Adjustment device for photographic enlarging or reproducing apparatus
BACKGROUND OF THE INVENTION 
     This invention relates to a device for displacing a body along a guiding column and is particularly suitable as an adjustment device for the projection head of an enlarging or reproducing apparatus. 
     In the known adjustment apparatus for adjusting the height of the projection head at a projection table there is provided a friction wheel rotatably journalled at the projection head. The whole device is mounted on a vertically disposed guiding column which may be formed of a square bar or a rod having circular cross-section. The friction wheel is pressed against the guiding column with a high spring force. This spring force is supported by a surface of the guiding column opposite the friction wheel. In this manner considerable friction forces are created at the area between the guiding column and the body sliding along the column. This friction force can be limited by arranging ball bearings between the body and the guiding column so that an adjustment is possible with a reasonable expenditure of force. 
     Such adjustment devices are expensive to manufacture with the necessary high precision. During assembly the friction wheel drive must be adjusted to the guiding column. 
     It is accordingly an object of the present invention to provide an adjustment device of the type discussed which is much simpler in construction than prior art devices. 
     A further object of the present invention is to provide an adjustment device as discussed which obviates the necessity of adjustment and which can be manufactured with considerably larger tolerances without affecting its operation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention the friction wheel is subdivided into two wheel portions. Due to a force acting along the axis of the friction wheel, the wheel portions are pressed against a guidance surface on the guiding column which forms an angle equal to or smaller than 90° with the friction wheel axis. 
     The forces pressing the friction wheels against the guiding column are directed almost exclusively in the direction of the friction wheel axis, and only small components of the forces press the sleeve against the guiding column. This permits elimination of an abutment for the friction wheel and a ball bearing between the abutment and the guiding column which are used in conventional devices. 
     At the same time the efficiency of the friction wheel drive is increased because the spring force available is almost exclusively used for generating the friction wheel pressure of the two opposing friction wheel portions. 
     In a preferred embodiment of the invention the guidance surfaces for the friction wheels are formed by the flanks of a longitudinal groove in the guiding column, the tapered sides of the wheel portions bearing against these flanks. 
     Pressure of the friction wheels against the flanks of the groove is obtained in a particularly simple manner in accordance with the invention. The wheel portions are pushed apart along their axis of rotation by a spring disposed between them. In this manner the pressure, as well as the counter pressure of the spring is utilized for pressing the friction wheels against the guiding column. 
     An advantageous embodiment of the invention utilizes a guiding column having a C-shape, the friction wheel extending partially into the open end of the &#34;C&#34; with its axis of rotation aligned between the tips of the &#34;C&#34;. In this manner the manufacturing costs for the guiding column can be substantially reduced. 
     Another embodiment of the invention provides that the guiding column has a shoulder portion extending outwardly between the friction wheels. The friction wheels press against the lateral surfaces of this portion of the column. In order to generate the pressure a spring which is not axially displaceable is associated with each wheel portion, the spring bearing axially against its wheel portion. 
     Further in accordance with the present invention the guidance surfaces and the surfaces of the wheel portions bearing thereagainst form an acute angle with each other. As a result, the wheel portions tend under the action of the axial spring to be wedged outwardly from the guiding column. This tends to pull the C-shaped sleeve on which the wheel portions are mounted more tightly against the guiding column. Because this arrangement is self-centering, time-consuming adjustment of the tolerance in the friction wheel or in the sleeve is eliminated from the manufacturing process. 
     The same effect may be obtained in accordance with another variation of the invention in that the side surfaces of the wheel portions are shaped as cones or truncated cones. 
     A further improvement of the invention is obtained in that friction-reducing elements are attached within the sleeve between it and the surfaces of the guiding column against which it bears. These sliding elements are made of synthetic material which considerably reduces the friction between the sleeve and the guiding column, and maintains centering of the sleeve on the guiding column. In order to operate the friction wheel drive and thereby to adjust the projection head secured to the sleeve there is provided a hand wheel which is connected to the friction wheel and rotatable therewith. 
     The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a guiding column and friction wheel drive in accordance with the invention; 
     FIG. 2 is a side elevational view of the apparatus of FIG. 1; 
     FIG. 3 is a cross-sectional view of a modified embodiment of the present invention; and 
     FIG. 4 is a cross-sectional view of still another embodiment of the guiding column and friction wheel drive in accordance with the invention. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is illustrated a guiding column 1 of C-shape. The guiding column is surrounded by a U-shape sleeve 2 so that the opening of the U of the sleeve coincides with the open side of the C-shape of the column. Between the ends of the U-shaped sleeve 2 there is journalled a shaft 3 in such a manner that it extends tangent to the guidance column along the open side of the C-shape. 
     Rotatably secured to the shaft 3 is a friction wheel means 4 which consists of two wheels or wheel portions 4a and 4b. The wheel portion 4a is fixedly secured to a hand wheel 5 for rotation therewith, while the wheel portion 4b is axially displaceable and rotatably mounted upon the shaft 3. The two wheel portions 4a and 4b are hollow and a helical spring 6 is disposed in the interior of the two wheel portions. The spring 6 tends to push apart the two wheel portions. The side surfaces 4a&#39; and 4b&#39; of the two wheel portions 4a and 4b are tapered to form conical or truncated conical surfaces. 
     The shaft 3 is so arranged in the U-shape sleeve that the friction wheel 4 extends into the opening of the C-shape column 1. Accordingly the surfaces 4a&#39; and 4b&#39; of the wheel portions 4a and 4b bear against the tip ends of the C legs of the column 1. Due to the tapered front surfaces of the wheel portions 4a and 4b the friction wheel tends under the influence of the spring 6 to move out of the C-shaped column so that the sleeve 2 is pulled against the guiding column 1. Within the corners of the U-shaped sleeve 2 there are disposed friction-reducing sliding elements 7 which are preferably made of a synthetic material. By means of the elements 7 the sleeve 2 presses against the guiding column 1 under the action of the spring 6 and under the action of the friction wheel 4 which due to its inclined surfaces tends to move outwardly. These friction-reducing elements in combination with the conical surfaces of the wheel portions 4a and 4b promote automatic centering of the sleeve 2 at the guiding column 1. 
     At the sleeve 2 there is secured a bearing pivot 8 upon which the projection head 9 (see FIG. 2) is disposed so that it can be inserted and locked. 
     In the embodiment of FIG. 3, the guiding column has a different profile. Here the groove is not formed by the opening of a C-shaped column. In order to increase the solidity of the column it has a closed surface which forms a groove 10 in the region of the friction wheel. The lateral surface or flanks 10a and 10b of the groove are slightly inclined. Otherwise, the construction of the adjustment device is identical with that of FIGS. 1 and 2. 
     FIG. 4 illustrates a variation or different embodiment of the adjustment device; here the guiding column 1 includes a shoulder portion 11 which extends from the guiding column into the region between the wheel portions 12a and 12b. The wheel portions 12a and 12b bear against the lateral surfaces 11a and 11b respectively of the portion 11. Each wheel portion is axially displaceable upon the friction wheel shaft 13 and is connected therewith by pins 14a and 14b to rotate therewith. Two helical springs 15a and 15b surround the shaft and bear on the one hand upon stops 16a and 16b respectively fixedly secured to the friction wheel shaft 13 and on the other hand against the wheel portions 12a and 12b. Thereby the springs press the lateral surfaces 12a&#39; and 12b&#39; of the wheel portions against the lateral surfaces 11a and 11b of the shoulder portion 11. The lateral surfaces of the wheel portions and the lateral surfaces of the shoulder portion 11 form an acute angle with each other. Accordingly, as in the embodiment of FIG. 1, the wheel portions tend to move away from the guiding column so that the sleeve is lightly pressed against the guiding column. The friction wheel shaft 13, rotatably mounted in the sleeve 2, is rigidly connected with the hand wheel 17. 
     The invention is not limited to the embodiments described above, but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.