R/C slip eccentric

[METHSSNIFFER]

Am I right in thinking isnt reverse changing the entry of steam to the piston from the valve chamber and isnt this change governed by the distance the valve rod as to move to close one port and open another ie exhaust and inlet. If this is correct then slip eccentric isnt required only a linkage connecting both valve rods. A servo fitted between the frames would throw the necessary distance required to reverse the direction of the valves

[LnBmad]

The issue there ash is to do with the slide bar. On other valve gears you move the slide bar up or down to move the valves to cover the correct port. Slip eccentric gearing has the slide bar as part of the eccentric, this means the slide bar rotates around the axel and so cannot simple be raised or lowered into gear.

[DolwyddelanLightRail]

For this to work with a motor in a tender/waggon behind the loco, the motor would have to act as a starter motor, such like on diesel engines where they engage into a mesh turn however much you want and then dis-engage. Fitting something like that to a small motor would be quite difficult I should imagine

[GTB]

Am I right in thinking isnt reverse changing the entry of steam to the piston from the valve chamber and isnt this change governed by the distance the valve rod as to move to close one port and open another ie exhaust and inlet.

There's more to it than that for slide valves. What you describe is just swapping ports and is what the reversing valve does on wobblers and piston valves like Accucraft.

With slide valves and piston valves, the valve has to move in sync. with the crank pin so it opens the correct port at each dead centre. In forward the eccentric is 90deg plus a bit ahead of the crank pin, in reverse it is the same angle behind the crankpin.

With slip eccentric the eccentric is loose on the axle and rotates between two stops that determine the direction. Roundhouse 'simplified Walschaerts' does the same thing as their slip eccentric gear, but using a reversing link driven by a fixed eccentric.

Anyone wanting to understand what goes on in the valve chest of a little loco can do worse than read the relevant chapter in Greenly. The first edition is available online to download and is better in my opinion, as later editions spend less time on simple valve gear and more on his own design.....

http://archive.org/details/modellocomotive00greegoog

Graeme