Technical Aspects Multiple Working

GrimShady · 21 Sep 2018

I've always been puzzled by multiple working with regard to different Class Locos/Multiple Units. I have some questions that I hope someone can shed some light on.

Take for example a 170 and lesser powered Sprinter unit. At any notch setting the 170 is more powerfull than let's say a 153 and will have different speeds at that setting, surely this means the Sprinter will in escence be dragged along to an extent, wouldn't this result in an overspeed situation being transmitted via the mechanical transmission to the engine?

How is it compensated for?

Regarding locomotives for example a Class 20 & 37, doesn't the same situation exist with the traction motors?

Elsewhere on this forum there's been talk of Class 68 & Class 88 in Electrical mode working in multiple, how would this work with the different power outputs at any notch setting and also wouldn't there be a delay in output from the Diesel Vs Electric loco?

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edwin_m · 21 Sep 2018

The coupler/coupling is able to haul/propel the train with one part entirely dead, so it doesn't matter too much if one half is exerting a bit more tractive effort than the others. There is just a tension or compression force in the coupling. With double headed locomotives there will be tension in the intermediate coupling when accelerating, otherwise the leading locomotive wouldn't be contributing to accelerating the rest of the train.

The speeds on the engines and traction motors will match the wheel speed through whatever gearing is provided. This may mean that one is doing more work than another, but again that doesn't matter too much unless one unit is being dragged at more than its maximum speed. This load imbalance may even be the reason why double headed locomotives aren't allowed twice the load of a single loco.

GrimShady · 21 Sep 2018

edwin_m said:
The coupler/coupling is able to haul/propel the train with one part entirely dead, so it doesn't matter too much if one half is exerting a bit more tractive effort than the others. There is just a tension or compression force in the coupling. With double headed locomotives there will be tension in the intermediate coupling when accelerating, otherwise the leading locomotive wouldn't be contributing to accelerating the rest of the train.

The speeds on the engines and traction motors will match the wheel speed through whatever gearing is provided. This may mean that one is doing more work than another, but again that doesn't matter too much unless one unit is being dragged at more than its maximum speed. This load imbalance may even be the reason why double headed locomotives aren't allowed twice the load of a single loco.

Okay, how would this work with a mechanical and hydraulic transmission? How is the engine on the mechanical unit protected from overspeed/shock if ihe hydraulic unit is faster/more powerful.

t_star2001uk · 21 Sep 2018

GrimShady said:
Okay, how would this work with a mechanical and hydraulic transmission? How is the engine on the mechanical unit protected from overspeed/shock if ihe hydraulic unit is faster/more powerful.

Do we have any mechanical transmission units left....

TRAX · 21 Sep 2018

GrimShady said:
Okay, how would this work with a mechanical and hydraulic transmission? How is the engine on the mechanical unit protected from overspeed/shock if ihe hydraulic unit is faster/more powerful.

The maximum speed of the less-capable unit becomes the maximum speed of the more-capable unit, thus of the entire train.

t_star2001uk said:
Do we have any mechanical transmission units left....

Class 172, 195...

t_star2001uk · 21 Sep 2018

172s are more of an automatic transmission rather than mechanical. There is no input from the driver where gear change is concerned. With a 172 and either a 165/ 168/ 170 the driver drives from the leading unit as per the driving instructions for the particular unit that is leading.

GrimShady · 21 Sep 2018

TRAX said:
The maximum speed of the less-capable unit becomes the maximum speed of the more-capable unit, thus of the entire train.

Class 172, 195...

It's still possible to exceed the speed limit of the lesser unit is it not? I'm sure I've read posts here about 170s leading 142s doing 90mph. Wouldn't this severely damage the 142s propulsion?

I appreciate all the responses gents!

randyrippley · 21 Sep 2018

GrimShady said:
Okay, how would this work with a mechanical and hydraulic transmission? How is the engine on the mechanical unit protected from overspeed/shock if ihe hydraulic unit is faster/more powerful.

It doesn't work, as BR found out the hard way in the early days of DMUs. There were examples of hydraulic units dragging machines with mechanical gearboxes and the driver forgetting the need to change gear - with resultant damage

TRAX · 21 Sep 2018

t_star2001uk said:
172s are more of an automatic transmission rather than mechanical. There is no input from the driver where gear change is concerned. With a 172 and either a 165/ 168/ 170 the driver drives from the leading unit as per the driving instructions for the particular unit that is leading.

In railway terms, a diesel-mechanical locomotive or MU is diesel powered with an automatic gearbox using similiar technology as on road vehicles (with actual gear changes). It's called mechanical because in essence it's what it is, just mechanics in the form of gears. No fluid (hydraulic) or electricity involved.

GrimShady said:
It's still possible to exceed the speed limit of the lesser unit is it not? I'm sure I've read posts here about 170s leading 142s doing 90mph. Wouldn't this severely damage the 142s propulsion?

I appreciate all the responses gents!

Yup that wouldn't be good for the 142.

It's as possible to exceed the speed limit of the lesser unit as much as it is when it's not coupled to a more-capable unit. If a driver knows what he should be doing, it shouldn't happen.

randyrippley · 21 Sep 2018

GrimShady said:
It's still possible to exceed the speed limit of the lesser unit is it not? I'm sure I've read posts here about 170s leading 142s doing 90mph. Wouldn't this severely damage the 142s propulsion?

I appreciate all the responses gents!

more likely to cause stability problems: the wheelset/suspension of the Pacers was the limiting factor. Anything faster was a derailment risk - though a leading bogied set with better high speed stability may have mitigated that

snowball · 21 Sep 2018

edwin_m said:
With double headed locomotives there will be tension in the intermediate coupling when accelerating, otherwise the leading locomotive wouldn't be contributing to accelerating the rest of the train.

Unless the power of the leading locomotive is used up overcoming air resistance - as I suppose might be conceivable at high speed if the less powerful one is in front?

edwin_m · 21 Sep 2018

randyrippley said:
It doesn't work, as BR found out the hard way in the early days of DMUs. There were examples of hydraulic units dragging machines with mechanical gearboxes and the driver forgetting the need to change gear - with resultant damage

Yes, the hydraulic classes of first generation DMU had a control for changing gear but it only did anything when there was a mechnical unit coupled up behind. After they overspeeded (oversped?) a few engines and I think caused a fire or two, they changed the coupling code so running the two types in multiple wasn't allowed.

randyrippley · 21 Sep 2018

I've read somewhere that the class 24/25 fleet were a good example of unbalanced locos causing problems: apparently the different sub-types had different power notch settings (differences in power, differences in traction motors, series vs parallel winding) with the result that two different versions would effectively fight each other. The story went that attempts were made to try to use matched pairs when in multiple

GrimShady · 22 Sep 2018

randyrippley said:
I've read somewhere that the class 24/25 fleet were a good example of unbalanced locos causing problems: apparently the different sub-types had different power notch settings (differences in power, differences in traction motors, series vs parallel winding) with the result that two different versions would effectively fight each other. The story went that attempts were made to try to use matched pairs when in multiple

Class 20s and 37/0s combo were once used on coal trains in Scotland during the late 80s to use the slow speed control of the 20 during loading operations. I wonder if the same difficulty was experienced?

During the days of the E&G push pull services, it wasn't uncommon for a 25 or 37 to substitute for a 27. In this case, if the 37 was on the rear, you'd have it shoving much more than the 27 was pulling given the 37s extra power.

t_star2001uk · 22 Sep 2018

TRAX said:
In railway terms, a diesel-mechanical locomotive or MU is diesel powered with a gearbox using similiar technology as on road vehicles (with actual gear changes). It's called mechanical because in essence it's what it is, just mechanics in the form of gears. No fluid (hydraulic) or electricity involved.

Sorry. I stand corrected. I was thinking 1st gen DMUs. It was a long day yesterday....

randyrippley · 22 Sep 2018

TRAX said:
In railway terms, a diesel-mechanical locomotive or MU is diesel powered with an automatic gearbox using similiar technology as on road vehicles (with actual gear changes). It's called mechanical because in essence it's what it is, just mechanics in the form of gears. No fluid (hydraulic) or electricity involved.

.

even that's not quite true.....
at least in the UK, 1950's mechanical shunter and DMU gearboxes tended to be either pre-selector boxes, or speed-actuated clutchless epicyclic units. Only a few had fluid flywheels. By then, epicyclics and pre-selectors were falling out of favour for road vehicles - which were moving toward automatics with fluid flywheels and multistage torque converters

37057 · 22 Sep 2018

t_star2001uk said:
172s are more of an automatic transmission rather than mechanical. There is no input from the driver where gear change is concerned. With a 172 and either a 165/ 168/ 170 the driver drives from the leading unit as per the driving instructions for the particular unit that is leading.

There's not much difference between a transmission in a 1st gen DMU and a 172 etc.

Both use fluid to transmit drive in the form of a fluid coupling or torque converter to drive mechanical gears. The former is semi-auto and the latter fully auto.

Hydrodynamic transmissions are still very much mechanical. Only difference is the speed change is done by transferring the fluid from torque converter to couplings.

apk55 · 22 Sep 2018

Modern DMUs use automatic transmission systems where the power unit electronics sorts out which mode of operation or gear is required depending on speed and driver demand. If coupled with a different type of unit the units could change mode or gear at different speeds.
To get high torque or tractive effort at low speed a torque converter is often used rather than multi speed gear box. However torque converters are rather inefficient. And you can only get full engine output at a limited speed range.

The first generation DMUs were more like a manual car where a the driver changed gears manually so you could over-speed an engine. There could be a problem with with different types not having the same speed range for each gear. Old Electric locomotives with manual tap changer or resistance control are also like this (class 86 & 87 for example).

An electric transmission is effectively like a continuously variable transmission so has the advantage that you can get full engine output at the wheels over a large speed range. The control system will sort out all the setting and all the driver will do is select a tractive effort setting. Therefore power sharing is good between units.

It would be nice if somebody could develop a contentiously variable mechanical transmission system for at least multipull units, they are currently available for cars but not as yet available at higher power ratings.

GrimShady · 22 Sep 2018

Very informative posts!

Does anyone know if how the 68 & 88 combo would function in diesel and electric?

Fisherman80 · 22 Sep 2018

I am trying to think if I ever witnessed a 141 working in combo with another class.

The modified 141 (Cummins engine/Voith transmission) did work a couple of times with a 142 as I recall......but I can never recall an unmodified (Leyland engine/SCG transmission) working in multiple with another class.

If anyone knows differently I would be interested to hear.

Technical Aspects Multiple Working

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