Needle roller bearings use rolling elements with a very small diameter-to-length ratio - typically 3:1 to 10:1 - creating a bearing with a very small radial section height relative to bore diameter. This compact cross-section allows the designer to support high radial loads in a bore where a conventional roller or ball bearing of equivalent capacity would require a significantly larger housing. The high roller complement and large contact area between rollers and raceways give needle bearings very high load ratings per unit cross-section area. Drawn cup needle bearings use the housing bore directly as the outer raceway - eliminating a separate outer ring and further reducing radial space requirements. Caged and full-complement designs are available - full complement carries higher loads but at lower speed limits due to roller-to-roller contact. Needle roller bearings require a hardened shaft surface or separate inner ring for the rolling elements to run against. Without adequate shaft hardness, the needles will rapidly brinell the shaft surface and cause rapid bearing failure. Combined needle roller and thrust bearing assemblies provide simultaneous radial and axial load capacity in a single compact package. Needle roller bearings are fundamental components in automotive manual and automatic transmissions, two-stroke engine cranks and small ends, printing machine eccentric mechanisms, textile rapier heads, hydraulic motor bores, compact industrial gearboxes, and all applications where the highest load capacity in minimum radial space is the governing design constraint.
