Abstract:
An isolation mount for use in motor vehicles in which the mount is sandwiched between the subframe and body. The mount includes an elastomeric upper mount and lower mount and a threaded fastener drawing the two elastomeric mounts together. The upper mount has an insert member with an oblong shape in the lateral displacement direction of the vehicle. The oblong shape increases the volume of elastomer that can be used to respond to lateral forces and reduces the stresses developed on the elastomer. The lateral response rate is stiffer than the fore and aft response rate and both are stiffer than the vertical response rate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an isolation mount used in securing a vehicle body to a support structure, such as a vehicle cradle mount or subframe, and for absorbing vibrations and movements between the two structures. 
     Subframe mounts are used extensively in unibody vehicles to isolate the vibration which is transmitted from the engine to the subframe and the body. The subframe mount also improves vehicle dynamics by providing vertical stiffness, lateral stiffness and fore and aft stiffness and their respective stiffness rates. The operator of the vehicle perceives that vibration isolation relates to ride quality and that improved vehicle dynamics translates into improved handling performance. 
     Typically, there are as many as four locations on the subframe where an isolation mount is utilized. The subframe is sandwiched between the upper portion and the lower portion of the vibration mount and the vehicle body rests on top of the upper mount. A bolt extends through an aperture in the frame and the isolation mount. The lower mount and then the upper mount are connected by a cage nut on the body to complete the attachment, of the body to the subframe. The mount isolates engine or transmission induced vibration that is transmitted along the subframe to the body. The mount also improves vehicle dynamics by controlling or attenuating relative movement between the vehicle body and subframe in the vertical mode or plane, that is up and down, relative movement, and also to control lateral mode or plane, that is side to side movement, and fore and aft mode or plane, that is front to back relative movement., 
     A typical design of a subframe isolation mount employs a relatively hard or high durometer rubber (typically 40 to 80 Shore A) as an isolating material. High durometer rubber for cradle or subframe mounts is an excellent material for improved handling in the lateral plane, especially when it is combined with rate plates to stiffen the response in the lateral plane and to a limited degree the fore and aft plane. However, since the solid elastomeric material is generally very stiff, it does not control or attenuate vertical forces from the subframe to the body very effectively. As a result, the isolation mount has a high lateral stiffness rate response which is desirable but it has a fore and aft stiffness rate response which is marginally acceptable and a vertical stiffness rate response which is low. Therefore, good ride and handling of a vehicle are compromised because of the stiffness properties of the solid elastomeric material. 
     Thus, there is a need for a vibration isolation mount that provides for ride quality that is satisfactory to the operator without sacrificing the handling characteristics of the vehicle in the lateral plane, fore and aft plane and vertical plane. Additionally, there is a need for a mount that is lighter in weight, improves durability and reduces both initial and high mileage noise, vibration, and harshness between a subframe and a body. 
     SUMMARY OF THE INVENTION 
     The present invention discloses an isolation mount for motor vehicles having a subframe or cradle and a body. The body has an aperture and the subframe has a through hole. The isolation mount includes a foamed elastomeric annular upper isolation member between the body and the subframe. The foamed elastomeric insert member is disposed in the annular upper member and extends therefrom. The insert member has an elliptical inner axial passageway. An elliptical isolation member extends from the upper isolation member and is contiguous to the elliptical inner axial passageway of the insert member. The elliptical isolation member has a cross sectional area and a passageway. The cross sectional area has a major axis and a minor axis. The major axis provides control or attenuation for lateral vibration and the minor axis provides control or attenuation for fore and aft vibration. An elliptical retainer member is disposed in the passageway. The retainer has a bore. The annular lower isolation member is mounted to the opposite subframe opposite the annular upper isolation member. The annular lower isolation member and the annular upper isolation member provide control or attenuation for vertical vibration. Thus, a simpler, more cost effective isolation mount is provided which improves vehicle dynamics between the body and the subframe in the vertical direction, lateral direction and fore and aft directions. 
     It is an object of the present invention to provide a foamed elastomeric isolation mount with an elliptical insert which attenuates the vibration transmitted from the subframe to the body as well as providing improved vertical mode, lateral mode, and fore and aft mode stiffness for improved vehicle dynamics. 
     It is another object of the present invention to provide an isolation mount with an elliptical insert which utilizes foamed elastomeric upper and lower mounts which isolate vibration transmitted from the subframe to the body, as well as providing low vertical stiffness, high lateral stiffness and moderate fore and aft stiffness to improve handling performance of the vehicle. 
     These and other features of the present invention will become apparent from the subsequent descriptions and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art upon reading the following specification and by reference to the drawings which include: 
     FIG. 1 is the desired relationship of vehicle load and displacement on the isolation mount in the lateral mode, fore and aft mode and vertical mode; 
     FIG. 2 is an exploded view of the isolation mount oriented for assembly to a vehicle according to the invention; 
     FIG. 3 is a vertical sectional exploded view of the isolation mount according to the invention; 
     FIG. 4 is a vertical sectional view of the assembled isolation mount. 
     FIG. 5 is a top sectional view along  5 — 5  in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As used herein, fore and aft mode shake means front to back movement or displacement which is along the longitudinal axis of the vehicle and transverse of the axis of a mount and is identified by the letter F in FIG.  2 . Vertical mode shake means up and down movement or displacement which is along the axial axis of a mount and identified by the letter V in FIG.  2 . Lateral mode shake mans side to side movement or displacement that is transverse of the longitudinal axis of the vehicle and is perpendicular to the fore and aft mode shake and identified by the letter L in FIG.  2 . Lateral shake stiffness is the displacement of a mount in the lateral mode shake direction divided by unit load. Vertical mode shake stiffness is the displacement of a mount in the vertical mode shake direction divided by unit load. Fore and aft shake stiffness is the displacement of a mount in the fore and aft mode shake direction divided by unit load. The desired lateral mode, fore and aft mode and vertical mode load versus displacement relationship for a vehicle is shown in FIG.  1 . 
     An isolation mount according to the present invention is designated by the numeral  100  as shown in FIGS. 2-4. The mount  100  includes an elastomeric upper mount  10 , an elastomeric lower mount  60  and a threaded fastener member  70 . 
     The upper mount  10  has a thimble member  26  with a flange portion  27  and an axially extending tubular portion  28 . The mount  10  has a foamed elastomeric annular portion  14  which preferably has a top portion  14 A with an inner diameter  16  and an outer diameter  18 . The top portion  14 A has an axial length  15 . Alternatively, the outer peripheral surface of the annular portion  14  may take any shape that can be used in the application, such as square, rectangular, polygonal, conical, triangular, elliptical or truncated conical or any other suitable shape. The annular portion  14  has an axially extending portion  20  which is a close blat sliding fit into the hole H in the subframe S. The axially extending portion  20  has an inner diameter  24  and an outer diameter  22  smaller than the outer diameter  18  of the top portion  14 A. The axially extending portion  20  has an axial length  25  that extends the axial length L 1  of the hole H in the subframe S. The bottom  15 A of the annular portion  14  is placed adjacent the one side A of the subframe S. The axially extending tubular portion  28  is preferably elliptical in shape. Alternatively, the portion  28  may be round, parabolic with rounded ends or any other shape suitable for practicing the invention. As illustrated in FIG. 5, if the tubular portion  28  is elliptical in shape, the space between the tubular portion  28  and a threaded fastener  70  can optionally be provided with an elastomeric material  50 . 
     An insert  30  is disposed in the annular portion  14  and in the axially extending portion  20 . The inset  30  has an outer diameter  32  and an inner peripheral shape  34 . The peripheral shape  34  is preferably, elliptical with a major diameter and a minor diameter which is similar to the shape of the annular axially extending layer  48 . 
     The insert  30  has an annular flange portion  40  and an axially extending tube portion  42 . Alternatively, the peripheral shape  34  may be parabolic with rounded ends or any other suitable shape that could be used in practicing the invention. The outer diameter  32  of the insert is larger than the outer diameter  22  of the axially extending portion  20 . The tube portion  42  also has an axially extending length  45  which is normally the same as the length L of the hole H in the subframe S for a purpose to be discussed later on. The thickness of the tube portion  42  may vary radially. The thickness is greatest adjacent to the axis of the minor diameter which corresponds to the direction of the lateral mode L and thinnest near the axis of the major diameter which corresponds to the fore and aft mode F. The insert  30  is disposed in the upper mount  12  such that it extends along the free end of the tube portion  42  but the flange portion  40  is disposed in the annular portion  14 . Thus there is a radial elastomeric layer portion  46  between the bottom surface  15 A of the annular portion  14  and the bottom  41  of the flanged portion  40 . Additionally, an annular axially extending elastomeric layer  48  is disposed against the elliptical inner peripheral shape  34  of the insert  30  and the tubular portion  28  of the thimble  26 . The annular layer  48  has an elliptical shape. 
     If a stiffer response is required in the lateral L direction, a greater compressive force is imposed on the layer  48  by utilizing a thicker tubular member  28  having a larger outer periphery used in order to provide an additional compressive force on the elastomer. As a result, the elastomeric response rate becomes stiffer. If the tubular member  28  is reduced in thickness, or has a smaller outer peripheral surface area, the compressive force on the elastomeric layer  48  is reduced and the response rate of the elastomer becomes softer. 
     The elliptical shape of the layer  48  in the lateral displacement mode L provides an increased amount of volume of elastomer than a prior art round shaped layer, to absorb radial forces in the lateral mode direction L. This has two effects: 1) the large volume of elastomer provides the design engineer with another variable with which to tune the response of the mount to lateral displacement L to improve vehicle ride and handling characteristics: and 2) the larger volume of elastomer to absorb the lateral displacement forces to reduce stress in the elastomer. Thus, the elastomer has more area to absorb the force generated by the lateral displacement and as a result, the elastomer is exposed to a narrower range of material stress variations. Those skilled in the art will recognize that alternatively, the shape of the annular axially extending layer  48  and the tubular portion  28  of the thimble member and the peripheral shape  34  may be substantially parabolic in cross sectional area with rounded ends near the fore and aft, F, displacement mode or oblong in cross section or oval in cross sectional shape or any other non-circular shape suitable for practicing the invention. 
     The surface area of the tubular portion  28 , in the lateral direction L, is preferably twice the surface area of the tubular portion  28  in the force and aft direction F. Alternatively, the ratio of surface area in the lateral direction L to surface area in the fore and aft direction F ranges from 1.05 to 4.0. The response of the mount to lateral forces is greater than the response to fore and aft forces, which is greater than the response to vertical forces. The vertical response ratio is lower or softer than the lateral or force and aft response rates. 
     The elastomeric lower mount  60  includes a lower annular portion  62 . The annular portion  62  has an inner diameter  66  and an outer diameter of  64 . A flat flanged annular member  68  is adjacent to the lower radial surface  65  of the lower annular portion  62 . The inner diameter  66  is slightly greater than the outer diameter  22  of the axially extending portion  20 . The lower mount has an axial length  63 . The length  63  and the length  15  may be equal to each other. Optionally, the ratio of length  63  divided by length  15  may be greater than 1.0 or less than 1.0 depending on the vertical response rate desired. 
     A threaded fastener  70  is inserted into the inner diameter  66  of the lower mount  60 , through the inner diameter  16  of the tubular portion  28  and a hole (not shown) in the body B. The head  72  of the threaded fastener  70  is adjacent to the flat flanged member  68  and the threaded portion  74  extends axially past the flanged portion  27  of the thimble  26 . A threaded nut  76  engages the threaded portion  74  and the upper mount  10  and the lower mount  60  are clamped together to place a compressive axial load on the mounts  10 ,  60 , respectively. 
     The upper isolator mount  10  and the lower isolator mount  60  are made of foamed elastomeric materials. Preferably, the members  14 ,  62 , respectively, are made of a foamed microcellular polyurethane material (MCU) which can be compression molded, cast or injection molded or processed by m ans well known in the prior art. Altematively, other foamed elastomers such as foamed fluorocarbon, foamed highly saturated nitrile, methyl acrylate acid polymer foam, silicone foam, EPDM foam, Neoprene® foam or the like and thermoplastic elastomers may be used in practicing the invention. Neoprene® is a registered trademark of DuPont. 
     Foamed microcellular polyurethane is a polymer product obtained from the interaction of the di-isocyanate glycol and a blowing agent. The glycol is usually a polyol which can be of either the polyester or polyether type. Both types generally have hydroxyl groups that are free to react with the di-isocyanate. The polyesters are low molecular weight macroglycols. The isocyanate reacts with water to produce carbon dioxide gas for foaming. Foam density is determined by the quantity of water present in the formulation and is characterized by the weight of the polyurethane material divided by the overall volume of the part. Once intimately mixed, the ingredients are discharged from a mixer and deposited into a mold where the complex chemical reactions take place to form the microcellular polyurethane. 
     The chemical reactions are primarily exo-thermic which converts the liquid into a foam. This technology is known in the prior art. See  Rubber Technology, Third Edition , edited by Maurice Morton-Van Norstand Reinhold, ISBN0-442-2642204, pages 555-560, which is incorporated by reference herein. The damping characteristics of a microcellular polyurethane foam are adjusted by the amount of gases trapped in the body of the polyurethane. Thus, the stiffness, and the vibration characteristics of the microcellular polyurethane can be adjusted by varying MCU density to meet specific application requirements. Microcellular polyurethane foam density varies from 0.3 to 0.8 grams per cubic centimeter. Preferably, the range of density is 0.4 to 0.6 grams per cubic centimeter. The typical characteristics of MCU elastomers as compared to solid rubber are primarily influenced by the microcellular structure of the material and not by the chemical backbone or primary polymeric material. Due to the microcellular nature of the MCU material, polyurethane cells will collapse on top of each other under compressing load. This in turn will provide for a higher vertical stiffness and hence permits tuning of the ride characteristics in the vertical mode direction. 
     The annular portion  14  preferably has a foam density which is greater than the density of the lower annular portion  62 . Preferably, the portion  14  and portion  62  are made of MCU material and portion  14  has a density which is 0.05 grams per cubic centimeter greater than the density of portion  62 . Alternatively, the foam density of portion  62  can be the same as or greater than portion  14 . 
     While the invention has been described in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment only. On the contrary, it is intended to cover all alternative modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims.