walschaert,s valve gear

[exairman]

I have now reached the point where l am able to run the Darj B class l am building. After various tribulations with the boiler(more of that later) the engine ran but was a very jerky and not continous in reverse. So back to the books, on reading l discovered that the return cranks on an outside admission system (slide valve) should be to the left of axle centre when the crank is at the bottom of it,s stroke, mine are to the right, if l were to change them it would no longer be prototypical. Drawings and photos clearly show the return cranks are the right of the axle centre. When l came to the books LBSC,s Model Locomotive Building the overall drawing of Tich at the front of the book shows the return crank to the right, later in the book in the construction bit it shows the crank to the left. A similar contradiction occurs in Martin Evans, book Manual of Model Steam Locomotive Construction. Now totally confused prefer it where l have got at it adds to the spindly appearance of the valve gear. Do l need to change it? Regards exairman

 

[goldstar31]

Here, I would strongly suggest that you read D.L.Ashton's stuff on this topic.

Don wrote originally two small books Stephensons and Walshaerts Gear- publishing them at his own expense. Subsequently, Don created a web site and created a combined book.
From my own contacts with him, he checked all his work with Manchester University prior to going into print and is the accepted guru on the subject.

Don posts regularly on Model Engineering Clearing House forum and I have no doubt that he will assist in your query.

From my last information, he is involved yet again with a full size loco as well as his many interests in such things as tugs, saxophones and writing music!

One sits at the feet of the Master- and marvels!

Good Luck

Norman

 

[exairman]

I forgot to mention l was viewing the loco from the left hand so l am talking about instead of read aft and of course for left fwd , sorry about that but l did say l was confused. Regards exairman

 

[Charles Lamont]

For a start, it depends which side of the engine you are looking at. Secondly it depends in whether the engine is inside admission (usual for piston valves) or outside admission (almost all slide valves). Thirdly it depends on whether you lower or raise the radius rod for forward gear.

Lets look at the left hand side of the engine and assume the more common arrangement in which you drop the radius rod for forward gear. If the crankpin is at the bottom, and you want to got forwards you have to admit steam to the front of the cylinder. A slide valve will need to be pulled back, so the expansion link will need to be rocked back at the bottom, so the return crank will be behind the axle, or leading the crankpin (going forwards). It will be 'on the right' on this side and 'on the left' on the right hand side of the engine. Well, actually, with the right-hand engine leading, the RH crankpin will be at the back with the return crank at the top. Conversely, back on the left now, an inside admission piston valve will need to be pushed forward, to let steam in to the front of the cylinder; so the return crank will trail the crankpin.

Depending on the valve gear layout, the return crank may not be at exactly 90 degress to the crankpin. Many designs have a sloping eccentric rod. Roughly speaking, still with the crankpin at the bottom and the expansion link at mid-swing, the return crank pin will fall on a line drawn through the expansion link eye and the axle centreline. For a slide vale the return crank would be shorter, and for a piston valve longer, than with an 'all square' layout.

 

[exairman]

Many thanks for that reply,it would appear that my version is the correct one or is it? If l were to reposition the return crank to the fore of the axle it shorten the eccentric rod and reverse the the lever or alter the position of the reversing rod from above the lever fulcrum to below it in order to maintain the instinctive movement of the lever. Still would seem all is in order as is, much to my relief. Now all l have to do is sort out why it is not working quite like it should. In LBSC,s book he state that when the valve is in the central position it should just span the ports and in fact the ports should a crack at both ends, is this correct?Regards exairman

 

[Charles Lamont]

In LBSC's book he state that when the valve is in the central position it should just span the ports and in fact the ports should a crack at both ends, is this correct?

Walschaerts gear is usually set for equal lead in mid-gear I think. That is to say, with the radius rod in mid position, so that it does not move, the valve should open each end by equal amounts (measure with feeler gauges). The amount of opening is the 'lead'. In full gear, both forward and reverse, the valve should open similar amounts at each end, but don't expect it to be exact. If properly designed and made, the forward events shold be better than reverse. (I have a vague idea Darjeeling Himalayan locos always point up the grade to keep water over the crown, so they won't have to do much work in reverse gear anyway.)

With the piston at dead centre, front or back, the expansion link should be at the centre of its swing, so moving between full forward and full reverse should not alter the position of the valve.

 

[WileE]

Charles Lamont
exairman has tried to reply to you but for some reason the system is not showing it. he would like to thank you for your help and advice and he is pleased to report that the Darj is now running really well, on cop air as the boiler has sprung a leak!!
Cheers
WileE and exairman

 

[crueby]

Goldstar - thanks for the reference to Ashtons web page - been researching the valve gear for an upcoming model, and had not run across that one yet. Lots of good info.

Here is a link to it:
http://www.donashton.co.uk/index.html