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FIELD OF THE INVENTION 
     This invention relates to a stay damper which assists in the opening of a vehicle rear door and support of the door in a fully open sate. 
     BACKGROUND OF THE INVENTION 
     With respect to a stay damper which assists in the opening of a swing-type vehicle rear door, JPH10-115340A and JPH11-201210A published in 1998 and 1999 by Japan Patent Office, respectively, propose a locking mechanism which automatically locks contraction of the stay damper from an elongated position. 
     The stay damper comprises a cylinder, a piston accommodated in the cylinder, and a piston rod connected to the piston and projecting from the cylinder in an axial direction. The base of the cylinder is connected to a vehicle body while the projecting end of the piston rod is connected to the door. The cylinder is charged with gas, and the piston rod is biased to project from the cylinder according to a gas pressure. 
     The piston rod is covered by a cylindrical cover. The cover is supported by the projecting end of the piston rod such that the cover can overlap the outer circumference of the cylinder and swing about the projecting end of the piston within a limited range. 
     A leaf spring is interposed between the piston rod and the cover. The leaf spring biases the cover to swing about the projecting end of the piston such that the tip of the cover displaces in a lateral direction. 
     When the door is closed, the stay damper is in a contracted state in which the piston rod is in the cylinder except for the projecting end thereof and the cover overlaps the outer circumference of the cylinder. 
     In contrast, when the piston rod is elongated to its maximum length as the door is opened, the cover does not overlap the cylinder any more and the tip of the cover is removed from the outer circumference of the cylinder. In this state, the leaf spring causes the cover to swing about the projecting end of the piston rod, and as a result, the cover is inclined with respect to the cylinder. Thus, the tip of the cover is offset from the cylinder tip in the lateral direction. 
     When the contracting force is applied to the piston rod, the tip of the cover abuts against the cylinder tip, thereby preventing the piston rod from contracting. 
     According to the above construction of the stay damper, when the door is fully open, the stay damper is automatically brought into a locked state in which contraction thereof is prevented and thereafter the door is supported in the fully open position by the stay damper. 
     When the door is to be closed, a lock release operation is required which is accomplished by shifting the tip of the cover from the lock position against the force of the spring in a lateral direction. After performing the lock release operation, the cover no longer interferes with the contracting piston rod and the stay damper contracts smoothly under an appropriate resistance due to the pressure of the gas in the cylinder. 
     SUMMARY OF THE INVENTION 
     However, the prior art stay damper which automatically locks in the elongated position has the problem described below. 
     Specifically, if the operator of the door does not know about the lock release process, the operator may not be able to release the lock of the stay damper in the elongated position, and hence the operator may not be able to close the door. If the operator attempts to forcibly close the door in this state, the stay damper may break. 
     It is therefore an object of this invention to solve the above problem related to the release of a stay damper from a lock position by a simple construction. 
     In order to achieve the above object, this invention provides a stay damper comprising a cylinder in which a gas is charged, a piston rod which projects from the cylinder according to a pressure of the gas, and a stopper which locks contraction of the piston rod from an elongated position. The stopper displaces between a lock position which locks contraction of the piston rod and a release position which permits contraction of the piston rod, according to an operation force applied from outside. The stay damper further comprises a restricting member which holds the stopper in the lock position when the stopper is in the lock position and holds the stopper in the release position when the stopper is in the release position such that the stopper displaces only when the operation force is greater than a restriction force of the restriction member. 
     The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a stay damper according to this invention including a partial longitudinal sectional view of the same. 
         FIG. 2  is an enlarged longitudinal sectional view of a stopper according to this invention. 
         FIG. 3  is similar to  FIG. 2  but shows the stopper in a locked state. 
         FIG. 4  is a cross-sectional view of the stopper taken along the line IV-IV in  FIG. 2 . 
         FIG. 5  is a cross-sectional view of the stopper taken along the line V-V in  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  of the drawings, a stay damper interposed between a rear door and a vehicle body of a vehicle comprises a piston rod  2  projecting from a cylinder  1 . The cylinder  1  accommodates a piston connected to the piston rod  2  and is charged with a gas which biases the piston so as to cause the piston rod  2  to project from the cylinder  1 . The stay damper assists the rear door to open through the piston rod  2  which projects as a result of the expansion pressure of the gas. The rear door is arranged to swing horizontally about a vertical axis. 
     A ball joint  1   a  is fixed to a base of the cylinder  1 . A ball stud fixed to the vehicle body is fitted in a spherical shaped recess of this ball joint  1   a  such that the base of the cylinder  1  is connected to the vehicle body and the cylinder  1  is capable of swinging about the ball stud fixed to the vehicle body. 
     A ball joint  2   a  is fixed to a projecting end of the piston rod  2 . A ball stud fixed to the rear door is fitted in a spherical shaped recess of this ball joint  2   a  such that the projecting end of the piston rod  2  is connected to the rear door and the piston rod  2  is capable of swinging about the ball stud fixed to the rear door. 
     The piston rod  2  is covered by a cover  3  having a cylindrical shape. A base  3   a  of the cover  3  is fitted to the projecting end of the piston rod  2  via a pin  2   c . More specifically, the pin  2   c  penetrates a base  2   b  of the ball joint  2   a  and the base  3   a  of the cover  3  in a lateral direction with respect to the direction of projection of the piston rod  2  such that the cover  3  can swing about the pin  2   c , or in other words, such that a tip of the cover  3  can displace in the lateral direction. 
     A stopper  10  is fixed to the tip of the cover  3 . 
     Referring to  FIG. 2 , the stopper  10  comprises a cylindrical part  11  which has a substantially identical inner diameter to the cover  3 , and a holder  13  having an elongated circular cross-section. The cylindrical part  11  is fixed to the tip of the cover  3 . The holder  13  covers a tip  1   b  of the cylinder  1  when the piston rod  2  is in the most elongated position. 
     Referring to  FIG. 4 , the elongated circular shape corresponds to two semi-circles facing each other and connected by two straight parallel lines. The length of the minor axis of the holder  13  is equal to the inner diameter of the cylindrical part  11 . In other words, the shape of the holder  13  corresponds to the shape of the cylindrical part  11  elongated in the swing direction of the cover  3 . 
     Since the cross-sectional shapes of the holder  3  and the cylindrical part  11  are different, a step  11   a  is formed between the cylindrical part  11  and the holder  3 . 
     On the opposite side of the holder  13  to the continuous part  13   b , a gap  13   a  is formed. 
     On the inner circumference of the holder  13 , a pair of projections  12  are formed inward so as to hold the outer circumference of the tip  1   b  of the cylinder  1 . Each of the projections  12  is in the form of a rib having a length almost equal to the length of the holder  3  in the direction of projection of the piston rod  2 . The pair of projections  12  are constructed in positions respectively facing a midpoint along a major axis of the elongated circular cross-section of the holder  3 . 
     According to the above construction, the stopper  10  elastically deforms so as to enlarge the gap  13   a  when an outward force is applied to the holder  13  from within the inner side thereof. More specifically, in  FIG. 4 , when a downward force is applied to the stopper  10 , the outer circumference of the tip  1   b  of the cylinder  1  pushes the projections  12  outward so as to enlarge the gap  13   a . Due to this outward force exerted on the projections  12 , the holder  13  elastically deforms and then displaces downward as shown in  FIG. 5 . 
     Herein, the relative position of the holder  13  and the tip  1   a  shown in  FIGS. 2 and 4  is referred to as a release position of the stopper  10 . The relative position of the holder  13  and the tip  1   a  shown in  FIGS. 3 and 5  is referred to as a lock position of the stopper  10 . The release position is a position in which the tip  1   b  is adjacent to the gap  13   a , and the lock position is a position in which the tip  1   b  is adjacent to an opposite part  13   b.    
     When the stopper  10  is in the release position, the piston rod  2  can intrude into the cylinder  1 , and when the stopper  10  is in the lock position, intrusion of the piston rod  2  into the cylinder  1  is blocked by the tip  1   b  which interferes with the step  11   a.    
     The projections  12  function to hold the stopper  10  in both the release and lock positions. When the tip  1   b  moves in the holder  13  from the release or lock position, it exerts a deforming pressure on the holder  13  via the projections  12  so as to enlarge the gap  13   a . The holder  13  exerts a resilient force on the tip  1   b  via the projections  12  as a reaction force. This reaction force works as a holding force for holding the tip  1   b  in the release position and the lock position respectively. When the stopper  10  is moved from the release position to the lock position, or vice versa, the tip  1   b  must push the projections  12  aside so as to move through the space between the projections  12 . This tip holding function of the projections  12  is realized by the design of the holder  13 . 
     To ensure that the holder  3  deforms according to an outward force by the tip  1   b  and exerts a resilient reaction force on the tip  1   b , it is necessary to constitute the stopper  10 , including the holder  13 , by an elastic material. In view of this requirement, the holder  13  is preferably constructed from a resin. For example, the cover  3  is manufactured by injection molding a plastic material and then press fitted onto the outer circumference of the tip of the cover  3 . 
     The stay damper constructed as described above is maintained in a contracted state when the rear door is closed. When an operator operates the rear door to open, gas pressure in the cylinder  1  assists the rear door to open via the piston rod  2 . 
     When the rear door reaches the fully open position, the stay damper reaches the most elongated position as shown in  FIG. 1 . Here, the holder  13  of the stopper  10  faces the outer circumference of the tip  1   b  of the cylinder  1  as shown in  FIG. 2 . The operator of the rear door then applies a downward force on the stopper  10  in  FIG. 2 . The tip  1   b  of the cylinder  1  pushes the projections  12  aside and the holder  3  deforms. In this state, the tip  1   b  of the cylinder  1  moves in the holder  13 . The tip  1  and the stopper  10  thus relatively displace from the release position shown in  FIG. 4  to the lock position shown in  FIG. 5 . In the lock position, the tip  1   b  is held by the projections  12  and displacement of the tip  1   b  from this position is restricted by the projections  12 . In the lock position, the tip  1   b  interferes with the step  11   a  of the cylindrical part  11  when the piston rod  2  contracts, thereby locking the contraction of the piston rod  2 . Hence, when the operator removes his/her hand from the rear door, the rear door is maintained in the fully open position. 
     In order to close the rear door in the fully open position, the operator first applies an upward force on the stopper  10  in  FIG. 5 . According to this operation, the tip  1   b  of the cylinder  1  again pushes the projections  12  aside so as to move through the space between the projections  12 . The tip  1   b  and the stopper  10  then relatively displace from the lock position to the release position shown in  FIG. 4 . Once the tip  1   b  has reached the release position, displacement of the tip  1   b  therefrom is again restricted by the projections  12 . Hence, the operator can remove his/her hand from the stopper  10  while keeping the stay damper in the release position. The piston rod  2  of the stay damper contracts smoothly according to a contraction force exerted by the closing operation of the rear door, performed by the operator. 
     If the operator did not push the stopper  10  to lock the contraction of the piston rod  2  when the rear door was fully opened, the rear door would not be maintained in the fully open position. When closing the rear door from the fully open position, in contrast, the operator pushes the stopper  10  in the direction opposite to the direction for locking so as to displace the stopper  10  to the release position. 
     Since the operator of the rear door performs the operation of the stopper  10  when the rear door is open, the operator of the rear door easily understand how to release the stopper  10  when he/she proceeds to close the rear door. This stay damper can, therefore, prevent an unfavorable situation in which the operator cannot close the rear door because the operator does not understand how to release the stopper  10  or in which the stay damper becomes defective due to an attempt to close the door forcibly while the stay damper is locked in the elongated position. 
     Further, according to this stay damper, since the projections  12  restrict the stopper  10  when it is in the release position as well as when it is in the lock position, the stopper  10  in either position does not move unless another external shifting force is applied, and hence unintended locking or releasing operations of the stopper  10  are prevented. Thus, the stay damper always operates as intended by the operator, and hence the rear door is opened and closed smoothly. 
     Although the invention has been described above with reference to a certain embodiment, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art, within the scope of the claims. 
     For example, in the embodiment described above, the projections  12  are formed in the shape of ribs, but the projections  12  may be formed in a semi-spherical shape. This shape reduces resistance to the relative displacement of the stopper  10  and the tip  1   b  between the lock and release positions, thereby reducing the operation force required to operate the stopper  10 . 
     In the embodiment described above, unlike the prior art device, a spring is not provided between the piston rod  2  and the cover  3 . It is also possible, however, to provide a spring between the piston rod  2  and the cover  3  as long as the resilient force of the spring does not exceed the restriction force of the projections  12 , in order to balance the forces required for the locking and releasing operations of the stopper  10  or in order to intentionally differentiate these forces. 
     In the embodiment described above, the gap  13   a  is provided in the holder  13  in order to promote elastic deformation of the holder  3 , but elastic deformation of the holder  13  can be achieved by other measures such as reducing the wall thickness of the holder  13 . 
     The embodiment describe above focuses on a stay damper for a rear door of a vehicle, but the stay damper according to this invention can also be applied to a hood for an engine room or a trunk lid of a vehicle as well as a vertical swing door of a hatchback style vehicle.

Summary:
A stay damper comprises a cylinder ( 1 ) in which a gas is charged, a piston rod ( 2 ) which projects under a pressure of the gas in the cylinder ( 1 ), and a stopper ( 10 ) which locks contraction of the piston rod ( 2 ) in an elongated position. The stopper ( 10 ) displaces between a lock position which locks contraction of the piston rod ( 2 ) and a release position which permits contraction of the piston rod ( 2 ) according to directional operation forces applied by an operator. The stopper ( 10 ) comprises a restricting member ( 12, 13 ) which holds the stopper ( 10 ) in the lock position when it is in the lock position and holds the stopper ( 10 ) in the release position when it is in the release position. Since the locking operation of the stopper ( 10 ) is performed manually, the releasing operation thereof is easily understood by the operator.