a preferred embodiment of the present invention is explained with figures . however , the scope of the invention is not limited by the illustrated embodiments of the figures . a lawn niower shown in fig1 ( a ) and 1 ( b ), which is the arm coupling type , comprises an operation ann 10 as a main body , a driven shaft 11 rotatably provided inside the operation arm 10 as shown in fig2 , a discoid metal cutter 13 installed on the driven shaft 11 at one end of the operation arm 10 , an arm side joint case 16 provided at the other end of the operation arm 10 with a large diameter than the operation arm 10 and either one of an engine 17 and an electic motor 18 selectively installed on the operation arm 10 through the joint case 16 . the operation arm 10 is formed of a hollow shaft . a holder 12 is attached to the one end of the operation arm 10 . the metal cutter 13 has a plurality of saw - edged cutting tooth on its outer peripheral and is rotatably attached to the holder 12 . the cutter 13 is connected to the end of the driven shaft 11 through a pair of bevel gears ( not shown ). the operation arm 10 has a cover 14 at its end to partially cover the cutter 13 so that the safety of work is secured . in addition , the cutter 13 may be replaced by a cutter composed of one or more strings extending radially from the center of rotation two handles 15 are installed on a center part of the operation arm 10 so that the user can work on mowing grass by grpping the handles 15 with both hands . in place of the two - handed type handles 15 as shown in fig1 ( a ) and 1 ( b ), a looped type handle may be used . furthermore , a loop - like belt can be attached to the operation arm 10 for shoulder carrying the mower . the arm side joint case 16 , as a main body side joint case , is provided on the opposite end of the operation arm 10 from the cutter 13 . the arm side joint case 16 is constructed by a cylindrical member of a large diameter than the operation arm 10 . either an engine 17 as shown in fig1 ( a ) or an electric motor 1 s as shown in fig1 ( b ) is selectively attached to the operation arm 10 through the arm side joint case 16 . the electric motor 18 has a power supply cable 20 with an electrical plug 19 which is inserted into the power supply outlet so as to supply electric power to the electric motor 18 . otherwise , a battery may be used as the power supply instead of the commercial power supply . by using the battery as the power supply , the lawn mower can be quietly driven by the motor in the outdoors with no power supply outlet . the engine 17 is a single cylinder 4 - stroke cycles engine . inside of an engine cover 21 , there are engine coxnponents such as a crankcase for rotatably supporting an engine output shaft 22 connected to a crankshaft shown in fig2 , and a cylinder in which a piston tavels reciprocally is provided in the crankcase . also , the engine 17 is equipped with a fuiel tank 23 storing fuel of gasoline . a cooling fan 24 generating engine cooling air is installed on the output shaft 22 . the cooling fan 24 comprises a disk 25 connected to the output shaft 22 and a plurality of fan blades 26 fixed to the disk 25 . further , the engine 17 is equipped with a recoil starter for turning the output shaft 22 manually so that the engine 17 can be started by pulling a recoil knob 27 as shown in fig1 ( a ). an engine side joint case 28 , as a power source side joint case , which can be attached to and removed from the arm side joint case 16 , is provided on the engine cover 21 . as shown in fig2 , this engine side joint case 28 is partly fitted to the arm side joint case 16 . on the outer surface of the arm side joint case 16 , one end of a link 32 is swingably connected to a supporting point 31 of the outer surface of the arm side joint case 16 and the other end of the link 32 is swingably connected to a middle portion of a tightening lever 33 . a detent projection 34 is provided at one end of the tightening lever 33 as a fastener member , so as to be selectively engaged with a projection 29 formed on the engine side joint case 28 . when the tightening lever 33 is pushed down to the arm side joint case 16 after engaging the detent projection 34 with the projection 29 , the tightening lever 33 tightly couples the joint case 16 of the operation arm 10 and the joint case 28 of the engine 17 . in addition , regarding the fastener member , it is not limited to the structure shown in fig2 . for example , bolts can be used to couple the joint cases 16 and 28 of the operation arn 10 and the engine 17 . furthermore , a connecting sleeve is rotatably attached to the outside of the arm side joint case 16 , and the engine side joint case 28 may be provided with a male screw to be coupled with a female screw which is provided inside the connect sleeve so that the operation arm 10 and the engine 17 can be screwed . the arm side joint case 16 , which is formed of a cylindrical member , incorporates a centrifugal clutch 35 inside . this centrifugal clutch 35 comprises a clutch drum 36 fixed to the driven shaft 11 and a clutch input shaft 38 rotatably supported by a supporting wall 37 . as shown in fig3 , a plurality of pivot members 42 are rotatably supported by a pin 41 on a rotor plate 39 fixed to the clutch input shaft 38 . spring force of an extension spring 43 is applied to each pivot member 42 so as to leave a frictional contact region 44 of the pivot member 42 from an internal surface of the clutch drum 36 . thus , when the rotational speed of the clutch input shaft 38 increases , the centrifugal force applied to the pivot members 42 moves the pivot members 42 toward the internal surface of the clutch drum 36 against the spring force of the extension springs 43 , whereby the clutch input shaft 38 and the driven shaft 11 are connected through the pivot members 42 . on the other hand , when the engine 17 is started by pulling the recoil knob 27 , the centrifugal clutch 35 has not been engaged so that the recoil knob 27 can be pulled without a big resistance being applied . an engaging groove 45 is provided across the end of the clutch input shaft 38 of the centriflgal clutch 35 in a diametrical direction an engaging projection 46 to be engaged with the engaging groove 45 is provided on the side of the disk 25 of the cooling fan 24 opposite to the fan blades 26 . thus , the driven shaft 11 and the engine output shaft 22 are coupled through the centrifugal clutch 35 with the engagement of the engaging groove 45 and the engaging projection 46 . in addition , it is also suggested that the engaging groove and the engaging projection can be provided vice versa as shown in fig2 , in order to attach the engine 17 to the operation arm 10 , first the engaging groove 45 and engaging projection 46 are engaged each other , then the engine side joint case 28 of the engine cover 21 is fitted to the arm side joint case 16 of the operation arm 10 , and fially the tightening lever 33 as the fastenmg member is manipulated to tighten the joint cases 16 and 28 . according to this structure , the lawn mower drives the cutter 13 by the driving force of the engine 17 so that grass can be mowed . as shown in fig1 ( b ), the electric motor 18 has motor cover 47 shown in fig4 , a cylindrical motor side joint case 48 , as the power source side joint case , which is the same size as the engine side joint case 28 is attached to this motor cover 47 . the mnotor side joint case 48 has a projection 49 with which a detent projection 34 of a tightening lever 33 installed in the arm side joint case 16 is adapted to engage . an engaging projection 51 is provided on the end of a motor output shaft 50 so as to engage with the engaging groove 45 provided across the end of the clutch input shaft 38 . according to this structure , the driven shaft 11 and the motor output shaft 50 are coupled through the centrifugal clutch 35 with the engagement of the engaging groove 45 and the engaging projection 46 . thus , as shown in fig4 , in order to attach the electric motor 18 to the operation arm 10 , first the engaging groove 45 and the engaging projection 51 are engaged each other , then the motor side joint case 48 of the motor cover 47 is fitted to the arm side joint case 16 of the operation arm i 0 , and finally the tightening lever 33 as the fastening member is manipulated to tighten the joint cases 16 and 48 . according to this structure , the lawn mower drives the cutter 13 by the driving power of the electric motor 18 so that grass can be mowed . according to the above disclosure , the user can selectively install either one of the engine 17 and the electric motor 18 on the operation arm 10 with the preparation of both the engine 17 and the electric motor 18 as attachments to a lawn mower . that is , the lawn mower can be used as whichever with an engine drive type and a motor drive type on the basis of the mowing environment for example , the engine 17 would be used as the power source in the situation where there is no power supply outlet or at the place where silence is not required . on the other hands , the electric motor 18 would be used as the power source in the situation where there is a power supply outlet . that is , the user can select either the engine 17 or the electric motor 18 as the power source based on the working environment . in addition , even if the user has purchased the lawn mower comprising the operation arm 10 and the engine 17 as a set , the lawn mower can be changed to the electric motor operation by purchasing the electric motor 18 , additionally . fig5 is a sectional view of a second embodiment of the lawn mower of the present invention . this view shows the same portion as fig2 . in this lawn mower , the centrifugal clutch 35 is incorporated inside the joint case 28 of the engine cover 21 . a large - diameter flange 52 having the engaging groove 45 is provided on the end of the driven shaft 11 . and , a large - diameter flange 54 having the engaging projection 46 is provided on the end of a clutch output shaft 53 secured to the clutch drum 36 of the centrifugal clutch 35 . the engaging projection 46 is adopted to couple with the engaging groove 45 . in this embodiment , the motor output shaft 50 of the electric motor 18 is coupled with a large diameter flange 52 of the driven shaft 11 directly . furthermore , the centrifugal clutch 35 may be installed to the extent of the operation arm 10 or may be installed inside the engine 17 . the present invention is not limited by the abovedescribed preferred embodiment ; it can be changed in various ways in the range that does not deviate from the subject matter . for example , this invention can be applied to such a lawn mower that the power source and the cutter are installed on a carriage .
a lawn mower comprises an operation arm , a driven shaft rotatably supported inside the operation arm and having two opposite ends , a cutter operatively connected to one end of the driven shaft , a power source operatively connected to the other end of the driven shaft for generating a drive power to be transmitted to the cutter through the driven shaft , wherein said power source is selective from an engine and an electric motor , an arm side joint case provided on the end of the operation arm , a power source side joint case provided on a housing of the power source and detachably and selectively connected to the arm side joint case , and a centrifugal clutch incorporated in the arm side joint case and having a first member connected to the driven shaft , a second member detachably and selectively connected to the power source and a third member provided to engage the first member and the second member according to the centrifugal force applied thereto .