Journal or sleeve bearings have long been used on turbochargers but lately improvements in ball bearing technology has made ball bearings a viable option. However, when it comes to choosing between a turbocharger with a journal or sleeve bearing and a turbocharger with a ball bearing, there's only one choice – the turbocharger with a ball bearing. In almost every aspect, the latter is the better choice. Understanding the advantages of the ball bearings over journal bearings requires that we first understand how the two bearings work. Let's begin with the traditional journal bearing system.
Journal bearings have traditionally been used on turbochargers. This bearing system consists of two short journal bearing at either end of the turbocharger's main shaft. These two bearing are usually copper bushes with oil holes. A thin film of oil is fed through the bushes to provide clearance between the bush and the main shaft, with the contact area between the bush and the main shaft being fairly large. In addition, a metal thrust bearing with a similar coat of oil is used to carry the thrust load on the main shaft.
In some designs the two bushes are replaced by a longer sleeve with a scalloped center area resulting in a very similar basic design.
The ball bearings on the other hand consists of either a single cartridge design with a row of tapered ball bearing on either end, or two separate ball bearing to support either end of the turbocharger's main shaft. These bearing are also lubricated by an oil feed but the actual rotating area between the main shaft and the bearing is greatly reduced. This means that less lubrication is required in the ball bearing design. In addition, the use of a tapered ball bearing eliminates the need for thrust bearings as the taper design carries the thrust load on the main shaft.
Advantages of Ball Bearings over Journal Bearings
From this description, we can easily see why ball bearings are better. Firstly, the ball bearings have a far smaller contact area and thus have a lower frictional drag coefficient. In turn, less frictional drag means better response and less power consumption. As a result, the turbo can spool up much faster, which reduces turbo-lag and offers a major performance advantage over journal bearing turbochargers at lower to mid turbocharger speeds.
And, secondly, the reduced contact area of the ball bearings means that it requires far less lubrication, allowing for lower oil pressure feeds. This makes the turbocharger more reliable in terms of lowering the chance of seal leakage, and reducing the chance of turbo failure during engine shut down. The elimination of the thrust bearing also makes the ball bearing turbocharger more reliable.