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CROSS REFERENCE TO RELATED APPLICATION 
   The present application is a continuation of U.S. patent application Ser. No. 10/706,539, filed on Nov. 12, 2003 now U.S. Pat. No. 6,988,851. 

   BACKGROUND AND SUMMARY 
   The present invention pertains to a manually operated, engine driven vibratory concrete screed and, more particularly, to an improved vibration isolation and control system for such a screed. 
   Vibratory screeds are used to smooth the surface of freshly poured concrete and eliminate air pockets within the concrete mass. One type of manually operated screed is driven by a small gasoline engine (e.g. 1 to 2.5 horsepower) that turns an eccentric exciter mechanism to impart a high speed vibratory force to a screed blade attached to the exciter mechanism. For example, an engine operating in the range of 5,000–7,500 rpm will generate in a centrifugal force in the range of about 245 lbs. to 550 lbs. This type of vibratory screed includes an operating handle connected through a frame piece to the vibratory exciter and engine. The machine is pulled over the surface of the concrete and a small amount of fresh concrete will build-up behind the blade to ensure that the surface is uniform and depressions are not created. The blade may be up to 24 feet in length and, although vibration of the blade helps make the concrete flow, the operator must still pull the machine. When the build-up of concrete behind the blade is uneven, there is a tendency for one end of the blade to lift and create an uneven surface. The operator must tilt the operating handle downwardly on one side to generate a force sufficient to counteract the upward movement of the blade. This requires the operator to exert a large amount of force on the handle. Also, the screed blade may have to be turned horizontally over the surface of the concrete, as when moving around a curve or a corner, requiring the operator to exert a large amount of force on the handle in a generally horizontal plane. 
   It is also necessary to isolate the transmission of vibration from the exciter and blade to the operator. Specifically, the frame that carries the operator handle is isolated from its connection to the blade or to the exciter mechanism with rubber or other elastomer vibration isolators. It is desirable to use as soft a vibration isolator as possible to provide maximum vibration isolation for the operator. However, because of the high loads that the operator must impose on the blade for the reasons discussed above, harder vibration isolators are required in order to provide an adequately stiff connection between the operator handle and the blade to transmit the required control force. Soft vibration isolators, e.g. those having a durometer of about 30 provide excellent vibration isolation for the operator, but are too soft to permit adequate force to be transmitted from the handle, through the isolators, to the blade. Soft isolators also amplify the distance through which the operator must move the operating handle to adequately control the blade. The operator handle may be as much as 3.5 feet from the vibration isolators such that a very small amount of movement at the isolator connection is magnified into a large amount of movement where the operator grasps the operating handle. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a vibration isolation system for a vibratory screed which includes a blade, a vibratory exciter mechanism driven by an engine and attached to the blade, and an operating handle frame connected to the exciter mechanism, comprises a bifurcated frame member having a pair of arms positioned to straddle the exciter mechanism for attachment on laterally opposite sides thereof; an elastomeric vibration isolator captured between each arm and a surface of the exciter mechanism, the isolator being confined to limit vertical compressive movement and to permit substantially greater horizontal shear movement; and a retainer attached to each of the arms or to the exciter, the retainer adapted to engage the isolator to limit the amplitude of horizontal shear movement. Preferably, each arm of the frame member includes an upper attachment surface, and the opposite sides of the exciter mechanism have mounting surfaces that are disposed generally parallel to the upper attachment surfaces, and the isolators are confined between the attachment surfaces and the mounting surfaces. 
   In a presently preferred construction, the isolators include rigid upper and lower end plates that have threaded connectors attached thereto, and the attachment surfaces and the mounting surfaces are adapted to receive threaded fasteners for attachment to the threaded connectors. Each of the upper attachment surfaces is formed integrally with a retainer. In the preferred embodiment, each of the retainers comprises a downwardly opening cup having an upper base surface that forms the attachment surface and a downwardly divergent side wall that is positioned to engage the isolator to limit the amplitude of horizontal movement. Each of the isolators preferably comprises a cylindrical body, and the retainer cup has a frustoconical shape that is coaxial with the cylindrical axis of the isolator in a no-horizontal-load rest position, the cup wall positioned to engage the isolator under a horizontal shear load to provide the amplitude limit. The elastomeric isolator is preferably made of a natural rubber material having a durometer of about 30. 
   The apparatus also includes an elastomeric support isolator that is attached at one end to the frame member between the frame arms and at an opposite end to the surface of the exciter mechanism. The exciter mechanism includes an exciter housing that is positioned between the arms of the frame member and has an upwardly extending exciter drive shaft. The engine is positioned directly above the exciter housing and includes a downwardly extending output shaft connected to the exciter drive shaft, and an engine output shaft housing connected to the exciter housing with a flexible connection. The flexible connection includes an elastomer housing and a plurality of elastomer shock absorbers surrounding the elastomer coupling. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a vibratory concrete screed incorporating the subject invention. 
       FIG. 2  is an exploded perspective view of a portion of the apparatus shown in  FIG. 1 . 
       FIG. 3  is a side elevation showing the mounting of the elastomeric vibration isolator of the present invention. 
       FIG. 4  is a vertical section taken on line  4 — 4  of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A vibratory concrete screed  10  includes a long blade  11  which may be made, for example, from an aluminum or magnesium extrusion. The blade may have a length of up to about 24 feet. The blade  11  is clamped to the underside of an exciter mechanism  12  which includes an eccentric device driven by an engine  13  to impart a horizontal vibratory motion to the blade  11 . A supporting frame  14  is attached to the exciter mechanism  12  and includes an operator handle  15 . The screed  10  is operated over the surface of freshly poured concrete by the operator pulling the blade from the operator handle  15 . The vibration isolation system of the present invention is intended to overcome the problems in prior art devices, discussed briefly above, while providing necessary isolation of vibratory force to the operator. These problems include control of the tendency of the blade to move upwardly when the build-up of concrete behind the blade is uneven, and the need to pull one end of the blade in a circular arc around the opposite end as for movement around a curve. Both of these operations require a large amount of force to be exerted by the operator and, if the vibration isolation device between the operator handle and the exciter is too soft, control becomes difficult. On the other hand, if the vibration isolating device is too hard, then the vibratory forces transmitted to the operator become too great. 
   The blade  11  is demountably attached to the bottom of the exciter mechanism  12  such that the working face  16  of the blade faces the operator grasping the handles  15 , whereby the screed is pulled over the surface of the freshly poured concrete. As best seen in  FIGS. 1 and 3 , the upper edge of the working face  16  of the blade  11  is provided with a horizontal mounting rib  17  that is received in a groove  18  in a casting that comprises a lower exciter housing  20 . The front of the blade  11  also includes an upper horizontal mounting rib  21  over which a pair of mounting clips  22  are attached to the housing  20  with machine screws  23  to clamp the blade  11  to the exciter housing  20 . 
   Referring also to  FIG. 2 , the engine  13  is mounted vertically above and directly to the exciter housing  20  and includes a direct driving connection between the engine drive shaft (not shown) and an eccentric exciter mechanism mounted within the housing  20  via a flexible elastomer coupling  24 . The flexible coupling  24  is enclosed in an engine output shaft housing  25  attached to the engine and overlying the exciter housing, the engine output shaft housing also enclosing three elastomer shock absorbers  26  equally spaced around the flexible coupling  24 . The shock absorbers  26  interconnect the engine output shaft housing  25  and the exciter housing  20 . Each of the shock absorbers  26  is attached at its lower end to a coupling surface  27  on the exciter housing  20  and at its upper end to the engine output shaft housing  25  with machine screws  28 . As shown in  FIG. 1 , in the assembled position, the interface between the exciter housing  20  and the clutch housing  25  is sealed with an annular seal  30 . The direct driving connection between the engine  13  and the exciter mechanism  12  eliminates the need for a gear box or transmission and also helps isolate the transmission of vibrations from the engine to the operator handle. 
   The main supporting frame  14  includes a bifurcated lower frame member  31  defining a pair of mounting arms  32 . Each of the arms  32  terminates in a downwardly opening cup  33  which encloses an elastomeric vibration isolator  34  and provides means for attaching the isolator to the arm  32 . The lower ends of the vibration isolators  34  are attached to a mounting surface  35  on the exciter housing  20  on opposite sides of the exciter mechanism. Referring also to  FIG. 4 , the vibration isolators  34  are of a conventional construction, but are mounted and restrained in a unique manner that isolates the transmission of vibration to the operator yet provides the operator with the ability to control blade movement when the operator is required to exert additional force to the operator handle  15 . Each vibration isolator  34  includes a cylindrical body of an elastomer material, preferably natural rubber, with a relatively soft formulation, preferably about 30 durometer. The flat opposite ends of the elastomer body  36  are molded or otherwise attached to rigid metal end plates  37  to which nuts  38  or other suitable internally threaded connectors are welded. Each of the vibration isolators  34  is connected to the mounting surface  35  on the exciter housing  20  with a machine screw  40  extending upwardly through the underside of the mounting surface and into threaded engagement with a nut  38 . Each of the cups  33  includes an interior upper attachment surface  41  which engages the upper end plate  37  of the isolator  34  when the latter is inserted into the cup. Connection between the isolator  34  and the frame arm  32  is completed with an upper machine screw  42  extending through the attachment surface  41  and into threaded engagement with the nut  38  at the upper end of the isolator. With this isolator mounting arrangement, the isolators  34  are confined to significantly limit vertical compressive movement, but are capable of undergoing substantially greater horizontal shear movement because of the substantially unconfined elastomer body  36  combined with the low durometer and high flexibility of the elastomer material. The downwardly opening cups  33  within which the isolators  34  are confined, each has a generally frustoconical downwardly divergent wall  43 . In the no-load at rest position, there is no contact between the cylindrical elastomer body  36  and the wall  43  of the cup. In this mode, which is the predominant operating position over most conditions of use, the low durometer elastomer bodies  36  are very effective in isolating the transmission of vibration back through the arms  32  and frame member  31  to the operator handle  15 . However, when the operator must exert substantial force on the operator handle, as discussed above, movement of the operator handle and frame relative to the exciter housing  20  and blade  11  will result in horizontal deflection of the elastomer bodies  36  until a portion of the inside surface of the frustoconical walls  43  come into contact with the elastomer bodies. This contact provides, temporarily, a more rigid connection between the operator handle  15  and the blade  11 , thereby permitting the operator to exercise direct and more positive control. The cups could also be formed integrally with and as a part of the exciter housing  20 , such that the cups would be upwardly opening. Furthermore, the cups could have a cylindrical or other shape and the elastomer isolator body have a frustoconical or other shape. The important feature is shear movement of the isolators be permitted, but confined to certain maximum limits. 
   To provide additional support and a more stable connection between the exciter housing  20  and the supporting frame  14 , an elastomeric support isolator  44  is attached between the frame member  31  and a rear support surface  45  on the exciter housing  20 . The support isolator  44  may be of a construction identical to the vibration isolators  34 . The upper end of the support isolator  44  is attached to an intermediate frame portion  46 , between the arms  32 , with a threaded stud (not shown) attached to the intermediate frame portion and threaded into the upper end of the support isolator  44 . Similarly, the lower end of the support isolator  44  is connected to the rear support surface  45  with a machine screw (not shown) extending upwardly through the surface  45  and into threaded engagement with the isolator  44 . However, the support isolator  44  need not be and is preferably not confined in a cup, as are the vibration isolators  34 . The support isolator assists in transmitting vertical downward movement imposed by the operator on the operator handle to the blade. 
   It should be noted that the flexible elastomer coupling  24  and the elastomer shock absorbers  26  that comprise the flexible connection between the exciter housing and the clutch housing  25  may be identical to the vibration isolators  34  and the support isolator  44 , except that the flexible coupling  24  and shock absorbers  26  are smaller in size. The durometer of these shock absorbers, however, may be somewhat higher for example, about 50.

Summary:
A vibratory concrete screed includes a vibration isolation system that minimizes the transmission of vibrations to the operator under normal operating conditions, but becomes more rigid during screed control forces applied to the blade through the isolation system when the operator applies greater forces to the operator handle. The system includes low durometer elastomer vibration isolators isolating the operator handle from the vibration exciter and screed blade in a manner that limits vertical compressive movement of the isolators, yet permits substantially greater horizontal shear movement to effectively isolate the operator from vibration. The isolator mounting arrangement also includes retainers that engage the isolator to limit the amplitude of horizontal shear movement when the operator applies a greater control force to the operator handle.