Bo+Bo

A Bo+Bo wheel arrangement is an electric locomotive, with two four-wheeled chassis or bogies with an articulated connection between them, with the drawbar forces taken through these bogies, and with all axles powered by individual traction motors.^[1]

Baltimore and Ohio LE-1 steeple cab in 1899

This is in contrast to the more common Bo-Bo arrangement where the two bogies are mounted beneath a shared frame, and the forces are taken through that frame.

Partial sectioned drawing of a small boxcab Baltimore and Ohio LE-2 [de] of 1903, showing the connection between bogies and drawbar

The type was important for early electric locomotives such as the first Baltimore and Ohio LE-1 [de] in 1899^[2] or the Italian E.430 of 1901. These had two separate chassis and half-cabs, linked by a pin connector. A canvas dodger joined the gap between the cabs.

Later designs resembled the Bo-Bo layout, where a single large boxcab spanned the two bogies and was pivoted upon them. As these pivots did not carry the tractive force, they could be of simple design.^[3]

LNER 6000, the prototype of the British Railways class 76, while on hire to the Dutch national railways in 1947

Typical mid-century Bo+Bo locomotives include the South African Class 1E and Class 2E or the Japanese National Railways JNR ED17 [ja]. Railways using Cape gauge such as the South African or Japanese narrow gauge systems with long locomotives found Bo+Bo advantageous over Bo-Bo as there was less overhang of the couplers and so less sideways misalignment on tight curves.

(Bo′Bo′)(Bo′Bo′)

ChME5 [de; ru] and the four-axled articulated bogie

In the 1980s an extension of the Bo+Bo concept was used by ČKD Praha in Czechoslovakia for the ChME5 [de; ru], a small class of a dozen large switching locomotives for the Donets Railway. Although of modest power, 2,000 hp (1,500 kW), they were heavy and also geared for a high tractive effort at low speeds; so they required good traction. Rather than more conventional three-axled bogies for a Co-Co arrangement, a four-axled bogie was required. This would have been an excessively long wheelbase and so hard on track curves. A solution was to articulate the bogie itself, making each bogie a Bo+Bo chassis.

The earlier Soviet ТЭМ7 [de; ru] and the high-speed TEP80 had already used a (Bo′Bo′)(Bo′Bo′) layout with sub-bogies. The ChME5 repeated this, and retained the large span bolsters and flexicoil-style secondary suspension of the ТЭМ7. For the ChME5 though, the vertical forces were taken through the over-bogie frames, but the tractive effort were taken by coupling the sub-bogies as a Bo+Bo.

References

1. Encyclopædia Britannica, Volume 18, 1969 Edition, p1118 Table IV, SBN(GB) 85229 004 7
2. Hollingsworth, Brian; Cook, Arthur (2000). Modern Locomotives. pp. 26–27. ISBN 0-86288-351-2.
3. Nock, O.S. (1984). "11. Non-steam developments on all four main line railways". British Locomotives of the 20th Century. Vol. 2 1930-1960. pp. 152–153. ISBN 0850595967.