Diesel loco vacuum brake operating pressures

[Squirrel]

Does anyone know of a reference document that covers vacuum brake operating pressures on diesel locomotives?

[leopardml2341]

Does anyone know of a reference document that covers vacuum brake operating pressures on diesel locomotives?

The readily available (secondhand) book Diesel Traction for Enginemen https://pictures.abebooks.com/BOOKSANDINK/md/md19699129150.jpg probably gives you what you're seeking (and more), don't have my copy to hand to be certain though.

Edited June 30, 2017 by leopardml2341

[Dagworth]

Diesel locos operated at 21inches of vacuum, same as steam locos.

 

The western region used to run at 25inches in both steam and diesel locos, though I'm not sure how long into the diesel era this lasted.

 

Andi

Edited June 29, 2017 by Dagworth

[Squirrel]

Thanks for that, though the 21" is no problem, it's the first application and normal braking pressures I am after

[Ken.W]

BR diesels weren't actually vacuum braked, they were equipped to operate vacuum train brakes but the locos themselves were air braked.

When the vacuum train brake was applied a relay valve applied the loco's air brakes in proportion to the reduction in vacuum in the train brakes.

Normal max pressure for the loco brakes was 70 psi (60 psi I believe for Cl.40s)

[leopardml2341]

BR diesels weren't actually vacuum braked, they were equipped to operate vacuum train brakes but the locos themselves were air braked.

 

Hence the need for these:

 

https://www.google.co.uk/imgres?imgurl=http%3A%2F%2Fpaulbartlett.zenfolio.com%2Fimg%2Fs1%2Fv20%2Fp1011465871-3.jpg&imgrefurl=http%3A%2F%2Fpaulbartlett.zenfolio.com%2Fbraketender&docid=kPtGiGolQMv9nM&tbnid=jhnToAor_gOokM%3A&vet=10ahUKEwjX_MX-yuTUAhUFPFAKHb7fDLsQMwgvKAAwAA..i&w=580&h=289&client=tablet-android-pega&bih=600&biw=960&q=diesel%20brake%20tender&ved=0ahUKEwjX_MX-yuTUAhUFPFAKHb7fDLsQMwgvKAAwAA&iact=mrc&uact=8

 

In an era of largely unfitted freight traffic.

 

Though why this one's being used on this train I'm uncertain:

 

http://www.davidheyscollection.com/userimages/000-0-a-t-ermel-Type2-4.jpg

[david.hill64]

I cannot claim to be an expert on vacuum brakes but I think your question may be impossible to answer in the form that it is put. The vacuum in the brake pipe could be anything between 21" (WR excepted) and 0 (ie atmospheric pressure). The force in the brake rodding is proportional to the reduction in brake pipe vacuum. The actual brake force depends also on the friction coefficient between the (usually) cast iron brakeblock and wheel and wheel-rail adhesion. The cast iron friction characteristic is highly non-linear with speed, so brake rate depended both on brake pipe pressure and speed.

 

A driver would be better placed to answer, but my understanding is that it was good practice to make a significant admission of air into the brake pipe first so that 'stiction' in the system is overcome. As the vacuum brake is fully proportional, vacuum could then be recreated to reduce the brake force.

 

The maximum brake force that could be achieved would be with the vacuum pipe at atmospheric pressure. Drivers would always in normal operation be recreating vacuum in the system as the train came to rest ('stopping on a rising brake') to prevent an excessive jerk as the train stopped.

 

In air brakes the distributors on each vehicle control air pressure in the brake cylinders. You do not need to reduce the pressure in the brake pipe to zero to get a full service application. Once the full service braking rate is achieved, any further reduction in brake pipe pressure (eg by selection of the emergency brake position) will not result in an increased brake force. So brake application force is not linear with air brake pipe pressure over the whole range of pressures. With a vacuum brake it is.

Edited June 30, 2017 by david.hill64

[Ken.W]

Thanks for that, though the 21" is no problem, it's the first application and normal braking pressures I am after

As  posted briefly above, the locos were actually air braked, which were applied in proportion to the application of the train's vacuum brakes. So at 21" vacuum the loco's brake was 0 psi, and at 0" vacuum the loco brake would be 70 psi.

 

The "Initial" first application position refers to train air brakes and wasn't used with vacuum brakes. When applying a vacuum brake vacuum would normally be reduced to at least 15", however as David has explained, other than if just a small reduction of speed's required, normal procedure would be to make a significant reduction, "take a big bite", generally down to at least 10", in order to "get a hold of them". Where an initial application was required to "gather them in" - close the buffers up - on a freight, particularly a part fitted one, a light application of the loco's direct air brake would be used instead, before application of the vacuum.

 

The technique David describes of 'stopping on a rising brake' still applies with any brake, and the less brake you have actually on at coming to a stand, the smoother the stop.

 

With the air brake, from 'running' at 5 Bar (72.5* psi) a full brake application is achieved at 3.5 Bar (50 psi), as David said. Use of the emergency position simply downs brake pipe pressure and applies the brake faster, or zeros it to allow pipe parting during shunting etc.

* 70 psi pre-metric

[Ken.W]

Hence the need for these:

https://www.google.co.uk/imgres?imgurl=http%3A%2F%2Fpaulbartlett.zenfolio.com%2Fimg%2Fs1%2Fv20%2Fp1011465871-3.jpg&imgrefurl=http%3A%2F%2Fpaulbartlett.zenfolio.com%2Fbraketender&docid=kPtGiGolQMv9nM&tbnid=jhnToAor_gOokM%3A&vet=10ahUKEwjX_MX-yuTUAhUFPFAKHb7fDLsQMwgvKAAwAA..i&w=580&h=289&client=tablet-android-pega&bih=600&biw=960&q=diesel%20brake%20tender&ved=0ahUKEwjX_MX-yuTUAhUFPFAKHb7fDLsQMwgvKAAwAA&iact=mrc&uact=8

In an era of largely unfitted freight traffic.

 

Though why this one's being used on this train I'm uncertain:

http://www.davidheyscollection.com/userimages/000-0-a-t-ermel-Type2-4.jpg

Not exactly, they were to increase brake force when working unfitted or part-fitted freights. The locos were still proportionately braked when on vacuum fitted trains, just not directly by vacuum.

 

The second photos interesting on several points

I've not previously seen a photo of a brake tender used on a passenger train, and as you say, why?

Also, why use a  25 to double head a 47 on a relatively short train (10 on) well within the 47's capability, particularly as the two can't work in multiple.

From the rear six vehicles, the train appears to be an East Coast Car Carrier, a route where double heading was uncommon.

 

I think, probably, the 47's failed and the nearest assistance has been in removing the 25 from a nearby looped freight, and the brake tender happens to have come with it

[The Stationmaster]

As  posted briefly above, the locos were actually air braked, which were applied in proportion to the application of the train's vacuum brakes. So at 21" vacuum the loco's brake was 0 psi, and at 0" vacuum the loco brake would be 70 psi.

 

The "Initial" first application position refers to train air brakes and wasn't used with vacuum brakes. When applying a vacuum brake vacuum would normally be reduced to at least 15", however as David has explained, other than if just a small reduction of speed's required, normal procedure would be to make a significant reduction, "take a big bite", generally down to at least 10", in order to "get a hold of them". Where an initial application was required to "gather them in" - close the buffers up - on a freight, particularly a part fitted one, a light application of the loco's direct air brake would be used instead, before application of the vacuum.

 

The technique David describes of 'stopping on a rising brake' still applies with any brake, and the less brake you have actually on at coming to a stand, the smoother the stop.

 

With the air brake, from 'running' at 5 Bar (72.5* psi) a full brake application is achieved at 3.5 Bar (50 psi), as David said. Use of the emergency position simply downs brake pipe pressure and applies the brake faster, or zeros it to allow pipe parting during shunting etc.

* 70 psi pre-metric

 

 

The Steam Loco Driver's Handbook suggested about a 7" to 10' reduction in the vacuum level to get the brakes gripping and then going right to 'full on' (i,e, reducing the vacuum to 0") for a full service application.  But while that is what the book said the reality could be considerably different as much depended on the real effective brake power of the train rather than any ideas of theory.  On the WR Mk1 London commuter sets with vacuum brakes you could drop the vacuum by anything between about 5-6" on a really good set but on a really bad one, it might need as much as a 12" reduction to get any braking effect at all.   It all depended on how new, or worn, the coaching stock brake blocks were and how much slack there happened to be in the brake rigging - a set fresh off brake blocks being changed would have lower power than one that had done a few trips to Oxford and back while one which was due (or overdue) its planned brake block and brake rigging attention could be something of a handful and make it far more difficult to graduate the brake.  The same applied if you needed to make a full application (which was ideally avoided) and then performance could vary even more wildly.

 

I normally braked on the same marks/feel of the road giving me my location - as most Drivers would - and that of course was how you quickly found out if you had a dodgy brake on the stock.  For example for the 60 mph at Ladbroke Grove (and in from there to Paddington) I would normally drop the brake to about 15-16" of vacuum passing Mitre Bridge - thereby reducing the speed from 85mph to 60mph over a distance of about 60 chains; but once the brakes had started to bite they could be blown straight back off and be fully released before reaching the 60 board.  Similarly running into Paddington with 40 mph through the approach pointwork only a small application was needed to stop in the right place and again plenty of time to make sure the brakes were fully released before coming to a stand.

 

The (G)WR 25" vacuum had been abolished on the WR by the Autumn of 1966 so it effectively ended with the end of WR allocated steam.

[leopardml2341]

Not exactly, they were to increase brake force when working unfitted or part-fitted freights. The locos were still proportionately braked when on vacuum fitted trains, just not directly by vacuum.

 

 

Not sure what you mean?

 

Yes, I know they were indeed used to increase brake force hence my reference to unfitted freights. The tenders themselves were vacuum braked, ergo the additional brake force can only come directly from the locomotive controlled vacuum brake (on the brake tender) {sic}

 

 

There's another topic here too:

http://www.rmweb.co.uk/community/index.php?/topic/115616-steam-and-diesel-vacuum-brake-operation/

Edited June 30, 2017 by leopardml2341

[LMS2968]

With a vacuum brake, it was usual to reduce the vacuum by on - off movements of the brake lever; it wasn't like driving a car where you can vary the brake force simply by varying the force on the pedal. Moving the brake lever part way gave you or all nothing, depending on the individual brake valve.

 

It was usual to reduce the vacuum (from 21") to between 12" and 15", occasionally perhaps 10". The lever would be returned to Off as the level approached, then moved fully to On again. The rapidity of the movements would decide the vacuum level and, with other factors, the brake force.

 

It was very unusual to drop the vacuum to zero. It took time to recreate vacuum from this level, and any release would start at the engine and work its way back to the rear of the train. There was therefore the possibility of the front of the train rolling free while the rear was still under maximum braking, and a breakaway was a real possibility. If all vacuum was lost, you were generally committed to coming to a stand before recreating vacuum.

[leopardml2341]

One used to be able to observe that M.O. from behind the driver's cab of a first generation DMU, but the vacuum brakes on those were directly operated and not by proportional air control.

 

Anyway, that's leading us a bit

Edited July 1, 2017 by leopardml2341

[Davexoc]

The Manual for Enginemen states that initial brake application reduces the vacuum to 18in. which is proportional to the 10psi in the air brake cylinders on the loco. This was always said to just get the blocks on the wheels, ie. take up all the slack and stiction in the brake rigging.

One interesting thing the manual reveals is the Empty/Loaded selectors on wagons. When Empty is selected the brake is on just a 15" diameter vacuum cylinder, whereas when Loaded is selected a second vacuum cylinder of  18" diameter also comes into play, thus more than doubling the brake force available.

 

Dave

[leopardml2341]

The Manual for Enginemen states that initial brake application reduces the vacuum to 18in. which is proportional to the 10psi in the air brake cylinders on the loco. This was always said to just get the blocks on the wheels, ie. take up all the slack and stiction in the brake rigging.

One interesting thing the manual reveals is the Empty/Loaded selectors on wagons. When Empty is selected the brake is on just a 15" diameter vacuum cylinder, whereas when Loaded is selected a second vacuum cylinder of  18" diameter also comes into play, thus more than doubling the brake force available.

 

Dave

Thanks for quoting 'The Manual'; still can't find mine - it's no doubt somewhere safe.....

[45125]

One used to be able to observe that M.O. from behind the driver's cab of a first generation DMU, but the vacuum brakes on those were directly operated and not by proportional air control.

 

Anyway, that's leading us a bit

 

On a vac only  loco the vacuum is dropped by brake valve, it is only on a dual braked loco that the vacuum is dropped by a proportional relay Valve (AV1 or 2(Westinghouse) or DV2 (D&M)).

[leopardml2341]

We need to be careful about only using the term 'brake' as there are both locomotive brakes and train brakes. The operation of each and the terms air and vacuum have specific meaning in each case.