Patent Publication Number: US-6209412-B1

Title: Operation-control lever unit for engine-powered working machine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an operation-control lever unit for regulating power of an engine of an engine-powered working machine to control operation of a working tool of the engine-powered working machine. The engine-powered working machine may include a carrying bush cutter, a chemical sprayer, a vacuum dust collector and so on. 
     2. Description of the Related Art 
     A carrying bush cutter having a circular cutter driven for rotation by an engine carried on the back of an operator is known from Japanese Utility Model Publication No. (SHO) 63-14035. The circular cutter is attached to one end of a hand-operating rod. The rod is provided with an operation handle near the other end thereof. In use of the known bush cutter, the operator swings the rod up and down and right and left while gripping the operation handle. With this swinging operation, bushes are cut or removed by the rotating circular cutter. 
     In order to control the rotational speed of the cutter, output power of the engine is regulated by a throttle lever provided on a grip portion of the operation handle. However, because the operator is forced to continue gripping of the throttle lever and the operation handle throughout the bush-removing work, a heavy work load is put on the operator. 
     According to somewhat successful prior improvements, a lock mechanism is associated with a throttle lever to temporarily lock the throttle lever in a desired position, so that the work load on the operator can be reduced. Typical examples of the prior improvements are disclosed in Japanese Utility Model Publication Nos. (SHO) 53-42661 and (SHO) 55-21536, Japanese Utility Model Laid-open Publication No. (SHO) 60-41539, and Japanese Patent Laid-open Publication No. (HEI) 8-303263. 
     According to the disclosed operation lever units, the throttle lever is displaced to a predetermined operating position, then locked in this operating position by activating the lock mechanism. The lock mechanism is released at need. 
     More specifically, the operation lever unit disclosed in Japanese Utility Model Publication Nos. (SHO) 55-21536 includes a throttle adjustment lever and a throttle release lever provided side by side on the operating handle. However, because these levers have different axes of pivotal movement, the operation lever unit is complicated in construction, requires an increased number of structural components, and is uneasy to manipulate. Additionally, due to wear and deformation occurring during a long period of use of the engine-powered working machine, a frictional force acting on the throttle lever to lock the same lever in position against pivotal movement tends to decrease and eventually allows the throttle lever to move from the locked position. The throttle lever may sometimes return to its original position when the operation lever unit is subjected to vibration. The operation lever unit has no means to adjust the frictional force acting on the throttle lever. 
     In the operation lever unit shown in Japanese Utility Model Laid-open Publication No. (SHO) 60-41539, a throttle lever and a lock lever are mounted on the same pivot pin. However, due to a strong spring force acting directly on respective pivoted portions of the levers, a great muscular effort or force is required to turn each lever. Thus, the manipulability of the operating lever unit is relatively low. If the spring force is weakened, the lock lever will fail to lock the throttle lever at a desired position with sufficient reliability. 
     The operation lever unit disclosed in Japanese Utility Model Publication No. (SHO) 53-42661 includes a locking pawl pivotally mounted between the pivot axis of a throttle lever and the pivot axis of a lock lever for operatively interlocking the two levers. Because of the presence of the locking pawl, the operating lever unit is complicated in construction and requires an increased number of structural components. 
     In the operation lever unit shown in Japanese Patent Laid-open Publication No. (HEI) 8-303263, a throttle lever and a lock lever are pivotally mounted on the same pivot pin. However, because the lock lever is normally held stationary against pivotal movement using the force of a spring only, this operation lever unit has the same problem as the operation lever unit disclosed in Japanese Utility Model Laid-open Publication No. (SHO) 60-41539 previously described. 
     SUMMARY OF THE INVENTION 
     The present invention is conceived to solve the foregoing problems associated with the prior art. 
     A more specific object of the present invention is to provide a operation-control lever unit for an engine-powered working machine, which includes a throttle lever, a lock lever and a throttle-lever holding mechanism of simple construction having a relatively small number of components. 
     Another object of the present invention is to provide an operation-control lever unit for an engine-powered working machine, which is simple in construction and is capable of reliably locking a throttle lever in a desired position to regulate output power of an engine of the engine-powered working machine while keeping good manipulability of a lock lever disposed close to the throttle lever. 
     To achieve the foregoing objects, an operation-control lever unit of the present invention for regulating power of an engine of an engine-powered working machine to control operation of a working tool of the engine-powered working machine includes an operation-control handle having a grip portion and an enlarged head portion at an end of the grip portion, a throttle lever pivotably mounted to the head portion of the handle and pivotally movable about its pivot axis within a predetermined angular range, a lock lever pivotably mounted by a support shaft to the head portion and pivotally movable about an axis of the support shaft, the lock lever being slidably movable along the axis of the support shaft, and a throttle-lever holding mechanism operative in response to pivotal movement of the lock lever in a locking direction to frictionally hold the throttle lever at a desired position within the predetermined angular range. The throttle-lever holding mechanism includes a first cam coaxial with the support shaft and formed integrally with the head portion of the handle, a second cam coaxial with the support shaft and provided on the lock lever, the second cam being co-active with the first cam to displace the lock lever along the support shaft in a first direction away from the first cam, and a resilient means disposed behind the second cam when viewed from the first cam and resiliently urging the second cam toward the first cam. 
     Because the first cam is integral with the head portion of handle, the throttle-lever holding mechanism is relatively simple in construction and has a small number of structural components. 
     Preferably, one of the first and second cams has an integral tubular portion coaxial with the support shaft and projecting toward the other cam. The other cam has an outer peripheral surface slidably received in the tubular portion. With this arrangement, the cams are protected against contamination with dirt and dust and can smoothly operate in response to pivotal movement of lock lever. 
     The first and second cams each have an annular cam surface having at least one radial ridge. At least one of the ridge of the first cam and the ridge of the second cam has a flat top surface. With this flat top surface, the throttle lever can be stably held in a locked position even when the lock lever is pivoted to some extent. The flat top surface is preferably perpendicular to the axis of the support shaft. The ridge may have a generally trapezoidal cross-sectional shape. 
     Preferably, the throttle-lever holding mechanism further includes a friction member disposed between the throttle lever and the lock lever and forced against the throttle lever when coaction between the first and second cams displaces the lock lever in the first direction against the resiliency of the resilient means The throttle-lever holding mechanism may also include a second friction member disposed opposite to the first-mentioned friction member with the throttle lever disposed therebetween. The first-mentioned friction member and the second friction member cooperate to grip the throttle lever therebetween when the lock lever is displaced in the first direction. The first-mentioned friction member and the second friction member are preferably rubber ring discs mounted on the support shaft. 
     The resilient means is disposed between the lock lever and the friction member and urges the lock lever in a second direction to move the second cam toward the first cam and also urges the friction member into contact with the throttle lever. The resilient means is preferably a conical spring washer mounted on the support shaft. 
     The resilient means may include a first resilient member disposed between the lock lever and the first-mentioned friction member, and a second resilient member disposed between the second friction member and a portion of the support shaft. The first resilient member urges the lock lever in a second direction to move the second cam toward the first cam and also urges the first-mentioned friction member into contact with the throttle lever. The second resilient member urges the second friction member into contact with the throttle lever. 
     As an alternative, the resilient means may be disposed between the lock lever and the friction member urge the lock lever in a second direction to move the second cam toward the first cam. The resilient means is operatively separated from the friction member. 
     In one preferred form of the invention, the resilient means is disposed between the lock lever and the friction member, and the throttle-lever holding mechanism further includes a friction adjustment device for varying a preloading on the resilient means to adjust a frictional force acting between the throttle lever and the friction member. The support shaft is a screw having a head slidably guided in a first portion of the head portion of the handle and a shank including a screw portion threaded with a second portion of the head portion. The resilient means, the friction member and the throttle lever are disposed between the head of the screw and the lock lever. The screw forms the friction adjustment device. By turning the screw, the screw moves in an axial direction. With this axial movement of the screw, the distance between the head of the screw and the lock lever varies with the result that a preloading on the resilient means is changed. Since the friction member is urged by the resilient means, the friction exerted from the friction member to the throttle lever can be adjusted by changing the proloading on the resilient means. The screw is preferably a hexagonal socket head cap screw. The friction adjustment device may further include a lock nut threaded with the screw portion of the screw to lock the screw in position against movement relative to the head portion. 
     The above and other objects, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which preferred structural embodiments incorporating the principle of the invention are shown by way of illustrative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatical view of a carrying bush cutter as it is used in a bush-removing work as an engine-powered working machine in which an operation-control lever unit according to the present invention is incorporated; 
     FIG. 2 is a perspective view of the operation-control lever unit including a grip handle; 
     FIG. 3 is an exploded perspective view of a main portion of the operation-control lever unit; 
     FIG. 4 is an enlarged view, with parts cutaway for clarity, showing a portion of FIG. 2 including a first cam and a second cam; 
     FIG. 5 is a longitudinal cross-sectional view of the grip handle, showing the internal structure of the operation-control lever unit; 
     FIG. 6 is a cross-sectional view taken along line VI—VI of FIG. 5; 
     FIG. 7 is a cross-sectional view taken along line VII—VII of FIG. 5; 
     FIG. 8 is an enlarged view of a part of FIG. 6, showing a throttle-lever holding mechanism of the operation-control lever unit; 
     FIG. 9 is a side view of the grip handle showing a lock lever of the operation-control lever unit in its throttle-lock position; 
     FIG. 10 is a longitudinal cross-sectional view of the grip handle showing the positional relationship between a throttle lever and the lock lever shown in FIG. 9; 
     FIG. 11 is a developed view showing respective profiles of the first and second cams; 
     FIGS. 12A,  12 B and  12 C are views similar to FIG. 12, but showing operation of the first and second cam which occurs in response to pivotal movement of the lock lever; 
     FIG. 13 is a perspective view showing another cooperating pair of cams prepared for comparative purposes; 
     FIGS. 14A,  14 B and  14 C are views corresponding to FIGS. 12A,  12 B and  12 C, respectively, but showing operation of the cams shown in FIG. 13; 
     FIGS. 15A,  15 B and  15 C are cross-sectional views illustrative of the manner in which a frictional force acting between the throttle lever and a friction member of the throttle-holding mechanisms can be adjusted; 
     FIG. 16 is a cross-sectional view showing a modified form of the throttle-lever holding mechanism as it is in a releasing position; 
     FIG. 17 is a view similar to FIG. 16, but showing the throttle-lever holding mechanism in a locking position; and 
     FIGS. 18 to  21  are diagrammatical views showing various modes of application of the engine-powered working machine in which the operating lever unit of the present invention can be used. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Certain preferred embodiments of the present invention will be described below in greater detail with reference to the accompanying sheets of drawings wherein like or corresponding parts are designated by the same reference characters throughout several views. 
     Referring now to FIG. 1, there is shown a carrying bush cutter  1  as it is used in a bush-removing work as an engine-powered working machine in which an operation-control lever unit according to the present invention is incorporated. 
     The carrying bush cutter  1  includes a frame  2 , a power unit such as a gasoline engine  3  mounted on the frame  2 , a flexible tube  4  connected at one end to a power output portion of the engine  3 , an elongated rigid hollow support rod  5  having one end (proximal end) connected to the other end of the flexible tube  4 , and a circular cutter (working tool)  6  rotatably attached to the other end (distal end) of the support rod  5 . The circular cuter  6  is connected in driven relation to the engine  3  via a flexible power transmission shaft (not shown) extending through the support rod  6  and the flexible tube  4 . Thus, the circular cutter  6  is driven in rotation by output power of the engine  3 . 
     A grip handle  7  and an operation-control handle  10  are provided on a proximal end portion of the support rod  7 . The operation-control handle  10  is disposed behind the grip handle  7  when viewed from the distal end of the support rod  5 . The grip handle  7  and the operation-control handle  10  extend orthogonally from an upper surface of the support rod  3 . 
     The frame  2  has a pair of belts or straps (one being shown in FIG. 1) for enabling the operator M to carry the engine  3  on its back, with a cushioning pad  9  disposed between the back of the operator M and the frame  2 . In use of the carrying bush cutter  1 , the support rod  5  is held on, for example, the right side of a body of the operator M, with the grip handle  7  and the operation-control handle  10  being gripped by the left hand LH and the right hand RH of the operator M, respectively. The support rod  5  is swung right and left and up and down about its proximal end so that bushes are cut or removed by the rotating circular cutter  6 . 
     The left hand LH of the operator M is used essentially for gripping the grip handle  7  and moving the support rod  5  right and left and up and down. The right hand RH is used for gripping the operation-control handle  10  and performing throttle adjustment to regulate output power of the engine  3 . The operation-control handle  10  constitutes a main part of the operation-control lever unit of the present invention. 
     As shown in FIG. 2, the operation-control handle  10  is in the form of the grip of a gun and includes a tubular grip portion  11  secured at a lower end to the support rod  5  via a bracket (not shown), and an enlarged head portion  12  secured to an upper end of the grip portion  11 . 
     The head portion  12  is a hollow member of split structure consisting of left and right head halves or members  13 ,  14  joined together back to back to form a hollow interior space or pocket in the head portion  12 . The head portion  12  has an elongate hole or opening  12   a  formed in a front wall thereof. A throttle lever  30  and a lock lever  40  of the operation-control lever unit are pivotally mounted to the head portion  12  within the pocket. The throttle lever  30  has a trigger-like lever portion  31  projecting forwardly and downwardly from the elongate opening  12   a  in the front wall of the head portion  12 . Similarly, the lock lever  40  has an actuating lever portion  41  projecting rearwardly and downwardly from an elongate hole or opening  12   c  (FIG. 5) formed in a rear wall of the head portion  12 . 
     An engine start switch  80  for starting and stopping the engine  3  (FIG. 1) is provided on the rear wall of the head portion  12  at a position above the opening  12   c  (FIG.  5 ). Additionally, a secondary throttle adjustment lever  60  is provided on one side (left side in the illustrated embodiment) of the head portion  12  for achieving fine adjustment of the position of the throttle lever  30 . 
     FIG. 3 shows in perspective the general arrangement of the operation-control lever unit of the present invention. 
     As shown in FIG. 31 the head members  13 ,  14  have a generally pentagonal dish-like form and each include a planar base  13   a ,  14   a  and a peripheral wall  13   b ,  14   b  integral with a peripheral edge of the base  13   a ,  14   a  and extending perpendicularly to the base  13   a ,  14   a . The peripheral wall  13   b  has four cutout portions  13   c ,  13   d ,  13   e  and  13   f  formed in consecutive four sides of the pentagonal head member  13 . Similarly, the peripheral wall  14   b  has four cutout portions  14   d ,  14   e  (two cutout portions corresponding in position to the cutout portions  13   c ,  13   f  being not shown) formed in consecutive four sides of the pentagonal head member  14 . When the two head members  13 ,  14  are assembled together to form the control head  12 , the cutout portion  13   c  of the peripheral wall  13   b  and the corresponding unillustrated cutout portion of the peripheral wall  14   b  jointly form the above-mentioned elongate opening  12   a  (FIGS.  1  and  5 ). Similarly, the cutout portions  13   d ,  14   d  jointly form an elongated hole or opening  12   b  (FIG. 5) from which the engine start switch  60  projects outwardly. The cutout portions  13   e ,  14   e  jointly form the above-mentioned elongated opening  12   c  (FIG. 5) which allows pivotal movement of the lock lever  40 . 
     The head member  13  has an integral cylindrical sleeve  15  projecting perpendicularly from an inside surface of the planar base  13   a . The sleeve  15  is located near a front end of the head portion  12 . The head member  14  also has an integral cylindrical sleeve  16  projecting perpendicularly from an inside surface of the planar base  14   a . The sleeve  16  is aligned with the sleeve  15 . The sleeve  15  of the head member  13  has a through-hole, while the sleeve  16  of the head member  14  has an internally threaded blind hole closed at one end by the base  14   a . The head members  13 ,  14  further have a pair of aligned cylindrical sleeves  17 ,  18  projecting perpendicularly from respective inside surfaces of the planar bases  13   a ,  14   a  at a position located near a rear end of the head members  13 ,  14 . The sleeve  17  has a through-hole, while the sleeve  18  has an internally threaded blind hole closed by the base  14   a . The sleeve  18  is longer than the sleeve  17 . Respective outer peripheral surfaces of the sleeves  17 ,  18  are cut or removed at diametrically opposite portions thereof so as to define a pair of parallel diametrically opposite flat surfaces extending obliquely to an axis of the grip portion  11  (FIG.  2 ). When the two head members  13 ,  14  are assembled together, the sleeves  15 ,  17  on the base  13   a  are in abutment with the sleeves  16 ,  18  on the base  14   a , respectively. 
     An internally threaded hollow cylindrical spring retaining lug  19  projects perpendicularly from the inside surface of the base  13   a  of the head member  13 . The spring retaining lug  19  is located near an upper end of the head member  13 . The cutout portions  13   f ,  14   f  ( 14   f  being shown in FIG. 7) formed in the respective peripheral walls  13   b ,  14   b  at a lower end of the head portion  12  have a semicircular shape and jointly form a circular hole in which an upper end portion of a metal pipe  20  is fitted. The pipe  20  is assembled with the head members  13 ,  14  by means of a screw  64 . The screw extends successively through a hole  14   g  in the head member  14 , a radial through-hole (not designated) in the pipe  20 , and a hole  13   g  (FIG. 7) in the head member  13  and is threaded into a nut  65  so that the head members  13 ,  14  and the pipe  20  are tightly fastened together. The handle portion  11  is fitted around the pipe  20 . 
     The head member  14  has a through-hole  21  formed in an intermediate portion of the base  14   a  which is offset from the center of the base  14   a  toward the front end of the base  14   a . The intermediate portion of the base  14   a  has an annular flange  21   a  projecting from an outside surface of the base  14   a  so that a longitudinal part of the through-hole  21  is defined by the annular flange  21   a . The through-hole  21  is clearance-fit with an enlarged head  61   a  of a support shaft  61 . The base  14   a  further has an arcuate oblong hole  22  extending arcuately about a central axis of the sleeve  16 . The arcuate oblong hole  22  is disposed rearwardly of the through-hole  21  when viewed from the sleeve  16 . 
     The head member  13  further has a first cam  23  formed on the inside surface of the base  13   a  in coaxial relation with the through-hole  21 . The first cam  23  is a cylindrical cam having an axial central through-hole  23   a  aligned with the through-hole  21  in the head member  14 . The cylindrical cam  23  has an end face formed with a cam surface  24  (FIG. 4) facing the base  14   a  of the head member  14 . The cam surface  24  is profiled in a manner as described below with respect to FIG.  4 . 
     The throttle lever  30 , the lock lever  40  and a throttle-lever holding mechanism  50  are disposed between the head members  13 ,  14 . The first and second cams  23 ,  47  form a part of the throttle-lever holding mechanism  50 . 
     The throttle lever  30  includes a hollow cylindrical head  32  formed integrally with an upper end of the lever portion  31 , and a sector member  33  projecting radially outwardly from the cylindrical head  32 . The sector member  31  has an arcuate guide grooves  34  extending arcuately about an axis of the cylindrical head  32 . The throttle lever  30  further has a support lug  35  projecting from an outer peripheral edge portion of one surface of the sector member  33  toward the head member  14 . The support lug  35  is cutout or removed at diametrically opposite portions thereof so as to define a pair of diametrically opposite flat surfaces  35   a  (FIG. 7) for a purpose described later. Thus, the support lug  35  has a non-circular cross section. 
     The throttle lever  30  further includes a tubular socket  36  projecting perpendicularly from the other surface of the sector member  33 . The socket  36  totatably receives therein an anchor pin  38  connected to one end of a control cable  37 , as shown in FIG.  6 . The other end of the control cable  37  is connected to the control shaft of a throttle valve (not shown) of the engine  3  (FIG.  1 ). 
     The lock lever  40  has a substantially rectangular flat base portion  42  integral with an upper end of the lever portion  41  and bent at right angles to the lever portion  41 , and a hook-shaped spring retainer  43  provided at a lower end of the base portion  42  adjacent the lever portion  41 . 
     The lock lever  40  further has a cylindrical head  44  of double tube structure including a pair of concentric inner and outer tubes  45  and  46  joined at one end. The outer tube  46  has an integral hollow cylindrical extension  46   a  projecting from the joined end toward the base  14   a  of the head member  14 . The double tube head  44  is formed integrally with a distal end of the base portion  42  at the extension  46   a  of the outer tube  46 . The inner and outer tubes  45 ,  46  are connected together by an annular end wall  44   a  (FIG. 6) which forms the bottom of the cylindrical extension  46   a . The lock lever  40  has a second cam  47  formed at the bottom of the cylindrical extension for coaction with the cam  23  formed on the head member  13 . The cam  47  has a cam surface  47  profiled in a manner described below with reference to FIG.  4 . 
     In an assembled condition where the throttle lever  30  and the lock lever  40  are pivotally mounted on the head portion  12 , a conical spring washer  52 , a metal washer  53 , and a rubber ring disc  54  are disposed between the double tube head  44  of the lock lever  40  and the sector member  33  of the throttle lever  30  in the order named when viewed from the double tube head  44 . Similarly, a rubber ring disc  54 , a metal washer  53  and a conical spring washer  52  are disposed between the sector member  33  of the throttle lever  30  and the base  14   a  of the head member  14  in the order named when viewed from the sector member  33 . The conical spring washers  53  form a resilient means of the throttle lever holding mechanism  50  which is disposed behind the second cam  47  (FIG. 4) when viewed from the first cam  23  and operates to urge the second cam  47  toward the first cam  23 . The rubber ring discs  54  form a friction member of the throttle lever holding mechanism  50  which is disposed between the throttle lever  30  and the lock lever  40  and is adapted to be forced against the throttle lever  30  when coaction between the first and second cams  23 ,  47  causes the lock lever  40  to be displaced toward the throttle lever  30  against the resiliency of the resilient means (biasing members of conical spring washers)  52 . 
     In FIG. 3, numeral  60  denotes a fine adjustment lever  60  attached by a screw  66  to the support lug  35  of the throttle lever  30  for achieving fine adjustment of the throttle position. The headed support shaft  61  has a long shank  61   b  extending through the conical spring washer  52 , metal washer  53 , rubber ring disc  54 , arcuate guide hole  34  of the sector section  33 , rubber ring disc  54 , metal washer  53 , conical spring washer  52 , metal washer  53 , double tube head  44  of the lock lever  40  and through-hole  23   a  of the cylindrical cam  23 . A nut  62  is threaded with an externally threaded fore end portion  61   c  (FIG. 8) of the shank  61   b  of the support shaft  61 , as shown in FIGS. 6-8. Two screws  63 ,  63  are inserted into the through-holes of the sleeves  15  and  17  and threaded into the internally threaded blind holes of the sleeves  16  and  17  to couple the head members  13  and  14  to form the head portion  12 . In an assembled condition, the peripheral walls  13   b ,  14   b  of the head members  13 ,  14  are interlocked with each other, as shown in FIG.  6 . 
     In FIG. 3, reference numeral  67  denotes a return spring acting between the throttle lever  30  and the head portion  12  to urge the throttle lever  30  toward an original unlock position. A tension coil spring  68  has one end connected to the hook-shaped spring retainer  43  of the lock lever  40  and the other end connected by a screw  69  to the spring retaining lug  19  on the head member  13 . The spring  68  urges the lock lever  40  in an original stand-by position in which the lever portion  41  is forced against the lower oblique flat surfaces of the sleeves  17 ,  18 , as shown in FIG.  5 . 
     As shown in FIG. 4, the cylindrical first cam  23  formed integrally with the inner surface of the base  13   a  of the head member  13  is a fixed cam, while the second cam  47  formed at the bottom of the cylindrical extension  46   a  of the double tube head  44  of the lock lever  40  is a movable cam. 
     Due to the axial central through-hole  23   a  of the fixed cylindrical first cam  23 , the cam surface  24  formed on an end face of the cylindrical first cam  23  has an annular shape. The cam surface  24  has alternate ridges  24   b  and grooves  24   a  arranged in the circumferential direction of the cylindrical first cam  23 . The grooves  24   a  and ridges  24   b  extend radially across the thickness of the cylindrical first cam  23 . The ridges  24   b  each have opposite sidewalls or flanks  24   c  and  24   e  and a top  24   d . The top  24   d  is flat and extends perpendicularly to an axis of the cylindrical first cam  23 . One flank  24   c , which faces in the counterclockwise direction shown in FIG. 4, is beveled or sloped. The other flank  24   e , which faces toward the clockwise direction shown in FIG. 5, is perpendicular to the flat top  24   d  and is parallel to the axis of the cylindrical first cam  23 . In FIG. 4, the clockwise direction is the same as the direction of pivotal movement of the lock lever  40  toward a locking position. This direction is hereinafter referred to as “locking direction”. 
     The cylindrical extension  46   a  of the double tube head  44  has an inside diameter determined to achieve a slid-fit connection between the double tube head  44  and the cylindrical first cam  23 . This means that an inner peripheral surface  46   b  (FIG. 8) of the cylindrical extension  46   a  is in slide contact with an outer peripheral surface  23   b  of the cylindrical first cam  23 . 
     The cam surface  48  of the movable second cam  47 , which is formed on the annular end wall  44   a  at the bottom of the cylindrical bore  46   a , has the same cam profile as the cam surface  24  of the first cam  23 . More specifically, as shown in FIG. 11, the cam surface  48  has alternate ridges  48   b  and grooves  48   a  arranged in the circumferential direction of the annular end wall  44   a , so that the ridges  48   b  of the second cam  47  can interdigitate with the ridges  24   b  of the first cam  23 . Beveled or sloped flanks  48   c  of the ridges  48   b  face toward the locking direction (indicated by the profiled arrow in FIG. 11) which is opposite to the direction of facing of the sloped flanks  24   c  of the first cam  23 . 
     When the lock lever  30  is manually turned in the locking direction (clockwise direction in FIG. 5) about the axis of the double tube head  44 , the cam surface  48  of the second cam  47  coacts with the cam surface  24  of the first cam  23  to displace the second cam  47  and the lock lever  30  along the support shaft  61  (FIG. 3) in a direction away from the head member  13  against the resilient force of the conical spring washers  52  (FIG.  4 ). 
     As shown in FIG. 5, the grip portion  11  has an elongated recess  11   a  formed in a rear surface of the grip portion  11  along an upper member of the grip portion  11 . The recess  11   a  accommodates within it the lever portion  41  of the lock lever  40  when the lock lever  40  is fully depressed, as shown in FIG.  9 . 
     The control cable  37  includes a control wire  37   a  connected at one end to the anchor pin  38  and, at the other end, to the control shaft of the throttle valve (not shown) of the engine  3  (FIG.  1 ). The control cable  37  further has an upper sheath or jacket  37   b  covering an upper portion of the control wire  37   a , and a lower sheath or jacket  37   c  connected to a lower end of the upper jacket  37   b  and covering the remainder of the control wire  37   a . The lower jacket  37   c  has a larger diameter than the upper jacket  37   b . The upper jacket  37   b  has at its upper end a guide sleeve  37   d  of metal. The metal guide sleeve  37   d  is held within a retainer pocket  13   h  formed integrally with the base  13   a  of the head member  13  at a portion above the cutout portion  13   f.    
     The pipe  20  has a radial hole or opening  20   a  located near a lower opening  11   b  of the grip portion  11 , so that the control cable  37  can be drawn out from the operation-control lever  10  through the radial opening  20   a  of the pipe  20  and the lower opening  11   b  of the grip portion  11 . A lower end portion of the pipe  20  is drawn out from the lower opening  11   b  of the grip portion  11  and is connected to the support rod  5 . 
     The cylindrical head  32  of the throttle lever  30  is slidably fitted around the sleeves  15 ,  16  for pivotal movement about a common axis of the sleeves  15 ,  16 . The return spring  67  is a torsion coil spring having a coiled portion  67   a  wound around the cylindrical head  32 . By the resiliency of the torsion coil spring  67 , One end  67   b  of the spring  67  is urged against an upper part of the peripheral wall  13   b  of the head member  13 . The other end  67   c  of the spring  67  is anchored to a base portion of the throttle lever  31 . Thus, the throttle lever  30  is urged to turn clockwise in FIG. 5 about the axis of the sleeves  15 , 16  by the resiliency of the torsion coil spring  67 , so that the throttle lever  30  is normally held in the stand-by position shown in FIG.  5 . 
     The support shaft  61  extending across the breath of the head portion  12  penetrates the arcuate guide hole  34  in the sector member  33  of the throttle lever  30 . 
     The inner tube  45  (FIG. 6) of the double tube head  44  of the lock lever  40  is slidably fitted around the shank  61   b  of the support shaft  61  so that the lock lever  40  is pivotally movable about an axis of the support shaft  61 . The lever portion  41  of the lock lever  40  project obliquely and downwardly from the elongated opening  12   c  of the head portion  12 . The tension coil spring  68  acting between the hook-shaped spring retainer  43  of the lock lever  40  and the spring retaining lug  19  on the head member  13  urges the lock lever  40  to turn counterclockwise in FIG. 5 about the axis of the support shaft  61 . Thus, the lock lever  40  is normally held in the stand-by position of FIG. 5 in which the lever portion  41  is forced against the lower oblique flat surfaces of the sleeves  17 ,  18 . 
     As shown in FIG. 7, the support lug  35  on the sector member  33  of the throttle lever  30  is loosely received in the oblong hole  22  in the base  14   a  of the head member  14  with its cross-sectionally non-circular portion (including the flat surfaces  35   a ,  35   a ) projecting from an outside surface of the base  14   a  of the head member  14 . The cross-sectionally non-circular portion including the flat surfaces  35  is received in a recess  60   a  of the fine adjustment lever  60  which is attached by the screw  66  to the support lug  35 . Since the recess  60   a  has a non-circular cross section complementary in shape to the non-circular cross section of the support lug  35 , the fine adjustment lever  60  is non-rotatable relative to the support lug  35 . For the purpose of manipulation, the fine adjustment lever  60  is located on the left side of the head portion  12 , as shown in FIGS. 2 and 9. 
     As shown in FIG. 8, the support shaft  61  comprises a hexagon socket head cap screw having an enlarged cylindrical socket head  61   a , a long shank  61   b  and a male screw  61   c  externally threaded on a fore end portion of the shank  61   b . The socket head  61   a  is slidably received in the through-hole  21  of the base  14   a  of the head member  14  and has a hexagonal hole  61   d  for receiving therein the tip of a tool, such as a hexagonal bar wrench. When the support shaft  61  is to be rotated, the hexagonal bar having one end snugly received in the hexagonal hole  61   d  is turned. The head  61   a  may be shaped into a hexagonal form which is slidably receivable in the through-hole  21  and hence is rotatable relative to the head portion  12 . 
     As previously described, the shank  61   b  of the support shaft  61  extends through the arcuate guide hole  34  of the sector member  33  of the throttle lever  30 , and an axial through-hole  45   a  of the double tube head  45  of the lock lever  40 . The male screw  61   c  of the support shaft  61  is threaded through an insert nut  70  press-fitted in the axial central hole  23   a  of the fixed first cam  23 . A fore end portion of the male screw  61   c  projects from the insert nut  70 , and the nut  62  is threaded with the projecting fore end portion of the male screw  61   c  to lock the support shaft  61  in position against movement relative to the head portion  12 . Thus, the nut  62  serves as a lock nut. The lock nut  62  is partly received in an annular recess  13   i  formed in the outside surface of the base  13   a  of the head member  13 . The insert nut  70  may be replaced by a sleeve (not shown). 
     The cam surface  24  of the fixed cylindrical first cam  23  and the cam surface  48  of the movable second cam  47  are held in pressure contact with each other with the outer peripheral surface  23  of the fixed cylindrical first cam  23  being in sliding contact with the inner peripheral surface  46   b  of the cylindrical extension  46   a  of the double tube head  44  of the lock lever  40 . Since the cylindrical first cam  23  is slidably received in the cylindrical extension  46   a , any foreign matter including dirt and dust is no longer possible to get into the cylindrical extension  46   a . Thus, the cam surfaces  24 ,  48  of the first and second cams  23 ,  47  are completely free from contamination with dirt and dust and can operate stably and reliably over a long period of use. 
     The conical spring washer  52 , the washer  53  and the rubber ring disk  54  are disposed between the enlarged cylindrical head  61   a  of the support shaft  61  and one side surface of the sector member  33  of the throttle lever  30  in the order named when viewed from the head  61   a  toward the sector member  33 . 
     Similarly, the rubber ring disk  54 , the washer  53  and the conical spring washer  52  are disposed between the other side surface of the sector member  33  and one end face of the double tube head  44  of the lock lever  40  in the order named when viewed from the sector member  33  toward the double tube head  44 . 
     The conical spring washers  52  are disposed behind the movable second cam  47  when viewed from the fixed first cam  23  and form a resilient means for resiliently urging the cam surface  48  of the second cam  47  into pressure contact with the cam surface  24  of the first cam  23 . The conical spring washers  52  also serve as a bias means for resiliently urging the rubber ring disks  54  against the opposite side surfaces of the sector member  33  of the throttle lever  30  when the lock lever  40  is displaced along the axis of the support shaft  61  toward the throttle lever  30  by coaction of the first and second cams  23 ,  47 . The rubber ring disks  54  form a friction means adapted to be forced by the biasing means  52  against the sector member  33  of the throttle lever  30  to frictionally hold the throttle lever  30  in a desired operating position. The first and second cams  23 ,  47 , the resilient means  52  and the friction member  54  jointly form the afore-mentioned throttle-lever holding mechanism  50 . 
     The resilient force exerted from the throttle-lever holding mechanism  50  onto the throttle lever  30  can be adjusted by turning the hexagonal socket head  61   a  of the support shaft  61  by use of a hexagonal bar wrench (not shown) while the lock nut  62  is kept loosened so that the support shaft  61  is displaced relative to the head member  13  in the axial direction of the support shaft  61 . In the normal condition, the resilient force is adjusted such that the rubber ring disks  54  are in light pressure contact with the opposite side surfaces of the sector member  33  of the throttle lever  30 . 
     Operation of the operation control lever unit  10  will be described below with reference to FIG.  9 . 
     In use of the carrying bush cutter  1  (FIG.  1 ), the operation control lever unit  10  is gripped for manipulation by one hand (right hand RH, for example) of the operator M (FIG.  1 ). In this instance, an upper part of the grip portion  11  and a lower part of the head portion  12  are held in a palm of the right hand RH. The index finger R 2  of the right hand RH is placed on the lever portion  31  of the throttle lever  30 , the thumb F 1  is placed on the back of the fine adjustment lever  60 , and the remaining fingers are used to grasp the upper part of the grip portion  11 . Then, the index finger F 2  is pulled to depress the throttle lever portion  31  until it reaches a desired position. Then the lever portion  41  of the lock lever  40  is depressed into the recessed portion  11   a  (FIG. 5) by a portion of the palm (a ball of the thumb, for example). As the lever portion  31  is depressed, the throttle lever  30  turns counterclockwise in FIG. 9 about the axis of the screw  63  (FIG.  5 ). This achieves adjustment of the position of the throttle valve (not shown) to thereby regulate the engine speed. 
     Stated more specifically, as the throttle lever  30  turns counterclockwise in FIG. 10 about the screw  63  against the force of the return spring  67  (FIG.  5 ), the control wire  37   a  of the control cable  37  is pulled upwardly. With this upward movement of the control cable  37 , the throttle valve incorporated in the carburetor of the engine  3  (FIG. 1) is turned in a direction to increase rotational speed of the engine  3 . The throttle lever  30  shown in FIG. 10 is in the full-throttle position which defines an upper limits of an adjustable range of engine speed. 
     As the lever portion  41  of the lock lever  40  is depressed by the palm part such as the ball of the thumb F 1 , the lock lever  40  turns clockwise in FIG. 10 about the support shaft  61  against the force of the return spring  68 . In this instance, the movable second cam  47  on the double tube head  44  turns about the support shaft  61  in the same direction as the lock lever  40 . Accordingly, by a camming action between the respective cam surfaces  24 ,  48  of the first and second cams  23 ,  47 , the rotary motion of the lock lever  40  is translated into a linear motion of the lock lever along the axis of the support shaft  61  in a direction toward the throttle lever  30 . 
     The lateral movement of the double tube head  44  causes the conical spring washers  52 ,  53  (FIG. 8) to be compressed into a substantially fully distorted flattened position between the hexagonal socket head  21   a  of the support shaft  61  and the washer  52 . With this distortion of the conical spring washers  52 ,  52 , the rubber ring plates  54 ,  54  are strongly forced against the opposite surfaces of the sector member  33  of the throttler lever  30 , so that the throttle lever  30  is held in a desired position against pivotal movement. One example of such desired position is the full-throttle position shown in FIG.  10 . 
     The oblong guide hole  34  of the throttle lever  30  extends arcuately about the axis of the sleeves  15 ,  16 , the throttle lever  30  can smoothly turn about the axis of the sleeves  15 ,  16  while the sector member  33  is guided by sliding engagement between the guide hole  34  and the shank  61  of the support shaft  61 . 
     As described above with reference to FIG. 7, the fine adjustment lever  60  is firmly secured by the screw  66  to the distal end of the support lug  35  projecting from the sector member  33  of the throttle lever  30  through the oblong guide hole  22  to the outside of the head portion  12  of the operation-control lever unit  10 . 
     In operation, a ball of the thumb F 1  is placed on the back of the fine adjustment lever  60 , as shown in FIG. 9, then forced forwardly by the thumb F 1  while a certain pressure is continuously applied from the index finger F 2  to the lever portion  31  of the throttle lever  30 . The forced forward movement of the fine adjustment lever  60  cause the throttle lever  30  to turn clockwise in FIG. 10 about the axis of the sleeves  15 ,  16  against a frictional force acting between the opposite surfaces of the sector member  33  and the spring biased rubber ring plates  54 ,  54  (FIG. 8) of the throttle-lever holding mechanism  50 . During pivotal movement of the throttle lever  30 , the support lug  35  on the sector member  33  is guided by and along the arcuate oblong hole  22  formed in the base  14   a  (FIG. 7) of the head member  14 . The sector member  33  slips on two opposed surfaces of the ruber ring plates  54 ,  54 . 
     Thus, when the engine  3  (FIG. 1) while running at a maximum speed is to be slowed down, the fine adjustment lever  60  is forced by the thumb F 1  in the forward direction to cause the throttle lever  30  to separate from the full-throttle position of FIG.  10  and returns toward its original idling position of FIG.  5 . With this pivotal movement of the throttle lever  30 , the throttle valve incorporated in the carburetor of the engine  3  is operated in a direction to lower the engine speed (corresponding to power of the engine  3 ). During that time, the lock lever  40  is continuously held in its fully depressed locking position shown in FIG. 10. 10. 
     The fine adjustment lever  60  is displaced in the forward direction of the head portion  12  by forcing or pushing it with the thumb F 1 . The thumb F 1  when used to push the fine adjustment lever  60  can produce a greater power than when used to pull the same lever  60 . Thus, manipulation of the fine adjustment lever  60  using the thumb F 1  can be achieved with utmost ease and high reliability even though the throttle-lever holding mechanism  50  continuously operates to frictionally hold the throttle lever  30  in position against pivotal movement while the lock lever  40  is in its fully depressed position. 
     Then, explanation will be given of the cam mechanism which forms an essential part of the throttle-lever holding mechanism  50  (FIG.  8 ). As described above, the cam mechanism is formed by the stationary cam  23  and the movable cam  47 . These cams  23 ,  47  have respective cam surfaces  24 ,  48  engaged with each other. 
     As shown in FIG. 11, the cam surface  24  of the stationary cam  23  have four grooves  24   a  and four ridges  24   b  provided alternately at equal angular intervals of 90 degrees. The ridges  24   b  have the same height. The top surfaces  24   b  of the ridges  24   b  and the bottom surfaces of the grooves  24   a  are flat, parallel with each other, and perpendicular to the axis of the annular stationary cam  23 . The flat top surfaces  24   d  have a predetermined width (corresponding to an extent in the circumferential direction of the cam surface  28 ). One sidewall or flank  24   c  of each ridge  24   b  is sloped, and the other flank  24   e  is perpendicular to the top surface  24   d  and the bottom surface of the groove  24   a . The ridges  24   b  have a maximum width (including the width of the associated sloped flanks  24   c ) which is smaller than the width of the grooves  24   a.    
     Similarly, the cam surface  48  of the movable cam  47  have four grooves  48   a  and four ridges  48   b  provided alternately at equal angular intervals of 90 degrees. The ridges  48   b  have the same height. The top surfaces  48   b  of the ridges  48   b  and the bottom surfaces of the grooves  48   a  are flat, parallel with each other, and perpendicular to the axis of the annular movable cam  47 . The flat top surfaces  48   d  have substantially the same width as the flat top surfaces  24   d  of the ridges  24   b . One sidewall or flank  48   c  of each ridge  48   b  is sloped, and the other flank  48   e  is perpendicular to the top surface  48   d  and the bottom surface of the groove  48   a . A maximum width of the ridges  8   b  (including the width of the associated sloped flanks  48   c ) is substantially the same as that of the ridges  24   b  and is smaller than the width of the grooves  48   a . The sloped flanks  48   c  of the cam surface  48  and the sloped flanks  24   c  of the cam surface  24  face in opposite directions so that they are slidably engaged with each other when the movable cam  24  is turned, relative to the stationary cam  23 , in a direction indicated by the profiled arrow shown in FIG.  11 . In the normal condition where the lock lever  40  is in its standby position shown in FIG. 5, the ridges  48   b  of the cam surface  48  are in mesh with the ridges  24   b  of the cam surface  24 . 
     A camming action of the cam mechanism which is induced by coaction between the cam surface  24 ,  48  of the stationary cam  23  and the cam surface  48  of the movable cam  47  will be described with reference to FIGS. 12A to  12 C. 
     In the normal condition in which the lock lever  40  is in the standby position shown in FIG. 5, the cam surfaces  24 ,  48  are held in mutual interdigitating engagement under the bias of the conical spring washers  52 , 52 , with the ridges  24   b  ( 48   b ) of one cam surface  24  ( 48 ) being received in the grooves  48   a  ( 24   a ) in the other cam surface  48  ( 24 ). 
     When the lock lever  40  is turned clockwise in FIG. 5 so as to control pivotal movement of the throttle lever  30 , the movable cam  47  starts rotating in the direction of the arrow shown in FIG.  12 A. Rotation of the movable cam  47  causes the sloped flanks  48   c  of the cam surface  48  of the movable cam  47  come into contact with the sloped flanks  24   c  of the cam surface  24  of the stationary cam  23 , then slide up along the sloped flanks  24   c . With this sliding movement of the sloped flanks  48   c , the movable cam  47  is displaced in the left-hand direction in FIG. 12B against the forces of the conical spring washers  52  (FIG.  8 ). 
     As a consequence of the leftward movement of the movable cam  47 , the cylindrical double tube head  44  (FIG. 8) of the lock lever  40  slides along the support shaft  61  in the leftward direction in FIG. 8 with the result that the rubber ring discs  54  are forced against the opposite surfaces of the sector member  33  of the throttle lever  30  by the forces of the conical spring washers  52 . Thus, a frictional force acting between the ruber ring discs  54  and the sector member  33  increases. 
     Continued rotation of the movable cam  48  causes the flat tops  48   d  of the respective ridges  48   b  of the movable cam  47  to come into sliding contact with the flat tops  24   d  of the ridges  24   b  of the stationary cam  23 , as shown in FIG.  12 C. In this instance, the movable cam  47  is displaced leftwards from its original position by a distance corresponding to the height of the ridges  24   b ,  48   b . The position shown in FIG. 12C corresponds to a locking position of the throttle lever  30  by the action of the throttle-lever holding mechanism  50  in response to pivotal movement of the locking lever  40 . In this locking position, the conical spring washers  52  are substantially fully deflected by the cylindrical double tube head  44  of the lock lever  40  so that the rubber ring discs  54 ,  54  firmly grip the sector member  33  with maximum friction to thereby lock the throttle lever  30  in position against pivotal movement. 
     Since the flat tops  24   d ,  48   d  of the ridges  24   b ,  48   b  of the cams  23 ,  47  have a certain length in the circumferential direction so that the locking condition of the throttle lever  30  is be kept even when the lock lever  40  is further turned to some extent. 
     FIG. 13 shows in comparative purposes a cam mechanism composed of two circular disc cams  123  and  147  each having on its one end face a cm surface  124 ,  148  including four ridges  124   b ,  148   b  formed contiguously in the circumferential direction. The ridges  124   b ,  148   b  have a triangular cross section and are separated by perpendicular walls  124   a ,  148   a . The disc cam  123  is regarded as a stationary cam corresponding to the cam  23  (FIG. 3) formed integrally with the head member  13 , and the disc cam  147  is regarded as a movable cam corresponding to the cam  48  (FIG. 3) integral with the lock lever  40 . 
     The cam mechanism shown in FIG. 13 operates as follows. 
     In the normal condition in which the lock lever  40  is in the standby position shown in FIG. 5, the ridges  124   b ,  148   b  of the cam surfaces  124 ,  148  of the cams  123 ,  147  are held in mutual interdigitating engagement under the bias of the conical spring washers  52 , 52 , as shown in FIG.  14 A. 
     When the lock lever  40  is turned clockwise in FIG. 5, the cam  147  turned in the same direction, causing the ridges  148   b  of the movable cam  147  slide up along the ridges  124   b  of the stationary cam  123 . When respective tip ends of the ridges  148   b  arrive at the corresponding tip ends of the ridges  124   b , as shown in FIG. 14B, the movable cam  17  and the lock lever  40  are displaced leftwards to a maximum extent away from the stationary cam  123 . In this condition, a throttle-holding mechanism including the cam mechanism  123 ,  147  exerts a maximum frictional force to the sector member  33 , thereby locking the throttle lever  30  in position against pivotal movement. The tip-to-tip engagement between the ridges  124   b ,  148   b  of the cams  123 ,  147  is unstable, ans so when the force exerted on the lock lever  40  changes due to a change in working condition, a change in working posture, vibrations of the engine, or external shock forces, the movable cam  148  tends to turn in the opposite direction under the bias of the conical spring washers  52 , causing the ridges  148   b  to slide down along the ridges  124   b  of the stationary cam  123 , as shown in FIG.  12 C. This means that the frictional force acting on the sector member  33  of the throttle lever  30  under the forces of the conical spring washers  52  decreases, allowing the throttle lever  30  to pivot in the direction to return to its original standby position. With this returning movement of the throttle lever  30 , engine speed is slowed down, making the operation of the circular cutter  6  (FIG. 1) unstable. Such unintentional slowing down of the engine speed does not occur in the device of the present invention because of the trapezoidal cross-sectional shape of the ridges  24   b ,  48   b  of the cams  23 ,  47 . 
     One of the cams  23 ,  47  may have ridges of a triangular cross-section similar to those  124   b ,  148   b  of the cams  123 ,  147 . The number of the ridges  24   b ,  48   b  should by no means be limited to four in the illustrated embodiment and may be two, three, five or more ridges may be employed. Additionally, the stationary cam  23  formed integrally with the head member  13  of the operation-control handle  10  may be replaced with a separate cam  23  firmly secured to the head member  13 . 
     FIGS. 15A-15C illustrate the manner in which a frictional force acting between the rubber ring discs  54  of the throttle-lever holding mechanism  50  and the sector member  55  of the throttle lever  30  is adjusted. In the initial condition (corresponding to the position shown in FIG.  8 ), the throttle-lever holding mechanism  50  is in the position (neutral position) shown in FIG.  15 A. 
     When the frictional force is to be adjusted, the lock nut  62  is loosened as shown in FIG.  15 A. Thus, by turning a hexagonal bar wrench  90  in either direction of the arrow with its one end  91  received in the hexagonal hole  61   d  in the head  61   a  of the support shaft  61 , the support shaft  61  is moved right or left relative to the insert nut  70 , thus enabling adjustment of the frictional force between the sector member  33  and the rubber ring discs  54 . In the neutral position, the conical spring washers  52  have a height A equal to the distance between the under surface  61   e  of the head  61   a  and the metal washer  53 . 
     When the friction between the sector member  33  and the rubber ring discs  54  is to be increased, the support shaft  61  is rotated by the bar wrench  90  in the clockwise direction indicated by the solid-lined arrow shown in FIG.  15 A. By virtue of threading engagement between the screw portion  61   c  and the insert nut  70 , the support shaft  61  moves rightwards relative to the insert nut  70  and the head member  13  against the forces of the conical spring washers  52 ,  52 , as indicated by the solid-lined arrow shown in FIG.  15 B. When the support shaft  61  is displaced rightward by a distance x from the neutral position of FIG. 15A, the conical spring washers  52  are deflected in a somewhat flattened position and has a height B. 
     With this rightward movement of the support shaft  61 , a preloading force exerted on the conical spring washers  52  is increased. Thus, the rubber ring discs  54 ,  54  are forced by the conical spring washers  52 ,  52  against the opposite surfaces of the sector member  33  under a greater resilient force than that applied at the initial state. Consequently, the friction between the sector member  33  and the rubber ring discs  52  increases, correspondingly. At the same time, the stationary and movable cams  23 ,  47  are subjected to a greater force tending to hold them together. 
     The foregoing friction increasing adjustment is particularly useful when the coefficient of friction of the rubber ring discs  50  becomes small due to aging or deterioration by time. Additionally, the friction can be adjusted only by turning the support shaft  61  to move the same in an axial direction without exerting any adverse effect on another mechanism. 
     When the friction between the sector member  33  and the rubber ring discs  54  is to be decreased, the support shaft  61  is rotated by the bar wrench  90  in the counterclockwise direction indicated by the broken-lined arrow shown in FIG.  15 A. By virtue of threading engagement between the screw portion  61   c  and the insert nut  70 , the support shaft  61  moves leftwards relative to the insert nut  70  and the head member  13  under the forces of the conical spring washers  52 ,  52 , as indicated by the broken-lined arrow shown in FIG.  15 C. When the support shaft  61  is displaced leftward by a distance Y from the neutral position of FIG. 15A, the conical spring washers  52  are allowed to axially expand and has a height C. 
     With this leftward movement of the support shaft  61 , a preloading force exerted on the conical spring washers  52  is lessened. Thus, the rubber ring discs  54 ,  54  are forced against the opposite surfaces of the sector member  33  under a smaller resilient force than that applied at the initial state. Consequently, the friction between the sector member  33  and the rubber ring discs  52  decreases, correspondingly. At the same time, the stationary and movable cams  23 ,  47  are subjected to a smaller force tending to hold them together. 
     The foregoing friction decreasing adjustment is particularly useful when the initially set friction is too large for the operator to manipulate the throttle lever  30 . Likewise the friction increasing adjustment mentioned previously, the friction decreasing operation can be achieved by merely turning the support shaft  61  and does not exert any influence on the operation or another mechanism. 
     After the foregoing adjustment, the lock nut  62  is threaded over the screw portion  61   c  of the support shaft  61  to lock the support shaft  61  at the desired position relative to the head portion  12  of the operation-control handle  10 . With this friction adjustment, it is possible to grip the sector member  33  between the rubber ring discs  52 ,  52  to frictionally hold the throttle lever  30  in a desired position. At the same time, a force required to turn the lock lever  40  to activate the throttle-lever holding mechanism  50  can be adjusted at a desired value. 
     Thus, the friction on the throttle lever  40  can be easily adjusted by displacing the support shaft  61  in the axial direction by turning the support shaft  61  in such a manner that the adjusted friction is suited for the operator. 
     FIG. 16 shows a modified form of the throttle-lever holding mechanism  50  according to the present invention, the modified mechanism  150  being in the state corresponding to the standby position of the lock lever  40 . In FIG. 16, these parts which are identical to those in the embodiment shown in FIG. 8 are designated by the same reference characters, and further description thereof can, therefore, be omitted. 
     The modified throttle-lever holding mechanism  150  includes a stationary cam  23  formed integrally with the head member  13 , and a movable cam  47  provided at the bottom of a first axial recess  144   a  formed in one end face of a cylindrical head  144  of the lock lever  40 . The axial recess  144   a  is slidably fitted over a peripheral surface of the stationary cam  23  which is cylindrical in shape. The cylindrical head  144  further has a second axial recess  144   b  formed in the opposite end face of the cylindrical head  144 . A conical spring washer  152  fitted around the shank  61   b  of a support shaft  61  is received in the second axial recess  144   b  and fixed in position by a stop ring  171  attached to the support shaft  61  such that the conical spring washer  152  preloaded between the cylindrical head  144  and the stop ring  171 . The conical spring washer  152  urges the cylindrical head  144  rightwards to keep the movable cam  48  in engagement with the stationary cam  23 . 
     The throttle-lever holding mechanism  150  further includes two rubber ring discs (friction members)  154 ,  154  fitted around the shank  61   b  of the support shaft  61  and disposed on opposite sides of the sector member  33  of the throttle lever  30 , and two metal washers  153 ,  153  fitted around the shank  61   b  and each disposed on the outer side of one of the rubber ring discs  152 ,  152 . One of the metal washers  153  is disposed between the head  61   a  of the support shaft  61  and one of the rubber ring discs  152 , and the other metal washer  153  is disposed between the other rubber ring disc  152  and the cylindrical head  144  of the lock lever  40 . The cylindrical head  144  further has a central annular third recess  144   c  facing the peripheral surface of the shank  61   b . The annular recess  172  is filled with an oil-impregnated sponge rubber or O-ring  172 . 
     In the standby position shown in FIG. 16, the metal washer  153  disposed between the sector member  33  and the cylindrical head  144  is separated by a space from the cylindrical head  144 . The force of the conical spring washer  152  does not act on the sector member  33  of the throttle lever  30 . The throttle lever  30  is held in the standby position of FIG. 5 by the force the return spring  67 . 
     When the lock lever  40  is depressed as shown in FIG. 9, the cylindrical head  144  turns about an axis of the shank  61   b  of the support shaft  61 . The rotational movement of the cylindrical head  144  is translated into an axial leftward movement of the cylindrical head  144 , as shown in FIG. 17, by a camming action induced by and between the respective cam surfaces  24 ,  48  of the stationary and movable cams  23 ,  47 . The cylindrical head  144 , as it is displaced leftwards against the resiliency of the conical spring washer  152 , comes into abutment with the confronting metal washer  153 , then forcing the same washer  153  leftwards. Consequently, the rubber ring discs  154  disposed on opposite sides of the sector member  33  is axially compressed between an under surface  61   e  of the head  61   a  of the support shaft  61  and the cylindrical head  144  of the lock lever  40 . Thus, the sector member  33  is firmly gripped between the rubber ring discs  154 ,  154  with the result that the throttle lever  30  is locked in position against pivotal movement. 
     With the arrangement of the throttle-lever holding mechanism  150 , the throttle lever  30  can be manipulated with a lesser force or pressure than the throttle lever operationally connected with the throttle-lever holding mechanism  50  of the first embodiment. The required manipulating force is at least greater than a combined force of the force of return spring  67  and the biasing force applied to the control cable  37 . 
     The conical spring washers  52 ,  152  of the throttle-lever holding mechanisms  50 ,  150  may be replaced with compression coil springs having a small axial length. In the embodiment described above, the operation-control lever unit is used in the carrying bush cutter. The operation-control lever unit according to the present invention may be employed in other engine-powered working machines, such as shown in FIGS. 18-21. 
     FIG. 18 shows a chainsaw  201  driven by an engine  203 . The engine-driven chainsaw  201  has a grip handle  210  projecting laterally from a body  203   a  of the chainsaw  201 , and an operation handle  210  projecting forwardly and upwardly from the engine  203 . The grip handle  207  is gripped by a left hand LH of the operator M, and the operation handle  210  is gripped by a right hand RH of the operator M. The operation handle  210  is equipped with the operation-control lever unit of the present invention described above for controlling rotational speed of the engine  203 . The chainsaw  201  has a cutting blade  206  with teeth on an endless chain projecting forwardly from the body  203   a  for trimming trees. 
     FIG. 19 shows a chemical sprayer  301  driven by an engine  303  carried on the back of the operator M via a frame  302 . The frame  302  also carries thereon a chemical tank  390  disposed below the engine  303 . The chemical tank  390  has a built-in pump driven by the engine  303 . The chemical sprayer  301  has a spray nozzle  306  attached to the top of a rigid pipe  305 , and a flexible hose  304  connecting the rear end of the pipe  305  and the chemical tank  390 . An operation handle  307  is provided on the rear end of the pipe  305  and is gripped by a right hand of the operator M. The operation handle  307  is equipped with the operation-control lever unit of the present invention described above. The pump driven by the engine  303  forces a chemical fluid to be drawn from the tank  390  and sprayed out from the spray nozzle  306  onto trees and plants. 
     FIG. 20 shows a blower  401  driven by an engine  403  carried on the back of the operator M via a frame  402 . The engine-driven blower  401  includes a blower pipe  405  having a nozzle  405   a  at a front end thereof, a flexible hose  404  interconnecting a rear end of the pipe  405  and a compressor  490  driven by the engine  403 . An operation handle  410  is provided on a rear end portion of the pipe  405  and gripped by a right hand RH of the operator M. The operation handle  410  is equipped with the operation-control lever unit of the present invention. Pressurized air supplied from the engine-driven compressor  490  is forced out from the nozzle  405   a  to collect dust, leaves, trash on the roads. 
     FIG. 21 shows a vacuum dust collector  501  driven by an engine  503  carried on the back of the operator M via a frame  502 . The vacuum dust collector  501  includes a vacuum generator  590  driven by the engine  503 , a rigid pipe  505  connected to the vacuum generator  590  via a flexible hose  504 , and a vacuum attachment  506  attached to a front end of the pipe  505  for collecting, by suction, dust, leaves and trash on the roads. An operation handle  510  is provided on a rear end portion of the pipe  505  and gripped by a right hand RH of the operator M. The operation handle  510  is equipped with the operation-control lever unit of the invention described above. 
     Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the present invention may be practiced otherwise than as specifically described.