient production of Sara Lee's bagels. The line begins with a batch of dough, and ends with cases of bagels ready for shipment to the company's markets in the Midwest and northeast.
Along the way, the dough is separated into balls, which are formed into bagels as they are extruded across a long spike, then cooked, cooled and packaged - all without any operator intervention, save for one manual "handoff" of a baker's peel board midway through the process.
At several points in the line, pairs of conveyors or other mechanical components must "mate" perfectly, with one component indexing a rank or file of bagels to where the next component expects to find it. At these points, a positioning error of even a fraction of a bagel diameter can result in bagels being lost or cooked improperly. To index the bagels accurately, each clutch/brake is constantly engaging and disengaging, to alternately halt and advance the various segments of the line - in some cases cycling as fast as twice per second, 24 hours per day.
The site of Sara Lee's first trial with an oil-shear clutch/brake was the bagel line's "bridge" conveyor. Running beneath two conveyors transporting four separate bagel "lanes," this conveyor bridges the line back together, after it temporarily diverges to simultaneously receive newly formed bagels from multiple extruders. Bagels drop onto the bridge conveyor from the two overhead conveyors in four parallel rows, which the bridge conveyor transports to a slotted peel board - a 22" x 17" target with four rows of six slots, accommodating two dozen bagels. Not only must this conveyor index precisely to ensure that all of the bagels find their slots properly, but it must move fast enough to match the advancement of the peel tray.
The clutch/brake at this point cycles every 1.5 seconds. Dry friction units lasted about two weeks in this service, but the oil-shear drive endured it - without error - for nearly two years. Following this success, Sara Lee added eight more oil-shear units, at positions including:
*The two parallel conveyors prior to the bridge conveyor. Both of these
clutch/brakes cycle twice per second, to keep the bagels evenly spaced as
they drop from the extruders.
*A robotic "grabber" carrying the individual bagels from the peel board to a
boiler. It takes a firm grip for the grabber to hold each bagel without
dropping it, but this requires the clutch/brake to position the grabber
perfectly, so a missed grab doesn't tear the bagel.
*A case erector in the packaging area. The case erector assembles cartons to
contain finished bagels that have already been packaged in plastic sleeves.
Each case holds 72 bagels, meaning the case erector must assembly as
many as 13,000 cases per day.
Multiple Savings with Oil-Shear
In operation, an oil-shear clutch/brake must squeeze a thin film of oil from between a series of alternating friction discs and steel drive discs before achieving full surface contact. These disc surfaces are dry in an ordinary clutch/brake. The oil absorbs the heat of engagement, which is dissipated through the housing. The oil also cushions the engagement and minimizes wear.
Actuation pressure can be provided by a compressed-air or hydraulically powered piston and/or springs, depending on the selected actuation logic. Sara Lee's units have air-actuated clutches and air/spring powered brakes.
Sara Lee uses five standard Posidyne* clutch/brakes, four of which are Model 02 units, providing 384 lb.in. dynamic torque capability, with cyclic inertia of 0.03 lb.ft.* and thermal capacity of 0.80 hp. The fifth unit is Force Control's new Model 1.5, which provides approximately the same dynamic torque, but with reduced inertia (0.012 lb.ft2) and increased thermal capacity (1.5 hp).
The other four units are Packaged Machinery Drives - a Force Control design that marries a Posidyne clutch/brake to a high-strength worm gear reducer within a unitized, sealed housing.
Posidyne* clutch/brakes range in capacity from 93 lb.in. to 24,000 lb.in. Basic thermal capacity ranges from 0.70 to 1.5 hp, and this can be significantly increased with fan or internal cooling.
Most of Sara Lee's clutch/brakes are direct-coupled to the output component. However, the Posidyne* 1.5 on the "grabber" drive uses a C-face output.
Sara Lee had been using air-actuated dry clutch/brakes, and the oil-shear units have lowered air and energy demands here, too. "It takes about 20 psi to engage each oil-shear clutch/brake, while a worn dry friction unit might have taken as much as 100 psi," Baxter says. "Since adopting the oil-shear units, we've reduced the load on our compressor."
One to Go
Baxter continues: "When Force Control first approached us, we were skeptical - the Force Control clutch/brakes carry a higher price tag, for a component we had come to view as a disposable commodity. However, the longevity we saw on the bridge conveyor unit was enough to change our thinking.
Once we reached the one-year anniversary with that unit, we undertook to upgrade all of our clutch/brakes to oil-shear. Now, the clutch/brake that drives a small sifter conveyor early in our process is the only dry friction unit we have left. Once that's replaced we'll be fully converted - and I've got a quote on my desk to do just that."
For more information, contact Stan Porter, Marketing Manager Force Control Industries, Inc., P. O. Box 18366, Fairfield, OH 45018. Phone 513/868-0900. Fax 513/868-2105.