Locking Mechanism For Vertically Adjustable Drawing Boards

Abstract

A parallel motion guiding mechanism supporting a drawing board and including two opposite levers is provided with a link between the levers pivotally connected with one of the levers and connected with the other lever for rotational movement and displacement longitudinally of the link. A locking mechanism is provided to optionally lock the link to said other lever in order to prevent a change of the mutual spacing of the levers and thereby to prevent movement of the parallel motion guiding mechanism.

Description

Drawing tables having vertically adjustable drawing boards, in which the supporting means for the drawing board comprises a parallel motion guiding means, are subject to the drawback that they lack an effective and easily operable locking mechanism for retaining the drawing board in the desired vertical position.

The main object of the present invention is to remove this drawback.

Drawing tables of the type described now commonly used are generally provided with a counterbalancing mechanism. Such a mechanism is adapted to retain the drawing board in the vertically adjusted position which necessitates a very accurately calculated construction in view of the fact that the inclination of the drawing board effects the position of the center of gravity and, thus, also the turning moment which is to be counteracted by the counterbalancing mechanism.

When certain works are performed it is desirable or even necessary that the surface of the drawing board is kept horizontal. Then, it is often necessary that the draftsman when working on the remote parts of the board rests against the board more or less heavily. In such a case the counterbalancing mechanism per se cannot prevent the drawing board from changing its position vertically. This may be obviated to a certain extent for instance by increasing the friction of the joints of the parallel motion guiding means. However, inherently connected with this measure is a substantial reduction or a complete loss of one of the advantages provided by the parallel motion guiding means, i.e., the easy and convenient adjustment of the drawing board vertically.

Attempts have also been made to lock one of the levers of the parallel motion guiding means to the frame supporting such means. These arrangements have turned out to be cumbersome to operate and, furthermore, difficult to fabricate due to the great forces necessary to keep the guiding means in a stationary position.

While maintaining all the advantages of the parallel motion guiding means the invention facilitates retaining the drawing board vertically in a completely stable position by using a locking mechanism which is easily available and conveniently operable. This is achieved by the fact that the locking mechanism prevents the levers of the parallel motion guiding means from changing their mutual spacing. Such a change of the mutual spacing is connected with every change of the position of the drawing board vertically.

Accordingly the invention provides a locking mechanism for vertically adjustable drawing boards supported by a parallel motion guiding means, characterized in that two opposite levers of said means are provided each with a guide member for a pivoted link which is longitudinally displaceable in relation to at least one of said guide members said one guide member being formed as or provided with means for optionally preventing displacement of said link. When the locking mechanism is engaged the link is prevented from displacement in relation to the guide member which means that there is no possibility of moving the levers of the parallel motion guiding means. However, when the locking mechanism is disengaged such levers are free to move and the drawing board may be raised or lowered in the conventional manner.

The invention now will be described in greater detail with reference to the accompanying drawing in which

FIG. 1 is an elevational side view of a drawing table having a parallel motion guiding means for the drawing board and provided with a locking mechanism according to the invention;

FIG. 2 is an elevational side view of the drawing table according to FIG. 1 showing the drawing board in a horizontal position;

FIG. 3 is a diagrammatical side view of a drawing table including another embodiment of the parallel motion guiding means and the locking mechanism provided therein the drawing board being shown in a position corresponding to that shown in FIG. 2;

FIG. 4 is a diagrammatical side view in accordance with FIG. 3 of the arrangement shown therein but with the drawing board in a raised horizontal position; and

FIG. 5 is a sectional view of a simple embodiment of the locking mechanism according to the invention.

In FIG. 1 there is shown a drawing board 1 which is connected with a plate bracket 2. Bracket 2 is provided with bearings for two levers 3 and 4 pivotally connected therewith and allows for adjustment of the drawing board to different inclined positions by means forming no part of the present invention. Lever 3 is angled and is pivoted at the lower end thereof to a pedestal 5 to which is pivoted also a third lever 6 for swinging movement vertically. With lever 3 is pivotally connected a triangular plate 9 at one corner thereof the plate having bearings for the pivoted connection of levers 4 and 6 at the other corners thereof. Drawing board 1 is supported at bracket 2 by levers 3 and 4 forming together with plate 9 and lever 6 a parallel motion guiding means, known per se, and may be adjusted vertically without changing the inclination of the drawing board.

In a bearing 7 provided for the connection of lever 4 to plate bracket 2 there is pivoted to said lever a link 8 at one end thereof. Furthermore, link 8 is guided by a guide member 10 provided on lever 3 said link being rotatably and displaceably guided on lever 3 by said guide member.

Guide member 10 is formed as a locking member allowing link 8 to be rotated and displaced in relation to lever 3 when the locking guide member is disengaged. One embodiment of this locking guide member is shown in FIG. 5. As will be seen therein link 8 is constructed as a double link comprising two link members 11 and 12 disposed one at each side of lever 3. Link members 11 and 12 are each provided with longitudinally extending slots 13. Transversely through these slots and an opening formed by lever 3 extends a bolt having a flat head 14, a portion 15 of square or rectangular cross-sectional form, and a threaded end portion 16, a nut 17 being screwed onto said latter portion. The nut is provided with an operating stick 18 by which the nut may be tightened against link member 11 which is thereby applied against lever 3, bolt 15 being forced in the opposite direction to be applied with head 14 thereof against link member 12 which is thereby applied against lever 3. Due to this engagement link members 11 and 12 are frictionally locked to lever 3 and are prevented to move in relation to said lever.

In the embodiment disclosed in FIG. 5 the frictional force is provided directly between lever 3 and link members 11 and 12 and also between link member 11 and nut 17 and between link 12 and head 14 of the bolt which is non-rotatably mounted in lever 3. Thus, there are four friction surfaces, which is advantageous, since the locking means is then able to maintain the parallel motion guiding means in a stationary position even if nut 17 is not heavily tightened. The pitch of the threaded end portion 16 may be great which means that the rotation of nut 17 for engagement and disengagement, respectively, may be relatively small. The change of the angular position of operating stick 18 and thus the change of the position of the outer end thereof will be correspondingly small which is advantageous considering the operation of the locking means.

It is not necessary to provide the locking mechanism on the angled lever 3 as shown in FIGS. 1 and 2. In FIGS. 3 and 4 there is shown diagrammatically an embodiment wherein link 8 is pivoted to lever 3 and is rotatably and displaceably connected with lever 4 by means of guide member 10 allowing the link to be locked to lever 4. In FIG. 3 the drawing board is shown in a lower position, guide member 10 being received by the outer end of the slot provided in link 8. In the position according to FIG. 4 the drawing board is elevated to an upper position link 8 being displaced in relation to guide member 10 which is received by the inner end of the slot.

The slot formed in the link must not necessarily be a straight slot. In the arrangement according to FIGS. 3 and 4 the link 6 projects from lever 4. This will be obviated by giving the link and the slot therein an arcuate form.

The link may be formed in other ways than that described above. Thus, it is possible to form the link for instance as a bar which is displaceable in a bore extending diametrically through a rotatable pin. In that case the locking mechanism may comprise for instance a stop screw screwed into a threaded bore extending at a right angle to the bore receiving said bar.

It is possible to modify the invention in other respects over those mentioned above. For instance, if it is sufficient that the drawing board is to be locked in a lower position only, then an abutment such as the end wall of the slot or an abutment lug provided on the link may engage guide member 10 and thus limit the downward movement of the parallel motion guiding means. Particularly if the link comprises a bar displaceable through a rotatable pin such abutment lug may be replaced by a nut screwed on the bar. In that case there is provided for an adjustment of the lower position of the drawing board which may be utilized to compensate for a non-accuracy, if any, in the manufacture and to lower the production cost by allowing a wider tolerance zone.

Claims

What I claim is:

1. A locking mechanism for vertically adjustable drawing boards, comprising a parallel motion guiding means including two opposite levers, means mounting said levers for vertical swinging movement, means mounting said drawing board on said parallel motion guiding means, a guide member on each of said levers, a pivoted link connected to one of said levers by the associated guide member for rotational movement in relation to the lever and connected to the other one of said levers by the associated guide member for rotational movement and displacement longitudinally of said link in relation to the lever, and manually operable means for optionally preventing displacement of said link in relation to said other lever.

2. A locking mechanism as claimed in claim 1 wherein one of the guide members for the link is provided at the end of one of said levers the other one being provided between the ends of the other lever.

3. A locking mechanism as claimed in claim 1 wherein there is provided in said link a slot extending longitudinally thereof from a location near one end of said link, and wherein said one guide member comprises a threaded bolt said link being arranged to be clamped between a nut screwed on said bolt and an abutment provided by said bolt.