US3799486A

US3799486A - Height adjusting mechanism

Info

Publication number: US3799486A
Application number: US00285527A
Authority: US
Inventors: R Mohr; A Olree
Original assignee: Steelcase Inc
Current assignee: Steelcase Inc
Priority date: 1972-08-31
Filing date: 1972-08-31
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26
Also published as: JPS4970757A; GB1401937A; CA986002A; DE2343328A1; US3799485A; CA1004126A

Abstract

The height adjusting mechanism includes a threaded spindle rotatably mounted in a chair base and a seat supporting column telescopically mounted in the base and operably threadably mounted on the spindle. To lock the spindle against rotation with respect to the base and thereby allow one to adjust the chair height by rotating the seat supporting column, one depresses upwardly a button at the bottom of the base. The button includes a pair of fingers which engage slots in a shoulder plate which is attached to the spindle. As the column is threaded downwardly and approaches its lowermost position, the bottom thereof engages the tops of the fingers and forces them out of engagement with the slots in the shoulder plate.

Description

United States Patent [191 Mohr et a1.

[ Mar. 26, 1974 HEIGHT ADJUSTlNG MECHANISM [75] Inventors: Robert G. Mohr, Grand Rapids;

Adrian R. Olree, Caledonia, both of Mich.

[73] Assignee: Steelcase lnc., Grand Rapids, Mich.

[221 Filed: Aug. 31, 1972 [21] Appl. No.: 285,527

[52] US. Cl. 248/406 [51] Int. Cl. Fl6m 13/00 [58] Field of Search..... 248/406, 405, 354 S, 188.4, 248/162; 297/339, 347; 108/147, 141;

3,164,357 1/1965 Hage et a1 248/406 Primary Examiner-Marion Parsons. Jr. Attorney, Agent, or FirmPrice, Heneveld, Huizenga & Cooper 5 7] ABSTRACT The height adjusting mechanism includes a threaded spindle rotatably mounted in a chair base and a seat supporting column telescopically mounted in the base and operably threadably mounted on the spindle. To lock the spindle against rotation with respect to the base and thereby allow one to adjust the chair height by rotating the seat supporting column, one depresses upwardly a button at the bottom of the base. The button includes a pair of fingers which engage slots in a shoulder plate which is attached to the spindle. As the column is threaded downwardly and approaches its lowermost position, the bottom thereof engages the tops of the fingers and forces them out of engagement with the slots in the shoulder plate.

16 Claims, 11 Drawing Figures PATENTED MAR 2 6 I974 SHEET 2 OF 3 FIG].

PATENTEBMAR26|974 3799486 SHEET 3 [1F 3 I I HEIGHT ADJUSTING MECHANISM CROSS-REFERENCE This application is related to patent application Ser. No. 285,188, filed on Aug. 31, 1972, by Richard H. Wolters and assigned to the assignee of this application. Mr. Wolters is the inventor of the specific means for normally causing the chair supporting column to rotate in conjunction with the threaded spindle which is disclosed in this application. The claimed subject matter of this application is the locking means for locking the threaded spindle against rotation with respect to the base and the combination of that locking means with the aforesaid holding means for normally causing the chair supporting column to rotate with the threaded spindle.

BACKGROUND OF THE INVENTION The present invention relates to height adjusting mechanisms for chairs. Normally, a bell nut is rotatably mounted on the chair base and a threaded spindle is threaded in the nut. The chair is mounted at the top of the spindle. Normally, the spindle and bell nut rotate together, generally because of some type of biased detent means therebetween. The height of the chair is adjusted to holding the bell nut and turning the chair to thereby thread the spindle upwardly or downwardly. One drawback to such a mechanism is that the spindle is exposed, at least above the bell nut. The spindle is unsightly and any grease or lubrication thereon becomes exposed to dust. Yet another drawback is that such devices are somewhat mechanically unsophisticated.

One prior art mechanism employs a spindle rotatably mounted on the chair base, the chair itself being mounted on a column which is in turn threaded onto the spindle. The bottom of the sheet metal tubular column is actually formed into the shape of a nut having downwardly deviating detents. These detents normally engage indentations in the threads of the spindle such that the column and spindle tend to rotate in unison. A locking member is slidably but nonrotatably mounted on the bottom of the spindle and can be depressed upwardly such that dimples thereon engage apertures in the bottom of the base. This locks the spindle against rotation with respect to the base. When the spindle is locked against rotation with respect to the base, the detents turn out of the indentations in the spindle threads and the column threads on the spindle. While this mechanism is more sophisticated mechanically, in that one merely pushes a button in order to achieve height adjustment while rotating the chair, it does suffer the drawback in that the formed nut at the base of the tubular column is more readily subject to bending than would be a conventional nut. This is particularly true if one carelessly rotates the column downwardly until it is jammed against the bottom of the base. Even a sturdier mechanism could suffer thread stripping or other damage under these circumstances. Indeed, the column might become jammed so'tightly that it would be difficult to start it threading upwardly again.

SUMMARY OF THE INVENTION In the present invention, the seat supporting column in a height adjusting mechanism of the type described above cannot be jammed against the bottom of the base because the system includes means for automatically disengaging a locking means when the column reaches its lowermost position. Thus, the locking means is fixed against rotation with respect to either the base or the threaded member and includes a detent member. The other of the base or threaded member includes a receiving means for receiving the detent member. The locking means is movable relative to the receiving means whereby the detent member can be moved into and out of engagement therewith. The chair support means, such as a column or the like, includes an abutment means in alignment with the detent member for forcing it out of engagement with the receiving means, against the force of one holding it therein, when the chair support means is threaded downwardly to its lowermost position. This makes it impossible for one to carelessly jam the chair supporting means against the bottom of the base when threading it downwardly. However, the detent member is preferably designed such that when the chair support means is rotated in an elevating direction, the detent will engage its receiving means even with the support in its lowest position and will cause the support to thread upwardly.

Preferably, the detent member includes a wedge and the receiving means includes a corrsponding cam. These members are arranged such that the abutment means forces the wedge into alignment with the cam as the chair support is lowered to its lowermost position. Further rotation of the chair support means causes the cam to slide over the wedge, forcing the detent member out of engagement with the receiving means. It is further preferable that the entire locking means be positioned within a downwardly opening cavity at the bottom of the base for purposes of convenient mounting as well as for the sake of appearance.

These and other objects and advantages of the invention will be more fully understood and appreciated by reference to the written specification and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a generally cross-sectional view of a chiar base embodying the height adjusting mechanism of the invention;

FIG. 2 is a cross-sectional view of the same chair base wherein the chair has been adjusted upwardly;

FIG. 3 is a bottom plan view of the detent washer which rests on top of the adjusting nut and upon which the chair supporting column rests;

FIG. 4 is a generally elevated perspective view of the height adjusting nut;

FIG. 5 is a side elevational view of the spindle;

FIG. 6 is a generally perspective view of the cylindrical sleeve of the chair supporting column;

FIG. 7 is a generally perspective view of the collar which is secured to the inside of thesleeve of the chair supporting column;

FIG. 8 is a perspective view of the locking button which is used to lock the spindle against rotation with respect to the base;

FIG. 9 is a generally perspective, cut-away view of themechanism at the base of the chair supporting column;

FIG. 10 is a cross sectional view showing the spindle locking means at the base of the chair base; and

FIG. 11 is the same view as FIG. 10, with the chair supporting column having been adjusted downwardly to its lowermost point.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment, spindle 10 is rotatably mounted in chair base 20 (FIG. 1). Chair supporting column 40 with its affixed cup 42 is telescopically received with base 20 and is operably, threadably mounted on spindle 10 through the interaction of nut 30 and detent washer 50. Spindle lock 60 at the bottom of base 20 includes a pair of upwardly projecting fingers 62 which can be pushed upwardly into engagement with a receiving slot 16 in shoulder plate 13 which is secured to spindle 10. The locking means 60 is fixed against rotation with respect to base 20 such that engagement of finger 62 with receiving slot 16 provides a means for holding spindle 10 against rotation with respect to base 20. This allows chair supporting column 40 to be adjusted upwardly or downwardly by rotation thereof. The bottom of chair supporting column 40 acts as an abutment means in alignment with finger 62 such that when column 40 is rotated downwardly to its lowermost position, the bottom thereof abuts the top of finger 62 and forces finger 62 out of engagement with receiving slot 16 in shoulder plate 13. This renders it impossible to further hold spindle 10 against rotation with respect to base 20 (compare FIGS. 10 and 11).

Spindle 10 comprises a steel shaft or the like having threads 11 thereon (FIGS. 5 and 9). An annular shoulder plate 13 is secured to spindle 10 near the base thereof. Shoulder 13 and a stem 14 projecting downwardly from annular shoulder 13 facilitate rotatable mounting of spindle 10 within base 20. Annular shoulder plate 13 includes a plurality of slots or notches 16 about its perimeter which facilitate the cooperation of shoulder plate 13 with spindle lock 60 to hold spindle 10 against rotation with respect to base 20 (FIGS. 10 and l l Spindle 10 includes a groove 12 extending vertically therein from generally its top to its bottom (FIG. 5) for cooperating with detent washer 50 to facilitate the slidable but nonrotatable mounting to spindle l0. Groove 12 is deeper than threads 11 such that a key tab received in groove 12 cannot slip out between threads 11.

Base 20 includes a hub 21 with outwardly radiating legs 22 (FIG. 1). Hub 21 is generally cylindrical, being hollowed out in the center from its top almost to its bottom for receiving spindle 10 and chair supporting column 40. The bottom wall 23 of hub 21 includes an opening 25 therein through which stem 14 of spindle 10 passes. Spindle bearing washers 24 are positioned between bottom wall 23 and shoulder plate 13 to provide a rotatable bearing means at the base of spindle 10. A pair of openings 26 positioned on opposite sides of bottom wall 23 facilitate cooperation of spindle lock 60 with spindle 10 (FIGS. 10 and 11).

Stem 14 of spindle 10 passes through bottom wall 23 and into a downwardly opening cavity 17 in the bottom of base 20. Spindle lock 60 is located generally within cavity 17. To prevent upward removal of spindle 10, a snap ring slips into a receiving groove at the base of stem 14 to lock spindle 10 in place.

A bearing sleeve 27 is inserted in hub 21 in snugly fitting relationship thereto. Bearing sleeve 27 is a selflubricating bearing sleeve made of nylon or the like which extends generally the length of hub 21.

Nut 30, which is threadably mounted on threads 11 of spindle 10, includes depressions 31 in the top surface thereof for cooperating with detent washer 50 (FIG. 4). In each side of nut 30 there is a pin hole 32 for receiving a pin 33 (FIGS. 4 and 1). Pins 33 facilitate operable connection of nut 30 to chair supporting column whereby column 40 is free to move vertically with respect to nut 30, but is fixed against rotation with respect thereto. Each pin 33 comprises a swage pin, a screw, or other similar fastener.

Column 40 comprises a metal sleeve 41 rigidly joined to a collar 42 at the bottom thereof by means of welding or the like (FIGS. 1, 6, and 7). It is telescopically received within bearing sleeve 27 and hub 21. Collar 42 includes a central opening 43 therein through which spindle 10 passes. Sleeve 41 and collar 42 include matching

apertures

44 and 45 respectively in the side walls thereof. Apertures 44 and 45 are elongated in a generally vertical direction (as shown) or are oversized round holes which give the same degree of vertical play for pins 33 as would an elongated slot. They are positioned on opposite sides of the column 40 in alignment with pin holes 32 in nut 30. It is through

mating slots

44 and 45 that each pin 33 passes (FIG. 1). In this manner, column 40 is locked against rotation with respect to nut 30, but is free to slide vertically a short distance with respect thereto clue to the fact that the apertures have a greater diameter at least in a vertical direction than do pins 33.

Mounted atop column 40 is a chair support 48 which may be of any conventional construction. It is secured to the top of column 40 by means of welding, bolting, or the like.

Collar 42 of column 40 rests directly atop detent washer 50 (FIG. 1). Detent washer 50 in turn rests atop nut 30. Preferably, detent washer 50 is made of metal, but it could be made of a self-lubricating material such as nylon or the like. It includes a key 51 projecting inwardly towards the center thereof for mating engagement with groove 12 of spindle 10 (FIGS. 3 and 9). This fixes detent washer 50 against rotation with respect to spindle 10.

Projecting downwardly from the bottom surface of detent washer 50 are a plurality of detents 52, on for each depression 31 in the top surface of nut 30 (FIGS. 3 and 9). Detents 52 are received within depressions 31 and normally prevent nut 30 from rotating with respect to washer 50 and spindle 10.

Spindle lock for locking spindle 10 against movement with respect to base 20 includes a button 61 formed of sheet metal or the like (FIG. 8). Button 61 includes a pair of upwardly projecting detent fingers 62 (FIGS. 1, 8, 10, and 11). By pushing button 61 upwardly, one can move the upper portion of each detent finger 62 into notch or slot 16 in shoulder plate 13 of spindle 10 (FIG. 10). Each detent finger 62 is wider at its bottom portion than at its top portion and juts inwardly at the base of its top portion to define a limit shoulder 63. Only the upper portion of detent finger 62 is sufficiently narrow to pass through a notch 16, and accordingly, limit shoulder 63 engages the bottom of shoulder plate 13 to prevent further upward movement of button 61.

A guide pin 68 projects upwardly from button 61 and is slidably received in a guide tunnel 69 in stem 14 of spindle 10. The cooperation of guide pin 68 and guide tunnel 69 serves to guide button 61 as it is pushed in or out, thereby minimizing jamming.

The top edge of each detent finger 62 is cut off at an angle on one side to define a sloping wedge 64. The bottom of column 40 acts as an abutment means and is aligned with the top of finger 62. As column 40 is adjusted downwardly within hub 21, its bottom surface abuts the very top of each detent finger 62 and forces detent finger 62 downwardly until wedge 64 is aligned with the edge of receiving notch 16 (FIG. 11). At this point, further rotation of a chiar and of column 40 causes the edge of receiving notch 16 to engage wedge 64 and, acting as a cam, actually forces button 61 downwardly against the pressure of a person who is depressing button 61 upwardly. In this manner, detent fingers 62 are forced out of engagement with notches 16, and further downward adjustment of column 40 is prevented. This feature prevents a person from carelessly adjusting column 40 downwardly to the point where it jams against shoulder plate 13 and possibly causes damage to the mechanism.

On the other hand, rotation of the chair and column 40 in an elevating direction will effect upward threading of column 40 even from its lowermost position. This is because wedge 64 is unidirectional, the left side of finger 62 (as viewed in FIGS. and 11) being left vertical, i.e., with a blocking configuration, rather than being cut at an angle at the top. Thus, the left side of notch 16 (as viewed in FIGS. 10 and 11) will engage only the vertical edge of detent finger 62, rather than a sloped wedge surface, and spindle 10 will thereby be prevented from rotation. It is significant that the tip of detent finger 62 can never be forced entirely out of notch 62 by column 40 (although it can be by the cam action of shoulder plate 13). One can always effect upward adjustment of column 40, even from its lowermost position.

A spring 65 mounted between button 61 and the bottom of bottom wall 23 normally biases button 61 downwardly (FIGS. 1, 10, and 11). Thus, detent fingers 62 are normally biased out of engagement with notches 16. A small retainer ring 66 is force fitted or otherwise secured to the very bottom of base at the edge of cavity 17. Its inside diameter is less than the outsidediameter of button 61 so that button 61 is prevented from falling out of the bottom of base 20.

Assembly is effected by threading nut onto spindle 10 from the top. Detent washer 50 is then slipped onto spindle 10 with key 51 in groove 12. Spindle 10 is then inserted upwardly through opening 43 in collar 42 of column 40. The

apertures

44 and 45 in the base of column are aligned with pin holes 32 in nut 30. Pins 33 are then inserted through apertures 44 and into pin holes 32 and are secured either by a forced fit or, in the case of screws, by threading. Finally, washer 18 is secured to the top of spindle 10 by screw 19 in order to prevent the accidental removal of column 40 from spindle 10.

The assembled column 40, nut 30, detent washer and spindle 10 are then inserted into hub 21 and bearing sleeve 27. Stem 14 is passed through stem opening 25 in bottom wall 23 until shoulder plate 13 comes to rest on spindle bearing washers 24 at the bottom of hub 21.

Spindle lock 60 is assembled to base 20 by positioning spring 65 within locking button 61 and inserting fingers 62 upwardly through openings 26 in bottom wall 23 of hub 21. Retainer ring 66 is then secured to the bottom edge of cavity 17 to hold. locking button 61 in place.

In operation, rotation of a chair mounted to chair support 48 rotates column 40 which in turn rotates nut 30. The rotation of nut 30 normally rotates detent washer 50 because of the engagement of detents 52 with depressions 31. Rotation of detent washer 50 results in the rotation of spindle 10 due to the engagement of key 51 with groove 12. Thus. rotation of the chair normally results in simultaneous rotation of column 40, nut 30, detent washer 50, and spindle 10. To adjust the height, the user depresses button 61 upwardly so that the upper portions of detent fingers 62 engage notches 16 in shoulder plate 13 of spindle 10. This holds spindle 10 against rotation with respect to base 20. Detent washer 50 is similarly held against rotation since it is locked against rotation with respect to spindle 10. The user then rotates the chair, thereby ro tating column 40 and nut 30. The rotation of nut 30 causes detents 52 to ride up out of depressions 31. The entire column 40 is thereby forced to move upwardly in a vertical direction, but is free to do so because

apertures

44 and 45 through which pins 33 project allow vertical movement. Since spindle 10 is fixed against rotation, nut 30 threads upwardly or downwardly thereon to thereby adjust the height. FIG. 1 shows the mechanism in its lowermost position and FIG. 2 shows the mechanism in its uppermost position.

If the chair is being adjusted downwardly, column 40 will eventually engage the top of detent fingers 62 (FIG. 11). As column 40 continues to move downwardly, it will force fingers 62 downwardly, in spite of the upward pressure of the user on button 61, until wedge 64 on each detent finger 62 is in line with the edge of notch 16 which acts as a cam. At this point, shoulder plate 13 will begin to turn, sliding upwardly along wedge 64. The mere fact that button 61 begins to be pushed in a downward position advises the user that he has reached the bottom. In any event, the slope of wedge 64 is sufficiently gradual that it tends to be overcome by the detent force of detent washer 50 on nut 30. Thus, rotation of column 40 again begins to rotate spindle 10, and shoulder plate 13 actually slides along wedge 64 and over the top of finger 62. This prevents further downward adjustment of column 40 altogether and thereby prevents column 40 from being jammed downwardly against shoulder plate 13.

On the other hand, column 40 can always be adjusted upwardly from its lowermost position since at least the tips of detent fingers 62 can be inserted into notches 16 even with column 40 in its lowermost position and since fingers 62 act as a positive stop against shoulder plate 13 when the chair is rotated in a raising direction.

Of course, it is understood that the above is merely a preferred embodiment of the invention and that many changes and alterations can be made thereof without departing from its spirit and broader aspects.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. In a height adjusting mechanism for a chair or the like including a threaded member rotatably mounted in a chair base; a chair support operably threadably mounted to said threaded member; and means for locking said threaded member against rotation with respect to said base whereby said support can be rotated and thereby adjusted upwardly or downwardly, the improvement in said mechanism comprising:

said locking means being fixed against rotation with respect to one of said base and said threaded memher; said locking means including a detent member; receiving means on the other of said base and said threaded member for receiving said detent memher; said locking means being movable relative to said receiving means whereby said detent member can be moved into and out of engagement therewith; and

said chair support means including abutment means in alignment with said detent member for forcing said detent member out of engagement with said receiving means, against the force of one holding it therein, when said chair support is threaded downwardly to its lowermost position.

2. The mechanism of claim 1 in which said detent member includes a wedge thereon; said receiving means including a cam; said wedge and cam being so arranged that said abutment means forces said wedge into alignment with said cam as said chair support is lowered to its lowermost position whereby further rotation of said chair support causes said cam to slide over said wedge and thereby force said detent member out of engagement with said receiving means.

3. The mechanism of claim 2 in which said receiving means comprises an aperture in a plate, said plate being secured to said other of said base and said threaded member; said cam comprising the edge of said aperture.

7. The mechanism of claim 1 in which said base includes a downwardly opening cavity at the bottom thereof; said locking means being disposed within said cavity.

8. In a height adjusting mechanism for a chair or the like including a threaded spindle mounted in a base, a support column operably threadably mounted on said spindle and a locking means for locking said spindle against rotation with respect to said base whereby said column can be adjusted upwardly or downwardly by rotation thereof, the improvement in said mechanism comprising:

a plate secured to one of said spindle and said base;

a receiving aperture in said plate;

a button fixed against rotation with respect to the other of said spindle and said column and having at least one detent finger projecting upwardly therefrom, said button being movable whereby said finger can be moved into or out of engagement with said aperture; and

the bottom of said column being in alignment with said finger whereby when said column is adjusted to its lowermost position, the bottom of said column abuts said finger and forces said finger out of engagement with said aperture whereby further downward adjustment of said column is prevented.

9. The mechanism of claim 8 in which said base includes a bottom wall; said plate being secured to said spindle and being rotatably carried on said bottom wall; said bottom wall including an aperture through which said detent finger projects; said base including a downwardly opening cavity below said bottom wall, said button being positioned within said cavity.

10. The mechanism of claim 9 in which said cavity includes retaining means around the peripheral edge thereof for preventing said button from falling out of said cavity.

11. The mechanism of claim 10 in which said finger includes a wedge at the top thereof, said wedge being positioned entirely through said aperture when said finger is pushed completely into engagement therewith; the bottom of said column forcing said wedge downwardly as it approaches its lowermost position until said wedge is in alignment with the edge of said slot whereby further rotation of said column causes said plate to ride up over said wedge and thereby disengage said finger from said slot.

12. The mechanism of claim 11 in which said button includes two of said fingers, one positioned generally on either side of said spindle.

13. The mechanism of claim 8 in which said finger includes a wedge at the top thereof, said wedge being positioned entirely through said aperture when said finger is pushed completely into engagement therewith; the bottom of said column forcing said wedge downwardly as it approaches its lowermost position until said wedge is in alignment with the edge of said slot whereby further rotation of said column causes said plate to ride up over said wedge and thereby disengage said finger from said slot.

14. The mechanism of claim 13 in which said button includes two of said fingers, one positioned generally on either side of said spindle.

15. The height adjusting mechanism of claim 13 comprising:

a nut threadably mounted on said spindle;

a detent washer resting on top of said nut including at least one downwardly projecting detent;

said nut including at least one detent receiving depression in the top thereof; said washer being positioned around said spindle and being slidable vertically with respect thereto, but

being fixed against rotation with respect thereto; said column resting on said washer;

said column including an elongated, generally vertical slot near the base thereof;

said nut including a pin projecting radially therefrom and engaging said slot; the vertical stroke of said wedge being greater than the vertical height of said elongated slot whereby one will not be able to apply sufficient upward pressure on said button to cause said column to rise upwardly without forcing said finger downwardly.

16. The mechanism of claim 15 in which said button includes two of said fingers, one positioned generally on either side of said spindle.

Claims

1. In a height adjusting mechanism for a chair or the like including a threaded member rotatably mounted in a chair base; a chair support operably threadably mounted to said threaded member; and means for locking said threaded member against rotation with respect to said base whereby said support can be rotated and thereby adjusted upwardly or downwardly, the improvement in said mechanism comprising: said locking means being fixed against rotation with respect to one of said base and said threaded member; said locking means including a detent member; receiving means on the other of said base and said threaded member for receiving said detent member; said locking means being movable relative to said receiving means whereby said detent member can be moved into and out of engagement therewith; and said chair support means including abutment means in alignment with said detent member for forcing said detent member out of engagement with said receiving means, against the force of one holding it therein, when said chair support is threaded downwardly to its lowermost position.

4. The mechanism of claim 3 in which said detent comprises a finger including a shoulder, said shoulder abutting said plate to limit the extent of engagement of said finger with said aperture.

5. The mechanism of claim 4 in which said base includes a downwardly opening cavity at the bottom thereof; said locking means being disposed within said cavity.

6. The mechanism of claim 5 in which said locking means is fixed against rotation with respect to said base and said receiving means is on said threaded member.

8. In a height adjusting mechanism for a chair or the like including a threaded spindle mounted in a base, a support column operably threadably mounted on said spindle and a locking means for locking said spindle against rotation with respect to said base whereby said column can be adjusted upwardly or downwardly by rotation thereof, the improvement in said mechanism comprising: a plate secured to one of said spindle and said base; a receiving aperture in said plate; a button fixed against rotation with respect to the other of said spindle and said column and having at least one detent finger projecting upwardly therefrom, said button being movable whereby said finger can be moved into or out of engagement with said aperture; and the bottom of said column being in alignment with said finger whereby when said column is adjusted to its lowermost position, the bottom of said column abuts said finger and forces said finger out of engagement with said aperture whereby further downward adjustment of said column is prevented.

15. The height adjusting mechanism of claim 13 comprising: a nut threadably mounted on said spindle; a detent washer resting on top of said nut including at least one downwardly projecting detent; said nut including at least one detent receiving depression in the top thereof; said washer being positioned around said spindle and being slidable vertically with respect thereto, but being fixed against rotation with respect thereto; said column resting on said washer; said column including an elongated, generally vertical slot near the base thereof; said nut including a pin projecting radially therefrom and engaging said slot; the vertical stroke of said wedge being greater than the vertical height of said elongated slot whereby one will not be able to apply sufficient upward pressure on said button to cause said column to rise upwardly without forcing said finger downwardly.