embodiments of the present invention will be described in detail hereinunder with reference to the accompanying drawings . fig1 to 12 illustrate a shield mounting structure for a helmet , having c - shaped mounting holes according to an embodiment of the present invention . in the figures , the reference numeral 1 denotes a helmet body ; numeral 2 denotes a base plate fixed to each of right and left outer sides of the helmet body 1 ; numerals 3 and 4 denote a shield and a shield keep cover , respectively . the helmet body 1 has a full - face type configuration which is generally known . the base plate 2 which supports the shield 3 is fixed with set - screws 5 and 5 &# 39 ; to each of right and left outer sides of the helmet body 1 . the base plate 2 is provided with a pivot shaft 6 serving as a pivotal center of the shield 3 , a resilient engaging piece 7 which not only restricts a pivoting range of the shield 3 but also limits the rotation of the shield , and an arcuately elongated projection 8 for abutting engagement therewith of a stopper part 18 projecting from a lower peripheral edge of the shield 3 , the pivot shaft 6 , resilient engaging piece 7 and projection 8 being formed integrally with the base plate 2 by resin molding . below and further below the pivot shaft 6 are disposed the resilient engaging piece 7 and the arcuately elongated projection 8 , respectively . the pivot shaft 6 is projectingly provided in a circular front shape and it is centrally formed with a through hole for insertion of the set - screw therein . on the front side of that through hole there is formed a recess in a stepped shape so that the head of the set - screw 5 can be fitted thereon , and an upper part of the pivot shaft is cut out linearly to form a retaining recess 9 , into which is fitted an engaging projection 21 . the resilient engaging piece 7 is formed into a generally c shape in front view by cutting off part of a circular ring . part of its continuous outer peripheral surface is connected to the base plate 2 , while both open side ends of the resilient engaging piece 7 are integrally formed with pawl portions 7 &# 39 ; extending radially outwards . the resilient engaging piece 7 is provided with guide projections 10 and 10 &# 39 ; extending respectively from the continuous side toward the open side and from the continuous side toward the outside . longitudinal side portions of the guide projections 10 and 10 &# 39 ; are formed as inclined faces 11 and 11 &# 39 ; which are inclined toward the surface of the base plate 2 so that the insertion and removal of the shield 3 can be done smoothly as will be described later . the arcuately elongated projection 8 is formed in the shape of an arc extending along the lower peripheral edge of the base plate 2 , centered on the pivot shaft 6 . halfway in the longitudinal direction of the projection 8 there is formed a stepped portion 12 for abutment therewith of the stopper part 18 of the shield 3 . the position of the stepped portion 12 determines a lower - limit position of the closing motion of the shield 3 . further formed integrally with the base plate 2 is an overhang portion 13 in a position lower than the arcuately elongated projection 8 , the overhang portion 13 having a through hole for insertion of the set - screw 5 &# 39 ; therein . in a rear side part of the overhang portion 13 there is formed a retaining hole 14 with which a locking piece 22 formed on the shield keep cover 4 comes into engagement disengageably . the shield 3 is for opening and closing a window opening 15 formed in the front face of the helmet body 1 and it is formed in conformity with a curved shape of the helmet body , using a transparent or translucent , or colored and transparent , synthetic resin plate . at each of right and left side portions of the shield 3 there are formed a mounting hole 16 for fitting on the pivot shaft 6 ; an arcuate guide hole 17 in which is fitted the resilient engaging piece 7 to determine a pivoting range of the shield 3 ; and a mounting portion 3 &# 39 ; having a stopper part 18 formed on an outer peripheral edge thereof . the mounting hole 16 is formed in a generally c shape by cutting off part of the peripheral edge of a circular hole having a diameter a little larger than the outside diameter of the pivot shaft 6 . the arcuate guide hole 17 , which is for determining a pivoting range of the shield 3 , is formed in an arcuate shape centered on the mounting hole 16 which is fitted on the pivot shaft 6 , and in one side portion thereof for engagement with the resilient engaging piece 7 in a closed state of the shield 3 there are formed fitting portions 19 for fitting therein of the pawl portions 7 &# 39 ; of the resilient engaging piece 7 . the width of the arcuate guide hole 17 is a little narrower than the outside diameter of the resilient engaging piece 7 in an external force - free state , and a depression 20 is formed halfway in the edge portion of the hole , so that when the shield 3 is rotated and the depression 20 formed in an intermediate position of the arcuate guide hole 17 reaches the position of the resilient engaging piece 7 , the engaging piece 7 which has been compressed inwards expands into an unloaded shape thereof and gets into the depression 20 , whereby the shield 3 is held in a predetermined open position . the shield keep cover 4 which covers the mounting portion 3 &# 39 ; of the shield 3 is formed of a synthetic resin material , and on an upper part of its inner surface opposed to the mounting portion 3 &# 39 ; there is formed an engaging lug 21 integrally , while on a lower part of the said inner surface there is mounted a locking piece 22 so as to be operable from the outside . the engaging lug 21 is for fitting into the retaining recess 9 formed in the upper side of the pivot shaft on the base plate 2 and it is formed projectingly in a - shape on the back side of the cover body . the locking piece 22 is a rectangular flat plate and the upper surface of its front portion which fits into and disengages from the retaining hole 14 is cut to form an inclined face , while on the opposite side ( rear portion ) there is formed a spring piece 23 projectingly in the shape of an arrow , the spring piece 23 urging the locking piece 22 in a direction to fit into the retaining hole 14 at all times . the locking piece 22 is fitted into a guide piece 24 formed on the back of the shield keep cover 4 so that the spring piece 23 is positioned in the rear portion which is smaller in width . as the spring piece 23 moves toward the narrower portion in the guide piece 24 , it is pressed inwards to store a biasing force . upon release of the backward moving force of the spring piece 23 , the locking piece 22 is moved in a direction to fit into the retaining hole 14 by the stored biasing force , and thus it comes automatically into engagement with the retaining hole . an operating plate 25 is fixed to the locking piece 22 in a position outside the shield keep cover 4 so that the operation for disengaging the locking piece 22 from the retaining hole 14 can be done from the exterior of the shield keep cover 4 . further , lugs 26 and 26 &# 39 ; for pressing the mounting portion 3 &# 39 ; of the shield 3 to the base plate 2 side are formed in suitable positions of the inner surface of the shield keep cover 4 . in the drawings , the numeral 27 denotes a guide for guiding the mounting portion 3 &# 39 ; of the shield 3 at the time of mounting of the mounting portion . the guide 27 is provided on the inner surface of the shield keep cover 4 so as to be positioned outside and in front of the arcuately elongated projection 8 projecting from the outer surface of the base plate 2 . the following description is now provided about mounting and removing operations for the shield in the shield mounting structure described above . first , the shield keep cover 4 is fixed to each of the right and left base plates 2 secured to the helmet body 1 , at two upper and lower points through engagement between the retaining recess 9 and the engaging lug 21 and also through engagement between the retaining hole 14 and the locking piece 22 , and an opening 28 is defined by both the outer surface of the helmet body 1 and the front edge of the shield keep cover 4 . as the mounting portion 3 &# 39 ; of the shield 3 is inserted into the opening 28 , the mounting portion 3 &# 39 ; is guided inwards of the arcuately elongated projection 8 by the guide 27 projecting from the inner surface of the shield keep cover 4 . in this operation , an upper part of the mounting portion 3 &# 39 ;, or the edge of the mounting hole 16 , comes into abutment with the outer peripheral surface of the pivot shaft 6 , so that the mounting portion 3 &# 39 ; is guided in a direction of abutment with the guide 27 . ( see fig4 .) with further insertion of the mounting portion 3 &# 39 ; of the shield 3 , the edge portion of the mounting hole 16 and the arcuate guide hole 17 move onto the pivot shaft 6 and the resilient engaging piece 7 , respectively , and the shield keep cover 4 is pushed up in the direction of z . in this case , the presence of the lugs 26 and 26 &# 39 ; gives assistance to this motion . consequently , the mounting portion 3 &# 39 ; further moves to the inside . ( see fig5 .) with still further movement of the mounting portion 3 &# 39 ; to the interior through the space between the base plate 2 and the shield keep cover 4 , the mounting hole 16 reaches the position of the pivot shaft 6 , the rear side of the arcuate guide hole 17 reaches the position of the resilient engaging piece 7 , and the stopper part 18 reaches the position of the front end of the arcuately elongated projection 8 , whereupon the mounting portion 3 &# 39 ; is fitted and set to the base plate 2 by the biasing force of the shield keep cover 4 . this is confirmed by making a set completion sound , which is a click . ( see fig6 .) mounting of the mounting portion 3 &# 39 ; of the shield 3 is completed in the above manner . now , the shield 3 can rotate vertically about the pivot shaft 6 to open and close the window opening 15 of the helmet body 1 . during vertical motions of the shield , the engagement of the mounting portion 3 &# 39 ; with the shield 3 is ensured by the engagement between the pivot shaft 6 and the mounting hole 16 , the engagement between the resilient engaging piece 7 and the arcuate guide hole 17 and further the engagement between the stopper part 18 and the arcuately elongated projection 8 . ( see fig7 .) for removing the shield 3 mounted to the helmet body 1 , first the shield is turned up to the upper - limit position of its opening motion . ( see fig8 .) then , the shield 3 is moved in a direction to get over the upper - limit position while lifting the mounting portion 3 &# 39 ; of the shield 3 in a leaving direction ( z direction ) from the outer surface of the base plate 2 , and the stopper part 18 is removed from the arcuately elongated projection 8 . ( see fig9 .) after the stopper part 18 is disengaged from the arcuately elongated projection 8 , the shield 3 is pulled out in the direction of arrow , so that the edge portion of the mounting hole 16 becomes disengaged from the pivot 6 and moves onto the upper surface of the pivot shaft , while the rear - side edge of the arcuate guide hole 17 is guided by the inclined face 11 of the guide projection 10 and moves onto the upper surface of the resilient engaging piece 7 . ( see fig1 .) then , in this state , the shield 3 is pulled out toward the opening 28 , whereby the mounting portion 3 &# 39 ; is pulled out completely from between the base plate 2 and the shield keep cover 4 . ( see fig1 .) referring now to fig1 to 21 , there is illustrated a shield mounting structure for a helmet according to another embodiment of the present invention , in which a completely continuous hole is formed as a mounting hole in a shield mounting portion , and an upper fixing position of a shield keep cover is spaced from a pivot shaft . more specifically , a mounting hole 30 formed in a mounting portion 29 &# 39 ; of a shield is a circular hole whose circumference is not cut out at all , while an arcuate guide hole 31 and a stopper part 32 are of the same construction as in the previous embodiment . as in the previous embodiment , a base plate 33 fixed to a helmet body 1 is provided with a pivot shaft 34 and a resilient engaging piece 35 , and an elongated projection 36 is formed on an outside portion of the base plate 33 except upper and front portions . the lower portion of the elongated projection 36 is formed in the shape of an arc centered on the pivot shaft 34 as in the previous embodiment , and a stepped part 37 for abutment therewith of the stopper part 32 is formed in an intermediate position of the elongated projection 36 . a retaining recess 38 is formed in an upper side face of the elongated projection 36 so that an engaging lug 42 of a shield keep cover 41 which will be described later is fitted therein . means for fixing the lower portion of the shield keep cover is the same as in the previous embodiment ; that is , the lower portion of the shield keep cover is fixed by the engagement between a retaining hole 40 formed in an overhang portion 39 extending from the lower portion of the base plate 33 and a locking piece 43 mounted to the shield keep cover 41 . further , a guide 44 is provided projectingly on an upper inside face of the shield keep cover 41 in conformity with the upper side end of the elongated projection 36 to guide the insertion of the mounting portion 29 &# 39 ; of the shield 29 . therefore , the space between both ends of the elongated projection 36 on the base plate 33 provides an opening 45 which is defined in cooperation with the shield keep cover 41 . for setting the mounting portion 29 &# 39 ; of the shield 29 in the above construction , the mounting portion 29 &# 39 ; is inserted inside the shield keep cover 41 along the guide 44 formed on the inner surface of the shield keep cover . ( see fig1 and 14 .) as the insertion is further continued , the mounting potion 29 &# 39 ; moves onto the upper surfaces of the pivot shaft 34 and resilient engaging piece 35 provided on the base plate 33 and pushes up the central portion of the said cover in the direction of z through lugs formed on the inner surface of the shield keep cover 41 , then further moves to the interior . ( see fig1 .) when the mounting hole 30 has reached the position of the pivot shaft 34 , the rear portion of the arcuate guide hole 31 has reached the resilient engaging piece 35 and the stopper part 32 has reached the lower end portion of the elongated projection 36 , they are engaged and set therein by the resilience of the shield keep cover 41 . at this instant there is made a click indicating that the setting was completed . ( see fig1 .) through the above operations the mounting of the mounting portion 29 &# 39 ; of the shield 29 is completed and the shield 29 is now capable of rotating vertically about the pivot shaft 34 , whereby the window opening of the helmet body 1 can be opened and closed . during the above vertical motion of the shield , the mounting portion 29 &# 39 ; of the shield 29 is never disengaged from the base plate 33 because of the engagement between the resilient engaging piece 35 and the arcuate guide hole 31 and the engagement between the stopper part 32 and the elongated projection 36 . removal of the shield 29 once set can be done in the same manner as in the previous embodiment ; that is , the shield is turned up to its upper - limit position , then is moved in a direction to get over the upper - limit position while lifting the mounting portion 29 &# 39 ; in a leaving direction ( z direction ) from the outer surface of the base plate 33 , and the shield is then pulled out from the opening 45 , whereby the mounting portion 29 &# 39 ; is disengaged and pulled out completely from the base plate 33 . ( see fig1 to 21 .) in the embodiment just described above , inclined faces 46 and 46 &# 39 ; are formed in the upper and lower peripheral edges , respectively , of the pivot 34 so that the insertion and removal of the mounting portion 29 &# 39 ; of the shield 29 can be done smoothly . since the shield mounting structure of the present invention is constructed as described in detail above , the shield mounting portion can be mounted and removed while the shield keep cover is left fixed to the helmet body or to the base plate secured to the helmet body . thus , the operation is superior in point of convenience . besides , since the shield which has been once set can be removed only after an operation which cannot occur in the normal state of use is applied to the shield in the upper - limit position of the shield opening motion , there is no fear of disengagement of the shield in use . moreover , since the mounting ( setting ) of the shield can be done by only inserting the shield mounting portion into the space between the base plate and the shield keep cover from the opening , the shield mounting structure of the invention is very easy to operate and is convenient . further , since the operation for mounting and removal of the shield keep cover is not necessary , there is no fear of loss of set - screws or breakage of convex - concave retaining members .