Gresley 3-Cylinder Steam Chassis

This steam engine chassis has rods for 3 cylinders in the Gresley pattern.
This means all three cranks are on the middle axle.
This was used in Gresley A1, A3 and A4 Pacifics, V2s and other engines.
These engines mostly has Greley's conjugated valve gear too.  This required
regular lubrication and maintenance.

On Thompson and Peppercorn chassises there is divided drive,
which means the centre piston acts on the front axle instead.
That was used on the Thompson B1, Peppercorn A1 ("Tornado") and others.
Thompson favoured a third set of Walschaert's valve gear in the middle, in
preference to Gresley's valve gear.

In order to avoid contact with the front driving axle, Gresley Pacifics have
the centre cylinder inclined at an angle of 1 in 8.
This means the crank angles are not precisely 120 degrees apart in real life,
perhaps being at 120 degrees, 117 degrees and 123 degrees.

There are quite a few modified parts in this design:
Wheel cut down from 32020 model team wheel hub, to make 5'4" wheel in 8mm:1ft scale.
Two 2.5M axles, cut from one 5M axle.
Short half bush (~3mm) cut from crenellated half bush.
15mm axle with stud on end, cut from 3M axle with stud on end.
1M axle with bezel, cut from axle pin.

The gear ratio from a 71427 or 43362 gearmotor is 5:1 overall.
I have run the chassis motor at 9V, which produces a scale wheel speed of 135mph,
faster than the design speed of the locos.

Mark Bellis
2nd May 2009