kcsivils Wrote:
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> How do pilot and trailing trucks perform the
> function they were designed for?
>
> What specific functions are they designed for?
>
> Why would a four wheel pilot or trailing truck be
> used instead of a two wheeled version?
First off, I'll limit this mostly to North American practice: European railroads usually spent a great deal of effort keeping their track top-notch and could (and did) get by using rigid-frame locomotives for a very long time. In essence you can use a flexible locomotive on iffy track, or a rigid engine on good track, but if you want to use a rigid engine on iffy track you're going to have problems. Especially in European practice, you see rigid wheels--especially trailing trucks--used very late, until the end of steam in some cases. Flexible lead and trailing trucks steady the locomotive and also spread out the weight. Early on they were always more or less rigid, sometimes with slight side-to-side play allowed, but basically rigid either way. In early North American railroads you see the use of 0-6-0 and 0-8-0 freight engines, or sometimes rigid-frame 2-6-0 freight engines, none of which were ever quite satisfactory. Unpowered axles could spread weight out but if mounted rigidly they didn't do a whole lot to help a locomotive pick its way over rough track.
The flexible 4 wheel lead truck was invented first. It was found that rigid-frame locomotives did not track well enough on the quality of railroads typical here in North America. The 4-wheel lead truck permitted better running over iffy track, leading to the 4-2-0 which itself led to the 4-4-0 american standard locomotive (and also the 4-6-0 by the end of the 1840's, but it didn't really catch on right away). They work via a few different setups. Sometimes you have them mounted to a fixed pivot--this was common on the 4-2-0's and on earlier 4-4-0's. On such 4-4-0's the lead drivers were usually flangeless so as to allow for curves. In other setups, appearing a little bit later on (but still more more less civil war era) you see the use of swinging or sliding bolsters to permit lateral motion of the lead truck for curves so all four drivers could be flanged. These lateral motion trucks also provided centering via spring controlled motion or sometimes just gravity. The hangers for the swing motion were arranged on an angle such that moving side to side also lifted against gravity. Swing motion trucks on passenger or freight cars typically work the same way. A radius bar could be applied so as to improve centering force; it was common for higher-speed engines but frequently omitted where sharp curves were present. For example, the D&RG 4-4-0's had swing motion lead trucks but no radius bars. Axle load reduction from using 4 instead of 2 lead wheels was also beneficial during the iron rails era, especially if higher speeds were expected. Later on, some 20th century engines were built with 4-wheel lead trucks solely as to reduce axle loads. This was especially associated with 1920's 3-cylinder engines like the 4-10-2's and 4-12-2's that didn't necessarily need to run at high speed but understandably had a lot of weight up front.
The two wheel bissel truck didn't see common use until the mid1860's, where you see the development of the 2-6-0 and 2-8-0 types. Like the 4 wheel swing motion truck, the bissel truck uses gravity to provide centering motion. The industry rapidly regarded it as good enough at steadying an engine on curves for slower freight engines, but not quite sufficient for higher speeds. Virtually all attempts by various railroads at using 2-4-0's and 2-6-0's and even the earliest 2-4-2's for high speed work failed due to the same complaint of insufficiently steady motion. On slower freight engines it worked well enough and left more weight on the drivers since it, itself, could not carry as much weight as a 4 wheel truck--an advantage for hauling heavy loads provided no great speed was required. Consolidations ultimately became the most common of all steam locomotives, but just about no-one ever regarded them as speedy.
Rear trucks could be for guidance while running in reverse. Many of the very first uses of rear trucks were for this purpose. These were usually two wheel once the bissel truck came available. As with lead trucks, rear trucks could be based on swinging or sliding motion for side to side play. Such early engines were often but not always tank engines, and during the post civil war era were often called "double-enders." Later on, 2-wheel rear trucks became standard for carrying larger fireboxes that couldn't fit over or between driving wheels. These expanded into 4-wheel (rarely, 6 wheel) tailing trucks once trailing truck axle loads started to exceed permissible amounts for a single axle. The 4-8-4 was not developed out of the 4-8-2 because there's anything special about the 4-wheel rear truck, but because putting upwards of forty tons or more on a single axle was not acceptable. The 4-wheel trailing truck was almost entirely used for purposes of axle load reduction. Many of the engines that had 4-wheel trailing trucks could not even run safely in reverse except at a crawl because the 4 wheel rear truck was quite often a hinged frame extension to make room for an enlarged ashpan and such engines tracked badly when backing up.
European practice saw usage of several designs like the cartazzi axle or the krauss-helmholtz arrangement, but those did not see much if any use in North America so I will omit them for the purpose of shortening an already long post. Suffice it to say they did the same jobs just using different means.
"TLDR:" Pilot and trailing trucks are for either steadying motion or reducing axle load, or both, and work via a variety of different designs.
EDIT: I didn't even get to mentioning weight equalization levers and distribution schemes. It could turn into a book. Books or at least long articles *have* been written on the matter.
Edited 2 time(s). Last edit at 10/17/2020 12:33PM by James.
