Continuing with the Linear Motion theme, up next, bushings, bearings and other linear mechanisms are examined. Beginning with an explanation of bushings versus bearings.
Dynamic bearing systems are, at their most basic level simply an engineered hole/shaft interface that is designed to allow for relative rotation between the components. Typically one of the parts is completely fixed while the other part rotates against it. Bearing system designs can range from very simple to very complicated, with many moving parts and components employed to transmit motion across the system. A quick overview of the two most basic types of bearing systems and their limits, will serve as a great introduction to the wide world of bearings and rotational motion.
Journal Bearing (Bushing/Shaft)
The simplest kind of bearing goes by a number of different names, such as the plain bearing, the journal bearing, or the bushing bearing. This type of bearing system is simply an engineered running clearance fit that is typically constructed from a bushing and a shaft (or “journal”). This simple hole/shaft interface is designed to facilitate rotational motion of the journal within the bearing hole typically with an external lubricating compound designed to fill in the clearance spaces between the shaft and hole. In this way, there are no intermediary rolling or sliding elements that allow motion. Journal bearings are typically built nowadays by leveraging advanced materials technologies such as oil impregnated bushings in order to build a joint that has a low rolling/sliding resistance friction associated with it.
These bearings are very simple to construct, but they are almost always employed alongside a comprehensive, reliable, active lubrication system. There are purely “dry” (no lubrication) journal bearings that can be specified for low load, low speed applications. These types of bearing systems typically employ polymer bushings that can be specified, but most industrial applications that use journal bearings are fully lubricated shaft/bushing systems. It is important to note that these systems must be located in areas of the machine that are reasonably accessible for preventative maintenance or repair if needed because the successful operation of these journal bearing systems are highly contingent on the presence of saturating lubrication at every single rotation of the shaft within the bushing. A failure of the lubrication system typically leads to fast, catastrophic failure of the entire machine.
If an active lubrication system is not feasible for a particular application that must sustain high speeds, as well as variable dynamic loading, it is recommended that a rolling contact bearing system design be considered.
Rolling (Ball) Bearings
A bearing is a mechanical component that usually possesses a rolling or sliding component that further reduces the friction between the moving shaft and the stationary hole or reference point. The most common and easily identifiable type of bearing is probably the ball or roller bearing. These types of bearings are comprised of an inner raceway that is usually directly (press) fitted to the inner shaft. The rolling elements (balls, or conical rollers) then roll along the opposite side of the inner raceway, and they are all contained within one unit by the outer raceway. It’s not difficult to visualize how this system allows relative motion between the inner and outer raceways, via the relative rolling of the rolling elements. The rolling elements in the bearing cartridge can be lubricated and sealed completely in between the inner and outer raceways of the bearing in order to prevent any exposure from environmental contaminants
The rolling elements of the ball bearings can be fabricated from a hardened tool steel alloys, and they can even be specified with additional coatings (such as Chrome) in order to further increase the wear resistance and corrosion resistance of the ball bearing. Stainless steel ball bearing systems can also easily be obtained if corrosion or degradation is a concern in the particular application. If significant thrust loads must be sustained, whereas a journal bearing would be almost useless, a cylindrical or needle rolling bearing system is the ideal bearing for the job and can readily be specified to meet a wide range of applications.
For lower cost applications where the bearing can be readily accessible for lubrication and maintenance, a journal bearing may be a perfectly suitable design and can be employed in many types of applications. However, if accessibility to the bearing would be impossible, and/or the bearing system must sustain radial high loads or thrust loads, the design engineer should proceed and begin exploring the wide world of rolling contact bearings. Either way, the odds are very good that a suitable bearing is out there waiting to be unearthed for each and every variant of mechanical application.
Thus begins our 4 part series on Working with Linear Bushings. Stay tuned for part 1, Working with Linear Bushings: Linear Guide Mechanisms!