Journal Bearing vs. Ball Bearing Performance
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The journal bearing has long been the brawn of the turbocharger, however a ball-bearing cartridge is now an affordable technology advancement that provides significant performance improvements to the turbocharger.
Ball bearing innovation began as a result of work with the Garrett Motorsports group for several racing series where it received the term the &#;cartridge ball bearing&#;. The cartridge is a single sleeve system that contains a set of angular contact ball bearings on either end, whereas the traditional bearing system contains a set of journal bearings and a thrust bearing.
Journal Bearing
Ball Bearing
Turbo Response &#; When driving a vehicle with the cartridge ball bearing turbocharger, you will find exceptionally crisp and strong throttle response. Garrett Ball Bearing turbochargers spool up 15% faster than traditional journal bearings. This produces an improved response that can be converted to quicker 0-60 mph speed. In fact, some professional drivers of Garrett ball-bearing turbocharged engines report that they feel like they are driving a big, normally aspirated engine.
Tests run on CART turbos have shown that ball-bearings have up to half of the power consumption of traditional bearings. The result is faster time to boost which translates into better drivability and acceleration.
On-engine performance is also better in the steady-state for the Garrett Cartridge Ball Bearing.
Reduced Oil Flow &#; The ball bearing design reduces the required amount of oil required to provide adequate lubrication. This lower oil volume reduces the chance for seal leakage. Also, the ball bearing is more tolerant of marginal lube conditions, and diminishes the possibility of turbocharger failure on engine shut down.
Improved Rotordynamics and Durability &#; The ball bearing cartridge gives better damping and control over shaft motion, allowing enhanced reliability for both everyday and extreme driving conditions. In addition, the opposed angular contact bearing cartridge eliminates the need for the thrust bearing commonly a weak link in the turbo bearing system.
Competitor Ball Bearing Options &#; Another option one will find is a hybrid ball bearing. This consists of replacing only the compressor side journal bearing with a single angular contact ball bearing. Since the single bearing can only take thrust in one direction, a thrust bearing is still necessary and drag in the turbine side journal bearing is unchanged. With the Garrett ball bearing cartridge the rotor-group is entirely supported by the ball bearings, maximizing efficiency, performance, and durability.
Ball Bearings in Original Equipment &#; Pumping up the MAZDASPEED Protegé&#;s heart rate is a Garrett T25 turbocharger system. With Garrett technology on board, the vehicle gains increased acceleration without sacrificing overall efficiency and it has received many rave reviews from the world&#;s top automotive press for it&#;s unprecedented performance.
Journal or sleeve bearings rely on fluid films &#; usually oil &#; to support rotating shafts. Ball and roller bearings provide the same support of shafts through mechanical means. Both bearings are used to counter radial and axial loads. This article will briefly describe both types, then cite some examples where one type might be selected over the other.
Ball and Roller Bearings
A typical ball or roller bearing consists of inner and outer raceways, a number of spherical, cylindrical, tapered, or semi-cylindrical elements separated by a carrier, and, often, shields and/or seals designed to keep dirt out and grease in. When installed, the inner race is often lightly pressed onto a shaft and the outer race is held in a housing. Designs are available for handling pure radial loads, pure axial (thrust) loads, and combined radial and axial loads.
Ball bearings are described as having point contact; that is, each ball contacts the race in a very small patch&#;a point, in theory. Roller bearings have line contact rather than point contact, enabling them greater capacity and higher shock resistance. Rolling-element bearings do not have infinite lives. Eventually, they fail from fatigue, spalling, or any number of other mechanisms. They are designed so that on a statistical basis, a certain number are expected to fail after a set number of revolutions has accumulated. This defines the useful life of the bearing.
Shaft and bearing alignment play a critical role in bearing life. For higher misalignment capacity, self-aligning bearings are used.
To increase radial-load capacity, the bearing carrier is eliminated and the space between the races is filled with as many balls or rollers as will fit &#; the so-called full-complement bearing. Wear and friction in these bearings are higher than those using carriers because of rubbing between adjoining rolling elements.
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In critical applications where shaft runout is a concern &#; machine tool spindles, for instance &#; bearings may be preloaded to take up any clearance in the already tightly-toleranced bearing assembly.
Journal Bearings
Journal or sleeve bearings make use of a pressure wedge of fluid that forms between the rotating shaft and the bearing. The portion of the shaft supported by the bearing is called the journal and is usually hardened for wear-resistance. Bearing pad material is usually a softer material such as tin- and lead-based babbitt, bronze, copper-lead, sintered powdered metal, carbon, PTFE, etc.
As the shaft rotates in the bearing clearance it tends to want to climb up the bearing wall, creating a region of high pressure in the oil film which supports the shaft journal. As the load on the shaft varies, so too does this region of high-pressure, making journal bearings quite tolerant of overloads and shock loads.
A journal bearing that has developed this full hydrodynamic lubrication could run forever as there is no wear. Wear occurs upon start-up, however, since there is no supportive oil film when the shaft is at rest. Thus, soft materials are preferred for bearing liners, particularly materials with low frictional coefficients. Some designs rely on an auxiliary pump that pressurizes the bearing until the shaft can begin producing the hydrodynamic wedge.
Journal bearings are affected by the misalignment between bearing and shaft as its presence tends to hamper the formation of the fluid film.
Choosing a Journal or Rolling-element Bearing
Journal bearings generally have lower initial costs than rolling-element bearings. These savings may be offset if external equipment such as pressurizing pumps need to be used. Journal bearings require less radial space than rolling-element bearings but need more length axially.
Journal bearings are more capable of managing shock and overload compared with ball and roller bearings. They are also less prone to fatigue. They can run quieter than rolling-element bearings, especially when the rolling-element bearings begin to wear. Also, because oil separates the journal and the bearing, dirt and other particles have less impact on a journal bearing&#;s operation.
Rolling-element bearings, because of their low starting friction, are the preferred choice for intermittent operations and cold environments. They are much more adapted to handling misalignment between shaft and bearing with self-aligning designs available to increase this capacity.
While journal bearings exist which can carry axial loads, rolling-element bearings can be designed to handle radial and axial forces in combination.
Worn journal bearings often require rebabbitting and renewal of the journal surface. Rolling-element bearings can usually be replaced as units.
As an example, at one time railway cars used journal bearings on the wheel axles. The same load was applied to the bearing whether the car was moving or still. Often, multiple locomotives were necessary to get the train moving, while one was sufficient to keep it rolling. Rolling-element bearings have all but replaced journal bearings on railcar axles.
On the other hand, if the loading in an internal-combustion engine is considered, the use of journal bearings for the crankshaft, connecting rods, etc. makes complete sense. At start-up, only cranking forces exist. But as combustion commences, the shafts are coming up to speed, and the now-hydrodynamic bearings are able to successfully shoulder the increased forces acting upon them.
Summary
This article presents a brief discussion of the differences between journal and rolling-element bearings. For more information on additional products, consult our other blogs. We are a deep groove ball bearing supplier. If you are interested in our products, please contact us now!
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