Hydrostatic transmission has a variable output (reversible) swashplate pump and a fixed swashplate motor. If the diesel engine dies and you try to tow the loco, the motor acts as a pump. It's tempting to think that if you just connect a bypass across the motor you can tow it willy nilly, but there is a complication. As well as the main variable swash pump there is an engine driven boost pump which keeps a minimum pressure in the system. There is a built in bypass on the motor, but without this boost pump running the manufacturer advises that towing at more than walking speed will cause irreparable damage to the motor. I’ve dreamed up a scheme with a battery operated boost pump, but I’ve no idea what flow is required, and it’s required to run for an hour or so to enable recovery. The supplier of the hydrostatic transmission is not being co-operative, probably because there is little or no profit in it for him.
Why do we want to be able to tow the loco? Well it’s a bit embarrassing to have an engine failure on a single track railway, and a considerable financial loss in ticket sales if you have to stop trains for half a day to mess about removing prop-shafts in the middle of no-where. Another possibility I’m looking at is a disengage-able gearbox between the motor and the final drive. I haven't yet found on that can accommodate two propshafts, one going to each bogie.. Dog clutches on the motor output is another possibility I’m looking at. At this stage I need to look at all options.
I’ve spoken to Alan Keefe, he uses hydrostatics and puts up with the inconvenience, or the JCB transmission I referred to earlier. I think that has 4 fixed ratios in either direction and a torque converter. It’s proving very difficult to get hold of anyone at JCB who knows what he’s talking about. First question they ask is what JCB digger is it fitted to, when I say I want to fit it to a locomotive they go silent. The factory which actually makes the gearboxes doesn’t even answer the phone, when you get on to head office they promise to ring you back, but don’t.
Shelagh originally had a hydrostatic transmission and a Ford 4D engine (I worked on it in the early 70s) It now has a more modern engine and a 4 speed fwd rev box and torque converter. It’s locked in 3rd gear I think as it is difficult to arrange a gearstick on a twin cab loco. The reverse is selected electrically. One of the drivers on the R&ER put it as 'it works, but it's always in the wrong gear'. That gearbox itself is probably 30+ years old. Just to be pedantic, Shelagh incorporates some parts of one of the original Heywood locos, but not very much. I spent a happy summer in my youth driving it. It didn't have servo assist on the swash control, you needed muscles on your muscles as the only way of slowing down was to heave back on the swash and engine brake. They now have continuous automatic brakes.
Thanks for the info on the 2 Romney locos. Something like the Twin Disc is what I’m looking for, but as far as I can tell they are well obsolete. It’s what is fitted to 2 other locos on the railway, one of which was the prototype for the RHDR, pretty much the same except only one gearbox on each bogie with chain coupling between the bogie wheels.
I’m waiting for Monday to phone the people Ady1 found, they seem to be agents for all sorts of transmission suppliers. So far I've found JCB, an Italian company (who are at least talking to me) and ZF, from whom I'm awaiting a reply.
Thanks for all the replies so far.