Needle Roller

Needle rollers are the load-carrying component of the bearing, and are the one common component of all needle roller bearing types. These allow the outer and inner rings to rotate almost friction-free. They are most frequently manufactured from a steel alloy.

Raceways (if present) serve to guide the rollers within the bearing. While the inner and outer rings are designed to fit closely, a necessary minimal amount of clearance is required to account for thermal expansion and other variances. These items are commonly manufactured from carbon chromium steel or other alloys to improve hardness, fatigue resistance, and dimensional stability.
Bearing cages separate rollers to keep the load evenly distributed around the bearing. They also reduce bearing noise, improve rolling conditions, and prevent sliding. Additionally, they contain rollers within a single assembly. Steel is the most commonly used cage material in needle roller bearings. Plastic cages can be used where operating conditions permit. Lubrication and surface treatments can reduce the amount of heat generated from friction. Some drawn cup needle roller bearings are designed without a cage, and contain a full complement of needle rollers. Full complement bearings combine maximum load carrying capacity with the low cost of a drawn cup bearing. An inward turned lip is used to retain the rollers in the bearing ring in this design. Testing has established the following coefficient of friction values for caged and full complement bearing designs:

Seals (if present) isolate the rollers, cage, and lubricant from contaminants and moisture in harsh environments. Seals must be able to withstand the oxidation, heat, and chemicals to which the bearing may be exposed. Sealed bearings will often supply an inner ring access hole in the outer ring, so the bearing can be relubricated. Seals are highly reliable when friction between the seal and bearing or shaft is kept to a minimum. Additional surface treatments or lubrication can be used to reduce this friction. Bearings with integral seals are limited to relatively low rotational speeds compared to open bearings. Seals are commonly made of acrylonitrile-butadiene rubber, polyurethane, or fluororubber.

Lubrication is required for all needle roller bearings. Annular grooves and a lubrication hole are may be incorporated into the outer ring of most heavy-duty, machined needle roller bearings raceways to help the lubricant proliferate the circumference of the bearing.
Synthetic oils are most common for quality bearings. Mineral oils are appropriate for high-speed use. Synthetic oils are good for moderate to high speeds. Petroleum exhibits good lubrication under heavy-load/high-speed conditions. Silicone oils offer good heat resistance and do not corrode rubber, but are more appropriate for low speeds. Oils can be dripped, centrifuged, or impregnated into bearings.
Greases are best for moderate to high rotation speeds. Typically, the high-speed torque of a greased bearing is lower than an oiled bearing, but the opposite is true at lower speeds. These are usually applied via grease pack, but grease plating is also available. Silicone greases offer good heat resistance and do not corrode rubber, but are best for low to moderate speed applications.
Dry films should only be used where 'wet' lubricants will prove unsuitable or will accumulate large amounts of dirt. Dry films are difficult to apply and produce wear flakes as the bearing rotates, possibly obstructing bearing operation. Non-contact seals rely on the gap control between rotating and non-rotating components. The gap can be positioned radially, axially, or a combination of the two. Greases may be placed within this gap to reduce contaminant permeability. Since there is no contact between components, there is no friction which must be accounted for in the seal's implementation. Non-contact seals are preferred for high RPMs. The most common type of non-contact seal is a labyrinth seal, and many on these seals are constructed of steel or another rigid metal.