As with any component of a steam locomotive, the design of the driving wheels changed over the years as a result of advancements in technology. Obviously, the main purpose of the driving wheels of a steam locomotive is to support the weight of the locomotive and transfer linear force from the pistons into rotational force applied to the rails. However, there are many other important aspects of steam locomotive driving wheels that may not be immediately obvious. For example:
Pretty much from the beginning of steam locomotives, the drivers were cast with solid spokes connecting the inner hub to the outer rim. A "tire" of stronger steel was heated and press-fitted onto the outer rim of the wheel.
Toward the end of steam, newer technologies were used to construct the inner portion of the driver wheels. The main reasons for the change in technology were to strengthen the driver and to provide better counterbalancing of the main rods. Both of these issues were addressed using "disc drivers" instead of spoked drivers. There were four types of disc driving wheels for steam locomotves. Each was made by a different company and each varied slightly from the others in their appearance. The four types were:
Oddly, as locomotives were shopped and had their wheels upgraded to disc drivers, the new disc drive wheels were not always applied to all of the wheels of a locomotive at the same time. A photographer named Eilenberger recorded Grand Trunk Western locomotive No. 6039 (a Mountain) at Elsdon engine terminal in March 1939 with boxpok drivers only on the second driver axle, while on September 21, 1941, it had the boxpok drivers on at least the second and third axles (and possibly the first, which is obscured in the photograph), but not on the fourth.
Traditional drivers of steam locomotives were cast as a spoked wheel. This design was strong and functional. Counterweighting was added to compensate for the weight of the connecting and main rods. As you can see in the photo, large, heavy rods required a large counterweight. This design successfully served its purpose for most of the steam era.
The word "Boxpok" is actually a shortened combination of two words: "Box" and "Spoke" or "Box-Spoke" and was a proprietary name for the General Steel Castings design. However, I'm not sure the words "box" or "spoke" were ever actually used to describe a "Boxpok" driver. This type driver had fewer "spokes" than conventional locomotive wheels and were construction out of box-like sections. The design proved to have greatly improved lateral strength and rim stiffness. Viewed from the side, the opening between the spokes was egg-shaped, rather than wedge-shaped. Being hollow made it easier to apply counterbalancing. Apart from the egg-shaped holes, the outer surface was smooth with no lips or raised edges.
The earlier, Type-A Boxpok wheels had a fewer number of larger holes. The later, Type-B Boxpok wheels had more, smaller holes.
The boxpok drivers proved an important modification in high-speed service. Many railroads employed Boxpok disc drivers during the 1930s and 1940s including the Union Pacific on their Big Boys.
Scullin disc drivers were introduced in 1932 which preceeded the Boxpok design. Scullin drivers were lighter than Boxpok drivers and as a result were easier on the tracks. They had a much lower mass than traditional spoke-and-rim castings. They are easily identified by their small round holes. They also had fewer holes than traditional Boxpok Disc Drivers. They were used primarily on NYC streamlined Hudsons.
Baldwin Disc Drivers looked very similar to Boxpok Disc Drivers. The most obvious visual difference are:
The Universal type of disc drives were rare. If this design was used it was when a locomotive was rebuilt. The Universal Disc Driver had larger, more triangular shaped holes than the other types of disc drivers.