Abstract:
A chair adjustment mechanism is disclosed which allows a change of position in an unlocked state and maintains a position in a locked state. The mechanism, when used in conjunction with an adjustable chair part, may not be unlocked without a force being applied opposite to a force urging the part to a default position.

Description:
FIELD OF THE INVENTION 
     The present invention relates to adjustable chair parts and more particularly to a mechanism to allow a chair part to be adjusted and the adjustment to be maintained. 
     BACKGROUND OF THE INVENTION 
     Chairs are known to have parts that are adjustable such as the arm rests, the seat and the backrest. The height of a backrest, for instance, may be adjusted up or down and, additionally, an angle a backrest makes with reference to the seat may be altered. In many chairs, a person seated in a chair who wishes to change the angle of a backrest moves a handle from a locked position to an unlocked position, releasing a mechanism that maintains the support bar for the backrest at a particular angle. The person may then find a new angle for the backrest support bar and, by returning the handle to a locked position, set the mechanism to maintain the new angle. 
     Typically, springs bias various adjustable parts to a default position. Unfortunately, when a part of an unoccupied chair is unlocked while significantly offset from its default position, a violent action may ensue as the part returns to its default position. This violent action could cause injury. It is desirable, then, to design a mechanism that avoids this problem. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention there is provided locking apparatus including a clutch plate and a locking bracket, the locking bracket having a passageway adapted to accept the clutch plate, the passageway including a first channel and a second channel, the first channel allowing movement of the clutch plate along the passageway, the second channel preventing movement of the clutch plate along the passageway. 
     In accordance with a further aspect of the present invention there is provided a chair including: a chair base; a chair adjustment mechanism, mounted to the chair base, including a locking apparatus comprising a clutch plate and a locking bracket forming a passageway receiving the clutch plate, the passageway including a first channel and a second channel, the first channel allowing movement of the clutch plate along the passageway, the second channel preventing movement of the clutch plate along the passageway; a seat mounted to one of the locking bracket and the clutch plate of the chair adjustment mechanism; a backrest; and a backrest support bar supporting the backrest, mounted to another one of the locking bracket and the clutch plate of the chair adjustment mechanism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the figures which illustrate an example embodiment of this invention: 
     FIG. 1 is a schematic side view of a chair embodying the subject invention. 
     FIG. 2 is a schematic underside perspective view of a chair adjustment assembly made in accordance with the subject invention. 
     FIG. 3 illustrates a partial underside elevation of the chair adjustment assembly, in an unlocked position, taken along lines  3 — 3  of FIG.  2 . 
     FIG. 4 is an exploded perspective view of a portion of the assembly of FIG.  2 . 
     FIG. 5 illustrates a partial section taken along lines  5 — 5  of FIG.  3 . 
     FIG. 6 illustrates a partial underside elevation of a chair adjustment assembly, in a locked position, taken along line  3 — 3  of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a chair  11  comprises a chair seat  13  and a backrest  15  supported on a backrest support bar  17 . The chair seat and backrest support bar are mounted to a chair adjustment mechanism  10  (chair adjustment mechanism  10  is mounted to a chair base  21 ). 
     Referencing FIGS. 2,  3  and  4 , chair adjustment mechanism  10  comprises a chair seat frame  18  with seat mounting flanges  19  for accommodating bolts to mount the chair seat to the seat frame  18  and a chair backrest support bracket  24  with a slot  25  for receiving the backrest support bar. The seat frame  18  and backrest support bracket  24  are pivotally joined by shaft  28 . Shaft  28  supports a main spring  44  that bears against the underside of the bracket  18  and a shaft  20  carried by the backrest support bracket  24  in order to bias the backrest support bracket to a default angle with respect to the seat frame  18 . 
     The seat frame  18  also carries a shaft  16  on which a locking bracket  12  is slidably mounted. A clutch plate  14  has a tubular end  14 A receiving shaft  20  and a slotted end  14 B receiving shaft  16 . Spacers  26 A,  26 B on shaft  20  between the sides of the backrest support bracket  24  and tubular end  14 A of clutch plate  14  maintain the clutch plate in a fixed position on shaft  20 . 
     A stirrup-shaped compression member  32  has a central opening which receives shaft  16 . The arms  32 A,  32 B of the compression member  32  pass through slots in seat frame  18  and terminate in openings  32 C,  32 D which receive a pin  36 . A handle  30  is pivotally mounted to pin  36  and terminates in a cam  31  which abuts a domed-end abutment  16 A of shaft  16 . A cam box  38  surrounds the cam  31  and is supported on pin  36 . 
     Referring to FIG. 4, locking bracket  12  has a passageway  23  for receiving clutch plate  14 . Passageway  23  has two channels, a first channel  23 A in which longitudinal movement of clutch plate  14  is allowed and a second channel  23 B in which longitudinal movement of clutch plate  14  is prevented. Prevention of movement in second channel  23 B is accomplished by endowing clutch plate  14  with a feature which, as shown, comprise teeth  29  which fit into a corresponding feature in second channel  23 B, which, as shown, comprise teeth  27 . 
     Returning to FIG. 3, when handle  30  is moved to an “unlocked” position, cam  31  draws compression member  32  outwardly causing the compression member  32  to compress a locking spring  34 . Compression of locking spring  34  allows locking bracket  12  to be urged by an unlocking spring  22  acting on spacer  42 B toward an “unlocked” position along shaft  16 . In the unlocked position, first channel  23 A (FIG. 4) of locking bracket  12  receives clutch plate  14  such that clutch plate  14  is free to move within passageway  23  of the locking bracket. With the clutch plate freed to move, as shown in FIG. 5, main spring  44  urges backrest support  24  to tilt with respect to seat frame  18  such that shaft  16  is maintained at an end of slot  25  in clutch plate  14  closest to slotted end  14 B. 
     Locking spring  34 , which acts opposite to unlocking spring  22  along shaft  16 , is stronger than unlocking spring  22 . Consequently, when handle  30  is moved to a “locked” position illustrated in FIG. 6, compression member  32  frees the locking spring  34  to act on spacers  40  and  42 A and locking bracket  12  to urge the locking bracket toward a “locked” position along shaft  16 . In the locked position, clutch plate  14  is received in second channel  23 B (FIG. 4) of locking bracket  12  such that longitudinal movement of the clutch plate through passageway  23  is prevented. 
     In operation, a user of a chair having chair adjustment assembly  10  may move handle  30  from a locked position to an unlocked position. The resulting action of cam  31  against domed-end abutment  16 A of shaft  16  draws compression member  32  outwardly, compressing locking spring  34 . Locking bracket  12  is then allowed to be urged by unlocking spring  22  to move from a locked position, which prevents longitudinal movement of clutch plate  14 , to an unlocked position, at which movement of clutch plate  14  along passageway  23  of locking bracket  12  is allowed. Despite unlocking spring  34  urging locking bracket  12  along shaft  16  to an unlocked position, locking bracket  12  will not move due to frictional force on locking bracket  12 , normal to shaft  16 , applied by main spring  44  via shaft  20  and clutch plate  14  unless an opposing force to that of main spring  44  is applied by the user. Thus, once a user leans back on the chair backrest to relieve this frictional engagement of the clutch plate in second channel  23 B of locking bracket  12 , unlocking spring  34  will move locking bracket  12  along shaft  16  causing clutch plate  14  to move out of second channel  23 B and into first channel  23 A. 
     While the clutch plate is in first channel  23 A, movement along passageway  23  of locking bracket  12  is allowed and the user of the chair may move the chair backrest to a new position. Once a new position has been decided upon, the user of the chair may move handle  30  from the unlocked position to the locked position. With handle  30  in the locked position, locking spring  34  pushes compression member  32  against spacers  40  and  42 A, and consequently locking bracket  12 . If teeth  29  of clutch plate  14  and teeth  27  of second channel  23 B are aligned, locking bracket  12  moves along shaft  16  so that clutch plate  14  leaves first channel  23 A wherein movement is allowed and enters channel  23 B wherein teeth  29  of clutch plate  14  and teeth  27  of second channel  23 B mesh and movement of clutch plate  14  through passageway  23  is prevented. If teeth  29  of clutch plate  14  and teeth  27  of second channel  23 B are not aligned, the locking bracket moves only when the tilt of chair back  17  is varied slightly to align the teeth. 
     Turning to FIG. 4 in conjunction with FIG. 1, slot  25  defines a range of longitudinal movement of clutch plate  14  through passageway  23  and thus defines a range of possible positions of backrest support bar  17 . Due to the clutch plate teeth used to lock backrest support bar  17  in a chosen position, a discrete number of positions, at which backrest support bar  17  may be locked, exist within this range. 
     In an alternative embodiment, spacers  40 ,  42 A and  42 B may be incorporated into the design of compression member  32  or locking bracket  12 . 
     Although springs are used throughout the above disclosure, other resilient members may be used in place of springs, such as resilient clips. 
     Optionally, a feature other than teeth is used in second channel  23 B to prevent movement of clutch plate  14  through passageway  23 . Thus, any suitable male feature may be chosen for clutch plate  14  and a corresponding female feature chosen for second channel  23 B. For instance, clutch plate  14  could have a series of semicircular protrusions which fit into semicircular indentations in second channel  23 B. 
     In an alternative embodiment, chair adjustment mechanism  10  includes a mechanism for adjusting the angle seat  13  makes with respect to chair base  21 . A clutch plate carried by one of a chair seat frame  18  and chair base  21  is received by a locking bracket carried by the other of chair seat frame  18  and chair base  21 . A handle is moved to an unlocked position, chair seat  13  is adjusted to a desired angle and the handle is returned to a locked position. 
     Other modifications will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.