Class 15X and Class 14X gear changes

[Aictos]

While I'm not too sure on the Class 14X fleets, I believe that the Class 15X fleets have 7 notches so when the driver changes up or down a power notch, what speed should they be at?

 

[driver9000]

Whatever speed is necessary to maintain control of the train. The transmission changes around 45mph and doesn't need power to be reduced. Sometimes you can happily cruise along in notch 3 other times you'll be in notch 7. It all depends on what you need from it.

 

[Llama]

The Voith transmission on 14x and 15x (not 158/159) units changes 'up' from using the torque converter to the fluid coupling at between ~30 (power notches 1-3) and ~45mph (notch 7) depending on the power notch selected. If traction power is taken yet the speed drops, the transmission will 'change down' to the torque converter at approximately 3-5mph lower than it would change 'up' at, so that the transmissions don't 'hunt'.

 

[The Lad]

It's a automatic gearbox. Proper DMUs had a more conventional gearbox which needed changing at 15/27/41 mph

 

[anamyd]

Delete

 

[507 001]

It's a automatic gearbox. Proper DMUs had a more conventional gearbox which needed changing at 15/27/41 mph

No it’s not.

 

[anamyd]

It's a hydraulic transmission.

 

[37057]

Which happens to operate automatically and is a box that contains gears then, no..?

 

[Nick Ashwell]

There are no gears in a hydraulic transmission like in a normal car gearbox

 

[37057]

They're a different type of gearbox but it'd love to know how a hydraulic transmission works without some form of gears or toothed wheels!

 

[anamyd]

They definitely still have gears in them. Something like 3 on the 2-speeds

 

[Aictos]

Thanks for explaining all, that's one more thing I've learnt

 

[hexagon789]

It's a automatic gearbox. Proper DMUs had a more conventional gearbox which needed changing at 15/27/41 mph

172s have an automatic gearbox; 14x, 15x, 16x, 170-171, 180 and 185 all have hydraulic transmissions.

They definitely still have gears in them. Something like 3 on the 2-speeds

There are no gears in the transmission itself, however there are generally gears to connect the engine, transmission, reversing gear and wheelsets to one another.

They're a different type of gearbox but it'd love to know how a hydraulic transmission works without some form of gears or toothed wheels!

Here goes:

The transmission as fitted to a Class 150 consists of 3 integral parts:

The Torque Convertor
The Fluid Coupling
The Reversing gear.

The Torque Convertor is used for staring off and lower speeds, it corresponds with the low gear ratios in a conventional 4-speed gearbox as fitted to most First Generation DMUs. The Fluid Coupling provides a direct drive for normal running.

Both Torque Convertor and Fluid Coupling can be filled with oil in order to transmit power or emptied of oil to when the train is freewheeling with the throttle closed.

Assuming the train is at a stand, when the power controller is opened a fill valve commences to fill the Torque Convertor with oil, the pump effectively converts the mechanical energy of the diesel engine into a flow of oil.

The oil impinges off the blades of a turbine arranged to rotate around the pump, the oil then return to the pump to begin the cycle again. The turbine is linked by the reversing gears to the output shaft.

A characteristic of the Torque Convertor is that maximum Torque, or turning effort, is produced when the turbine is at a stand still, the oil exerting a force on the the turbine blades proportional to the speed of the pump.

Thus, on starting, the converter multiplies torque in a way no different in principle to the selection of low ratio first gear in a conventional mechanical gearbox.

Finally, when the train reaches a speed where the torque multiplication ceases - comparable to the speed where a driver of a First Gen DMMU would've changed into fourth gear, the control system will commence to empty the Torque Converter and begin to fill the adjacent Fluid Coupling. This has two elements and resembles a tennis ball cut in half with the two halves facing each other, each half containing a series of cup-shaped recesses. The engine-driven half - called an impeller - accelerates the oil, throwing it from the impeller cuts into similar cups in the driven member thereby causing it to rotate. The driven member is connected to the Torque Converter turbine by a shafting passing through the hollow centre of the Torque Converter/Fluid Coupling input shaft.

The emptying of the Torque Converter and filling of the Fluid Coupling takes smoothly and the flow of power is not interupted. Under normal circumstances the change-over from one to the other is difficult to detect. The transmission can freewheel in motion if the power controller is closed, a valve operating to empty the Fluid Coupling/Torque Converter; both will be emptied when the power controller is closed and the train is at a standstill.

There is no direct mechanical link between the Fluid Coupling or, for that matter, between the various elements inside the Torque Converter. The working fluid (oil) forms the only connexion between the driving and driven members.

Finally, when in the Torque Converter stage, the throttle notch directly controls the speed of the engine, each notch corresponding to a particular speed. When on the Fluid Coupling however, the engine speed is directly proportional to the speed of the train and the throttle controls the load on the engine rather than having a direct bearing on the engine speed.

 

[37057]

My argument is that you still have constant mesh gears driving the output shaft of the transmission from the torque converter / couplings, so still a (mechanical) gearbox! Hydraulic or mechanical transmission is like another HST loco-or-unit case! They're just boxes of gears, bearings and shafts at the end of the day whether the fluid couplings are house inside it or not... and whether you call it a transmission or a gearbox. This comes up quite often on this forum.


Gears in a gearbox (or transmission)...

 

[najaB]

They're just boxes of gears, bearings and shafts at the end of the day whether the fluid couplings are house inside it or not... and whether you call it a transmission or a gearbox.

That's true, but the main point being made is that when power is being delivered a gearbox has a direct and uninterrupted mechanical connection from engine to wheel where a hydraulic transmission does not.

That's a pretty significant difference - if somehow the wheels were suddenly stopped while in motion, with a gearbox there would be bits of metal flying through casings, that would be much less likely with a hydraulic transmission.

 

[Billy A]

They're a different type of gearbox but it'd love to know how a hydraulic transmission works without some form of gears or toothed wheels!

The two speed one on the class 150 has several gears inside, but it doesn't change gear as such. It uses a torque converter to start and get going, but then switches to a fluid coupling (basically a torque converter missing one element) driving the same gear. The fluid coupling slips much less than the torque converter so you get what amounts to a higher gear without actually changing gear as such. More modern DMUs use a three speed Voith transmission which has a second fluid coupling driving a higher gear so they do actually change gear, albeit purely hydraulically because they drain the low gear coupling and fill the high gear one.

 

[Billy A]

They're a different type of gearbox but it'd love to know how a hydraulic transmission works without some form of gears or toothed wheels!

The two speed one on the class 150 has several gears inside, but it doesn't change gear as such. It uses a torque converter to start and get going, but then switches to a fluid coupling (basically a torque converter missing one element) driving the same gear. The fluid coupling slips much less than the torque converter so you get what amounts to a higher gear without actually changing gear as such. More modern DMUs use a three speed Voith transmission which has a second fluid coupling driving a higher gear so they do actually change gear, albeit purely hydraulically because they drain the low gear coupling and fill the high gear one.