In 1940 Britain was fighting for its life and a multitude of good and not so good ideas were put forward to develop new armored vehicles to meet the needs of home defense and the British Army overseas. Lincolnshire was a hub of development at the time from a variety of companies and one of them was the firm of Ruston-Bucyrus. Founded in 1930 the firm was a merger of the Lincolnshire firm of Ruston and Hornby (who had already had some experience with track laying vehicles in WW1 during the development of early British tanks) and the American firm of Bucyrus-Erie of Bucyrus, Ohio.
They specialized in vehicles for earth-moving and excavation mainly track based although there were a couple of walking machines. By 1940 they were already placed under contract to build some British Cruiser tanks but they also tried to develop their own expertise in cranes for a more military use and the vehicle chosen to prototype the design was an old rusting RB-10 grab crane.
Looking at the need for new tracked vehicles for the British Army, the Chief Engineer for Ruston's Bill Savage appointed Ruston-Bucyrus Designer Fred Staddon to take a spare old rusty RB-10, previously used on site for moving scrap metal on rail cars and make it into a crawler capable of carrying a field gun. Losing no time Staddon has drafted up this official sketch of his design:
In case you can't read what it says the sketch is dated 18/10/40 at the bottom and numbered 40RBL78 'Proposed Layout of MA Field Gun'
Now the big problem herein was not putting weight on the tracks - the 'chassis' was a very sturdy welded design and normally with crane boom and bucket weighed in at a full 8 tons. This design was still in production into the 1960's it was so useful and robust but it was slow, 2.5 miles per hour in top gear slow in fact. (In low ratio the slow speed is a blistering 0.8mph but gears aren't in game so we need not concern ourselves with that)
Despite being rated in crane terms as 'fast' this is more to do with the speed at which it could 'slew' (that is to rotate) and operate the bucket rather than movement on tracks. To solve this problem the engineers at Ruston-Bucyrus very cleverly 'gearing up' the propelling machinery to reach a nausea inducing 15 miles per hour. Not bad from a small diesel motor:
Like all this type of crane there is no suspension to speak of or even a hull as the 'turret' rotates on a central mount between the tracks, the engine is in the rear of the cab. (known as the 'Lincoln Cab' (square) style.
Given that this is the chassis (seen from a rather ungainly underneath angle - note the toothed ring is on the far side here)
has no suspension and that this geared up machine still retained this dog-tooth gearing system underneath it is hardly surprising it was described as 'virtually unsteerable' at this speed and dangerous to bystanders. One wonders how much more hilariously dangerous it would have become with the planned armored body and field gun.
Armour wise it's obviously not going to be well armored with minimal bullet proof levels of armor 5-10mm max. the Field gun also (look at the size of the crane) isn't huge and isn't mentioned but its 1940 - there aren't many field guns of the right size and weight to choose from such as a 25pounder for example.
Anyhow the project failed and was not adopted and this chassis was sent back to the yard to load scrap "to the dismay of those drivers who thereafter had to negotiate the concrete loading ramp with this errant machine".
This crane then seems to have vanished probably living out it mechanical life before joining the scrap metal itself in those rail cars. A shame really to lose a testament to the skill of the engineers at Ruston's and a piece of the history of British vehicle development at this key juncture in history when fighting for its life all manner of things were considered.
There is however the possibility that this machine could live again in world of tanks as either a low tier arty (maybe replace the horrible Birch Gun) or even as a TD (look at the depression from that gun -45? ) Certainly it would be dreadful to fire on the move but with a fast rotating turret and decent ground speed it more maneuverable and useful than many of its tier compatriots.
Engine: 33hp 3VRON 3 cylinder diesel engine upgradeable to a petrol unit
Speed: 2.5mph original to 15mph improved
Ground Pressure: 1440 pounds per sq. ft
Various websites related to crane manufacture
'Lincoln Excavators'- 1930-1945
This video shows the rotation speed of the cab (turret) and vehicle in motion: https://youtu.be/PVAADLz-u7U
http://i.imgur.com/tXylWig.jpg - RB-11 Instruction Manual