Abstract:
A composite crankshaft counterweight of a two-cycle internal combustion engine includes a “T” shaped counterweight surrounded by a cup-shaped retainer that holds light weight inserts against the counterweight. Together with the counterweight the inserts create a full circle that reduces the empty volume in a crankcase of a two-cycle engine.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to a crankshaft counterweight for an internal combustion engine.  
         BACKGROUND OF THE INVENTION  
         [0002]    A crankshaft for an internal combustion engine typically comprises a shaft having at least one eccentric crankpin. A rod connects the engine&#39;s piston to the crankpin so that linear movement of the piston is translated to rotation of the main shaft. In order to balance rotation an eccentric mass is attached to the shaft to counterbalance the mass of the crankpin, rod, and the piston. The crankshaft can either have double counterweights or be cantilevered. With a double counterweight crankshaft, force from the piston is applied at a point, or at points that distribute the force evenly between the bearings supporting the shaft. Typically, a balanced crankshaft has, for each piston, two closely spaced counterweights, joined by a crank pin to which a rod is connected. Each counterweight is attached to a shaft. A cantilevered crankshaft has only one counterweight and shaft.  
           [0003]    For two-cycle engines, it is common to use a “T” shaped counterweight. A crank pin is attached near the bottom of the stem of “T”. One shaft (in the case of a cantilevered crankshaft) or two shafts (in the case of a balanced crankshaft) are attached to the counterweight(s) at its dimensional center. A “T” shaped counterweight is not strictly or literally in the form of a “T.” Rather, the “T” description suggests a relative distribution of mass to a side opposite of the crank pin and perpendicular to its axis of rotation. Shapes that are closer to that of a triangle or a triangle with two squeezed sides are also considered “T” shaped counterweights. “T” shaped counterweights are typically made from a single piece of metal, but have also been constructed using “laminations” or a multiple, stacked plates cut in a “T” shape and joined by, for example, pins. Examples of these types of counterweights are shown in U.S. Pat. No. 4,342,236. Most of the mass of a “T” shaped counterweight is concentrated where it is most effective, while reducing the overall mass of the counterweight.  
           [0004]    To transfer a charge of a fuel and air mixture from a carburetor into a cylinder, a process referred to as scavenging, two-cycle engines typically draw into the engine&#39;s crankcase the charge using a pressure decrease generated by upstroke of the piston. The down stroke of the piston then compresses, and thereby pressurizes, the charge prior to an intake port opening on the cylinder. The relatively higher pressure of the charge causes it to flow into the relative lower pressure cylinder. Increasing the pressure differential enhances scavenging of the cylinder. Better scavenging tends to improve engine performance and reduce emissions. Because the volume that the piston displaces during movement is fixed, decreasing the volume of voids within the crankcase will tend to increase pressure within the crankcase and thus improve scavenging.  
           [0005]    Some of the volume of the voids within the crankcase is necessary to accommodate the sweep of the counterweight. Because the swept volume of the counterweight in particular “T” shaped counterweights is significantly greater than the actual volume of the counterweight, it is desirable to fill this void. One prior art approach is to extend the shape of the counterweight to form a full circle. Most of circle is relatively thin, but it includes a much thicker, “T”-shaped portion forming the counterweight and support for attaching a crank pin and crankshaft. A drawn metal cup having a circular shape is attached to the edges of the extended counterweight to enclose voids on either side of the thick “T” shape and the extended edge.  
           [0006]    This example of a “full circle” counterweight has several problems. First, the counterweight is made of forged steel in order to make it into a circular shape with most of its mass concentrated in the thick “T”-shaped area. Forging the counterweight is a relatively expensive process. Second, because the steel extends to the edges of the circle, the overall mass of the counterweight is increased. Further, the mass is added in places where it is not useful for counterbalance. Not only is the additional weight an extra load on the engine, it is also detrimental to balance of the crankshaft.  
           [0007]    Another example, useful for laminate-constructed counterweights, is to cut openings in each of the inner layers of the laminate to form in each layer a “T” shape surrounded by a circle, and attach complete, circular shaped layers on the outside to seal the voids created by the openings in the inner layers on the outer layers, thus creating sealed voids. However, manufacture and assembly of this type of full circle counterweight is relatively complicated, and extra mass is added in places where it is not useful for counterbalance.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention relates to an improved full circle counterweight for a crankshaft of an internal combustion engine that overcomes one or more of the problems with prior art.  
           [0009]    The invention, as defined by the claims, is disclosed below as part of a detailed description of an example of two-cycle engine for a power tool having a full-circle counterweight assembly.  
           [0010]    The full circle counterweight assembly in the disclosed example includes a counterweight that is shaped generally like that of a conventional “T”-shaped counterweight. Like a conventional “T” shaped counterweight, it includes openings for receiving a crank pin and a shaft. The T-shaped metal counterweight is partially encased within a retainer in the form of a drawn metal cup. Two lightweight spacers, made for example from plastic, fill voids on opposite sides of the stem of the “T” shaped counterweight. Edges of the cup are folded over to retain the inserts within the cup, and to retain the cup on the metal counterweight. As compared to the prior art, little additional weight is added to make the counterweight full circle. Further, as compared to prior art, the full circle counterweight assembly tends to be more easily and less expensively made. The metal counterweight, cup and lightweight inserts can be assembled according to standard, relatively low cost methods. The assembly is relatively simple, as it involves relatively few parts and steps. The full circle counterweight finds particular advantage in two-cycle engines of the type used in power tools and equipment, as such engines must be both lightweight and easily manufactured to reduce cost, while meeting increasingly stringent emission requirements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a view of a full circle counterweight assembly.  
         [0012]    [0012]FIG. 2 is a cross-section of the full circle counterweight assembly of FIG. 1, taken along section line  2 - 2 .  
         [0013]    [0013]FIG. 3 is a cross-section of the full circle counterweight assembly of FIG. 1 taken along section line  3 - 3 .  
         [0014]    [0014]FIG. 4 is a cross-section of an engine including the full circle counterweight of FIG. 1.  
         [0015]    [0015]FIG. 5 is a partial cross-section of the engine of FIG. 4, taken through the full circle counterweight and crankcase.  
         [0016]    [0016]FIG. 6 is a partial cross-section of the crankshaft of the engine partially shown in FIG. 5, taken along section line  6 - 6 . 
     
    
     DETAILED DESCRIPTION  
       [0017]    In the following detailed description of an example of an embodiment of the invention, like reference characters refer to like parts.  
         [0018]    Referring to FIGS. 1, 2 and  3 , full circle counterweight assembly  10  includes a “T” shaped counterweight, which is generally designated  12 , and a retainer in the form of drawn metal cup  20  that holds inserts  18  next to the stem of the T-shaped counterweight. The “T”-shaped counterweight includes a stem portion  12   a  in which is formed openings  14  and  16  for a shaft and crankpin, respectively, and a cross portion  12   b,  which contains most of the mass of the counterweight and is disposed on the side of the dimensional center of the counterweight opposite opening  16 . As can be seen from the drawings, cross-portion  12   b  has a semi-circular or curved outer diameter that fits the curvature of retainer cup  20 . Ends of either one or two shafts (not shown) are fitted into opening  14  in a conventional manner. Axis  15  is the center of the retainer cup  20 , shaft(s), and the axis of rotation of the counterweight assembly  10 . A crank pin (not shown) is fitted into opening  16  in a conventional manner. Conventional methods, including relatively low-cost methods, may be used to fabricate the counterweight of metal. The counterweight could be, if desired, of laminate construction.  
         [0019]    Inserts  18  are, as compared to the metal counterweights, made of a much less dense material, and thus relatively lightweight. They may be made, for example, from a plastic such as nylon or other material that has a sufficiently high melting point to resist melting in the engine, tends not to degrade in the presence of the fuel and oil within an engine&#39;s crankcase, and that is of density substantially less than of the metal used to form the counterweightSuitable materials with lower density than the counterweight for inserts  18  include materials such as plastic, aluminum, and magnesium, composite materials, and materials with closed cells, such as honeycombs, foam, or closed cell plastic.  
         [0020]    Cup  20  may be made from a thin durable metal such as steel. The diameter of the retainer cup is approximately the diameter path of the rotating counterweight. To hold the inserts  18  within the retainer cup, and to hold the retainer cup on the counterweight  12 , the edge of the retainer cup  20  is bent over edges of the counterweight  12  and inserts  18 . During assembly crankshaft opening  14  is aligned with a corresponding opening  21  in the retainer cup&#39;s back wall  20 . Depending on the size of the opening, the retainer may take on a form closer to that of a ring. Inserts  18  are then placed into the open regions in retainer cup  20  so that an insert  18  is placed on each flank of the stem of the “T” shaped counterweight  12 .  
         [0021]    After the inserts are fitted, an extended edge of retainer cup  20  is bent inward towards the center of retainer cup  20  and down over counterweight  12  and inserts  18  to form a retaining edge or lip  22 . The length of retaining edge  22  need only be of sufficient length to retain inserts  18  firmly in position, and to keep the retainer firmly attached to the counterweight  12 . Outer edges of inserts  18  and counterweight  12  may be notched to a depth of the thickness of the lip  22  to receive the lip, thus reducing the profile of the counterweight assembly allowing clearance of the void within the crankcase required for the counterweight to be reduced. To further reduce weight, inserts  18  may be cored to remove mass, by milling for example, to create voids  24 . These voids face inwardly, toward the retainer cup and sit against the cup&#39;s back wall.  
         [0022]    Referring to FIGS. 4, 5 and  6  two-cycle engine  26  is an example of one use of counterweight assembly  10 . The engine includes a piston  28  mounted for reciprocating movement within a cylinder  29  formed in cylinder block  30 . The piston includes a wrist pin  32  for connecting it to an eyelet  34  of rod  36 . On an opposite end of rod  36  is a second eyelet  38  for connecting to crankpin  40 . The crank pin is connected to counterweight assembly  10 . Connected to the counterweight assembly is shaft  42 . Also shown is a muffler  44  for receiving exhaust from cylinder  29 . Shaft  42  transmits power through a transmission system to a working element that is not shown. The shaft and full circle counterweight assembly is mounted within crankcase  46 . As shown in FIG. 6, the crankshaft with illustrated engine  26  has two full circle counterweight assemblies  10  and two shafts  42  joined back-to-back to form a balanced crankshaft. In the particular example in FIG. 4, the engine is mounted to a chassis  48  of a chain saw of known construction. A saw, not shown, depends from one side of the engine and chassis, as does a pair of handles. No particular configuration of engine or tool is intended to be implied by the example. For example, the counterweight assembly  10  may be used with either cantilevered or balanced crankshafts. Other examples of tools are vegetation trimmers, brushcutters, hedge trimmers, lawn mowers, edgers, and leaf blowers, for which it is advantageous to use two-stroke engines for weight and cost considerations.  
         [0023]    Lightweight inserts  18  fill what would otherwise be void within the crankcase  46 . Crankcase is preferably constructed to provide only the clearance necessary for the various moving parts. The inserts thus reduce the volume within the crankcase that would otherwise be necessary for clearance. A full circle counterweight with light weight inserts and a retainer holding them to a T-shaped counterweight thus contributes to increased power and lower exhaust emissions through improved crankcase compression in crank case scavenged engines, while maintaining good rotational balance for a crankshaft and good manufactureability.