How to build a T3 - Part 4
Step 5: Getting it Moving
Now we can add the wheels to the body. For this, we use the Wheel keyword.
[Note: if you put the wheel in a separate file from the body, you will need to use the "Rebuild Stock Index" command so that the editor knows where to find the wheel.]
The axles are at L= 249,419,and 549, H=55. So we have:
Wheel 0/249/55 55/0 KPEV_T3_11spoke_110D
Wheel 0/419/55 55/0 KPEV_T3_11spoke_110D
Wheel 0/549/55 55/0 KPEV_T3_11spoke_110D
Wheel 0/249/55 55/0 KPEV_T3_11spoke_110D_L
Wheel 0/419/55 55/0 KPEV_T3_11spoke_110D_L
Wheel 0/549/55 55/0 KPEV_T3_11spoke_110D_L
Note that this is just for test purposes: we will need to change it in a moment.
For the next few stages, we shall be working on the left-hand side of the loco only. Once we've got the motion sorted out on one side, we can do the other.
If you have a slow computer, you may find that leaving the D3D preview open with animation switched on slows down the response of other programs. You can use the red button in the toolbar to stop the animation when you don't need it.
Step 6: Coupling Rods
The coupling rod is the rod that joins the three driving axles together. Its movement is defined by a fixed point on the wheel - the crankpin. To represent this, we need to define the coupling rod as a component, and then make a new component that calls the wheel as a simple include and the coupling rod as a Motion1.
Name KPEV_T3_driver Component Motion1 0/0/-23 KPEV_T3_cpl_rod Include 0/0/0 KPEV_T3_11spoke_110D [end] Name KPEV_T3_cpl_rod Component ... [end]
[Notice how I use the prefix KPEV_T3_ in each name, so that there's no danger of my little T3 coupling rods accidentally ending up on someone else's BR 9F.]
The new KPEV_T3_driver component can then be called instead of the centre driving wheel. It's best to use the centre drivers as the reference frame, because they are also connected to the connecting rod and the eccentric rod. The coupling rod is simply made of four boxes, taking the crankpin of the centre drivers as its origin. You could improve the appearance a bit by making the crankpin bushes octagonal instead of rectangular.
Step 7: Connecting Rod and Piston Rod
From the drawings, we see that the connecting rod is 158 cm long between pivot centres. The centre line of the piston rod is H=55 (in line with the wheel centres) W=105. To make life easy, the cylinders are horizontal.
For the connecting rod, we need to use a Motion2, which defines an object that moves to intersect a line. Thus we have to make a KPEV_T3_conn component, and add to the KPEV_T3_driver component the line:
Motion2 0/0/-23 -158/55/0 KPEV_T3_conn
The piston rod and crosshead moves with the far end of the connecting rod. Thus we need a simple Motion1 statement inside the connecting rod component.
Motion1 0/158/0 KPEV_T3_pistrod
Step 8: The Valve Gear
The T3 is fitted with Allan motion. This is a bit different from the Stephenson gear fitted to most British locos. There are two eccentric rods, 180 degrees out of phase, driving the two ends of the expansion link.
First we have to build the return crank and eccentrics. These are rigid with the centre drivers, so they have to be in the KPEV_T3_driver component. The eccentric assembly is simply a cylinder: the actual eccentrics would be too small to see properly.
The eccentric rods are also rather too small to build up in the same way as the coupling and connecting rods, so the easiest thing is to use a transparent texture.
This is simply a 128x8 pixel bitmap, using black (treated as transparent if the filename starts with "$") to form a long, narrow shape with widened ends. If we stick this on a 3cm high panel, we get an eccentric rod that looks as though it's only 1.5 cm thick.
The eccentric rods are both defined by Motion2 keywords relative to the drivers. In reality the ends attached to the expansion link move in complicated arcs, but to a reasonable approximation we can say they are moving along horizontal straight lines.
The swing link supports the lower end of the expansion link and one of the eccentric rods. Thus its motion is defined by one fixed point (the reverser bracket on the footplate) and one moving point (the end of the first eccentric rod). For this, we need to use a Motion3 relative to the eccentric rod.
The expansion link is trickier, and it's necessary to cheat a bit.. None of the Rail3D motion commands will reproduce this motion exactly, so the trick is to make the expansion link in two pieces, again using transparent textures, each of them using a Motion3 relative to its eccentric rod and an artificial fixed point in the middle of the expansion link.
To link the expansion link to the valve rod, we need a radius rod with a Motion2 relative to an arbitrary point on the expansion link. As usual in Allan motion, the valve rod is inclined along a line passing through the driving wheel axle.
For motion2, we have to define the line of the valve rod in the form
H= a + L*b/100
Measuring the intercepts off the drawing, a=112, b=-18
The valve rod itself is a Motion2 (with the same line) off the end of the radius rod, and finally the reversing link is a motion3 off the compensating link, in much the same way as the swing link.
With the addition of the reversing gear above the footplate (very crudely blocked in for the moment), this completes the motion for the first side. Of course, we still need to add the cylinder and valve chest...
Download mock-up with completed motion components.
Step 9: Cylinders
At this point, before copying the motion components to the other side, it might be a good idea to put in the cylinders and see if the piston rod and valve rod are in the right place.
The valve chest is a box with the cylinder forming its outer lower edge, and stretching from L=124 to 174. However, the easiest way to build it is to make a cylinder of radius 74, centre at H=100, W=72, and then discard all but the relevant quadrant. The cylinder iself is easily formed with the cylinder tool (L=94 to 185, W=105,H=55, R=24). The front end is red, the rest black.
Everything seems to line up, although the crosshead looks a bit far away: time to double-check the measurements! Indeed, a small error shows up at once - the throw of the cranks should be 27cm, not 23. I must have measured it from rail height! This is easily fixed in the driver component, but the crankpins on the wheels will have to be fixed too. While we're about it, lets fiddle with the colours a bit more: German locos have red underframes, after all.
With the further addition of slidebars and a stuffing gland (just a box around the point where the piston rod enters the cylinder), it looks like this:
Coming next: detailing the boiler...