SKF NA4904 Nadellager (20x37x17)

As a needle roller bearing, this bearing carries high radial loads in a very compact section. The dynamic load rating of 19.4 kN characterises this bearing. The static load rating is 22.4 kN. Where radial space is tight and the load is high — on connecting rods, gear shafts and pivot points — that is the reason for this design. The machined needle roller bearing carries load through a set of long, slender cylindrical rollers running on the raceways in line contact.

Because the needles are a multiple of their diameter in length, a large load-carrying length arises over a very small section height: the bearing position builds barely higher radially than the needle diameter and still takes a radial force for which a ball bearing would need a markedly larger outside diameter. The machined and ground ring is what sets this version apart from the drawn cup; it is rigid in form, dimensionally accurate and is fitted into a machined housing bore without taking on the raceway form of that bore.

Two points govern the layout. First, in versions without an inner ring the needle set runs directly on the shaft: the shaft surface is then the raceway and has to be treated like a bearing component in hardness, surface finish and dimensional tolerance; where that cannot be realised, a separate inner ring is used. Second, this design takes no axial force — such ribs as are present guide the needle set, not the shaft — so axial location calls for a solution of its own.

Because the contact pressure is distributed over a long contact line, misalignment has a stronger effect than with short rolling elements and forces the load out to the needle ends; shaft and housing bore must be aligned to one another accordingly. The open form leaves the needle set exposed for lubricant supply and heat dissipation and at the same time requires the construction to protect it against contamination.

The normal radial clearance is matched to common fits and operating temperatures. A material attribute is not held as a separate catalogue field for these items; the material information is to be taken from the specification table of the individual item.