Vacuum Cylinders

Hi does anyone have any experience of operating a passenger train were the coaches in the train are fitted with different size Cylinder e.g. 18" & 21" what effect would this have on braking etc ....?????

 

A greater pressure will be applied where the piston area is greater because pressure=force*area. The force will be the same because it is atmospheric pressure acting on the piston whatever the size.

 

If the cylinder is correctly sized for the vehicle it shouldn't make any difference. It is down to the leverage of the braking system and the weight of the vehicle. If this is wrong then, you will have a situation where a vehicle is over-braked (more likely to lock the wheels) or under-braked (greater application necessary therefore chance of locking wheels on other vehicles).

 

Right I think I understand.... However we were running a restored vintage coach (on LNER goods underframe) with 3 Mk1 and crews were complaining that the Vintage coach (18") was braking harder than the Mk1 on the same Vacuum values.

 

no it isn't, pressure is force/ area, hence its unit- 1 pascal= 1 N/m^2. as the diameter of the cylinder increases the pressure created will reduce.

 

In this situation wouldn't the formula be rearranged to force = pressure x area? So, with the pressure being constant, as the area of the piston increases, so does the total force acting upon it.

 

Takes me back to school does this!

John Elliott Jnr is right. The force exerted by the piston in the smaller cylinder will be less than that in a larger cylinder for the same pressure. The force will increase or decrease as the pressure in the braking system changes.

The pressure on the piston will increase as the vacuum below the piston in the vacuum cylinder is destroyed by the admittance of air into the train pipe. Equally the pressure will decrease again when the driver's brake valve is put into release and the vacuum ejector or pump restores the vacuum in the train pipe.

 

Two questions initially come to mnd:

1) What evidence do the crews have for this or is it simply opinion?
2) Was the LNE underframe originally vac braked or has it been added during the conversion to a coach?

I'm guessing that the vintage coach is a long wheelbase 4wh vehicle with a tare weight of about 13 tons. On this basis we have an area/weight ratio of 18x18/13 = 24.9 (assuming a single cylinder). The Mk 1 will have a ratio of 21x21x2/34 = 25.9 which is pretty similar (unless it's on Commonwealth bogies). In fact the LNE underframe would be contributing marginally less brake power on this basis. Forget about the units as this is irrelevant in the argument. We are only looking at ratios. The only unknown that could have an influence is the arrangement of the rigging and the leverage obtained. If my guess is wrong and (say) the LNE underframe is a bogie vehicle with 2 cylinders then a similar calculation will give you the answer.

 

The underframe was originally Vac fitted however I did miss out one bit of information in that the underframe was lengthened by 2ft. I mention this as it resulted in some of the Roding being modified, length wise so would this have had any affect ?

 

If you've only added lengthened tie rods to the system it is very unlikely to have altered the leverage produced by the linkage. Any change in the angularity of the rod is going to be marginal. If any levers have been altered then that is a different matter as this would affect the brake force. I can't see the LNE designing a goods vehicle with a high brake force. It has to cater for both loaded and empty situations and they would probably design for the latter - locked wheels would be more of a concern than lack of optimum braking on a fully loaded vehicle.

 

Whilst I admire the technical answers here I wonder if the real reason for the variation might be that the 4 wheel chassis has a newly overhauled vacuum cylinder with new rings everything on the brake gear freshly greased etc, wheras the Mk1s might not?

Obviously I don't know for definite as I don't even know for certain which railway we're talking about. But we're talking about old equipment which has wear in it so the exact science may be a little out, personally I'd concentrate on the practical solutions such as perhaps adjusting the brakes by taking a turn or two off a turnbuckle rather than the mathmatical ones if it was my carriage... if it can be proved that the loco crew are right of course and not just looking for non existant problems with C&W stuff.

 

Sorry but I just thought a little update was in need here. Having spent at least a second thinking about mathmetical equations as to why a vacuum cylinder was not working I decided to take an alternative route!

So, having finally managed to get the cylinder dropped out of its casing not helped by a dodgy batch of studs all of which were flawed and snapped, it was time to make it work again.

It was quickly obvious that it had seized because water had got into the cylinder and been left for a few months given the fact that the vehicle in question wasn't a regular worker.

Off with the relief valve first to make sure there's no water or muck still in there. Then a little bit of brute force on the piston rod to free the piston up. Then with a little bit of a well known brand of petroleum jelly and a wire brush the inside of the piston was cleaned up and lubricated.

Onto the hole for the vacuum relief valve with 'Henry' the hoover and hey presto the piston started moving up and down the cylinder more and more smoothly.

New housing found cleaned up and painted inside and out, old housing removed after a bit of a fight with the bolts holding the bearing for the trunnion and new one fitted with plenty of grease on the trunnions... then off to the pub unfortunately having run out of time. Cylinder back on next time, I promise no equations!