Side bearings for railroad car are well known in the art. On a railroad car, wheeled trucks are provided toward and support opposite ends of a railcar body for movement over tracks or rails. The railroad car is typically provided with a side bearing disposed to opposite lateral sides of a longitudinal axs of the railroad car between a centerpiece or bolster of a wheeled truck and a portion of the body of the railroad car. As known in the art, and during movement of the railcar, each side bearing acts as an energy absorption apparatus and furthermore serves to control or restrict "hunting" movements of the railroad car.
Hunting is a phenomenon created by the wheeled trucks during movement of the railroad car over tracks or rails. The coned wheels of each truck travel a sinuous path along a tangent or straight track as they continually seek a centered position under the steering influence of the wheel conicity. In traveling such a sinuous path, a truck will yaw cyclically with respect to the car body about a vertical axis defined by a vertical centerline of the truck bolster. Of course, the truck also yaws or rotates quasi-statically with respect to the car body in negotiating curved track. As a result of the afore-mentioned cyclic yawing, "hunting" can occur as the yawing becomes unstable due to lateral resonance that can develop between the car body and the truck. As will be appreciated by those skilled in the art, excessive "hunting" can result in premature wear of the wheeled truck components including the wheels, bolsters and related equipment. Hunting can furthermore cause damage to the lading being transported in the car body.
Typically, a side bearing includes a base housing or cage which mounts to the bolster of the wheeled truck, a metal top plate and a spring disposed between the top plate and the base. In one form, the top plate and base are arranged in telescopic relation relative to each other. The spring of each side bearing is required to absorb the vertically directed forces placed on the side bearing by the car body as the railcar moves over the tracks. Moreover, the spring of each side bearing places a predetermined preload on the top plate. When assembled on the railroad car, this vertical force or preload of the spring acts essentially to keep the top plate in constant frictional contact with an underside of a portion of the railroad car body to restrict yaw axis motion of the wheeled truck. Thus, these type of energy absorption devices are sometimes referred to as constant contact side bearings. For purposes of this description, however, the term "side bearing" will be used throughout.
Recently different forms of thermoplastic elastomers have been used as the spring in side bearings to develop the required preload force to overcome hunting and to absorb the vertical forces imparted to the railcar side bearing. One such elastomeric spring is marketed and sold by the Assignee of the present invention under the tradename Tecs Pak. This form of resilient spring is formed from a HYTREL thermoplastic elastomer manufactured and sold by the DuPont Company.
As will be appreciated by those skilled in the art, as the wheeled truck yaws back and forth, the metal top plate of the side bearing slides across and relative to the underside of railroad car body. The resulting friction produces an opposite torque which acts to inhibit yaw motion. On certain side bearing applications, truck-hunting causes an excessive amount of heat to be produced at the interface between the top plate and the underside of the car body. The propensity of some wheeled trucks to hunt is beyond the ability/design of the side bearing to prevent. Such applications can be related to the railroad car design, truck type and other related design considerations.
The high levels of heat created by the friction between the metal top plate and underside of the car body are transferred to the elastomeric spring and tend to melt at least an upper area or surface of the elastomeric material forming the spring. Melting of the elastomeric spring significantly reduces the ability of the spring to apply a proper preload or force to the top plate of the side bearing thereby decreasing vertical suspension characteristics of the side bearing, thus, resulting in enhanced hunting of the wheeled truck. Of course, enhanced hunting or sideways movement of the wheeled truck increases the degree of sliding or relative movement between the top plate and undersurface of the car body leading to increased heat levels and further destruction of the elastomeric spring.
Thus, there is a need and a desire for a railroad car side bearing utilizing an elastomeric spring wherein the side bearing is configured to withstand the relatively high forces and heat generated by hunting movements of the wheeled truck relative to the railroad car body.