EMU, Variable Load Valve

[Adam Turner]

Hello,

Does anybody know what a Variable Load Valve does?

Am I correct in thinking this is what you need to adjust (via a nut) to adjust the brake pressure?

And I believe the averaging valve sends the variable load valve its signal (averaging valve gets its signal from secondary suspension).

But where does the variable load valve send its feed to?

Thanks for any help!
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After looking over this forum some mor, you chaps seem to be into the actual physical design of a train rather than its technical runnings. If this is the case, would anybody be able to advise me on a more technicaly advanced forum? Thanks!

 

[rail-britain]

From : railway-technical.com

Variable Load Valve
Also known as a Retainer, in the US. A valve used to vary brake application individually on each vehicle depending on the weight of the vehicle, it can be manually or automatically operated. In the manual version - used on freight vehicles only - a lever at the side of the wagon must be set for the required position.

Automatic versions of variable load valves are now often used. A simple version is operated by a lever connected between the valve mounted on the car underframe and the bogie frame. As the car load increases, the lever detects the depression of the car body and valve relative to the bogie and adjusts the setting of the valve in direct proportion.

On vehicles with air suspension, the lever is used to adjust a levelling valve which changes the air pressure in the suspension system so that the car body maintains a constant height, regardless of the load. Changes in the suspension air pressure are detected by a separate variable load valve and the brake application adjusted to suit.

The variable load system can also be used to adjust train acceleration so that it is constant regardless of load

 

[dan_atki]

Hi, welcome to the forums.

I did find this after a bit of a search: http://www.railway-technical.com/brake1.shtml#variableloadvalve

Looks like there is both a manual and an automatic aspect to it.

Regarding the technicalities of rolling stock - there are a few people on here who may be able to help you out in a bit more detail than that link would

 

[Adam Turner]

Wow, excellent, thanks so much!
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Also, does anybody know whether there is a page out there for a specific class of train? For instance, if I were to want to research a 321? Thank You

 

[rail-britain]

It really depends on what you want to know about the Class 321 EMU...
Design specification, technical specification, history, operational use, and so on

 

[laseandre]

uklines.net has some technical info on classes of trains, but no history or anything.

http://www.uklines.net/rs/321.htm < Page on 321s.

 

[Adam Turner]

Thanks for your help guys.

But what I am looking for is a list of components on the train, and a brief description of their function.

i.e. Primary Suspension - Suspension between track and bogey.

Secondary suspension - suspension between bogey and coach

etc

: )

 

[dan_atki]

uklines.net has some technical info on classes of trains, but no history or anything.

well not yet anyway :razz:

Seriously, though, in time it will have comprehensive information for each class currently in service as well as those no longer in service.

I know it is often frowned upon but Wiki is probably the best resource for the history of a specific class (321 page: http://en.wikipedia.org/wiki/British_Rail_Class_321) still, I'd be weary about some of the statistics on there - in compilation for figures for the stock pages on UKLines I found many discrepancies between Wiki, other sites, and books. If you can I'd recommend trying to confirm what info you find on there with another site/source.

Technical wise, The Railway Centre is a great resource for everything you'd want to know about classes both in and out of service (321 page: http://www.therailwaycentre.com/Recognition Tech Data EMU/EMU_321.html).

You'd also find that some TOCs such as Arriva Trains Wales and First Capital Connect have technical data about their fleets on their sites too.

 

[rail-britain]

It might better to get in contact with the Fleet Manager of the TOCs that operate these specific units

Ian Brookes at London Midland might be your best bet
London Midland
PO Box 4323
Birmingham
B2 4JB

London Midland won't have Class 321 units for much longer, so you'll need to be quick
I would have given you details for First Capital Connect, but they don't seem to have Class 321 EMUs anymore
First ScotRail have the Class 320s and 322s, quite similar to the 321s

 

[Adam Turner]

Thanks for all your help guys!

 

[driver9000]

Variable load valves on all air-suspension vehicles is essentially the same. there are push rods between the bogie and body. When the vahicle load its pushes the ords down opening an inlet valve to allow air to pass into the suspension bags to keep the solebar at a constant height to the rail. When the vehicle empties the opposite happens with the rods being pulled up opening a vent to allow air out to atmosphere (you can hear the hissing as the vehicle empties). The primary suspension on a Sprinter for example is a series of rubber blocks around the axlebox, and the air-suspension is the secondary suspension.

http://vicdriver.fotopic.net/p48138411.html

If you note above the axlebox end you can see the primary suspension blocks.

 

[TheSlash]

Variable load valves work off the pressure in the air suspension.
The air suspension is regulated by something called the horizontal levelling valve, which regulates the air flow to air bags to keep the vehicle body level.
I think both these valves just look at one bogie, so you would have 2 per coach, working independant of each other.
Horizontal levelling valves work by keeping the air suspension bags at the same height. So if you have all the weight on one side of a vehicle, the Horizontal Levelling valve detects this and adds extra air pressure to the heavy side.
Variable load valves are a large spring. The pressure of the air suspension pushes down on the spring, which opens the brake cylinder port further. The more pressure in the air suspension, the more it pushes down on the spring and the wider the port opens, this lets more air into the bake cylinder.
So basically a variable load valve gives you the same braking performance whatever the vehicle loading.

It's all tricky **** and i'm thankfully i never had to set up the ride height on an air suspension vehicle!!

 

[O L Leigh]

I think some of the respondents are confusing the Variable Load Valve (VLV) with the Self-Levelling Valve (SLV), though all the correct information is there.

Basically, the SLV keeps the vehicle at a constant height irrespective of the load by adjusting the pressure inside the secondary air-bag suspension. On most UK stock this is done automatically using a self-levelling bar with one bar operating each bag. One end of the bar is attached to the bogie and the other end to the SLV which is mounted on the underside of the carriage body. As more people get on the vehicle their weight pushes the carriage body down, lifting the bars up to open the SLV's in one direction to let more air into the bags until the distance between the carriage body and the bogie is restored. When all the people get out again, the carraige body rides up which drops the bars to open the SLV's the other way to vent the surplus pressure to the atmosphere.

Where the VLV comes in is to ensure even braking performance throughout the train irrespective of how heavy each vehicle is. The way this works is a bit complicated, but it is tied-in with the air suspension. Bear with me.

Each air suspension bag has an associated air tank known in the biz as a surge tank (these are normally seen under the carriage body just in-board of the bogie). This acts as a damper for each air-bag to prevent too much rebound, and as an expansion chamber to reduce the risk of over-pressure causing a blow-out. Air is taken from each of the four surge tanks under a vehicle to an Averaging Valve (AV) that produces one average pressure from the feeds it receives from the four surge tanks. The heavier the vehicle is, the higher the average pressure will be.

Air is then sent from the AV to the VLV which, on EMU stock like the Cl321, lives in the brake chest along with the 3-step relay. The VLV takes the pressure from the AV and feeds out an equivalent greater volume of air to the 3-step relay which will produce a brake cylinder pressure in proportion to the pressure sent by the VLV. So, the higher the pressure from the AV and the VLV, the higher the brake cylinder pressure will be in each brake step.

Slash: I'd bow to your greater expertise on the basis that the inner workings of the VLV was never explained to us, but I have to say that your description doesn't sound entirely correct. The reason I say this is that the VLV has a fail-safe setting. If there is no input from the AV or VLV (e.g. the air suspension system leaks and has needed to be isolated), you will get the maximum possible brake cylinder pressure in each step as if the vehicle was full and standing. If it relied on air pushing against a spring to open a valve I don't see how it could do that.

O L Leigh

 

[driver9000]

They lumped it all in to one system for us and called it Variable load valve, as far as we were concerned it was all one system - not that they went into great detail though

 

[TheSlash]

The VLV looks at the Horizontal Valve to determine it's brake force. I'll see if i can find the drawings to check for fail safe.
Just when you think you've sussed it, there is also another bit, the constat discharge. This constantly lets air out of the air system {only alittle bit} so that the valve is constantly moving and doesn't become siezed.