Needle bearings have rollers with low profile. The difference of a needle roller bearing and a roller bearing is the ratio of diameter and length of their rollers, when this ratio of the diameter and the length of roller of a roller bearing lies in the range of 0.4 to 0.1, then that roller bearing is called a needle roller bearing which are used to lessen the friction of a rotating surface.
These bearings have a large surface area which is in contact with the bearing’s outer surfaces compared to a ball bearing. Further, there is less added clearance (i.e., the difference between the diameter of the shaft and the same of the bearing) so they are much more small. A particular structure of a needle bearing consists of a needle cage which orients and contains the needle rollers themselves also, there is an outer race (which is sometimes called the housing itself). The Radial needle bearings are cylindrical and use rollers which are parallel to the axis of the shaft whereas the Thrust needle bearings are flat and use radial pattern of needles. The Full complement bearings possess solid inner and outer rings and some rib – guided cylindrical rollers. Since these bearings have the maximum possible number of rolling elements, they have extremely large radial load carrying capacity and also, are suitable for particularly compact designs. Needle bearings are widely used in automobile components such as transmission, rocker arm pivots, pumps and compressors. The drive shaft of a rear – wheel drive vehicle generally has atleast eight needle bearings, four are present in each U joint, and often more if they are particularly long, or operate on steeply inclined slopes.
Marks' Standard Handbook for Mechanical Engineers provides additional clarification on the topic of needle roller it says, a needle bearing is a roller bearing with rollers whose length is atleast four times its diameter.