Steam Engine Pilot Truck

Bobby Ogage posted:

I have read that the principal function of a pilot truck on a steam engine is to guide the engine's drivers into and out of curves in the track. The pilot truck may also provide some support for the front end of the engine.

What is the mechanical mechanism on the pilot truck that applies a force to the engine's driving wheels as it enters and exits curve tracks?

In a two axle (4 wheel) Engine Truck, there are two rockers, that transmit the initial movement of the Engine Truck wheels, to the truck bolster, and then to the frame center pin, thus forcing the locomotive's frame to the right or left as the case may be.

Can someone show a photo of what the mechanism looks like?

There are excellent reference drawings in the various Locomotive Cyclopedias from, say 1938, 1941, or 1944.

Here you go.

Excellent information Jim....thank you!

Yes - thanks.  I must study that.

My question is: is this true for early 4-4-0 types, where the lead truck is relatively far forward?

bob2 posted:

Yes - thanks.  I must study that.

My question is: is this true for early 4-4-0 types, where the lead truck is relatively far forward?

I don't think that the information that I have goes back much more than the turn of the twentieth century. I'll look and post if it does...but, please don't hold your breath.

Hot Water & Big Jim,

Thanks for your replies.

bob2 posted:

Yes - thanks.  I must study that.

My question is: is this true for early 4-4-0 types, where the lead truck is relatively far forward?

The answer is yes and no.  The earliest 4 wheel lead trucks (on either 4-2-0's or 4-4-0's) simply had a kingpin in the bolster, although a variety of designs were used for the truck itself.  Bissell realized that this geometry was wrong (in sharp curves, the flange on the front drive wheel on the INSIDE of the curve is being forced up against the railhead) so he originated the idea of locating the pin behind the truck, and using an inclined plane centering device to transfer the weight of the front of the locomotive to the middle of the truck (where the kingpin used to be located).  This was patented in 1857.  Others soon substituted swing links for the inclined planes.  These inventions eventually led to the development of the two wheel lead truck and the modern type of 4 wheel lead truck with a centered kingpin and rockers for lateral movement and providing the guiding/self centering forces. 

If you are interested in early locomotive development, I would suggest the book "American Locomotives, An Engineering History, 1830-1880" by John H. White, Jr.

Thanks.  I always wondered about that - it makes sense.  I havea 4-4-0 under construction (D16sb) and will consider some sort of mod to make it work that way.

Bob2, I have some 12" gauge antique park train steamers, all of which are 4-4-0's.  The simple truck with no mechanism for lateral movement is pretty common for those locomotives, but it can definitely cause problems on sharp curves.  As I noted before, the lead truck forces the front of the locomotive to the centerline of the curve, and the locomotive will try to align the center point between the drivers to the centerline of the curve, too.  Since the drivers on a 4-4-0 have relatively little lateral motion, the flange of the inside front driver and the flange of the outside rear driver tend to be pushed up against the railhead.  In the forward direction, this doesn't really hurt the tracking ability of the engine since the lead truck is doing its job (although it does make the flanges on the front drivers wear a bit).  But in reverse, it can cause the flange on the outside driver to dig in and climb the rail, causing a derailment.  This usually isn't a problem unless it is a very sharp curve, and it is worse if the locomotive is pulling a heavy load (again, this is with the locomotive running backward).  I've seen attempts to solve the geometry problem with a simple lead truck by moving the location of the pin back toward the rear axle of the truck, but that doesn't completely fix the issue and it adds the problem of the weight on the lead truck not being equal on the axles - the lead axle will have less weight on it than the back axle.  Bissell's original solution would work nicely, but I've never seen it replicated on a 12" gauge 4-4-0... someday I might give it a try.

On the other hand, Ottaway made allowances for lateral movement of the lead truck by having the pin slide sideways in a slot.  This worked OK for the tight curves that they were designed for, but it really didn't have the self-centering properties and the ability to guide the locomotive around the curves like the prototype lead trucks... it was totally up to the flange on the outside lead drive wheel to guide the engine around the curves, much like an 0-4-0.  They got away with it because of the relatively light weights and low speeds of these trains, but I can see how it would be completely unacceptable with the forces developed by a full size locomotive.  My Ottaway was missing the lead truck when I bought it (it was a pile of parts) so I got to build one from scratch.  Although it has the same overall dimensions as a stock Ottaway lead truck, I decided to make it with swing links.  It doesn't have the positive self-centering action that the rockers do, but I feel they are a big improvement over either the simple lead truck or the Ottaway lead truck with the slot. 

Truck with swing links (Retrofit for 1946 Ottaway):

Truck with pin only (1906 International Miniatures):

 I Hope this information helps on your D16sb project.  Is it an O gauge model?  I realize that it isn't practical to put swing links or rockers in a locomotive that size, but there are still ways to allow sideways movement of the truck, and I'm sure it will help it to negotiate sharp curves.