referring to the drawings , an adjustment means for a backrest in accordance with the preferred embodiment in the present invention includes a seat brace 10 , a back brace 20 installed to the seat brace 10 and an adjustment unit 30 disposed between the seat and back braces 10 and 20 . the back brace 20 can be driven via operation of the adjustment unit 30 , and then , moves with respect to the seat brace 10 . particularly referring to fig5 , the seat brace 10 , which is provided on the bottom of a seat 91 of an office chair ( not numbered ) and adjoins a back 92 of the office chair , includes two lateral walls 11 , a lower support plate 12 and an upper support plate 13 . the lateral walls 11 are parallel to each other , and a space ( not numbered ) is defined therebetween . the lower support plate 12 is connected to the bottom of the lateral walls 11 respectively and below the space . the upper support plate 13 is connected to the inner side of the lateral walls 11 respectively and provided in the space . an upper surface 121 and a lower surface 131 are defined on the lower and upper support plates 12 and 13 , respectively , and parallel to and spaced from each other . at least one cushion blocks 14 are disposed between the lower and upper support plates 12 and 13 and allow maintaining the distance from the lower support plate 12 to the upper support plate 13 . a top plate 101 is provided on the top edge of the lateral walls 11 respectively and above the space . in this embodiment , the top plate 101 is parallel to the bottom of the seat 91 . the upper surface 121 of the lower support plate 12 is angled at 25 - 35 degrees to the top plate 101 , and equally the lower surface 131 of the upper support plate 13 is angled at 25 - 35 degrees to the top plate 101 . a limited slot 122 is defined on the lower support plate 12 longitudinally . the upper support plate 13 is preferably in a form of a u - shaped plate and includes a yawn 132 . two projections 133 are formed on two sides of the upper support plate 13 and extend toward the inside of the yawn 132 , respectively . the back brace 20 is approximately l - shaped and includes a first end 21 and a second end 22 bent to the first end 21 . the first end 21 is adapted for assembly with the seat 92 ( as shown in fig5 ). the second end 22 is installed between the lower and upper support plates 12 and 13 of the seat brace 10 for tight assembly of the seat and back braces 10 and 20 . a top side 221 and a bottom side 222 are defined on the second end 22 and are parallel to each other . a thickness of the second end 22 is equal to the distance between the top and bottom sides 221 and 222 and also equal to the distance between the upper and lower surfaces 121 and 131 . accordingly , it will increase face friction between the lower and upper support plates 12 , 13 and the second end 22 , respectively , and prevent unexpected movement of the second end 22 between the lower and upper support plates 12 , 13 . therefore , the back 92 and the back brace 20 will not freely slide on the seat brace 10 without operation of the adjustment unit 30 . a coupled hole 223 is formed on the second end 22 and adapted to couple with a coupled element 224 . the coupled element 224 extends through the limited slot 122 of the lower support plate 12 and , then , is coupled to the coupled hole 223 for restricting movement of the back brace 20 . in this embodiment , the coupled element 224 has threads . a locating portion 225 is formed on the top side 221 of the second end 22 and longitudinally extends from the edge of the second end 22 toward the first end 21 . the adjustment unit 30 is pivotally installed to the lateral walls 11 of the seat brace 10 . two through holes 111 are defined on the lateral walls 11 respectively . an end of the adjustment unit 30 inserts through the through holes 111 respectively , and another end of the adjustment unit 30 is exposed outside for users to operate . an actuated element 31 includes an engaged end 311 formed on an end thereof and a connected end 312 opposite to the engaged end 311 . a first through - hole 313 longitudinally penetrates the actuated element 31 , and two openings of the first through - hole 313 are respectively formed on the engaged and connected ends 311 , 312 . the first through - hole 313 is adapted for inserting the end of adjustment unit 30 therethrough . a second through - hole 314 axially penetrates the connected end 312 and is perpendicular to the first through - hole 313 . a pin 315 is inserted through the second through - hole 314 and a hole 301 that is defined on the end of the adjustment unit 30 for fixing the actuated element 31 to the adjustment unit 30 . furthermore , the actuated element 31 is disposed in the space of the seat brace 10 and inserted through the yawn 132 of the upper support plate 13 so that the engaged end 311 can cooperate with the locating portion 225 of the second end 22 . when a user turns the adjustment unit 30 , the second end 22 of the back brace 20 is driven to move between the lower and upper support plates 12 and 13 with respect to the seat brace 10 so that the position of the back 92 of the office chair will be adjusted to a desired position . additionally , when the back brace 20 is driven to move by operation of the adjustment unit 30 , movement of the back brace 20 can be upward and rearward simultaneously or downward and frontward simultaneously . the cause of the above - mentioned is the inclined angle between the second end 22 of the back brace 20 and the top plate 101 of the seat brace 10 . the engaged end 311 of the actuated element 31 is preferably in a form of a gear , and the locating portion 225 of the second end 22 is preferably in a form of a row of teeth and engaged with the engaged end 311 of the actuated element 31 so that the second end 22 of the back brace 20 is driven to move with respect to the seat brace 10 . a turning handgrip 32 is installed to another end of the adjustment unit 30 opposite to the actuated element 31 for an easy grip by users . the turning handgrip 32 has an outer diameter which is larger than that of the actuated element 31 as to save effect in operation . referring to fig3 and 4 , when users turn the turning handgrip 32 to overcome friction between the lower and upper support plates 12 , 13 and the second end 22 , respectively , the engaged end 311 of the actuated element 31 is operated to drive the locating portion 225 of the second end 22 for movement of the second end 22 between the lower and upper support plates 12 , 13 . therefore , users can adjust the back brace 20 frontward or rearward with respect to the seat brace 10 with a single hand . furthermore , when the adjustment unit 30 is turned in operation , the back brace 20 is driven to move simultaneously . accordingly , when the adjustment unit 30 is stopped turning , the back brace 20 will be stopped to a desired position . therefore , users do not need to further push or pull the back 92 of the office chair using the back or another hand of the user or the like .
an adjustment device includes a seat brace including lower and upper support plates fixed thereon , respectively . a back brace includes a second end tightly disposed between the lower and upper support plates of the seat brace , with friction generated between the lower and upper support plates and the second end of the back brace , respectively . a locating portion is defined on the second end of the back brace . an adjustment unit includes an end pivotally connected to the seat brace and another end opposite to the actuated element and exposed outside of the adjustment device for use . an actuated element is mounted on the adjustment unit relative to the locating portion and connected to the locating portion .