|
|
|
|
By: Marv Dehne Here at SL-Montevideo Technology, Inc., we manufacture motors for a wide range of applications, including windshield wipers on commercial aircraft, industrial robots, medical equipment, manufacturing equipment, etc. We recently faced a problem in lubricating the bearings used on a diesel turbine fuel atomizer motor operating at 20,000 rpm. Ten different types of standard bearings and four types of lubricants were tried on the application, but none would withstand the high speed and temperature of the application for more than a short period of time without seizing. The brush-type DC motor is used on a tank engine and has an operating temperature range from minus 65 degrees F to 300 degrees F. The high temperatures are caused by the fact that the fuel atomizer motor is located right next to the gas turbine motor used to power the tank. The tolerances required for these bearings are exceptionally tight because the motor has a balanced armature. Engineers tried a range of bearing designs, including ribbon and crown retainers. Each time, the heat generated in the application quickly broke down the lubricant, and the bearings would seize. Our difficulties were attributed to the fact that the cage, as classically designed in rolling bearings, imposes very large relative loads on the balls or rollers at high speeds. With plentiful oil or automatically-replenished grease lubrication offering large damping capability, these loads can be tolerated without damage to the bearing. But with a single initial charge of grease, or with solid lubrication, these designs are often disastrous. The problem was solved by switching to a new bearing with a unique cage design. It has reduced clearance at the guide land to promote film formation and increased clearance at the pockets to reduce bearing-pocket forces. Thanks to the new bearings, which reduce ball-cage contacts, lubricant life has been dramatically increased: the new bearings have lasted for two years in the field with no noticeable wear. "The whole idea is the geometry of the bearing cage," explained Lew Sibley at Tribology Systems, Inc., in Paoli, PA, the bearing's designer, "so there is a minimum of interaction of the balls." Using modern computer bearing dynamic analysis, Tribology Systems computed the motions and forces of all the elements in a bearing in very small time steps to establish the basic dynamic stability of the bearing, and the wear rates of the solid-lubricant or boundary-lubricant films and other wearing parts in the bearing. This analysis showed that even at moderate speeds, large cage forces in standard design ball or roller bearings excessively stress the lubricant films in the bearing and cause large cage stresses. These stresses can be significantly reduced with special designs of the cage and other bearing components, which increases lubricant life and prevent the failure of cages that occasionally fracture in service. The SL-MTI motor used the patented AEROFLOAT cage design from Tribology Systems, which uses advanced aerodynamic lubrication theory to make the cage have as little intrusion as possible on the dynamic motions of the balls. The bearings accomplish this by 1) increasing the cage pocket clearance in relation to the ring guide land clearance by a factor much greater than that used for classical cage design in the bearing industry, and 2) making the tight-clearance land surface with a converging film shape of sufficient width (according to hydrodynamic lubrication theory) that it provides a significant bearing load capacity with extremely low friction coefficient. The cage literally floats in the bearing, and exerts a minimal amount of interaction with the balls, according to fully dynamic bearing analysis, but still maintains separation of the balls as required in high-speed applications. The wear pattern on the cage guide land surface of bearings shows contact only at cage bar locations, indicating the significant centrifugal warping of the cage structure. This warping at high speeds automatically forms a "converging wedge" configuration that generates the air or grease-lubricated hydrodynamic film. The cage is typically unstable at low speeds, when the boundary lubricated guide land friction is high, but becomes stable at high speeds when this warping generates a low-friction gas-lubricant film on the cage guide surfaces. The speed of this application is high enough and the composite solid-lube cages are flexible enough that this same type of warping occurs and is enhanced by wider guide land surfaces than in standard design bearings, so that a hydrodynamic grease-lubricant film is generated at these tapered land cage guide contacts to float the cage with very low sliding friction. Narrow-bearing hydrodynamic lubrication theory predicts that the load capacity of this film increases as the cube of the bearing contact width, so that a significant hydrodynamic film thicknesss can be generated at cage guide lands by increasing their width only by a factor of two or three, especially if the pocket clearances are also increased so that the loads on the cage are drastically reduced. For SL-Montevideo, the new bearing technology solved a difficult high temperature, high speed bearing application problem. This new bearing technology has also been proven in other high speed motor applications, flywheel primary bearings, gas turbine engines, diesel engines, various automotive applications and flywheel back-up for magnetic bearings on touch-down. Applications for which the technology is being considered include rolling mill bearings in the paper, aluminum and steel industries and paint facility and clean room environments, overhead trolley bearings in production facilities, and in military cruise missile engines. That's the good news. The not-so-good news is that these bearings are not yet in commercial production, and only a few hundred of them have been placed in service, according to Sibley. The bearings so far have been custom-tooled in small batches because bearing production runs typically start at 30,000, and demand has not yet justified such an investment. The bearing's cost and availability should improve with time and demand. Marv Dehne is purchasing agent with SL- Montevideo Technology, Inc., in Montevideo, Minn. For more information about this article, he can be reached at (320)269-5583. For information about the patented Aerofloat bearing and cage design, contact Lew Sibley, Tribology Systems, Inc., 239K Madison Ave., Warminster, PA, 18974. Phone (610)889-9088. _______________________ Reproduced with permission of LUBES 'N' GREASES magazine, October 1996. |
||||
|
|
||||
|
©2000 Tribology Systems, Inc. All rights reserved. |
