The Klein-Lindner axle was an invention to allow a steam locomotive with a long coupled wheelbase to negotiate tight curves. While the distance between the flanges on a locomotive's wheels is less than the track gauge, that only gives so much room -- restricting either the locomotive's rigid wheelbase or the minimum radius it can safely negotiate. Try to push those limits too far, and the pressure of flanges against rails will cause the flange to hop the rail, and you have a derailment. Gauge widening on curves can help a little, but only so much. One could articulate the locomotive by breaking its driven wheelbase into several sections and powering each independently, but articulated locomotives are much more complex and slip much more readily than a rigid-framed locomotive.

Lateral motion devices were invented to allow the driving wheels to move from side to side in a limited fashion, but there is again a restriction on how much motion can take place, since the side rods do not allow much more than an inch or two of motion. The Klein-Lindner system is an attempt to relax those restrictions.

It consisted of not one but two axles; an outer, hollow one to which the wheels are affixed, and an inner one attached to the side rods and cranks, and supported by the locomotive's springing. The Klein-Lindner system, incidentally, requires an outside framed locomotive, as otherwise there is no way to support the inner axle.

There is a sizeable gap between the inner and outer axles, and this allows the outer to slide and rock relative to the inner. At the center of the inner axle, halfway from each end, is a ball-shaped bulge that fits fairly snugly within the outer axle. A protrusion or key fixed to that ball goes through a slot cut in the outer axle, and it is this key and slot that allows sideways movement but not rotational movement.

The outer axle is guided by being attached, through a sleeve and bearings, to an A-frame connected to a pivot closer to the center of the locomotive, restricting its movement to radial swinging only. Thus, the outer locomotive axles are allowed to guide themselves into curves. Some kind of self-centering is applied to damp yawing movements.

Thus, the Klein-Lindner axle allows for a quite substantial amount of sideplay and radial steering of the leading and trailing axles of a locomotive with a long coupled wheelbase. It is sometimes combined with allowing the center axles of the locomotive to slide sideways.

The system was commonly used on narrow gauge locomotives produced in continental Europe, especially Germany and Italy, including many locomotives for export. Common wheel arrangements on which this system was employed were 0-8-0 and 0-10-0 in the Whyte notation.

The problem with the system in general was that it made a formerly simple and unfortunately inaccessable component of the locomotive, the axles, much more complicated and prone to breakage, lack of lubrication and other problems. Probably for these reasons, it was not adopted by British or American locomotive builders, who tended to favor articulated locomotives for such applications -- the British Garratts, the Americans Mallets.

A number of locomotives equipped with Klein-Lindner axles are still in existence, including a fair number in museums and preserved railways, but a number of German-built locomotives still operate on sugar plantations in Indonesia.

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