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.
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 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.
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!
Filed Under: Bearing Tips