Why is it called an idler pulley? Because it doesn't have a load to pull. It just goes round and round, happily and idly along, not pulling a load and doing no work at all.

sorry but it is that simple,, i worked in the industrial machine ind. for 37 years,, i have bought bearings that cost $22,000.00 all the way down to $2.00 ,, the bearing houses know what they are doing just take a bearing in and fine an old guy( the young ones don't know sh** yet, tell then what you want to use the bearing for and tell them you want to upgrade the bearing,,there are 4 to 5 up grades the higher you upgrade the more it cost and the better it is,,jim
PS
Ball bearings, , are probably the most common type of bearing. They are found in everything from inline skates to hard drives. These bearings can handle both radial and thrust loads, and are usually found in applications where the load is relatively small.
In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it spin very smoothly. But it also means that there is not very much contact area holding that load,

Roller bearings like the one illustrated below are used in applications like conveyer belt rollers, where they must hold heavy radial loads. In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load out over a larger area, allowing the bearing to handle much greater loads than a ball bearing. However, this type of bearing is not designed to handle much thrust loading.
A variation of this type of bearing, called a needle bearing, uses cylinders with a very small diameter. This allows the bearing to fit into tight places.
Cutaway view of a roller bearing
Photo courtesy The Timken Company


Ball Thrust Bearing

Ball thrust bearings like the one shown below are mostly used for low-speed applications and cannot handle much radial load. Barstools and Lazy Susan turntables use this type of bearing.
Ball thrust bearing
Photo courtesy The Timken Company


Roller Thrust Bearing

Roller thrust bearings like the one illustrated below can support large thrust loads. They are often found in gearsets like car transmissions between gears, and between the housing and the rotating shafts. The helical gears used in most transmissions have angled teeth -- this causes a thrust load that must be supported by a bearing.