Looking at photos of many steam locomotives, It seems that the lead truck (pilot truck, front truck?) always has inside bearings, while the trailing truck (rear truck) always has outside bearings. WHy the difference?
Enzoamps Looking at photos of many steam locomotives, It seems that the lead truck (pilot truck, front truck?) always has inside bearings, while the trailing truck (rear truck) always has outside bearings. WHy the difference?
It wasn't always the case...
But the trailing axle is under the firebox and is subject to heat and ash and coal dust. By locating the bearings outside, they run cooler and are less likely to get ash dropped on them.
The leading and trailing wheels are generally much smaller than the coupled wheels so the bearing rotates faster at a given speed. The load on the leading truck axles is generally a lot less than that on the coupled axles, which helps the bearing life. The trailing axle is often loaded more than the leading axles, and as a result the trailing axle sometimes has a larger diameter wheel than the leading axle, which reduces the bearing maximum speed.
Many locomotives around the "super power" era had outside bearings on the leading axles. When roller bearings were introduced, they were sometimes used on the leading and trailing axles only, partly because the smaller bearings were cheaper, but also because it increased the reliability of the locomotive.
Peter
M636C Enzoamps Looking at photos of many steam locomotives, It seems that the lead truck (pilot truck, front truck?) always has inside bearings, while the trailing truck (rear truck) always has outside bearings. WHy the difference? It wasn't always the case... But the trailing axle is under the firebox and is subject to heat and ash and coal dust. By locating the bearings outside, they run cooler and are less likely to get ash dropped on them. The leading and trailing wheels are generally much smaller than the coupled wheels so the bearing rotates faster at a given speed. The load on the leading truck axles is generally a lot less than that on the coupled axles, which helps the bearing life. The trailing axle is often loaded more than the leading axles, and as a result the trailing axle sometimes has a larger diameter wheel than the leading axle, which reduces the bearing maximum speed. Many locomotives around the "super power" era had outside bearings on the leading axles. When roller bearings were introduced, they were sometimes used on the leading and trailing axles only, partly because the smaller bearings were cheaper, but also because it increased the reliability of the locomotive. Peter
Alfred Bruce in his book argues that adoption of outside bearings on the trailing truck (which he calls a "trailer truck") adds lateral stability. Those outside part of the truck frame in which the trailing wheels are sprung connects to the frame with heart-shaped rollers. That shape lifts the side of the locomotive in the direction of the truck swing, which provides centering force. The rear drivers are prevented from lifting up on account of the equalizer beams keeping the drivers on the rails.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
The engine trucks on many steamers had outside bearings:
prr-s6404l06.jpg (1200×491) (rr-fallenflags.org)
selector The engine trucks on many steamers had outside bearings: prr-s6404l06.jpg (1200×491) (rr-fallenflags.org)
True, that!
The reason for outside bearing on the trailing truck are said by Alfred Bruce to be lateral stability, especially for the wide-firebox-over-the-trailing-wheels arrangement from the USRA designs and later.
A reason for inside bearings on the lead (engine) truck could be clearance, especially between a 4-wheel engine truck and the cylinders. This picture appears to be the Pennsy 2-10-4 that was imposed on the railroad by the War Production Board in the 1940s. For a two-wheel lead truck as seen here, there does not seem to be any problem clearing the cylinders.
GN's two classes of 4-8-4's had outside bearing lead trucks:
An outside bearing is a LOT easier to work on. I think those are roller bearings.
I believe you could change the bearings on those trucks just by sliding the bearings off of the axle stub. NOT something you could do if the bearings were inside the wheels.
Ed
7j43k GN's two classes of 4-8-4's had outside bearing lead trucks: An outside bearing is a LOT easier to work on. I think those are roller bearings. I believe you could change the bearings on those trucks just by sliding the bearings off of the axle stub. NOT something you could do if the bearings were inside the wheels. Ed
Yes they are roller bearings and the move to roller bearings prompted the move to outside bearings on lead trucks.
And, as explained above, outside bearings on trailing trucks are about all of the things mentioned, stability, heat, cleanliness.
Very early trailing truck setups had inside bearings, that did not last long.
Sheldon
Paul Milenkovic Alfred Bruce in his book argues that adoption of outside bearings on the trailing truck (which he calls a "trailer truck") adds lateral stability. Those outside part of the truck frame in which the trailing wheels are sprung connects to the frame with heart-shaped rollers. That shape lifts the side of the locomotive in the direction of the truck swing, which provides centering force. The rear drivers are prevented from lifting up on account of the equalizer beams keeping the drivers on the rails.
While an outside beaing trailing truck would provide more stability, this was not considered important in some areas.
In Germany, trailing trucks were almost exclusively equipped with inside bearings, right up to the end of construction in the 1950s (using roller bearings). There were a couple of exceptions, the class 05 4-6-4 and the class 06 4-8-4. 05 002 exceeded 124 mph, so I guess its truck was satisfactory. But basically, among engines with tenders, only these four locomotives had outside bearing trucks.
I think Delta trailing trucks used heart shapes in the linkages which were behind the trailing axle. However, what about the Pennsylvania Railroad "KW" trailing truck? This was loaded in front of the trailing axle, and the truck frame acted as an equalising beam (while the Delta truck needed a separate equalising beam which sat inside the cast frame. I don't think the KW truck had any equivalent of the heart shapes to provide a centering action and may have relied on spring centering.
For trailing trucks, besides the eccentric shaped rollers already described, there were also inclined rollers, where the round rollers had to roll "uphill". And also links. The point of these three, and perhaps others, was to use the weight of the locomotive (by lifting it), to apply return pressure on the trailing truck. I believe the roller style was used on lead trucks, too. And maybe links.
Inside bearing trailing trucks could look pretty elegant, I think. As a modeler, I kind of wish I had a good excuse to get something so equipped.
Exceptions are always interesting. And I always thought the Atlantic was a very handsome locomotive.
It seemed a lot of 2-6-2 and 2-8-2 locos were built for logging and mining with inside bearing trailing wheels. The trailing axle was more for guiding reverse moves as these operations often lacked turn-around facilities.
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