September 21, 1998 Design News

DESIGN APPLICATIONS From the regional editors

Spherical roller bearing solves longevity problem

Eccentric locking collar offers greater shaft holding power

Charles J. Murray, Senior Regional Editor

Wokingham, U.K.--Even under the best conditions, vibration can wreak havoc on a machine's bearings. But when those bearings are also subjected to dust, dirt, tangled vines, and acidic tomato juice, their chances for a long, reliable life plummet.

When engineers from Johnson Farm Machinery Inc. (Woodland, CA) recently redesigned a tomato harvester, however, they needed to find a way to enable their machine to deal with all those elements. The tomato harvester, which gently separates tomatoes from the vine, employs a shaker mechanism that simultaneously vibrates and rotates.

Problem is, rotation and vibration can shorten the life of a conventional bearing. To meet the needs of the machine, Johnson engineers sought two basic design elements: an efficient method of locking the bearing to the rotating shaft; and a bearing design that offered longevity. Previous bearings used on the machine lasted no more than a single season, and Johnson engineers wanted to extend that time.

To accomplish that, they employed a spherical roller bearing with an eccentric locking collar. The locking collar, known as Twist Lock, is believed to be the only one of its kind domestically available in a spherical roller bearing.

The Twist Lock technique, available on the Rex 3000 Series Twist Lock spherical roller bearing from the Rex Division of Rexnord Corp., offers significantly more shaft holding power than set screw techniques. Instead of employing a collar that wraps around the OD of the inner race, it uses a collar that has the same diameter as the inner race. As a result, the collar actually contacts the shaft.

The key to the Twist Lock design is a pair of cam surfaces--one on the inner race, the other on the face of the collar. When the bearing is installed, the user twists the collar, causing the mating surfaces of the cams to move in opposite directions. This creates a strong clamping force on the shaft. A set screw on the other side of the bearing provides additional holding force.

Another reason for the bearing's greater holding power is its contact surface area. Instead of contacting the shaft at two small points, it spreads the contact over a much broader area. "With the cam lock, the collar actually contacts the shaft," notes Jim Baker, a product specialist for the Rex Bearing Division of Rexnord Corp. "Instead of just the tips of the set screws making contact, you have about 50% of the collar actually making contact. So you have about 10 times the contact surface area."

Equally important, the Twist Lock technique combines with the spherical roller bearing design to provide greater capacity and longevity. Baker estimates that the spherical roller bearing offers 3.4 times the dynamic capacity of corresponding ball bearings. Although the new bearings provide clear operating advantages, it is often difficult to change a design once it's in the field. To simplify replacement of old ball bearings during retrofit and avoid redesigning the machine to accommodate the new bearings, Rexnord engineers designed a special casting to produce a modified, piloted flange housing that bolted right in place of the old ball bearing unit. This allowed Johnson to upgrade the bearings without a dimensional change in the equipment.

While the previous bearings needed annual replacement, the initial Rex bearings have offered more than two years of satisfactory operations. When the bearings were inspected after two years of tests, engineers found them to still be in good condition. Johnson has since standardized on the bearing and a heavier duty seal to ensure longer service. 

Additional details?Contact Russell Dick, Rexnord PT, Kingswood House, 12 Shute End, Walkingham, Berkshire, U.K. RG40 IBJ (P).

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