Setting return crank

[warthog1950]

Greetings, I'm not having any luck setting the return crank and D slide valves on my Simplex. I can get it to run air in forward, but then it will only turn the wheels approximately half a turn in reverse. can anyone please help?

[pondok]

Hi warthog,
Usually good timing it only depends on 3 things:
1- Angle of the return crank
2- Length of eccentric rod
3- slide valve dimensions

With the engine in front dead centre on one side, wheel eccentrics lined up with wheel centres, with the engine in true mid gear (radius rod in the centre of the expansion link pivot) is it possible to move the return crank on the pin so that the front face of the slide valve is just about to crack open the front port?

If it is, and assuming you've managed that with the return crank small end following the big end (not leading!) in the forward direction of travel (ie anti clockwise on the LH side), then so much the better, but not finished yet.

If you then fix the return crank in that position and roll the engine forwards to rear dead centre on the same side, the rear face of the slide valve should now just be about to crack open the rear port ("edge to edge"). If it doesn't, IN THEORY, there is an issue with the dimensions of either the valve itself, or the eccentric rod and/or return crank.

If you post a pic of the drawing or close-ups of the valve chest in front and rear dead centre, maybe we can tell you more. I do find trying to describe this stuff in words aint easy - much easier to use pictures.

cheers
andy

[baggo]

Hi warthog,

As Andy says, it's difficult to help without having the loco in front of you. Are you following Martin Evans's method for setting the crank and the eccentric rod length?

I would suggest that possibly the eccentric rods are the wrong length. It is still possible to then set the valves in forward gear so that the loco runs ok in forward gear but when you put it in reverse the valve timing will be miles out.

Incorrect setting of the return crank will only affect the valve travel in full gear and the error will be the same in forward and reverse i.e. it will run the same in forward and reverse.

Have you tried running the chassis with just one cylinder at a time? It's possible the problem may be with one side and not both.

Andy - for slide valves the return crank leads the crankpin when running forwards, not follows it.

John

[pondok]

Yes sorry quite right John, my text should have read

"If it is, and assuming you've managed that with the return crank small end leading the big end (not following!) in the forward direction of travel (ie anti clockwise on the LH side), then so much the better, but not finished yet."

Post some pics Warthog.

cheers

[baggo]

Hi warthog,

If you look on Dr John's website on page 10 of the Simplex section, he shows a way to set the return cranks using a simple jig:

www.dr-john.org/

He also suggests that the dimension given for the eccentric rods is too long although I've checked all the valve gear using Charles Dockstaders software and the dimensions seem pretty good. However, there was some confusion regarding the position of the motion brackets in the ME articles although this should have been corrected on the drawings?

John

[steam4ian]

G'day all.

I post my comments more as questions rather than staements to be followed because as may become obvious I have no practical experience with this issue.

I understand that the return crank setting is inviolate. That is it is set with pin at 90deg. to the crank pin. If the piston is at TDC the crank pin will be at 9 o'clock and the eccentric pin will exactly be at either 6 or 12 o'clock depending on inside or outside admission. So the return crank must be adjusted to set the pin to exactly 90 deg. to the crank pin.

Then if the eccentric rod is the right length and the link has the right backset, raising the radius rod from full forward to full reverse and back again should not move the valves.

IMHO, for Warthog to check his setting he should roll the loco so the piston is at TDC and check for valve movement when shifting from full forward to full reverse; the valve/s should not move; then repeat the same for the piston at BDC. With the loco on a true susrface a rough indication of return crank status would to have piston at TDC and use a square to check the 90 deg angle alignment of the axle centre and the eccentric pin. If that is well then the problem is the eccentric rod length, the motion barcket position or the backset.

Hope this doesn't confuse.

Regards
Ian

[warthog1950]

Greetings, Thank to you all for the really useful information. I really found it useful, and will let you know how I get on.
Regards, Warthog

[steam4ian]

G'day Andy (Warthog)

One correction occurred to me after my post; did anybody detect the "deliberate" mistake?

My comments were based on the "square" valve gear arrangement common on US locos. That is, eccentric rod is horizontal when the link is at either end of its travel.
Many English locos have the link mounted above square, I guess to minimise width so my angles will have to be adjusted by the amount the eccentric rod is above the horizontal.

Regards
Ian

[drjohn]

Hi Ian

Simplex is a "all square" loco. The calculations for Simplex (and any other all-squares) etc are on my site under "tips"

DJ