The Streetcar Program
The Streetcar Program
As noted above, the goals of the ERPCC were to develop a streetcar that had a smooth, high level of performance on city streets, which was quiet, and which provided a higher level of passenger comfort with style, yet was affordable, this latter goal to be achieved through standardization, primarily of trucks and carbody elements, where most TRC patents applied.
PCC Trucks And Their Advanced Features
The principal design goals for the trucks of ERPCC/TRC was weight reduction and noise reduction, minimal maintenance, improved ride quality, and use of rubber to minimize transmission of vibration to the car body. The techniques employed were so basic that they are still state of the art today.
ERPCC's primary focus was on making use of rubber in rail truck design. Rubber-connected steel pieces can transmit forces applied in the compression direction while having flexibility to forces applied in the shear direction. Since no slippage occurs between any of the surfaces no wear occurs and no lubrication is required. Utilization of these features would allow creation of a truck design that would be light in weight and light in maintenance. Three prototype sets of trucks were built, utilizing rubber to the maximum extent. The first design, not given a model letter, provided experience that was used for trucks A and B, the first of which had worm gear drive and was used under test car 5200, and the second of which had hypoid gear drive and was used under prototype car 5300. Production trucks continued under model letter B., with one exception to be mentioned later.
Trucks basically fall into two main categories, which are rigid frame and flexible frame. Flexible frame trucks are commonly found under freight cars and saw much application on streetcars, the fleet of Pittsburgh low floor cars being a good example. Rigid frames normally require two sets of springs, small primary springs which deflect to accommodate track irregularity, and secondary springs with a softer characteristic that determine ride quality as felt by passengers. TRC felt that use of a resilient wheel, for which it was awarded a number of patents, coupled with a single set of springs would suffice for streetcar use. The truck frame connecting the axles would be flexible so that it could accommodate track irregularity. The first production truck patent, #2244502, Rail Trucks, applied for on May 31, 1935, describes a flexible frame connecting both axles, the flexibility being achieved by use of rubber in the diagonally placed joints in the rectangular frame as shown in figure 1. This design feature - use of rubber to accommodate motions between truck elements - became the underlying basis for most all TRC trucks, and is found today on almost all modern railroad and transit car trucks in some form or another. In this patent the flexible frame in turn supports a rigid frame through soft springs, and will be recognized as the B-2 design used on most production PCC streetcars. This scheme, which minimized unsprung weight proved to have some unforseen issues which eventually resulted in major changes in design, but performance in relatively low speed streetcar applications was quite adequate to make it the most used North American PCC streetcar truck. (The use of swing links did have a positive affect on weight transfer - see note 1)
Similarly, another TRC patent provided a baseline for most modern trucks. Patent 2590033, Springs For Rail Trucks, applied for on September 4, 1945, does away with the use of swing links for the allowing of lateral motion of the car body. This patent describes the need for lateral motion allowance in rail trucks, and provides a method of using the secondary springs to allow both vertical and lateral motion of the car body. From earliest times the provision for lateral motion was accomplished by using vertical links called swing hangers, or swing links. Typically they were placed inside the truck, but on some trucks they were outside, as seen on some locomotive trucks and in the electric traction industry on cars that Brill and Cincinatti Car built for high speed service on interurban lines in the early 1930s. By eliminating the swing hangers truck weight could be significantly reduced, an ERPCC goal.
Because the B-1 and B-2 trucks were so strongly focused on minimizing unsprung weight, both trucks carried the traction motors on a frame supported above the suspension springs that by their nature could only allow a minimal amount of lateral motion. Therefore, lateral motion followed tradition and swing links - but with greased bearings - were employed to provide the amount of motion needed.
key truck patents:
2244502 -, 5-31-35, 6-3-41, Rail Trucks, has B-2 truck configuration and articulation through rubber.
2184102 - 7-22-37, 12-19-39, Rail Truck Suspensions, has B-1 truck configuration and articulation is through rubber, truck shown is wider gauge B-1
2590033 - 9-4-45, 3-18-52, Springs for Rail Trucks, has B-3, mentions rubber articulations.
2231195 - 5-25-36, 2-11-41, Rail Trucks, has outside frame RR truck with B-1 type bolster and rubber at j ournals allowing horizontal steering.
2590033 - Springs For Rail Trucks,9-4-45, describes need for swing links, patents use of secondary spring deflection in place of swing links
Note 1 - Weight transfer occurs when a vehicle is pushed by a force at some point above ground (or rail head) level. For most vehicles that level is below that of the axle's connection to the vehicle body, and the body will tend to rotate around that point, reducing the effective weight forward of that point, usually carried on an axle and wheels. As has been demonstrated by specially-designed road vehicles, it is possible to reduce the load on the forward wheels completely, lifting them off the ground. In a railcar truck, this shifting of effective weight from front to rear wheels will make the forward wheels more prone to slip. The closer the point of the force application point is to rail head level, the less reduction in effective weight will be seen at the forward wheels. Use of swing links moves the point of force application down towards the rail head, a positive achievement when high rates of acceleration and braking are a goal of the car design.
Further work is to be done.