Stripping the gearbox of Class 10X DMMU's

[strange6]

I have been told that it's quite easy to do this on these DMU's, especially if you don't let the revs drop in between changes and / or you get yourself in a pickle when braking but not coming to a stop and then selecting the wrong gear to get back the power. Does anybody have any experience or know of anybody who has damaged said mechanism on said units?
Any advice would be appreciated because I don't want to land myself with a large bill!

 

[flymo]

Are these boxes of the pneumocyclic type? If so they may be similar to the ones used on Leyland Atlanteans of yesteryear. I had loads of experience on them working on the buses in the 80's

Of course if they are not then disregard the following.

Working on the buses way back when these boxes were everywhere there was an electro-pneumatic (EP) valve chest used to control the air pressure to operate the brake bands around the annulus of each gear allowing the appropriate gear ratio to be selected. Each valve in the EP chest was controlled by a solenoid which moved a plunger and allowed air pressure through to an actuator situated in the lower side of the 'box. The actuator operated the brake band onto the annulus mechanically. Different gears had different sized actuators. I think the lower gears had larger sizes to allow more air to compress the brake band onto the annulus with more force to prevent slippage at low speed - high torque.

The problem was allowing the air pressure in one gear to drop fast enough so that when the next gear up was selected only one band was operated at one time. If more then one band was operated then excessive wear of the brake material on each band would severely shorten the life of the 'box. I recall on the buses where a fully automatic box was fitted, there were 'dump valves' fitted onto 1st, 2nd & 3rd gears to quickly release the pressure and get the bus moving along more quickly. On the semi-automatic 'boxes some had dump valves fitted some did not, meaning the driver had to operate the selector with more skill to allow a smoother change. The throttle was released during the shift time as keeping it pressed down would also increase the wear on the brake bands. In the auto boxes this was done automatically via an air cylinder on the throttle linkage

There was also a direct change system on older buses where the selector itself was a large 'air switch' that directly controlled the air going to the brake bands and the air exhausted via the gear shifter meaning a short delay was necessary to change gear smoothly.

I remember so many trips on the old DMU's and the changing of gears was such that you could hear what sounded like the EP valve releasing the pressure but more slowly back through the EP valve and piping so I'm guessing here that the reason why there is a small delay in releasing one gear before selecting another is to allow the air pressure to drop sufficiently to release one band before allowing the next band is operated.

I hope that is as clear as mud....

 

[O L Leigh]

Why would you be faced with a big bill?

DMUs had epicyclic mechanical boxes that almost certainly had an electro-pneumatic shifter of some design or other so that all the boxes would shift together down the entire length of the train, but I think the actual arrangements would be slightly different to those used on bus transmissions. It's a shame that the main website of The Railcar Association is currently offline for refurbishment as I'm sure it would give much more information.

I don't ever recall reading any accounts of stripping gears on DMUs that would suggest it was a common problem. Perhaps the control gear prevented shifting until the engine revs drop, but I suppose it must have been at least possible. What you tended to hear more about is engine damage due to overspeeding in the wrong gear, especially when they were worked in multiple with diesel-hydraulic units.

O L Leigh

 

[strange6]

Why would you be faced with a big bill?

For a few hundred quid, I'm going to be let loose on one in a couple of weeks time, under strict supervision of course.

 

[O L Leigh]

Then you've nothing to worry about because you'll be under instruction.

O L Leigh

 

[strange6]

Then you've nothing to worry about because you'll be under instruction.

O L Leigh

It's got dual controls then I assume?

 

[ralphchadkirk]

It's got dual controls then I assume?

It'll be a normal cab most likely. Certainly the units we use for experience days have no dual controls, save for an emergency vac valve on the secondmans side. So the instructor will be able to stop the train if he has to.

 

[strange6]

It'll be a normal cab most likely. Certainly the units we use for experience days have no dual controls, save for an emergency vac valve on the secondmans side. So the instructor will be able to stop the train if he has to.

Thanks. I really did think nobody would take me seriously about the dual controls!

 

[ChrisCooper]

You're not going to ruin one just like that. As with any mechanical transmission, bad driving will reduce the life of it, but not significantly unless it's all the time. It's like a manual box on a car, even good drivers crunch the gears at times, but it doesn't do any real damage. If you are under instruction they will be able to guide you and stop you doing anything awful anyway. With the epicyclic boxes, the main points of wear are the brake bands, which like the clutch on a car will wear down over time anyway and need replacing. Bad driving will speed that up.

The 1st generation DMUs use Self Changing Gears boxes which are similar to the pneumocyclics Leyland used on the Atlantean and Leopard, and were also fitted to Bristols and Daimlers. Unlike the Leylands they were all electro-pnumatic, hence being ideal for trains as it makes control from one cab easy. The only real difference between the bus and train versions was that the bus version had a built in reverse gear whereas the train version had a seperate reversing mechanism (needed so all 4 gears would work both ways).

AFAIK the train controllers have an electro-mechanical lock which prevents the gear selector being moved unless the throttle is at idle. Other than that there is nothing else, it's just up to the driver. Basically though just watch the rev counter and wait for it to settle back to idle before shifting, then wait a second or two for the gear to engage before slowly reapplying power. Once you get the hang of it you won't need the rev counter (you don't get one on a bus, you just have to listen to the engine). Changing down can be the hardest, but unless the line you are going to be driving on is particularly steeply graded the chances are you won't have to change down when moving. I've known even good drivers mistime down changes on buses and nearly catapult the passengers through the front window (one nearly catapulted me not long ago whilst trying to change from 4th to 3rd at about 35mph).

 

[strange6]

You're not going to ruin one just like that. As with any mechanical transmission, bad driving will reduce the life of it, but not significantly unless it's all the time. It's like a manual box on a car, even good drivers crunch the gears at times, but it doesn't do any real damage. If you are under instruction they will be able to guide you and stop you doing anything awful anyway. With the epicyclic boxes, the main points of wear are the brake bands, which like the clutch on a car will wear down over time anyway and need replacing. Bad driving will speed that up.

The 1st generation DMUs use Self Changing Gears boxes which are similar to the pneumocyclics Leyland used on the Atlantean and Leopard, and were also fitted to Bristols and Daimlers. Unlike the Leylands they were all electro-pnumatic, hence being ideal for trains as it makes control from one cab easy. The only real difference between the bus and train versions was that the bus version had a built in reverse gear whereas the train version had a seperate reversing mechanism (needed so all 4 gears would work both ways).

AFAIK the train controllers have an electro-mechanical lock which prevents the gear selector being moved unless the throttle is at idle. Other than that there is nothing else, it's just up to the driver. Basically though just watch the rev counter and wait for it to settle back to idle before shifting, then wait a second or two for the gear to engage before slowly reapplying power. Once you get the hang of it you won't need the rev counter (you don't get one on a bus, you just have to listen to the engine). Changing down can be the hardest, but unless the line you are going to be driving on is particularly steeply graded the chances are you won't have to change down when moving. I've known even good drivers mistime down changes on buses and nearly catapult the passengers through the front window (one nearly catapulted me not long ago whilst trying to change from 4th to 3rd at about 35mph).

The Greater Manchester Atlanteans (Northern Counties bodied) of later vintage had fully automatic pneumo-cyclic gearboxes that could also work in semi- automatic mode for 2nd and 3rd gears. Even when the driver selected 2nd and 3rd gear over-drive, the gears changed down automatically thus preventing engine damage. Clever boxes they were

 

[matchmaker]

What you tended to hear more about is engine damage due to overspeeding in the wrong gear, especially when they were worked in multiple with diesel-hydraulic units.

O L Leigh

Was it possible for DMMU to work in multiple with DHMU?

 

[western Champion]

It was certainly possible to do what they called "racing" gear changes in other words selecting the next gear and taking power before the revs had dropped back to idling. Some of the Tyseley four car sets had gear box protection to stop the driver doing this and they were a total pain to drive especially when trying to climb a hill.

I wouldn't say it was a common practice but it wasn't unknown that if a driver thought he wasn't going to stop at a particular station for example he would select 1st gear (max 15mph) at say 40 mph and that would help slow the set down.

I also remember one driver (it wasn't me honest ) who after hearing the above being talked about in the messroom decided to try it at Shenstone. He selected 1st at 60 mph ! there was a screaming noise followed by a loud bang and he successfully stopped but when he came to get going again there was no power in any gear. On examination the gear boxes were found in the four foot in bits !

He got away with it, you wouldn't today.....

 

[O L Leigh]

Was it possible for DMMU to work in multiple with DHMU?

For a time, yes.

The BedPan and Lea Valley line units were DHMUs (later Cl127 and Cl125 respectively) due to the need to keep to faster timings. They still had gearchange controls and gear indicators because they were compatible with DMMUs and these would need to be used by the driver if there was a DMMU in the formation. But in practice many drivers simply forgot which resulted in damage to the DMMU due to overspeeding in the wrong gear.

In the end they were simply given a different multiple working code to prevent them from working in multiple (from Blue Square to Orange Star), even though the units themselves were not modified in any way to enforce this and remained technically compatible for multiple working in spite of the different code.

O L Leigh

 

[matchmaker]

For a time, yes.

The BedPan and Lea Valley line units were DHMUs (later Cl127 and Cl125 respectively) due to the need to keep to faster timings. They still had gearchange controls and gear indicators because they were compatible with DMMUs and these would need to be used by the driver if there was a DMMU in the formation. But in practice many drivers simply forgot which resulted in damage to the DMMU due to overspeeding in the wrong gear.

In the end they were simply given a different multiple working code to prevent them from working in multiple (from Blue Square to Orange Star), even though the units themselves were not modified in any way to enforce this and remained technically compatible for multiple working in spite of the different code.

O L Leigh

Thanks, very interesting. Was not aware of this before.

 

[red circle]

For a time, yes.

The BedPan and Lea Valley line units were DHMUs (later Cl127 and Cl125 respectively) due to the need to keep to faster timings. They still had gearchange controls and gear indicators because they were compatible with DMMUs and these would need to be used by the driver if there was a DMMU in the formation. But in practice many drivers simply forgot which resulted in damage to the DMMU due to overspeeding in the wrong gear.

In the end they were simply given a different multiple working code to prevent them from working in multiple (from Blue Square to Orange Star), even though the units themselves were not modified in any way to enforce this and remained technically compatible for multiple working in spite of the different code.

O L Leigh

I think it was only the Bedpan units that were geared compatible and later changed to red triangle,I remember the lea valley Rolls Royce sets well,in place of the gear lever they had two levers ,one for for/rev and one for drive/neutral.
I once travelled pass in the cab from Stratford ll to Silvertown with a driver who had 'the hump'to say the least,"I should have crossed this off years ago"
followed by a one second gearchange and loud bangs from under the floor,I think he was trying to make the unit fail bigtime.One thing that did impress me was that he hit every platform with a 'full pint' and stopped dead on the mark.

 

[ChrisCooper]

The Greater Manchester Atlanteans (Northern Counties bodied) of later vintage had fully automatic pneumo-cyclic gearboxes that could also work in semi- automatic mode for 2nd and 3rd gears. Even when the driver selected 2nd and 3rd gear over-drive, the gears changed down automatically thus preventing engine damage. Clever boxes they were

GM did tend to be more advanced. It's different to Nottingham where the late build NCT Atlanteans still had the direct pnumatic control. Get a lovely squeek from the air when changing gear though. Always was a common sound around the Old Market Square. Also got it with the Barton Leopards too.

 

[strange6]

GM did tend to be more advanced. It's different to Nottingham where the late build NCT Atlanteans still had the direct pnumatic control. Get a lovely squeek from the air when changing gear though. Always was a common sound around the Old Market Square. Also got it with the Barton Leopards too.

The GM boxes were installed for fuel efficiency and they worked too: on average, if you kept the bus in fully automatic mode, you would save 20 percent or so in a day's running. Most drivers did run in fully automatic all of the time except if starting on a hill were they would overdrive with 2nd and 3rd to stop the box from searching gears when climbing. There were a few drivers though who liked the overdrive for performance and to be as noisy as possible by revving the thing!