Timing trouble (Troubled times!)

[ukjimoo]

Dear all

I finished my 2” DNY traction engine about a year ago and have steamed it for quite a few hours now so I decided it was time to set the advance on the eccentrics to try and achieve a nice smooth slow idle.
Some how I don’t think ive got it quite right so wondered if any one could offer any advice. This is Stevenson valve gear by the way.

When the engine was brand new I set the forward eccentric 90degs ahead of the crank and the reverse eccentric 90degs behind and ran it on air. It ran very well and I could get quite a slow idle and the engine would even run as low as 30psi.
Now having set the advance I find that even at 50psi and full throttle the engine does not run very happily. Therefore It seems i must have done something wrong!

This is what I did.
First I checked that the valve uncovered both ports by an equal amount in each direction of its traval.
Then I marked BDC on the flywheel. I found this by marking the fly wheel at the point when the crosshead stopped moving at the end of its backwards sweep and the point when it started going forward again and found the centre of the two points.
I put the reversing lever in full forward and turned the crank over in the forward direction until BDC was met. At this point I advanced the forward eccentric in the same direction until the rearmost port was just seen to crack open and locked the eccentric.
Then I put the reversing lever in full backwards and turned the crank backwards until BDC was met. At this point I advanced the backwards eccentric in the same backward direction until the rearmost port was just cracked and locked the eccentric.
I then had a look to see what the front end port was doing at FDC and it was pretty much the same – about a thou or two error compared to the rear.

I did notice that now the eccentrics are advanced, if I put the reversing lever in the middle position and turn the crank, the valve now moves back and forth and uncovers the rearmost port slightly.
Before i messed around with the setting, it stayed dead still and covered both ports.
I am inclined to think the ports should always stay covered when the reversing lever is in the middle but thats my very uneducated guess!

[baggo]

Hi Jim,

I think you should be setting the valve with the reverse lever in mid gear, not full gear. Stephenson's gear gives a variable lead depending on the cut off setting. If the eccentric rods are 'open' as opposed to 'crossed' the lead increases towards full gear. The amount of increase is also dependent on factors such as the length of the eccentric rods. Setting the gear in full forward and full reverse will not take into account the lead and will probably give an incorrect result.
I am not sure if your method will actually work very well as most methods for setting Stephenson's gear I have read say to set the angle of the eccentrics using a protractor or some method to ensure that both eccentrics are set at exactly the right angle (The correct angle should be mentioned on the drawings or in any construction notes) The valve is then set to give equal opening at each end of the cylinder. Using your method may result in the eccentrics being at different angles and the timing being way out?
Also, altering one eccentric position will also affect the other so you would probably have to repeat the operation several times.
I think the ports should just open in mid gear as a well made engine will actually run in mid gear. It may not self start in mid gear but should run if given a push!
Hope that helps,
John

[JohnP]

If one of Charlie Dockstader's valvegear programs suits the arrangement of Stephenson's valve gear used, I'd get the dimensions into that and spend some time playing with it. You can see the effects of advancing the eccentrics and how the steam distribution behaves at various reverser settings. I'd expect the ports to both be uncovered as Baggo says when the flywheel is rotated on mid gear. One thought - getting the dead centre is never the easiest of things - are you sure they are right? Are front and rear dead centre marks 180° apart?

JohnP

[baggo]

Hi again Jim,
I've just been reading the valve setting method suggested by Martin Evans for Rob Roy and it's exactly how you've done yours so the method is ok.

JohnP's got a point though about measuring TDC and BDC. Your method may not be accurate enough. It's very difficult to judge exactly when the crosshead stops moving. The method usually described involves putting a mark on the slide bar aligned with the front or rear edge of the crosshead when the crosshead reaches just BEFORE TDC or BDC. The flywheel is then rotated CLOCKWISE until the crosshead reaches the mark and a mark made on the edge of the flywheel against a pointer temporarily attached to the chassis. The flywheel is then turned past dead centre and then the crosshead brought up to the slidebar mark again but by rotating the flywheel ANTICLOCKWISE. The flywheel is again marked. Dead centre is then exactly halfway between the two marks, as you have done it. The difference is that the crosshead is brought up to the mark from both directions which takes up any play in the little end. Also it's much easier to judge when the crosshead reaches the mark than it is to judge when it stops.

Hope that makes sense!

I've just had a play with the Dockstader valve gear program and setting the eccentrics to 90° does result in the valve remaining stationary (or near enough) in mid gear.
I've also had a read of Bill Hughes book on the Allchin re setting the valve. He actually says to use an angled template to set the eccentrics but also says that in mid gear the ports should just crack at dead centres. It might be worth resetting the valve so that this happens and try again. If neither port opens in mid gear when the valve is centred the valve is probably slightly too long and needs a bit taking off each end.

[ausdan]

Ive have my Hunslet disassembled while I build a new boiler, and re making the valve gear in places, and will be going through the motion (mind the pun)so once again a good thread well timed

This is some more reading that may help
www.jghtech.com/html/dead-center-1.html
and another
www.steamingpriest.com/project_stevenson.html

[baggo]

Hi Jim,
another few thoughts!
Is the engine the John Haining one in ME 1979? If so, do you have the relevant articles?
Quoting from the valve setting description:

"Both eccentrics are now set as shown on the drawing in advance of the crank by 90° each but both will require moving forward by a few degress to give the required amount of lead or opening to steam at the commencement of the outward piston stroke, not less than 1/64" or more than about 1/32" for this engine.
It is very important to set the slide valve for equal lead so that with the crank on alternate centres the port shows the same amount of opening at each end in forward and reverse gear"

My interpretation of that is that in full gear each port should be open by between 1/64" and 1/32" when the piston is at dead centres rather than just cracked open. Looks like you probably haven't rotated the eccentrics enough or maybe even too far?

[ukjimoo]

Many thanks everyone for your help on this.

Baggo, i will try your method for finding the FDC and BDC. Sounds much more accurate than what i was doing.
I do have the John Haining articles and had read that part but it made no sense to me at all. It could mean the width of the port opening at BDC or the amount of linear travel of the valve when advancing the eccentric when setting up???

I will go with your suggestion and try it.

I know the engine works well on air at 90degs but would you expect the engine to work better on air with the advanced eccentrics or do you need the expansive properties of steam?
In other words if i have improved the running by setting correctly will i see the improvement by testing on air?

Jim

[baggo]

Hi Jim,

I must admit I had to read it a few times before I thought I understood it!
Lead is defined (by Martin Evans anyway) as the amount that the port is open at dead centre i.e a valve gear with no lead will open the valve at dead centre, one with lead will have opened the valve by an amount equal to the lead at dead centre.

I did wonder myself if the engine will only run properly on steam at the correct valve setting. When you had the eccentrics set at 90° the air would be admitted quite a bit after dead centre so would run freely. Now you've advanced the eccentrics the air is admitted quite a bit before dead centre and, I suppose, try and oppose the motion? Perhaps the effect is not so marked with steam?

Hopefully some-one else will chip in and help us out here!

John

[ausdan]

hes another link to a free PDF file (13pages) about valve gear
www.7plusrailroader.com/linkmotion/LinkMotion.pdf

[Donald G]

Hi, when I built my DNY about 10 years ago, I timed it by putting it in full forward gear, and then setting the forward eccentric to give about 0.015" opening on the port, check this also at back dead centre, and when it is as equal as you can get it, do the same in reverse, and set the back eccentric.

My engine will idle very slowly, but also has very good pulling power, especially on the winch where I have moved close on a ton car . I had to block the wheels of the engine as it wanted to go towards the car.

Once set up right it will go well I am sure

Donald G