Repair & renovation images
I spent some time getting my Live Steam Mallard working, and I have learnt a lot getting there. I thought it worth sharing the process. The loco was running better backward than forward, and whistling slightly all the time. There were two problems, one with the adjustment of the timing screws, and the other in the valve that directs superheated steam to whistle or thrust. For those who do not already know, the steam valve operates by having a cylindrical chamber. In the floor of the chamber are two holes, one leads to the whistle, the other to the piston valves for thrust. Pressed against the floor by a spring is a cylindrical piston which also has two holes drilled through it. The piston is rotated by a drive shaft that is keyed into the bottom of the piston. The angle of the piston selects whether the whistle holes or the drive-piston holes in valve-piston and floor align. The valve-piston as I extracted it is in the first microscope photo below. You will see that it is scratched, both radially and in parallel lines. The parallel ones must be original! At any rate, this accounts for the whistle drooling all the time. I cleaned it up with some 1200 wet&dry, no more uncertainty in the whistle or loss of steam, that I can see. Note also the wedge-shaped groove on the piston hole, the method of adjusting thrust.
steam valve 'piston' before:
after:
The next photo shows the shaft that governs the timing to cylinders. The central black sprocket is driven from the wheels. There are 5 screws in the sprocket, one holds onto its shaft, the others form two adjustment and lock screws pairs. The final photo shows same closer up, and with the black sprocket rotated. Observe the stainless steel pin that protrudes through a slot from the brass sprocket immediately behind the black sprocket, concentric with it but on a separate concentric shaft. The pin is free to advance or fall back about 15 degrees as the relative rotation of the two shafts and two sprockets changes. Observe that two screws define the limit of lead or lag between the shafts. These adjust the piston timing. Adjusting the forward and reverse screws optimises the phase of steam delivery. The forward one needed 0.75 turns clockwise to get forward running well.
The last picture shows the steam distribution valve. The small black button-shaped disk (slightly out of focus at the front of the photo) directs steam, introduced into the chamber through the small asymmetrically-placed hole at the top of the chamber, into the four holes visible around the drive shaft, while the remaining valves not receiving steam are vented to the chimney (through the hole in the shaft in the centre?) that also drives the little button-distributor.
Hope this is useful to someone.
Hi Jonathan. I read your article with a lot of interest. I have had exactly the same problem. The steam whistle kept whistling due to scratches in the rotary steam valve piston.
I bought the Mallard new. I lubricated, serviced, timed it, and it got running really well. However, as quick as after 20 runs this problem occurd. Was this problem developing this fast for you as well? What can we do to prevent it from scratching (again)? This is almost the only part of the loco which cannot lubricate without dismantling the loco for a part.
Has there maybe been a tiny "thing" between the valve and the surface wich caused the scratches?
Kind regards, Ryan