User:Huakehe/sandbox/Needle roller bearing

A needle roller bearing is a kind of bearing that uses needle-like rollers to provide anti-friction between components in machines that function under high loads and unstable condition, especially in automobiles and industrial machinery. In its design, the needle rollers provide a greater surface area in contact with the races, so they can support a greater load.

Unlike a ball bearing and other kinds of roller bearing s, a needle roller bearing employs rollers that are significantly longer and thinner, with length typically at least four times longer than its diameter. Commonly used in automobile industries and industrial machinery, needle roller bearings are capable of functioning under non-standard condition of high load capacity, high speed, and unstable bases. A highly accurate manufacturing process is required which raises the bearings' prices.

History
In 1925, George Hoffmann, a German engineer, patented "needle roller bearing," a kind of bearing that contains relatively longer and thinner rollers than cylindrical rollers. In his investigation on the properties of cylindrical roller bearings, he used longer and smaller-diameter rollers, which at full capacity of load, only rotates on their own axes instead of around the race (the track for rollers to move between the inner and outer rings). By discovering and employing this property, Hoffmann invented the needle roller bearings, which are largely used until today.

Apart from the conventional needle roller bearings, in 1955, R.H. White invented the thrust needle bearings, a special subcategory of needle roller bearings. When improving the early automatic transmission, White developed this kind of small diameter roller bearing to function as a longitudinal anti-friction between joint components of machines. Noticeably, it can undertake much more axial load than a regular needle roller bearing.

Components
The components of a typical needle roller bearing include:
 * an inner and an outer concentric cylindrical track with different diameters that form a race


 * multiple rollers whose length are generally significantly greater than diameter (rollers longer and thinner than cylindrical rollers) rotating between the two tracks
 * two side surfaces (or a cage) that fix the rollers functioning within the two tracks

Classification
Based on the design and arrangement of the rollers, needle roller bearings can be divided into three general categories: loose-needle bearings, caged-needle bearings, and thrust bearings. Needle rollers also have specialized designs to fit different usages.

Loose-needle bearings
Rollers are filled in the race (space between the inner and outer track) without a retainer. If working under high speed, this kind of bearing has the danger of loose-needle rollers falling out. However, when using on a hardened shaft that provides more stability, loose-needle bearings are more economical.

Caged-needle bearings
Rollers are fixed on a retainer between the inner and outer track. This kind of bearing can function under higher speed, since the cage reduces the radial load capacity of the needle rollers.

Thrust bearings
The head of the rollers are arranged to point towards the center of the two concentric tracks. This kind of bearing is designed for undertaking considerable axial load.

Rounded rollers
They are broadly used in various fields, since their two rounded ends provide smaller contacting surfaces with the side walls of the retainer (or the cage) that reduce friction.

Flat rollers
They have a longer effective load-carrying length, aiming to increase the bearing's load capacity.

Rollers with appropriate crowning
They can be used in a wide range of conditions by made to an appropriate degree of roundness at both ends.

Advantages:

 * Needle roller bearings have higher load capacity due to their larger amount of rollers comparing to other kinds of roller bearings and their larger contacting (loading) surfaces comparing to the ball bearings.


 * They are more suitable in functioning on a rough shaft, due to the rollers' advantageous larger area of support under oscillating or unstable movement


 * They have generally lower costs comparing to other types by price per pound of capacity

Disadvantages:

 * Loose needle rollers may fall out under high speed condition.
 * Needle roller bearings require high accuracy in manufacturing, since the needle rollers are small in diameter, and usually function under high loads.


 * They are comparatively expensive if using in a standard condition with low load and speed.

Application
There are two fields that largely use needle roller bearings: automobile industry and industrial machinery.

Automobile industry
Needle roller bearings are used in multiple components in engines, such as valve train, piston and crank, balance shaft, and compressor components, and etc.. In both automatic and manual transmission, needle roller bearings are employed to provide anti-friction under high loads. In several components in steering systems and drive-lines, they have the property of noise reduction due to their stability.

Industrial machinery
Needle roller bearings are used in a wide range of engineering fields including locomotive valve gears, gudgeon-pins of large compression-ignition engines, compressors and pumps, rolling mills, wind power generation, and etc..

Lubrication
Lubrication in needle roller bearings requires more attention than other types of roller bearings, since the large numbers of rollers increase the friction between the rollers and the tracks during high speed contact.

A Lubricant create an oil surface that separate the rollers and the raceway, and there are two categories of bearing lubrication: grease lubrication and oil lubrication. Grease is a suitable lubricant for bearings working at relatively low speed (the speed of the inner track less than 500 feet per minute), while oil is more preferable since it softens the frictional contact at all speeds.

Manufacturers
Companies that manufacture needle roller bearings include SKF, NTN Bearing, JTEKT, NSK Global, AST Bearings, RBC Bearings, and etc..