Near-infinite-life seal contains shaft lubricant

St. Louis, MO-- Did you ever drink a SLURPPY(TM)? It's a frozen carbonated beverage mostly consisting of ice, water and syrup. The company that makes and sells SLURPPY wanted to improve its dispensing machines by extending the useful life of the machines' gearmotors. They needed a parallel-shaft gearmotor with a continuous duty life of five years and zero lubricant leakage.

Jason Hill, chief gear products engineer for Emerson Specialty Motor Division, realized that achieving the life- and zero-leakage goals could not be done with conventional lip seals. Limited to a life of about 5,000 hours, such seals could never attain the five-year (43,600-hr) lifetime required by Emerson's customer. And engineers certainly could not turn to costly exotic materials for use in a beverage dispensing application. Gearmotor shaft bearings represented another point of potential failure. Clearly, getting to the required unit life without using expensive anti-friction components challenged Hill and his associates.

During early experiments, Hill tried to use iron-graphite or PM bearings in the gearmotor housing, but these components wore out shafts. Joseph Tevaarwerk, a tribologist on Emerson's staff, suggested that Hill use a solid bearing (a brass bushing). Instead, Hill decided to try using diecast zinc as a hydrodynamic sleeve bearing for the shafts of his company's three-stage gearmotor. This approach worked. Then, using formulae developed by Tevaarwerk, engineers optimized the diametral clearance for the integral diecast zinc bearings, as well as the amount of bearing area needed to promote hydrodynamic operation. Lubricant type and viscosity was also an important consideration.

Other Applications
* Food processing
* Pharmaceutical
production

The bearings--simple zinc sleeves--are integral to the gearmotor's housing.
Sealed into the housing, an FDA-approved, edible, semi-fluid grease made by
KlYber Lubrication N.A., Inc., Londonderry, NH, lubricates the bearings. The
housing leaves the factory about 1/3 full of grease, and during operation the
tacky grease climbs the gear train to lubricate bearings and shafts. Ramps cast
into the housing and "windows" cast into the bearings employ gravity and
gear-train motion to passively promote hydrodynamic bearing operation.

Examine the illustration of the bearing system for a moment. As the gears (molded unfilled nylon and acetal components) rotate, the ramp scrapes lubricant off the gear face. Gear action, and gravity, direct lubricant through the opening cast into the bearing (the window). Hill places shaft loading on the bearing opposite the window. Wear debris flushes from the bearing and collects in the "pocket."

To prevent lubricant leakage out of the gear housing and into the motor, Hill and Tevaarwerk designed an anti-leakage device, a drop-in replacement for a conventional lip seal. Stationary in the gear housing, this molded acetal part has a diametral clearance to the motor shaft, and a truncated internal helix. The ball bearing on the motor shaft traps the part.

When the motor isn't running, the semi-fluid lubricant's surface tension is such that it either can't breach the shaft-to-device clearance or does so at a slow pace. When the gearmotor begins running, the helical groove in the anti-leakage unit channels anything in the gap, from the motor side of the gearmotor toward the gearbox sump. "Because there aren't any rubbing parts, seal life goes from about 5,000 hours to something approaching infinity," says Hill.

Finally, when Hill and his colleagues first became involved with the new dispensing machine, the customer wanted a new system with a 1/3-hp gearmotor. The existing dispenser used a .25-hp motor. Be-cause the custo- mer wanted to increase product tank size by 20%, they believed more power was needed. The customer gave Hill test data that showed beverage viscosity versus motor amperage on a competitive unit. "That turned on the light for me," says Hill.

He tested the competitive motor and developed an amps versus torque curve, which enabled him to relate motor torque to beverage viscosity. Doing so made it clear that the customer did not need a 1/3-hp motor. "In fact they really needed a 1/8-hp motor," he says. "That change to a weaker motor opened the floodgates, and enabled me to use plastic gearing."

A NEMA 48, four-pole, split-phase motor powers the gearmotor; it's rated at 230/220V, 50/60 Hz and operates at 39C. The three-stage gearset produces a reduction of 16:1; input speed is 1,725 rpm. Altogether, the plastic gears and innovative bearing and sealing techniques helped reduce gearmotor cost by 25%.

Additional details...Contact Stephen Robin, Emerson Specialty Motor Div., 8100 West Florissant Rd., St. Louis, MO 63136, (314) 595-8013.

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