Abstract:
A truck cab suspension system provides improved durability of the truck cab structure by reducing twisting forces applied to the cab frame from the suspension system. The suspension system provides support to the rear sill of the truck by positioning two air springs on either side of the vehicle center line under the sill and two shock absorbers aligned along the vehicle center line between the air springs. The forces applied to the cab frame rear sill balance one another avoiding twisting.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This non-provisional application claims the priority of provisional application no. 60/289,021 filed May 4, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     The present invention relates to a cab suspension system for trucks and, more specifically, relates to a suspension providing minimum bending moment or twist to a truck cab rear sill. 
     2. Description of the Problem: 
     It is desirable to isolate vehicle cabs and their occupants from shock and vibration of a truck&#39;s chassis. This provides for the occupants&#39; comfort and extends the life of the cab by reducing stress which promotes structural damage to the cab, particularly to spot welds used to attach thin sheet metal components used for cab components such as the floor and the vehicle&#39;s skin to the cab&#39;s frame. 
     The load carried by a truck is supported by a frame which is resiliently carried by suspension springs and supported by the truck&#39;s wheels. Wheel suspension springs have very high spring rates which makes the ride jarring to the operator and to structures supported by the frame. To dampen the vibration and shock transmitted to the cab, suspension systems have been employed utilizing conventional suspension components such as air springs and shock absorbers to isolate the cab structure from shock and to dampen motion of the cab, respectively. 
     Cab suspensions have been situated close to the center line of the vehicle, and have positioned the air spring and primary damping elements of the suspension inward from the frame rails and underneath the rear sill of the cab. Due to space constraints and the desirability of enhancing damping to cab roll, prior art suspension systems have positioned shock damping devices offset from the centerline of the rear sill toward the front of the vehicle. Off centerline shock damping induces a bending moment or twist to the cab rear sill, inducing stress in the cab, particularly between the sill and sheet metal floor panels welded to the sill. 
     An example of such a cab suspension system is described in U.S. Pat. No. 5,109,939 to Conaway et al. In order to position suspension elements close to the vehicle center line under the cab, a rigid base member perpendicular to and breaching the gap between the frame rails is placed under the rear of the cab. A rigid impact plate is provided above the base member and parallel thereto for attachment to the cab bottom. Between the base member and the impact plate are a pair of air springs oriented for vertical compression and aligned across the direction of elongation of the vehicle. A pair of shock absorbers are located toward the front of the vehicle just forward from the air springs. The shock absorbers are aligned on one another perpendicular to the direction of elongation of the vehicle and both are inwardly canted from bottom to top and pivotally mounted to the impact plate. The central axes of the shock absorbers and the support member and impact plate generally form a trapezoid, controlling roll, but resulting in twisting forces transmitted through the shock absorbers about the cab&#39;s rear sill. 
     According to the invention there is provided a vehicle having a suspended cab. The vehicle has a chassis with elongated side frame rails and cross members connected between the side frame rails. One cross member provides support for the back of a vehicle cab. A cab attachment plate is supported above the cross member and provides for attachment to a cab rear sill in the floor of the cab. A pair of air springs is disposed, latitudinally aligned between the attachment plate and the cross member, for supporting the cab. Fore and aft shock absorbers, aligned along the vehicle center line are attached between the cab attachment plate and the cross member member for damping vertical movement of the cab. 
     SUMMARY OF THE INVENTION 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a side elevation of a truck illustrating positioning of the suspension system of the present invention between the cab and the frame; 
     FIG. 2 is a perspective view of the suspension system of the present invention showing placement thereof relative to the truck cab and frame, both of which are shown in phantom; 
     FIG. 3 is a perspective view of the suspension system from an upper front quarter perspective, with the suspension system placed in its operating environment between a frame cross member and a rear sill structural member of a truck cab; 
     FIG. 4 is a perspective view of the suspension system from an upper rear quarter perspective with the suspension system placed in its operating environment; 
     FIG. 5 is an upper rear quarter perspective view of the suspension system; 
     FIG. 6 is a side elevation of the suspension system in its operating environment; 
     FIG. 7 is an upper rear quarter perspective view of the suspension system in an alternative embodiment; 
     FIG. 8 is a side elevation of the suspension system of FIG. 7; 
     FIG. 9 is a bottom view of the alternative suspension system; 
     FIG. 10 is a rear elevation of the alternative suspension system; and 
     FIG. 11 is a side elevation illustrating operating principals of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures and in particular to FIG. 1 there is illustrated a truck  10  including a cab  12  supported on a vehicle frame  13 . Frame  13  provides for the attachment of wheels  11  and includes a pair of side rails  14 , which run substantially the length of the vehicle, Cab  12  is suspended above frame  13  on cab suspension elements  15  and  16 . The forward cab suspension  15  is conventional and secures the forward portion of the cab  12  to the frame  13 . Forward cab suspension  15  may include a vibration isolating sub element. Rear cab suspension  16 , according to a preferred embodiment of the present invention, is attached to a rear sill of the cab  12  in a way largely eliminating the application of twisting motions to the sill to prolong the service life of spot welds of the re sill. 
     Referring now to FIG. 2, the positioning of rear cab suspension attachment plate  20  between a cab  12  and frame  13  of truck  10  is better illustrated. Attachment plate  20  is mounted above a cross member  18  which in turn is positioned extending between parallel side frame rails  14 . Attachment plate  20  is positioned directly below the bottom of cab  12  to support the cab from below. Attachment plate  20  may be connected by appropriate fasteners to the framing for cab  12 . 
     FIG. 3 illustrates rear cab suspension assembly  16  in greater detail. Cab suspension assembly  16  is mounted on top of a cross member  18 . Cross member  18  is positioned latitudinally oriented between the frame siderails at a position under the intended installation point for the aft portion of vehicle cab  12 . Cross member  18  includes two end brackets  19  for attachment, one each, to each of the respective side rails. End brackets  19  are positionable along the frame side rails allowing the cross member  18  to be correctly positioned. A C-channel  21  is mounted between the brackets  19  and provides a support point to the suspension assembly  16 . Cab suspension assembly  16  underlies a rear sill  22 , which is part of the structural assembly of cab  12  and is one member to which the cab floor is mounted. 
     Cab suspension assembly  16  includes a support plate  24  resting on and attached to cross member  18  and attachment plate  20 , which is positioned above and parallel to the support plate in supporting contact with rear sill  22 . Attachment plate  20  is suspended above support plate  24  by a pair of air springs  26  and  28 , which are aligned with one another in the direction of elongation of cross member  18 . Air springs  26  and  28  have vertical axes perpendicular to attachment plate  20  and support plate  24  and are attached to the attachment plate and support plate by conventional means. 
     Attachment plate  20  and support plate  24  each have associated brackets  32  and  34 , respectively, for the attachment of a forward shock absorber  30 . Forward shock absorber  30  is mounted to exert damping force in the plane of the longitudinal center line A of the vehicle, being mounted predominantly vertically, but inclined toward the rear of the vehicle at the top. Attachment of the bottom of the shock absorber  30  to bracket  34  is by a pivot mount  38 , while the top of the shock absorber is mounted in bracket  32  by a similar pivoting mount  36 . 
     Referring to FIGS. 4 and 5 suspension assembly  16  is presented in perspective from the rear quarter, looking down. In FIG. 4 the suspension assembly is presented in its preferred operating environment, and in FIG. 5 cab suspension assembly  16  is illustrated free of other vehicle structural elements. The remaining major elements of suspension assembly  16  are now illustrated, including a pan hard rod  52  providing side to side positional stability of the cab  12  to counter roll occurring during cornering. An aft or rear shock absorber  40  dampens oscillation of cab  12 , generating forces aligned with a plane including the center line A of the vehicle. Panhard rod  52  is pivotally mounted in brackets  48  and  50 , with bracket  48  extending downwardly from attachment plate  20  along one end of the cross member  18  and bracket  50  extending upwardly from support plate  24  toward the opposite end of the cross member. Rear shock absorber  40  is mounted between brackets  42  and  44 , with bracket  42  extending rearwardly from attachment plate  20  and bracket  44  extending rearwardly and downwardly from the back of support plate  24 . The top end of shock absorber  40  is attached to bracket  42  by a pivot mount  46  and the bottom end of the shock absorber is attached in bracket  44  on a pivoting mount  47 . Shock absorber  40  is inclined toward the front of the vehicle from bottom to top, meaning that damping force exerted by the shock absorber is in the plane including the vehicle center line and intersects the line of force from the forward shock absorber above the assembly. Shock absorbers  30  and  40  are located between air springs  26  and  28  on the center line A of the vehicle. Front shock absorber  30  is typically located somewhat higher than rear shock absorber  40  to provide room below the assembly  16  to accommodate the rear end of a transmission housing (not shown). Air springs  26  and  28  are aligned perpendicular to and on either side of the center line. 
     FIG. 6 presents a side elevation of cab suspension assembly  16  illustrating the cooperating inclination of the front and rear shock absorbers  30  and  40 . It may be seen in the figure that the top ends of shock absorbers  30  and  40  are attached by brackets  32  and  42 , respectively, which depend from opposite edges of attachment plate  20 . The top ends of shock absorbers  30  and  40  are inclined toward one another, resulting in damping forces generated by the shock absorbers converging on one another, approximately above the center line B of sill  22 , which is also aligned with a bolt  202  used to attach the sill to attachment plate  20 . 
     Referring now to FIGS. 7-10, an alternative embodiment of a cab suspension assembly  116  is illustrated, modified for greater spacing between a cab  12  and cross member  18 . A pair of air springs  126  and  128  are positioned between a support plate  124  and an attachment plate  120 . Forward and rear shock absorbers  130  and  140  are mounted by brackets, including brackets  142  and  144  for shock absorber  140  and brackets  132  and  134  for shock absorber  130 . Shock absorbers  130  and  140  function in the same manner as the shock absorbers described in connection with the first embodiment and again are positioned on the vehicle center line A. 
     The air pressure of the air springs  126  and  128  may be adjusted through a displacement sensitive valve  70 , which depends from attachment plate  120  and which is actuated by a rod arm  68 . Rod arm  68  is connected at one end to an arm of support plate  124  and at its opposite end to an valve  70  actuation arm  71 . An air line  74  is connected from valve  70  to air springs  126  and  128  by connectors  74  and  76 , respectively. Valve  70  receives air from compressed air source (not shown) commonly available on trucks and delivers the air to the springs if displacement is less than a datum. Air may be discharged from the springs through valve  70  when displacement exceeds the datum. 
     Valve  70  depends from an extension of attachment plate  120 . A valve actuation arm  71  extends forward from valve  70 , and moves up and down depending upon the changes in spacing between attachment plate  120  and a support plate  124 , which are transmitted to the arm by a push rod  68 . 
     A panhard rod  152  is mounted between a downward oriented bracket  148 , depending from attachment plate  120 , and a upward oriented bracket  150 , positioned on support plate  124 . 
     FIG. 11 illustrates location and orientation of shock absorbers  30  and  40  (and in a way equally applicable to shock absorbers  130  and  140 ) to effect a minimum offset between the line E of mounting fasteners  200  between attachment plate and a cab rear sill and the load line F of forces generated by the shock absorbers. Shock absorbers  30  and  40  generate forces in line with the central axes of the shock absorbers, which intersect at point C. The effective load line F, a line intersecting point C and perpendicular to attachment plate  20  should have a minimum offset from the line E of the mounting fasteners to minimize twisting about the fasteners. 
     The invention provides cab suspension with a minimum of twisting about the rear sill element of the cab frame. The size of the mechanism in minimal and it is mechanically simple, while providing good mechanical isolation against vibration and shock. 
     While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.