Vacuum Brake arrangements for a 4-wheel coach

[billbedford]

Usually the vacuum cylinder ins mounted in trunnions bolted to the inside of the solebar and the outside of the closest longitudinal bearer. Since the whole cylinder is mounted below the floor, with room it to rotate in the trunnions, at least half of it is visible in a side view. 

 

All the brake yokes should be of the same pattern, with, in your case, the inner yokes having a longer pull rod. 

 

As you have drawn this you will not be able to mount the wheelsets into the bearings without splitting the linkages somewhere around the axles. 

 

 

[Compound2632]

8 minutes ago, Edwardian said:

So the cylinder needs to come lower?

 

Ah, so that's where my brake cylinder post is. Confused!

 

The top of the cylinder is about level with the top of the solebars, i.e. just under the floor. Certainly not plugged into a hole in the floor!

[Edwardian]

6 minutes ago, Compound2632 said:

 

Ah, so that's where my brake cylinder post is. Confused!

 

The top of the cylinder is about level with the top of the solebars, i.e. just under the floor. Certainly not plugged into a hole in the floor!

 

I agree, it's not right.

 

My question is whether the angle of the linkage to the centre crank will look OK

 

 

Edited April 7, 2021 by Edwardian
spelling!

[Edwardian]

2 hours ago, billbedford said:

Usually the vacuum cylinder ins mounted in trunnions bolted to the inside of the solebar and the outside of the closest longitudinal bearer. Since the whole cylinder is mounted below the floor, with room it to rotate in the trunnions, at least half of it is visible in a side view. 

 

All the brake yokes should be of the same pattern, with, in your case, the inner yokes having a longer pull rod. 

 

As you have drawn this you will not be able to mount the wheelsets into the bearings without splitting the linkages somewhere around the axles. 

 

 

 

I agree, I just don't have a means of explaining how it should look to the designer (who is not knowing of railways and has even less of a grasp of brake arrangements than I do).

 

This is useful, because I can see it isn't right, but the explanations help me to understand exactly why. Communicating a correct answer is proving problematic. 

[billbedford]

Do these help?

 

They are from wagons, so you will have to ignore the hand levers. 

[Edwardian]

Thank you, Bill, yes, I think they might

[Miss Prism]

4 hours ago, Edwardian said:

My question is whether the angle of the linkage to the centre crank will look OK

 

I think the brakes in the diagram should be in the off position, so the tumbler should be oriented the other way.

 

[Edwardian]

OK, folks.  Please ignore the unequal yokes and the fact that the vac cylinder is still disappearing up through the floor, the rodding and centre crank has been amended and I hope these features are not reasonably prototypical.

 

[Edwardian]

Quick question, if I may?

 

Would you say that the vac cylinder now sits low enough, or does it need to come down further?

 

Thanks

 

 

[Compound2632]

4 minutes ago, Edwardian said:

Quick question, if I may?

 

Would you say that the vac cylinder now sits low enough, or does it need to come down further?

 

Thanks

 

 

 

The cylinder must be around 16" or so from base to the top of the dome, so given that the top is level with the top of the solebar, the bottom must be well below the bottom of the solebar - 4" - 5" depending on the depth of your solebars. It should certainly be visible when looking at the carriage side-on.

[Miss Prism]

The central brake shaft axis is always a lot lower on GWR things:

 

http://penrhos.me.uk/Photos/U4 W39.jpg

http://penrhos.me.uk/Photos/S11 15.jpg

https://didcotrailwaycentre.org.uk/shopimages/articles/extra/290_02.jpg

https://didcotrailwaycentre.org.uk/shopimages/articles/extra/no-416-dean-4-wheel-brake-third_a_127_20201120-121051.jpg

 

[Compound2632]

4 minutes ago, Miss Prism said:

The central brake shaft axis is always a lot lower on GWR things:

 

Other railways are available.

[Edwardian]

1 minute ago, Miss Prism said:

The central brake shaft axis is always a lot lower on GWR things:

 

http://penrhos.me.uk/Photos/U4 W39.jpg

http://penrhos.me.uk/Photos/S11 15.jpg

https://didcotrailwaycentre.org.uk/shopimages/articles/extra/290_02.jpg

https://didcotrailwaycentre.org.uk/shopimages/articles/extra/no-416-dean-4-wheel-brake-third_a_127_20201120-121051.jpg

 

 

Indeed. I suppose here I'm in the Hattons space of 'generic' (it's a freelance coach, albeit based on an actual design).

 

After looking at varied prototypes, and considering the feedback here, I plumped for the transverse rod at the apex of the V hanger immediately above the lower footboard, as, for instance, with these MGN examples. 

 

[Compound2632]

5 minutes ago, Miss Prism said:

The central brake shaft axis is always a lot lower on GWR things:

 

Also, and seriously folks, the steel solebars on those GWR 4-wheelers are rather shallower than was usual for wooden solebars - around 9" vs 12"? - thereby exaggerating the appearance of the V-iron, although it does look to be the case that the cross-shaft is at axle height.

[billbedford]

1 hour ago, Edwardian said:

After looking at varied prototypes, and considering the feedback here, I plumped for the transverse rod at the apex of the V hanger immediately above the lower footboard, as, for instance, with these MGN examples. 

 

You have to be careful here. The first vacuum brakes were not automatic and used twin pipes and smaller cylinders. They were universally replaced, fairly quickly, by what we know today as automatic brake with the larger cylinders. This drawing looks like it has been taken from a GA of the 1880s with the earlier brakes. 

 

Below is a drawing taken from a Midland horsebox of the 1890s/1900s which shows the later arrangement of the brake cylinder. Note in this example both pull rods are mounted above the cross shaft with a double link to the cross shaft crank.  

[Compound2632]

@billbedford, agree; James has a drawing from the same office, of 1900, which (naturally enough) has the same layout of cylinder and V-hanger.

[Edwardian]

Fair point.  These are early1870s coaches assumed vac fitted during the 1880s 

[billbedford]

OK, This is part of a GN coach with Smith's brakes:

 

[Guy Rixon]

5 hours ago, billbedford said:

 

You have to be careful here. The first vacuum brakes were not automatic and used twin pipes and smaller cylinders. They were universally replaced, fairly quickly, by what we know today as automatic brake with the larger cylinders. This drawing looks like it has been taken from a GA of the 1880s with the earlier brakes. 

 

Below is a drawing taken from a Midland horsebox of the 1890s/1900s which shows the later arrangement of the brake cylinder. Note in this example both pull rods are mounted above the cross shaft with a double link to the cross shaft crank.  

The right-hand rod is a push-rod, not a pull-rod. This is necessary since the brake shaft rotates clockwise. In fact, you can see that the push-rod has significant vertical depth on the centre, to stiffen it.

[Compound2632]

3 minutes ago, Guy Rixon said:

The right-hand rod is a push-rod, not a pull-rod. This is necessary since the brake shaft rotates clockwise. In fact, you can see that the push-rod has significant vertical depth on the centre, to stiffen it.

 

Both rods pull. When the brake is applied, by destroying the vacuum in the train pipe, the piston rises causing the cross-rod and tumbler to rotate anticlockwise (from the side we're looking at, on the GN drawing), pulling on the pull-rods which in turn, via the suspended links, cause the yokes to be pulled inwards towards the axle. The linkage is arranged so that everything is under tension when the brake is applied, otherwise they might buckle: the forces being significantly greater than in the hand-operated brake gear of an ordinary wagon.

[Guy Rixon]

4 hours ago, billbedford said:

OK, This is part of a GN coach with Smith's brakes:

 

I would draw everybody's attention to the lower pull-rod. It is almost horizontal and passes below the left-hand axle. C.f. the freelance coach where the rod comes up at a steep angle from the centre crank and engages above the axle. In my experience, it is much more common to have the rods as horizontal as possible. 

[Guy Rixon]

2 minutes ago, Compound2632 said:

 

Both rods pull. When the brake is applied, by destroying the vacuum in the train pipe, the piston rises causing the cross-rod and tumbler to rotate anticlockwise (from the side we're looking at, on the GN drawing), pulling on the pull-rods which in turn, via the suspended links, cause the yokes to be pulled inwards towards the axle. The linkage is arranged so that everything is under tension when the brake is applied, otherwise they might buckle: the forces being significantly greater than in the hand-operated brake gear of an ordinary wagon.

That's true on the GN coach, where the rods connect to opposite sides of the centre crank. On the MR horsebox, however, both rods connect on the same side of the crank, so one of them is pushed.

[Compound2632]

30 minutes ago, Guy Rixon said:

That's true on the GN coach, where the rods connect to opposite sides of the centre crank. On the MR horsebox, however, both rods connect on the same side of the crank, so one of them is pushed.

 

Sorry, you're right. I've looked at a couple of other Midland NCPS drawings of similar vintage (early 1880s). It is a most peculiar arrangement; there is an arm attached to the cross-shaft upon which a tumbler is pivoted, and the yokes are arranged the same way round on each axle (rather than being mirror images). The rod that is a push-rod is quite noticeably fish-bellied.

[billbedford]

8 hours ago, Guy Rixon said:

That's true on the GN coach, where the rods connect to opposite sides of the centre crank. On the MR horsebox, however, both rods connect on the same side of the crank, so one of them is pushed.

That is not correct. There is a compensation link at the top of the cross shaft crank. The crank would be doubled and the link pivots about it's centre. So when the brakes go on, the cross shaft rotates clockwise and the link anti clockwise dividing the brake force equally between the two pull rods. 

[Compound2632]

2 hours ago, billbedford said:

That is not correct. There is a compensation link at the top of the cross shaft crank. The crank would be doubled and the link pivots about it's centre. So when the brakes go on, the cross shaft rotates clockwise and the link anti clockwise dividing the brake force equally between the two pull rods. 

 

That's what I thought, at first. That compensation link pulls on the bottom and pushes on the top - what it does do is equalise the force in the two rods. But looking at the way the yokes are linked up, the rod on the left has to pull to pull the brakes on, that on the right has to push.