You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a carrier for door drives of lifts. 
     2. Discussion of the Prior Art 
     Carriers of the kind which are mounted on the roof of an elevator cage at the door side, serve for the reception of all important components for the door drive, such as, for example, translation gear, belt drive, motor bearer with drive motor, guide rails, etc. In general, in the case of door drives for elevators such as have become known by, for example, U.S. Pat. No. specification 4,149,615 or European patent specification 0, 513,509, the carriers are conceived on each occasion only for a specific door system. In the case of different door systems with different entrance widths, carriers of different lengths matched to the entrance widths are required for the aforesaid door drives, whereby warehousing is more costly and the door drives are more expensive. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of providing a carrier of the kind stated in the introduction, which does not have the above-mentioned disadvantages. 
     Pursuant to this object, and others which will become apparent hereafter, one aspect of the invention resides in a carrier having consists of a base part and a length adapter, which are connectable together in such a manner that carriers of different lengths can be produced. 
     The advantages achieved by the invention are to be seen in that one and the same carrier can be used for different door systems with different entrance widths (for example, 600 to 800 millimeters). With the carrier the drive components can be arranged in such a manner that it is possible to reach all important components from the storey. The carrier according to the invention has a geometric form which offers mechanical protection for the belt drive unit, whereby, in particular, a protection against objects, dirt, water, etc., falling down as well as against wanton destruction by elevators users is guaranteed. The carrier is adjustable not only horizontally, but also vertically, so that an adaptation to different cages and compensation for tolerances are possible. Costs can be saved by this carrier and warehousing simplified. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is more closely explained in the following by reference to an example of embodiment in conjunction with the drawings, in which: 
     FIG. 1 shows an elevation of the carrier according to the invention, with mounted door drive components; 
     FIG. 2 shows a partially sectioned side elevation of the carrier with door drive components, in a scale enlarged relative to FIG. 1; 
     FIG. 3 shows an elevation of a base part of the carrier; 
     FIG. 4 shows a cross-section of the base part according to FIG. 3; 
     FIG. 5 shows an elevation of a length adapter of the carrier; and 
     FIG. 6 shows a cross-section of the length adapter according to FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A cage roof is designated by  2  and the front side of an elevator cage  1  by  3  in FIG.  2 . Two C-section profile rails  4 ,  5 , to which two support brackets  6  and  7  are fastened, are arranged on the cage roof  2 . The support brackets  6 ,  7  are screw-connected with the C-section profile rails  4 ,  5  by means of rotationally fast screws  8 , which are guided in the C-section profile rails, and nuts  9 . A carrier  10 , which consists of a base part  11  and a length adapter  12  and is more closely described in the following by reference to FIGS. 3 to  6 , is fastened to the support brackets  6 ,  7 . The carrier  10  is screw-connected with the support,  7  merely by way of the base part  11 , whereas the length adapter  12  is connected with the base part  11 . The most important components of a door drive, by means of which a door, consisting of two door leaves  13 ,  14 , of the elevator cage  1  can be opened and closed, are arranged at the carrier  10 . In FIGS. 1 and 2 these components fastened to the carrier  10  are illustrated merely by dotted lines and are briefly described in the following. 
     An electric motor  15  is screw-connected with the base part  11  of the carrier  10  by way of a motor bracket  16  and drives a belt pulley  18  by way of a belt  17 . The belt pulley  18  is rotatably mounted by means of ball bearings  19 . 1  on a pin  19  fastened to the base part  11  and connected rotationally fast with a toothed belt pulley  20 . A further toothed belt pulley  21  is rotatably mounted on the length adapter  12 , which is screw-connected with the base part  11 , and stands under the effect of a tensioning device, which is not illustrated, so that a toothed belt  22  guided over the toothed belt pulleys  20  and  21  always has a sufficient tension. Guide rails,  23 ,  24  are screw-connected with the base part  11  by way of screws  25  and spacers  26 , are. Support rollers  29  are guided on the guide rails  23 ,  24  and are rotatably arranged at guide carriages  27 ,  28  by way of ball bearings  29 . The guide carriages  27 ,  28  are firmly connected with the door leaves  13 ,  14  by way of screw bolts  30 . 
     Door drives of the kind briefly described in the foregoing are known. Reference is therefore made to, for example, European reference EP-A 0 332 841 and EP-B 0 513 509 for further not-illustrated details such as, for example, the movement transmission from the toothed belt  22  to the guide carriages  27  or  28  or the entraining equipment for the cage door and the shaft door. 
     According to FIGS. 3 and 4, the base part  11 , which consists of, for example, sheet steel, of the carrier  10  has a substantially U-shaped cross-section. In order to better protect the belt drive unit, the one U limb  11 . 1  of the base part  11  is longer than the other U limb  11 . 2 . A prolongation  11 . 3 , which is bent over at right angles and rests on the front side  3  of the lift cage  1  (FIG.  2 ), is provided at the other U limb  11 . 2 . Two rows of first bores  35  and two rows of second bores  36  are provided in the base part  11 . Moreover, the base part  11  similarly has elongate holes  37 , square passages  38  and rectangular passages  39  arranged in rows. A shaped-out portion  11 . 5  of trapezium-shaped cross-section is provided in the region of a lower longitudinal edge  11 . 4  of the prolongation  11 . 3 . The trapezium-shaped shaped-out portion  11 . 5  has a row of threaded holes  40 , which are, for example, reinforced by means of drawn-through elements and are intended for the fastening of the guide rails  23  and  24 . The square passages  38  are provided for the axle of the electric motor  15  and a belt pulley arranged on the axle. The rectangular passages  39  serve as cutouts for an infrared incremental transmitter of the door drive control. The elongate holes  37  are intended for the fastening of the motor bracket  16  and a carrier plate for the door drive electronic unit. The bores  35  and  36 , elongate holes  37 , passages  38  and  39  and threaded holes  40  preferably have a predetermined constant spacing from one another. The most diverse variants of the arrangement of the most important components are possible by the multiplicity of the different holes or passages, so that an optimum adaptation to construction conditions can be achieved. 
     The length adapter made of, for example, sheet steel has, according to FIGS. 5 and 6, a U-shaped cross-section which is smaller than the U-shaped cross-section of the base part  11 . In order to better protect the belt drive unit, the one U limb  12 . 1  of the length adapter  12  is longer than the other U limb  12 . 2 , wherein the U limbs  12 . 1  and  12 . 2  of the length adapter  12  are approximately equal in length to the U limbs  11 . 1  and  11 . 2  of the base part  11 . Two rows of further bores  41 , which are congruent with the first bores  35  of the base part  11 , are provided in the length adapter  12 . The length adapter  12  is used with greater door widths, wherein it can be adjusted in steps corresponding to the predetermined hole spacings and can be screw-connected with the base part  11 . Rectangular passages, which are necessary for the mounting of the above-mentioned belt tensioning device, are designated by  42 . The invention is not limited by the embodiments described above which are presented as examples only but can be modified in various ways within the scope of protection defined by the appended patent claims.

Summary:
A carrier for the reception of all important components of a door drive. The carrier has a base part and a length adapter, which are connectible together in such a manner that carriers of different lengths can be produced so that the carrier can be used in different door systems with different entrance widths.