Bushing for vehicle suspension
FIELD OF INVENTION 
     This invention relates to a transit bus, and particularly to a substitute for a torque or radius rod, useful as an improved repair or replacement part as well as an original equipment part. 
     BACKGROUND OF THE PRIOR ART 
     The torque rod or radius rod system in a transit bus is a crucial element in the bus suspension system and provides the securing interface between the chassis and the bus body, which are also connected to the axle by the bus shock absorber and leaf spring system. [Fleet Equipment, July, 1985, pages 65,66 ] The rod provides a secure connection between the chassis and frame or body and maintains the alignment of fixed points in the two members. In the prior art the torque rod or radius rod is a rod having bushings at two opposite ends. Typically the bushing comprises a split metal ring insert surrounded by a compressed rubber interface enclosed within the integrally cast circumferential end of the rod. The bushing is subjected to continual internal stress because the bushing/rubber/rod assembly is designed to be maintained in fixed relationship. Forces are absorbed into the rubber interface, and the twisting actions are absorbed by the rubber&#39;s elastic properties. The rubber interface resists both rotation and conical movement of the ring with respect to the torque rod. Typically, this design provides a rotation of +/-15 degrees and a conical angle of +/-5 degrees. Once the split ring becomes moveable, the ring will rotate and erode the surrounding rubber, eventually rendering the rod ineffective. 
     Transit buses usually include multiple torque rods, from 6 to 12 per bus, and because the rods are subjected to considerable strees, breakdown is frequent; however, replacement is a difficult and labor intensive job. For example, many garages schedule the replacement of a single torque rod for 4.5 hours of labor which involves the difficult task of installing the rod over a dowel in an interference fit and aligining the two supported points of the rod. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide an improved torque rod that is easy to install and more durable in its use. The present invention provides a rod having a bushing concentrically encircled by a polyurethane elastomer interface in which a precision fit between the bus chassis and body is achieved and saving in labor required for installation or replacement are realized. 
     These objects will be more fully appreciated with reference to the following description of the preferred embodiment taken in conjunction with the drawings in which: 
    
    
     DESCRIPTION OF THE DRAWINDS 
     FIG. 1 is a side view of a conventional torque rod. 
     FIG. 2 is a front view of the rod of FIG. 1. 
     FIG. 3 is a cross sectional view of a representive connecting portion of the torque rod. 
    
    
     DESCRIPTTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, an extended torque rod is shown which typically includes an integrally cast rod segment 1 that terminates in bushing segments 2 and 3 at either end that are respectively affixed to the bus chassis anad frame. 
     Usually the bushing ends are identical. As shown in FIG. 3 the bushing comprises a metal sleeve 10 mantained in place through an interface 11 maintained in the cast rod segment 12. 
     In the invention, an out-of-round torque rod is reclaimed and an elastomert polyurethane interface is boned to the rod, but permits free rotation of a concentric inner sleeve that is held in an interference fit to a dowel on the axel or frame. 
     In a representative example, the original bushing and compressed rubber interface are discarded from a used and worn rodd and the inner circumferential surface of the rod is cleansed and abraded by a process such as sand-blasting. A new bushing ring is bored for a slight interference fit (0.0001 to 0.0003 inch) to the bus frame and chassis dowel. The center of the out-of-round torque rod is determined by centerline to centerline measurements in a conventioanl center determination method; and the new bushing ring is concentrically placed therein in the center-line thereof (13 in FIG. 2). The ring and rod are then maintined securely in their precise alignment throughout the succeeding steps in which a polyurethane interface is poured and cast. The relative size of the bearing ring thickness and polymer interface between the bearing asnd torque rod opening are typically predetermined, in accordance with existing engineering design of any particular vehicle and standard motor vehicle engineering practices. As referred to herein, a replacement rod for a full-sized 40&#39;, 102&#34; bus is considered. For other type of vehicles, standard engineering practices in the automotive industry will predetermine specific parameters. Typically, however, the polymer interface, radially measured is considerably thicker than the thickness of the bushing ring. For example, if the bushing ring were of a metal 0.125 inch thick, the thickness of the polymer interface would be approximately more than 3 to 5 times greater. Similarly, the means for maintaining a mold for casting polyurethane in any particular rod, i.e. such as to prevent spillage or leakage of the liquid polyurethane resin from the volume intended tobe cast, is evident and takes optional forms depending on the shape of the rod involved and/or technician preference. 
     Before the interface is cast, however, a bonding agent is applied to the abraded surface of the rod and a parting compound is applied to the outer surface of the bushing ring so that respectively, the polyurethane compounnd 11 becomes bonded to the rod 12 but permits the bushing ring 10 to be freely rotating therein. The polyurethane compund is poured into, and fills the space between the rod and ring; the compound is thereupon allowed to cure. 
     In the formulation of a typical casting mixture, D-5QM Cyanaprene (American Cyanamid Co.) is a preferred urethane composition and is mixed with a suitable stoichiometric amount of Cyanaset M-Urethane (American Cyanamid Co.E, a curing agent. The bonding agent preferred is Thixon 416 (Whittaker Corporation/Dayton Chemical Division and the parting compound preferred is &#34;Never-Seez&#34;(Bostick Corporation). After an initiial &#34;setting&#34; stage of 1 hour at 212° F., curing takes about 16 hours at a controlled temperature of 212° F. In the curing of the urethane, an elevated temperature will generally produce a harder polyurethane while a cure period of shorter duration or lower temperature will result in a softer composition. 
     After curing, the rod is then installed on the bus. the full uninterrupted bushing, as opposed to the bus. The full uniterrupted bushing, as opposed to the split bushing used in the prior art, is free to rotate with respect to the hardened urethane interface. In the installation of the rod, one end of the rod is installed and secured relatively immovably in an interference fit in the dowel of the bus, and because the bushing permits free rotation of the rod, the installler may simply holdd the rod and turn it to match the corresponding dowel at the other end. This can be performed in about 45 minutes in contrast to the significant alignment effort required to install or replace a conventional split ring/rubber insert rod. In the rod herein, the unbroken bushing ring is able to move with respect to the polyurethane adjacent the bearing surface. Thus, the rod serves a purpose comparable to the prior art rod of maintaining the frame and chassis a predetermined degree apart and in the predetermined degree of respective alignment; however, the full radial movement permitted by the bushing ring may produce less stress on the components of the bus. For example, when a pothole is struck, the rubber in a prior art split ring rod will be subjected to the stress of impact; however, th efree insert of the rod herein is able to rootate in response to evident stress and permit other components of the bus suspensison system to absorb the shock. Similarly, acceleration and braking of the bus produces a movement and shock load in one lateral direction absorbed by the rod. The only movement of the rod is centerpoint-to-centerpoint resulting in the bus shock absorber and leaf spring suspension system absorbing up and down shock. 
     As noted above, in the polymer formulation for a typical casting mixture, a Cyanaprene D-5QM polyurethane resin composition is preferred. This composition is an elaslomer having 49-53 Shore D hardness when cured with a Cyanaset M Curative [methylenebis (o-chloroaniline)] [See Technical Data Sheet, Cyanaprene D-5QM, American Cyanamid Company, Wayne, N.J. ]. Represntative Chemical specifications for the Thixon (™) adhesive agent for bonding castable urethane compounds to metal are set forth in Thixon (™) Adhesives Data Sheet, Dayton Chemicals Division, Whittaker Corporation, West Alexandria, Ohio. This adhesive is a solvent borne bonding agent. 
     In an evalaution of the poured polyurethane rod in comparison to a prior art rubber sleeve rod, a hardness test measured with a Type A Durometer indicated a hardness of 71 for the rubber rod and a slightly harder measurement of 89 for the cast polyurethane rod. Preferably the durometer of the cast polyurethane should be at least equivalent to that of the compressed rubber in an OEM rubber sleeve torque rod. 
     While variations of the polyurethane formulation and the specific bonding and release agents employed are possible within limits predetermined by specific applications, such application variations are not intended to be excluded from the following.