There have been numerous attempts in the past, almost from the very beginning, to design a successful ball bearing system for small turbochargers. The minimal friction loss and rapid acceleration potential of ball bearings has made them attractive for automotive turbocharger application. Almost all double ball bearing systems that have been designed by various turbocharger manufactures have failed to meet necessary durability requirements. The one system that has met the necessary durability requirements is the Comp Turbo TRIPLEX CERAMIC™ triple ball bearing system. This system has now been in commercial production for over seven years and has established an enviable reputation for performance and durability. It continues to outperform competitive system in the field in Comp Turbo turbochargers.
The TRIPLEX CERMAIC™ bearing system utilizes three full compliment, angular contact ball bearings with ceramic balls. Two bearings are mounted back-to-back in the (cool) compressor end of a rotatable steel cylinder with one bearing mounted slidably against a pre-load spring in the (hot) turbine end of the cylinder. The steel cylinder rides on an outer lube oil film in the bearing housing that cushions the bearings against shock and vibration. All three inner races are clamped against a shoulder on the shaft and rotate with it. When the turbine end of the shaft expands axially due to heat conducted from the hot turbine wheel, the turbine end bearing can move axially with the shaft due to the sliding fit in the bearing carrier. Axial thrust is taken in both directions by the tandem bearings. The TRIPLEX CERAMIC™ bearing system is illustrated in Fig. 1.
Compared with steel balls, ceramic balls in ball bearings have proven to have a number of advantages. Bearing service life is two to five times longer and, since they are lighter and run at lower temperatures, allowable running speeds are as much as 50% higher. The surface finish of the balls is almost perfectly smooth so that friction losses are lower. They exhibit reduced skidding and have a longer fatigue life. These advantages have been established by a well-known ball bearing manufacturer and have been borne out by Comp Turbo turbochargers' operational experience. The minimal friction losses of the ceramic balls are enhanced by the absence of a cage in the full compliment bearings and the resulting mechanical efficiency of the system can approach 99%.
The acceleration rate of a turbocharger is a function of the rotor inertia and the frictional drag of the bearing system. Floating sleeve bearing systems have significant frictional looses due to oil film shear in the inner and outer diameter clearances and in the loaded and unloaded sides of the stationary thrust bearing. (See our Bulletin No. 1 for calculations of frictional losses in sleeve bearing systems.) Since the only frictional losses in the TRIPLEX CERAMIC™ bearing system are from rolling friction, the acceleration rate of the turbocharger rotor is maximized.
Sleeve bearing systems in turbochargers cannot operate without a continuous supply of lube oil from the engine. In very cold weather, lube oil becomes very viscous and, if it does not reach the turbocharger sleeve bearing promptly on engine startup, the bearings will fail. Comp Turbo Technology, Inc. has developed the first automotive turbocharger that does not require lube oil form the engine. The ball bearings are lubricated by a high temperature grease and they can be operated in very cold temperatures without problems.
Since there is no lube oil to drain back to the engine crankcase, the turbocharger can be mounted on the engine at a variety of angles, including vertically. Fig. 2 illustrates a Comp Turbo oil-less turbocharger mounted on a snowmobile engine in a vertical position.
Removing lube oil from turbochargers has additional advantages in that there can be no oil leakage past the turbocharger seals into the compressor and/or the turbine housing. Also, there is no lube oil in the bearing housing that can carbonize if the engine is shut down hot. Since there is no lube oil shear in the bearing system, frictional losses are reduced an additional amount from bearing systems that use lube oil.
The oil-less bearing system uses the identical parts used in the oil-lubricated TRIPLEX CERAMIC™ bearing system, except that the cylindrical bearing carrier is mounted in the bearing housing on two widely spaced "O" rings. The bearing housing is water cooled and the space between the "O" rings is open to the cooling medium. Thus, the O.D. of the bearing carrier is cooled to carry away heat generated in the ball bearings. Since the cylindrical bearing carrier is easily removed from the bearing housing, the ball bearing can be re-greased at appropriate intervals to extend their service life indefinitely.
The TRIPLEX CERAMIC™ bearing system is available in both the lube oil version and the oil-less version and represents a major step ahead in automotive turbocharging. Both systems are proprietary and protected by recently granted patents. These developments have projected Comp Turbo Technology, Inc. to be well ahead in ball bearing technology for turbochargers.