TURBOCHARGER THRUST BEARING COMPARISON
The floating sleeve bearing systems used in many current turbochargers must include a stationary thrust bearing that carries rotor thrust loads transmitted to it from collars mounted on the rotor shaft. The design of the stationary thrust bearing is the result of many years of development to arrive at configurations that were capable of carrying the rotor thrust loads at high speeds without failure. Their capability to function satisfactorily at the very high speeds and high boost pressures required of turbochargers used in racing applications might be considered borderline.
The stationary thrust bearings are the source of the largest horsepower loss in the floating sleeve bearing systems. A formula for calculating the power loss in a stationary thrust bearing can be derived from basic theory resulting in the following expression for HP.
HP = .1363
N = shaft speed – RPM
R2 = outside radius of thrust surface – in.
R1 = inside radius of thrust surface – in.
C = oil film thickness – in.
(The derivation of the formula can be furnished upon request.)
Note that the HP loss is a function of the fourth power of the radii.
The dimensions of a typical stationary thrust bearing in a turbocharger running at 90,000 RPM producing a boost pressure of approximately 45 psig might be as follows:
N = 90,000 RPM R2 = .555" R1 = .338" C = .0005 in2
The calculated HP loss of the loaded side of the thrust bearing is:
HP1 = .1363 = 1.806 HP
Since the thrust bearing has an unloaded side with an assumed clearance of .004", the loss attributed to this side is:
HPUN = .1363 = .374 HP
Thus, the total loss of this thrust bearing is 1.806 + .374 = 2.18 HP which is significant. Since the loss is proportional to the square of the speed at 100,000 RPM, the HP loss goes up to 2.69 HP. The mechanical efficiency of the turbocharger using floating sleeve bearings and the typical thrust bearing described above would be approximately .969.
The Comp Turbo turbochargers use angular contact, full compliment ball bearings with ceramic balls. There is no cage necessary to space the balls, and any friction loss attributed to a cage is eliminated. In the patented TRIPLEX CERAMIC™ ball bearing system, the rotor thrust is carried by a single angular contact bearing that has a static thrust load capacity of 607 lbs. (Barden Catalog). Since the thrust loads in turbochargers usually range between 50 and 100#, the single angular contact bearing can carry these thrust loads easily within its load-carrying capacity, making it ideal for use in racing turbochargers. The TRIPLEX CERAMIC™ ball bearing system has a calculated loss of approximately 1 HP, making the mechanical efficiency approximately .991.
The power loss in a turbocharger bearing system has a direct effect on the rotor acceleration. The turbocharger with the highest mechanical efficiency will accelerate the fastest, such as the Comp Turbo turbochargers with the TRIPLEX CERAMIC™ ball bearings. These turbochargers also produce a higher boost pressure since the high mechanical efficiency results in more turbine power available to drive the compressor.
Experience the acceleration and the outstanding performance of Comp Turbo Technology racing turbochargers!