Single car unit No: RDB 975385 (formerly a Cravens parcels car No:M55997) was converted as a test bed for hydrostatic drive in 1980 and was known as 'Laboratory 9  Hydra'.

 Although the design philosophy of a rail vehicle fitted with hydrostatic transmission Hydra had already been recognised, the mechanical design considerations in producing an improved design of transmission for increased mechanical reliability coupled with the inclusion of a hydrostatic braking facility. These modifications have been incorporated in a re-built transmission, two of which have been fitted to the test vehicle “HYDRA” to create a totally hydrostatically driven vehicle.

The modifications also include fitting more powerful prime movers to provide a more rigorous test of the transmission, auxiliary circuits to power friction brakes and cooling fans, a hydraulically operated friction brake, a microprocessor controller for the engine/transmission/brake package and a computer controlled data acquisition system.
The hydrostatic transmission has undergone an extensive development and endurance running program of up to 42,500km. Very few mechanical problems were experienced, these being mainly due to defects in bought-in components. The pressurised reservoir was however underspecified due
to the presence of a pressure peak not previously detected, so a suitable replacement was designed. Endurance running revealed a weakness in the motor design resulting in increased internal leakage. The problems arose too late to allow a major investigation of the cause, but high standards had been maintained to avoid oil contamination and it is thought the cause could be the unusual conditions of use.

It was fitted with hydrostatic transmission having the normal Leyland 680 six cylinder engine driving two Rexroth Hydromatic axial piston pumps. Each pump supplied fluid to a Volvo fixed displacement compact axle-end motor which drove the axle directly. The maximum system pressure was in the order of 400 bar.

This arrangement was only fitted to one bogie, initially the other one retaining its normal drive but with the cardan shaft disconnected but as described above it was later modified

 The unit was finally scrapped by Vic Berry at Leicester.

Author's collection

A better version of the article's photo of Hydra in the RTC Yard

Author's collection

Hydra sits in the sidings outside the Advanced Projects Lab in the early 1980's

Author

And here it is at the Mickleover test track 

Colin Marsden

A nice colour picture - this is at Etwall station on the Mickleover line in August 1984

Tony Woof