Bavarian State 4-2-2 Locomotives in Germany

Class AA 1 (Locobase 20644)

Data from "A Locomotive with Auxiliary Gear", American Engineer, Car Builder and Railroad Journal, Volume 71, No 7 (July 1897), p. 229. (Many thanks to Alexander Blessing for his 10 September 2019 email describing this unusual locomotive and providing the cite for the journal.) Works number was 3900 in 1896.

Locobase 5922 describes the von Borries compound Eight-wheelers that entered production in 1896. Alexander Blessing alerted me to a one-off experiment that bid fair to solve a long-running contradiction. In steam locomotives, traction power increased in locomotives of equal cylinder volume and boiler pressure if the cylinders turned drivers with smaller diameter But locomotives with tall drivers worked more easily at high speeds because they turned fewer revolutions per mile traveled, reducing piston speed.

Krauss's Linz shops reconciled the two by dedicating different adhesion axles to each task. The B IX Eight-wheeled class had two axles of 1,870 mm drivers for high speed work. Linz kept one of those, but fitted a smaller, independent axle between the coupled set and the leading bogie. A pair of trailing wheels carried the firebox.

The "Vorspannachse" (often translated as "dolly axle", which usually means an unpowered set of wheels) was a bit more than that as it had a separate set of cylinders to power the smaller, 1,000 mm (39.4 in) drivers. Each cylinder measured 266 mm in diameter and had a stroke of 460 mm (10.47 x 18.11 in). Space for these under the compound cylinders was found by inclining the latter at a 1 in 16 slope.

Blessing's description of this second set of cylinders as "boosters" is apt, considering its primary role. (Boosters in later US practice used a geared steam engine in the trailing truck or tender to turn one or more axles to increase starting tractive effort.). The small drivers normally rode about 30 mm/1.2 in) above the rails thanks to a springing design linked to the main driving axle that pushed the axle up against stops.

A great deal of thought evidently informed the mechanism that engaged the auxiliary driving axle. A steam-powered piston measuring 410 mm (16.2 in) in diameter pressed the axle down to contact the rails; a ball-socket joint with bronze foot pressed against two plates. The force of the piston lifted the engine about 25 mm (1 in) while compressing springs by 50 mm (2 in), maintaining a constant pressure. Levers linking the axle with the main driving axle springs distributed the load.

The amount of adhesion thus secured was about double that of the main drivers alone. Of particular note to the American author was the speed range in which the auxiliary axle could be lowered. In one trial, the crew lowered the axle while running up to 75 kph (46.6 mph). The operation was "so smooth that persons standing on the engine and not watching the driver did not notice the change over." [last 5 words emphasized in the original].

Most of the time the auxiliary axle was lowered while the locomotive was at rest and it was used for only short distances. (Thus serving the same purpose as the later boosters.) Certainly the mechanism seemed complicated, but, argued the American, no more so than the de Glehn compounds used. Wikipedia acknowledged that the mechanism increased maintenance cost, but the engine in fact consumed less steam.

Even so, it was unique (possibly deserving a new Whyte arrangement as a 4-(2)-2-2) and when the AA 1 suffered an accident in 1907, the engine was rebuilt as a superheated Eight-wheeler; see Locobase 20645.