When should steam enter the cylinder

My understanding is that, on full size, a major reason for having lead (steam entering before T or BDC) is to buffer the mass of the moving piston and, in our sizes, it is not necessary.

As far as shunting or starting is concerned, forget about positive or negative lead. As the piston leaves DC the angle of the crank changes such that the crank is exerting increasing leverage and (if one ignores connecting rod angularity errors) this will be minimal at DC and maximum at mid stroke.

You might like to think of lead in a steam engine being something like advancing ignition in an IC engine

Coltwold ~ Yes indeed I have tried to ignore lead, as many have said in a model it is best ignored. However, part of this exercise is to eradicate it somehow, preferably by tweaking the valve dimensions. we see.

The answer was under my nose. The simulation programme includes a few files one of which is by Don. Careful study shows the valve just cracking open 1 or 2 degrees before TDC which make absolute sense particularly for starting which unlike the internal combustion engine is not running and therefore has no momentum to bring it over TDC.

Don~ Based on calculations I did before it was in the low 70's. However, the loco does perform well with a load, but does make me curious how much better it might be if I could achieve a longer cut-out. This I dont think is possible without reworking the LSBC expansion link as I have maximised the movement of the die block to both top and bottom.

One thing I wished the simulator programme had is a setting for slack or slop in the linkage. Most valve gear if not all must posses some (?) This slack will effect the timing of steam entering the cylinder (delay) and presumably true cut off too.

Finally, someone did write book on this ~ a very good book it is too, isn't it Don !
but boy this self study stuff is hard going.... oh for evening classes

Keith, Allan's program does have the facility to introduce backlash. It's under View/Edit/Conditions.

My feelings on lead is that it's perhaps worthwhile setting it to about 10thou. This then helps to compensate for any 'slop' in the motion etc. when things get a bit worn.

John

Thanks Keith that is exactly the information I was looking for.

This is all way over my head just now.... So much to learn....hopefully it will become a little clearer when/if I reach this stage...

Pete

Thank you Don and Keith for your comments and observations.
This is my first venture into Walschaerts' gear and as with others before my head had been spinning trying to comprehend the factors needed to have things working properly.
This thread along with the previous one has been very helpful in clarifying issues that were mystifying if not totally bamboozling. Now things are starting to fall into place......I think Time to reread Don's book.

Jim

Keith,

'First things first' is spot on - the problems all arise at 'After that......'.

Your supposition of a right-angled triangle is correct but its orientation to maintain the gear's capacity to support equal leads at the valve also depends on the adjacent right-angled triangle which reaches the expansion link tailpin. In turn this assumes that the expansion link is square to the radius rod front pin when the crank is at dead centre. Whilst this means that there is only one correct measurement for the return crank and eccentric rod as a working pair, in practice one wonders just how accurately the return cranks were set in practice. At valve setting any adjustment was made at either the eccentric rod or sometimes the radius rod - neither being strictly correct if it was the return crank fixing at fault. Having often been through the long-winded practice of reconstructing the whole gear to accommodate a minor dimensional alteration (on the drawing, I might add!) I do notice that the revision of the return crank length is extremely minimal. There are already minor timing issues at the return crank, so it is a bit pedantic to worry about by how much a right-angled triangle misses the 90 degree mark. There are angles to four figure decimal degrees on some drawings. OK, so those guys knew trigonometry, but what was actually produced and accepted?

Bill Hall's simulator refuses to allow user input here, which can sometimes annoy, but any simulator gives precise results from what is input - perhaps to a finer degree than we can achieve in practice, though I have tested this out on an engine produced using laser cutting and digital readout throughout. When the reversing lever was dropped into full forward gear the clocked valve travel was within 2 thou of the simulator result! Considering the number of variables and coordinates involved this seems remarkable.

Don

Morning Ian,

There's no such animal as a simple electrical engineer! However, your conception is right. This rather extends Keith's original question but I'm sure that he will agree that separating the point of admission from the other events can add to the confusion you mention. It arises because Walschaerts' gear separates the two functions, each full of timing distortions, and then tries to combine them.

Consider the simple eccentric for Stephenson's gear at 90 degrees to the crank on dead centre. If we now advance the eccentric until its centre has moved linearly by the amount of lap + lead we have created a right-angled triangle with the centre of the axle. The longest side is the eccentric throw, the shortest side is lap + lead, and the other side is the quadrature. The act of advancing the eccentric has already combined the two components precisely and we only need to treat the angularity of rods.

Unfortunately, in both the separated functions of Walschaerts' gear we see introduced distortions adding to the angularities, creating a timing minefield. Blow this lot up to accommodate longer valve travels and the problems of asymmetry magnify accordingly. The best we can do is some astute juggling to counter one distortion with another.

It is quite rare to encounter a Walschaerts' design that can approach the results of the best Stephenson's layouts, yet the model engineer often perceives Walschaerts' as the simpler gear of the two because it is more readily seen in action.

Don