Plain bearing
A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on[3] or the ways on the bed of a lathe.
If an integral bearing wears out, the item may be replaced or reworked to accept a bushing.
[8] Common designs include solid (sleeve and flanged), split, and clenched bushings.
The flange is used to positively locate the bushing when it is installed or to provide a thrust bearing surface.
The most common method is a tab on the parting line edge that correlates with a notch in the housing to prevent axial movement after installation.
[citation needed] In general, the harder the surfaces in contact the lower the coefficient of friction and the greater the pressure required for the two to gall or to seize when lubrication fails.
Babbitt bearings are designed to not damage the journal during direct contact and to collect any contaminants in the lubrication.
[18] A common plain bearing design utilizes a hardened and polished steel shaft and a softer bronze bushing.
[19] A cast iron bearing can be used with a hardened steel shaft because the coefficient of friction is relatively low.
[20] In harsh environments, such as ovens and dryers, a copper and graphite alloy, commonly known by the trademarked name graphalloy, is used.
Solid plastic plain bearings are now increasingly popular due to dry-running lubrication-free behavior.
Solid polymer plain bearings are low weight, corrosion resistant, and maintenance free.
After studies spanning decades, an accurate calculation of the service life of polymer plain bearings is possible today.
Designing with solid polymer plain bearings is complicated by the wide range, and non-linearity, of coefficient of thermal expansion.
Plastic bearings are subject to the same design cautions as all other plastic parts: creep, high thermal expansion, softening (increased wear/reduced life) at elevated temperature, brittle fractures at cold temperatures, and swelling due to moisture absorption.
Plastic bearings are now quite common, including usage in photocopy machines, tills, farm equipment, textile machinery, medical devices, food and packaging machines, car seating, and marine equipment.
Common plastics include nylon, polyacetal, polytetrafluoroethylene (PTFE), ultra-high-molecular-weight polyethylene (UHMWPE), rulon, PEEK, urethane, and vespel (a high-performance polyimide).
[35] An example of a sintered metal bearing in action can be seen in self-lubricating chains, which require no additional lubrication during operation.
Another form is a solid one-piece metal bushing with a figure eight groove channel on the inner diameter that is filled with graphite.
If the plain bearing is not designed to run in the dry or boundary condition, it has a high coefficient of friction and wears out.
At low rotational speeds the lubrication may not attain complete separation between shaft and bushing.
The attitude angle and eccentricity ratio are dependent on the direction and speed of rotation and the load.
In some cases the frequency of the whirl coincides with and "locks on to" the critical speed of the machine shaft; this condition is known as "oil whip".
In this design, shims are installed between the two halves of the bearing housing and then the bore is machined to size.
The disadvantage of this design is its lower load carrying capacity, as compared to typical journal bearings.
The groove abruptly stops in order to create a downward force to stabilize the journal.
