Class A Roller Bearings (NW Mailing List)

NW Mailing List nw-mailing-list at
Mon Apr 9 06:18:48 EDT 2007

"Can anyone on the list enlighten a couple of NWHS members regarding how the
first style of driving axle bearings worked on the Class A?"


I assume you're referring to the "floating driving axle" arrangement used on
the early Class A's. According to the reprint of Jeffries' book on N&W
steam, these bearings were used on only the first few A's (I'm not sure of
the roaed numbers) and conventional roller bearings were fitted to later

In the floating driving axle arrangement, the roller bearing on each side of
the axle is located within the hub of the driver. The center race of the
bearing is fitted over a hollow, non-rotating tube. The tube passes through
(and may be a part of) the driving boxes, which fit into the frame openings
and support the locomotive's weight via the springs. The axle serves to
connect the two drivers together, transfer the torque from one side to the
other, and hold the drivers in alignment, but does not carry the weight of
the locomotive, and does not contact the roller bearings.

It was certainly an interesting arrangement, and allowed VERY large roller
bearings to be fitting to the engine. One thought I've had is this
arrangement may have been devised to allow roller bearings to be retrofitted
to non-roller bearing locomotives. Fitting roller bearings to steam
locomotives originally equipped with conventional bearings can be near
impossible due to the size of the frame openings for the driving axles.
(Read about problems retrofitting roller bearings to a locomotive on the
Smokey Mountain Railway a few years back for more information.) Roller
bearings require much more space than conventional bearings and the floating
driving axle arrangement would have been one way around this, although it
would have probably required new drivers for the engine.

Hugh Odom

The Ultimate Steam Page

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