Modelmaking-Final Year: Train of Thought: Technical train things.

http://www.sa-transport.co.za/trains/narrow-gauge/ng_info_pictures/bagnall_0-4-2_line_drawing_aj.GIF

Steam Locomotive Valve Gear Animation

http://home.roadrunner.com/~trumpetb/loco/

TECHNICAL STUFF

This page will explore the technical side of steam locomotives and stationary engines.

The animation shown above shows how a steam locomotive Walschaert valve gear works. This is an interesting animation as it has the same principles as the slide valve we will be basing part of our model on, even if it does not have all the same parts; the intended result of transforming longitudinal motion into rotary motion remains the same.

This link: http://home.roadrunner.com/~trumpetb/loco/ leads to a site which not only provided the above animation but which also proves a useful source for links to other sites regarding steam locomotives.

Steam Locomotive Workings Illustration

http://home.roadrunner.com/~trumpetb/loco/locoworks.html

1. Water Compartment 2. Coal Bunker 3. Worm Coal Conveyor 4. Reverse Lever (Johnson Bar) 5. Stoker 6. Throttle Lever 7. Water Gauge 8. Firebox 9. Crown Sheet
10. Safety Valve 11. Turbine-Generator 12. Boiler Water 13. Steam Dome 14. Throttle Valve 15. Boiler Tubes 16. Dry Pipe 17. Water Delivery Check Valve
18. Sand Dome 19. Flue Tubes 20. Superheater Tubes 21. Smokebox 22. Blast Pipe 23. Steam Chest 24. Cylinder 25. Piston
26. Crosshead 27. Main Rod 28. Side Rod 29. Sand Pipe 30. Ashpan Hopper 31. Brick Arch 32. Grate 33. Injector
For valve gear components, see Walschaert Diagram. (54K GIF)
1. Water Compartment2. Coal Bunker3. Worm Coal Conveyor4. Reverse Lever (Johnson Bar)5. Stoker6. Throttle Lever7. Water Gauge8. Firebox9. Crown Sheet
10. Safety Valve11. Turbine-Generator12. Boiler Water13. Steam Dome14. Throttle Valve15. Boiler Tubes16. Dry Pipe17. Water Delivery Check Valve
18. Sand Dome19. Flue Tubes20. Superheater Tubes21. Smokebox22. Blast Pipe23. Steam Chest24. Cylinder25. Piston
26. Crosshead27. Main Rod28. Side Rod29. Sand Pipe30. Ashpan Hopper31. Brick Arch32. Grate33. Injector

The following passage has been cut and pasted from this site, http://home.roadrunner.com/~trumpetb/loco/cbal.html, as it is not only very interesting from an engineering point of view, but an important part of how we will set up our own slide valves. The things we have to know in order to make a good model!

"Why Are the Main Driver Counterbalances Not Symmetrical?
First, recall that locomotive drivers are quartered; ie: the right/left connecting rod pins are oriented 90 degrees apart. As one crank is on dead center, the other is at the position of maximum torque, and the power strokes are evenly spaced throughout the driver rotation. It appears that most locomotive drivers are quartered with the right side leading by 90 degrees, as illustrated. (At least one example however, PRR K-4s #1361, has 'left hand lead'.)
Main Driver Axle Simplified Force Diagram

Main Driver Axle Simplified Force Diagram

What may not be obvious is that the counterbalances are doing more than simply statically balancing the crank pin, side rods, main rod, and eccentric crank on their respective sides. The main driver counterbalances are "tipped" to provide a force component on one side (eg: left) to cross-balance a rocking couple caused by the opposite (eg: right) side rods and eccentric crank about their own counterbalance weight. This cross balancing renders the axle/driver assembly in dynamic balance as well as static balance. This concept is applicable for all axles, but apparently was done only for the main axle where the additional mass and larger moment arm of the main rod and eccentric crank is much more significant.
Additionally, notice that the eccentric crank tips a different direction on each side of the locomotive (when the side rod is down the eccentric crank slants toward the front of the locomotive for normal inside admission valve gear). The main axle counterbalance position takes this into consideration also, contributing further to irregularity of the counterbalance weights.
A site visitor (thanks, Tim) comments that few if any American locomotives were cross-balanced before around 1930 (?). Our example Soo #2714 Pacific may have gotten the cross-balanced counterweight design with the Boxpok main driver conversion (ca. 1951?).However, another visitor (thanks, John) cites Union Pacific/AlCo 4-12-2s, designed during 1925, as having noticable evidence of cross-balancing on both main driving axles ("tipping" of the counterbalance lunes).
In the context of balance, it should be mentioned also that for this type of two cylinder (single cylinder per side) locomotive, the counterbalance weights balance all of the rotating mass per the above, but balance only a portion of the reciprocating mass.
The rotating mass (crank pins, each end of the side rods, back portion of main rod) can be theoretically perfectly balanced with counterbalance weight. However, the reciprocating mass (piston assemby, piston rod, crosshead, union link, bottom portion of combination lever, front part of main rod) cannot be balanced with the driver counterbalances. Any attempt to balance reciprocating mass only results in trading off horizontal unbalance for a vertical unbalance component (overbalance). This overbalance force, referred to as hammer blow, increases with the square of the velocity and is an undesirable characteristic of steam locomotives that was damaging to railbeds and bridges at higher speeds."

Slide Valve Eccentric

http://www.mgsteam.btinternet.co.uk/svalve.htm

The slide valve is not only one of the most important inventions of the Victorian era, but an important part of any self-respecting stationary steam engine. The above link leads to an interesting site:

Quote: "THE SLIDE VALVE. The admission of steam into the cylinder has to be controlled so that live steam (steam from the boiler) pushes the piston backwards and forwards to obtain a continuous rotary motion to the crank. The mechanism that controls the steam admission is called the valve gear or valve mechanism. The simplest type of valve is the slide valve which consists of a hollowed out block of metal which forms the shape of a 'D' when it is cut through in section as shown."

The same site explains other parts of the steam engine such as the Corliss valve:
Section of a Corliss valve in the cylinder

Section of a Corliss valve in the cylinder

http://www.mgsteam.btinternet.co.uk/cvalve.htm

Together, the slide valve and the Corliss valve work together to drive the steam engine.

http://www.greatwestern.org.uk/basic6.htm
The site which is linked above is entitled 'The Great Western Archive' and shows how the valve gear works on a moving steam locomotive.