Abstract:
A level comprises a frame defining first and second parallel channels and first and second rails fitted slidingly in the first and second channels respectively. The level is adjustable in length by sliding the rails in the respective channels of the frame.

Description:
This invention relates to a level. 
     The conventional carpenter&#39;s level, which is used for testing whether a nominally horizontal (or vertical) surface is in fact horizontal (or vertical), within an acceptable tolerance, comprises a frame, typically made of a metal such as aluminum alloy, having at least one flat guide surface and two bubble vials mounted in the frame. The level is positioned with the guide surface against the surface to be tested. One vial is oriented relative to the guide surface for testing a nominally vertical surface and the other is oriented for testing a nominally horizontal surface. The typical level is about 2 feet long, although longer levels, e.g. four feet or six feet long, are also available. 
     The fact that conventional levels are available only in increments of about 2 feet in length gives rise to difficulty or inconvenience in using the conventional level. Imagine, for example, that you are installing a shelf inside a closet having a back wall about 3′6″ wide between two side walls. You wish to attach a support to the back wall for supporting the rear edge of the shelf. The support should be horizontal, and you wish to mark the position of the support at two locations, adjacent the side walls respectively. Obviously, since the back wall is only 3′6″ wide, you cannot use a four foot level because there is not sufficient space between the two side walls. If you attempt to use a 2 foot level, you must place the markings in two steps, which is inconvenient and can cause errors. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a level comprising a frame defining first and second parallel channels, and first and second rails fitted slidingly in the first and second channels respectively, whereby the level is adjustable in length by sliding the rails in the respective channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which 
     FIG. 1 is a perspective view of a level in accordance with the invention in a retracted condition, 
     FIG. 2 is a similar view of the level in a partially extended condition, 
     FIG. 3 is a perspective exploded view of the level. 
    
    
     DETAILED DESCRIPTION 
     The illustrated level comprises a frame  2  and two rails  4 A and  4 B. In the following description, the suffix A or B is used when it is necessary or helpful to distinguish between the two rails or elements that are associated with the two rails. Otherwise, no suffix is used. 
     The frame  2  is made from a length segment of an aluminum alloy extrusion and is generally I-shaped in cross-section, having two spaced parallel guide members  8  connected by a web plate  12  so that two channels  20  are defined on opposite sides respectively of the web plate  12 . Each member  8  has at each edge a lip or flange  16  projecting toward the other member  8 . The frame has two parallel guide surfaces  24 , only one of which can be seen in the drawings. 
     Each rail  4  includes a slide  28  and a frame extender  32  which is attached to the slide. 
     The frame extender  32  is made of a length segment of the same extrusion as is used to make the frame  2  and consequently it includes two parallel guide members  36  and a web plate  40 . The guide members  36  and the web plate  40  define two channels  42 ,  44  on opposite sides respectively of the web plate  40 . Each guide member  36  has at each edge a lip or flange  52  projecting toward the other guide member  36 . The frame extender has two parallel guide surfaces. 
     Each slide  28  includes a web plate  46  and two flanges  48  which extend perpendicular to the web plate  46 . The slide has a rebate  60  at the base of each flange. The cross-sectional configuration of the slide is selected relative to that of the channels of the frame extender so that the slide  28  can be inserted in the channel  42  in only one orientation, and in this orientation the web plate  46  of the slide is held away from the web plate of the frame extender by the flanges  48 . The slide  28  is retained in position relative to the frame extender by pins (not shown). The slide does not obstruct the channel  44 . Each rail also includes an end cap  50 , which is attached to the frame extender and the slide. 
     The cross-sectional configuration of the slide  28  also allows the slide to be inserted in the channel  20  of the frame with the web plate  46  of the slide held away from the web plate  12  of the frame  2  by the flanges  48 . The slide constrains the frame extender into alignment with the frame  2 , with the two guide surfaces of the frame extender coplanar with the guide surfaces  24  of the frame. The slide  28  is slidable in the channel  20 , allowing adjustment in the length of the level between a retracted condition, in which the frame extenders abut the frame  2 , as shown in FIG. 1, through a partially extended condition, shown in FIG. 2, to a fully extended condition in which only a relatively short segment of the length of the slide is accommodated in the channel  20 . 
     The length of each slide  28  is approximately equal to the sum of the length of the frame  2  and the lengths of the two frame extenders  32 . The slide of the rail  4 A then extends through the channel  20 A of the frame  2  and into the channel  44  of the rail  4 B when the level is in the retracted condition. This allows the maximum difference in length between the retracted condition and the fully extended condition. 
     The frame also includes, in each channel  20 , a pair of biasing means such as leaf springs  52  (only the leaf springs  52 A are shown). The leaf springs  52 A are attached to the lower guide member  8 A and urge the slide  28 A toward the upper guide member  8 B. Conversely, the leaf springs in the channel  20 B are attached to the upper guide member  8 B and urge the slide  28 B toward the lower guide member  8 A. The leaf springs take up clearance between the slides  28  and the frame  2 . As shown in FIG. 3, leaf springs  52  preferably are formed of a single strip and include two wings  74 ,  76  extending from a central mount  72  in guide member  8 A. Central mount  72  may include two rivets  73 ,  75  for affixing springs  52  in place. Slides  28  include a sliding surface  49 L that abuts and slides against leaf spring  52  as the level is moved between the retracted and extended conditions. Slides  28  include a sliding surface  49 U that directly contacts and slides against a sliding surface  21 U in guide member  8 B. Springs  52  urge sliding surfaces  49 U of slides  28  and sliding surfaces  21 U of guide member  8 B together. Springs  52  increase the friction between the sliding surfaces which frictionally holds the level in any relative position between and including the retracted and extended conditions. 
     The web plate  12  is formed with an aperture in which two indicators such as bubble vials  56  are mounted. One of the vials is oriented for testing a vertical surface and the other is oriented for testing a horizontal surface. The bubble vials  56  project from the web plate  12  into the channels  20  of the frame  2 . Since the web plate  46  of the slide  28  is held away from the web plate  12  of the frame  2  by the flanges  48 , the web plate  46  remains clear of the bubble vials  56  when the slide is inserted in the channel  20 . Similarly, the web plates  40  are each formed with an aperture in which two bubble vials  58  are mounted. The bubble vials  58  project from the web plate  40  into the channels  42 ,  44  of the frame extender. Since the web plate  46  of the slide  28  is held away from the web plate  40  of the frame extender by the flanges  48 , the web plate  46  of the slide  28  remains clear of the bubble vials  58 . Moreover, when the level is compressed from its extended condition to its retracted condition, and the slide  28  of one rail enters the channel  44  of the other rail, the web plate  46  remains clear of the bubble vials  58  of the other rail. The bubble vials  58  are thereby protected from damage by impact with the slide. 
     The web plates  12  and  46  are formed with openings which serve to reduce the weight of the level and facilitate gripping and handling of the level. The openings also provide viewing access to the bubble vials, so that regardless of the degree of extension of the level, the user can see at least one vial oriented for testing a vertical surface and one vial oriented for testing a horizontal surface. 
     The manner of use of the level, for testing a nominally vertical or nominally horizontal surface, or for marking segments of a horizontal or vertical line on a vertical surface, will be well understood by a person skilled in the art. 
     In use of the level illustrated in the drawings, for example in a confined space, the user extends the rails from the frame sufficiently to span the desired distance without interference with other structures, and the user can then determine whether a nominally horizontal (or vertical) surface is in fact horizontal (or vertical) or can mark a horizontal (or vertical) line on a vertical surface between two structures that would interfere with a longer level. Further, it is possible to manipulate the level into a confined space in its retracted condition and then extend it for use, e.g. in marking a vertical or horizontal line on a wall. 
     It will be appreciated by those skilled in the art that under some conditions aluminum does not slide smoothly against aluminum but sticks due to galling. Various treatments are available for relieving galling. 
     It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof.