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: 
TECHNICAL FIELD 
     The disclosure generally relates to the field of mechanical locking systems for floor panels and building panels and production methods to insert a tongue into a groove. 
     FIELD OF APPLICATION 
     Embodiments of the present disclosure are particularly suitable for use in floating floors, which are formed of floor panels which are joined mechanically with a locking system integrated with the floor panel, i.e. mounted at the factory, are made up of one or more upper layers of wood or wood veneer, decorative laminate, powder based surfaces or decorative plastic material, an intermediate core of wood-fibre-based material or plastic material and preferably a lower balancing layer on the rear side of the core. Floor panels with a surface layer of cork, linoleum, rubber or soft wear layers, for instance needle felt glued to a board, printed and preferably also varnished surface and floors with hard surfaces such as stone, tile and similar materials are included. Embodiments of the disclosure may also be used for joining building panels which preferably contain a board material for instance wall panels, ceilings, furniture components and similar. 
     The following description of known technique, problems of known systems and objects and features of the disclosure will therefore, as a non-restrictive example, be aimed above all at this field of application and in particular at panels formed as rectangular floor panels with long and shorts edges intended to be mechanically joined to each other on both long and short edges. 
     The long and short edges are mainly used to simplify the description of embodiments of the disclosure. The panels may be square. Embodiments of the disclosure are preferably used on the short edges. It should be emphasised that embodiments of the disclosure may be used in any floor panel and it may be combined with all types of known locking system formed on the long edges, where the floor panels are intended to be joined using a mechanical locking system connecting the panels in the horizontal and vertical directions on at least two adjacent sides. 
     BACKGROUND 
     Laminate flooring usually comprise a core of a 6-12 mm fibre board, a 0.2-0.8 mm thick upper decorative surface layer of laminate and a 0.1-0.6 mm thick lower balancing layer of laminate, plastic, paper or like material. A laminate surface comprises melamine-impregnated paper. The most common core material is fibreboard with high density and good stability usually called HDF—High Density Fibreboard. Sometimes also MDF—Medium Density Fibreboard—is used as core. 
     Laminate floorings are joined mechanically by means of so-called mechanical locking systems. These systems comprise locking means, which lock the panels horizontally and vertically. The mechanical locking systems are usually formed by machining of the core of the panel. Alternatively, parts of the locking system may be formed of a separate material, for instance aluminium or HDF, which is integrated with the floor panel, i.e. joined with the floor panel in connection with the manufacture thereof. 
     The main advantages of floating floors with mechanical locking systems are that they are easy to install. They may also easily be taken up again and used once more at a different location. 
     DEFINITION OF SOME TERMS 
     In the following text, the visible surface of the installed floor panel is called “front side”, while the opposite side of the floor panel, facing the sub floor, is called “rear side”. The edge between the front and rear side is called “joint edge”. By “horizontal plane” is meant a plane, which extends parallel to the outer part of the surface layer. Immediately juxtaposed upper parts of two adjacent joint edges of two joined floor panels together define a “vertical plane” perpendicular to the horizontal plane. By “vertical locking” is meant locking parallel to the vertical plan. By “horizontal locking” is meant locking parallel to the horizontal plane. 
     By “up” is meant towards the front side, by “down” towards the rear side, by “inwardly” mainly horizontally towards an inner and centre part of the panel and by “outwardly” mainly horizontally away from the centre part of the panel. 
     By “locking systems” are meant co acting connecting elements, which connect the floor panels vertically and/or horizontally. 
     Related Art and Problems Thereof 
     For mechanical joining of long edges as well as short edges in the vertical and in the first horizontal direction perpendicular to the edges several methods may be used. One of the most used methods is the angle-snap method. The long edges are installed by angling. The panel is than displaced in locked position along the long side. The short edges are locked by horizontal snapping. The vertical connection is generally a tongue and a groove. During the horizontal displacement, a strip with a locking element is bent and when the edges are in contact, the strip springs back and a locking element enters a locking groove and locks the panels horizontally. Such a snap connection is complicated since a hammer and a tapping block may need to be used to overcome the friction between the long edges and to bend the strip during the snapping action. 
     Similar locking systems may also be produced with a rigid strip and they are connected with an angling-angling method where both short and long edges are angled into a locked position. 
     Recently new and very efficient locking systems have been introduced with a separate flexible or displaceable integrated tongue on the short edge that allows installation with only an angling action, generally referred to as “vertical folding”. Such a system is described in WO 2006/043893 (Välinge Innovation AB). 
     Several versions are used on the market. One of the most used versions is shown in  FIGS. 1   a - 1   d . A flexible tongue  30  is during locking displaced in a horizontally extending displacement groove  40  and into a tongue groove  20  of an adjacent panel. The displaceable tongue locks the edges vertically and a strip  6  with a locking element that cooperates with a locking groove  14  locks the panels horizontally. The locking is a combination of vertical displacement and turning similar to a scissor action. The tongue is gradually displaced inwardly during locking from one inner edge to an outer edge as shown in  FIG. 1   d  such that the tongue is bent in the length direction. Such systems are referred to as vertical snap systems and they provide an automatically locking during the folding action. 
     Although such systems are very efficient, there is still a room for improvements. 
     High locking force can only be accomplished with high snapping resistance when the tongue is pressed inwardly and bent in the length direction. This creates separation forces that tend to push the panels apart during folding. The locking may lose its strength if the flexibility and pressing force of the tongue decreases over time. 
     The flexibility must be considerable and allow that a flexible tongue is displaced in two directions about 1-2 mm. The material, which is used to produce such tongues, is rather expensive and glass fibres are generally used to reinforce the flexible tongue. 
     It would be a major advantage if snapping could be eliminated in a system that locks automatically during folding. 
     SUMMARY AND OBJECTS 
     An overall objective of embodiments of the present disclosure is to provide a locking system for primarily rectangular floor panels with long and short edges installed in parallel rows, which allows that the short edges may be locked to each other automatically without a snap action that creates a locking resistance and separation forces of the short edges during folding. 
     A specific objective is to provide a locking system with a separate displaceable tongue that may be bent in length direction with a lower separation force and that comprises means that prevent the tongue to slide back into the groove after locking 
     The above objects of embodiments of the disclosure may be achieved wholly or partly by locking systems and floor panels according to the disclosure. Embodiments of the disclosure are evident from the description and drawings. 
     An aspect of the disclosure is building panels provided with a locking system for vertical locking of a first and a second building panel by a vertical displacement of the panels relative each other. A displaceable tongue is attached into a sidewardly open displacement groove provided at an edge of the first panel. Said tongue cooperates with a tongue groove provided at an adjacent edge of the second panel for locking the edges vertically. A strip protrudes below the displacement groove and outwardly beyond the upper part of the edge or below the tongue groove and outwardly beyond the upper part of the adjacent edge. The displaceable tongue comprises a pulling extension at its outer part configured to cooperate with a pulling protrusion formed at an edge of the adjacent panel such that the displaceable tongue is pulled out from the displacement groove and into the tongue groove when the edges of the panels are displaced vertically against each other. 
     Said pulling protrusion may be part of the tongue groove. 
     The pulling extension may be inclined in relation to a main horizontal plane of the panels. 
     The pulling protrusion may be inclined in relation to a main horizontal plane of the panels. 
     The displaceable tongue may be provided with a locking hook that prevents the tongue to slide back into the displacement groove after locking. 
     The locking hook may lock against an outer part of the displacement groove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will in the following be described in connection to exemplary embodiments and in greater detail with reference to the appended exemplary drawings, wherein: 
         FIGS. 1   a - d  illustrate locking systems according to known technology. 
         FIGS. 2   a - e  illustrate a short edge locking system according to the disclosure. 
         FIGS. 3   a - 3   c  illustrate a short edge locking system according to preferred embodiments of the disclosure. 
         FIGS. 4   a - c  illustrate preferred embodiments of short edge locking systems with a separate strip. 
         FIGS. 5   a - e  illustrate a locking system according to an embodiment of the disclosure with a locking hook that prevents unlocking. 
         FIGS. 6   a - d  illustrate a tongue according to an embodiment of the disclosure with increased flexibility related to bending in length direction. 
         FIGS. 7   a - d  illustrate a method according to an embodiment of the disclosure to insert a tongue into a groove. 
         FIGS. 8   a - c  illustrate an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     To facilitate understanding, several locking systems in the figures are shown schematically. It should be emphasised that improved or different functions may be achieved using combinations of the embodiments. 
     All embodiments may be used separately or in combinations. Angles, dimensions, rounded parts, spaces between surfaces etc. are only examples that may be adjusted within the basic principles of the disclosure. 
       FIGS. 2   a - 2   e  show a first preferred embodiment of a short edge locking system provided with a flexible and displaceable tongue  30  in an edge of a first panel  1  inserted in a horizontally extending displacement groove  40 . The displaceable tongue  30  has a pulling extension  31  comprising a tongue pulling surface  32  and tongue locking surface  33 . The second adjacent panel  1 ′ has a pulling protrusion  21  with a groove pulling surface  22  that is also a part of a tongue groove  20  comprising a groove locking surface  23 . The pulling surfaces  22 ,  32  cooperate during the vertical displacement and pull the displaceable tongue  30  into a tongue groove  20 . The pulling extension comprises a tongue locking surface  33  that locks against a groove locking surface  23  and prevents vertical displacement of the edges in a first vertical direction. A locking strip  6  and a lower part  39  of the adjacent panel locks the edges in a second vertical direction. A locking element  8  and a locking groove  14  locks the edges horizontally together with the upper edges. The vertical connection may be used without the horizontal locking as shown by  FIG. 2   e . Short edges may be locked horizontally by, for example, friction between long edges. 
     The tongue may be attached into a displacement groove  40  formed on the panel comprising the strip  6 , the strip panel, or on the panel comprising the locking groove, the groove panel, as shown in  FIGS. 3   a - 3   c . The pulling protrusion  21  may extend upwardly or downwardly and the displacement groove may be inclined against the horizontal plane HP. 
       FIGS. 4   a - 4   c  show that the strip  6  may be formed as a separate material. The pulling protrusion  21  may be flexible and this may eliminate production tolerances and facilitate the displacement of the tongue  30  into the tongue groove  20  during folding. 
       FIGS. 5   a - 5   e  show that the displaceable tongue  30  may comprise a locking hook  34  that may serve as a friction connection to prevent the tongue  30  from falling out from the groove  40  but also to prevent the tongue from sliding back after locking. The locking angle A 1  is preferably about 45 degrees or higher. A higher angle facilitates displacement into the tongue groove  20  but also backward displacement. This may be prevented by a hook connection  34  that preferably locks against an upper or lower part of the displacement groove  40 . The hook connection is pressed into the groove by a hammer that inserts the tongue  30  into the groove  40  during production. The hook  34  slides against a bevel formed at the displacement groove  40  as shown in  FIG. 5   c . The upper part of the locking element is preferably located vertically below the tongue locking surface  33  as shown in  FIG. 5   d . This gives a stronger locking. The locking system may have a geometry that allows locking and unlocking with angling. 
       FIGS. 6   a - 6   c  show that the displaceable tongue  30  turns and bends in the length direction during folding when an inner short edge of the tongue, as shown in  FIG. 6   b  is in locked position and an outer short edge of the tongue  30  is in unlocked position as shown in  FIG. 6   d . The locking function may be improved if cavities  35  are formed on the displaceable tongue  30 . Locking may also be improved if the locking surface  32  at an edge has a lower angle than at an inner part as shown in  FIGS. 6   b  and  6   c . The cavities  35  may be formed at tongue section where the locking hooks  34  are formed. The displaceable tongue  30  comprises preferably a polymer material and is preferably formed by injection moulding. 
       FIGS. 7   a - 7   d  show that the locking hook  34  may comprise a hook part  34   a  that is used to press the hook connection upwards by inserting rails  36  during the insertion of the tongue  30  into the displacement groove  40 . 
       FIGS. 8   a - 8   c  show that the locking hook  34  may be used to prevent unlocking in any locking system where a tongue is displace in a groove from an inner position to an outer position. The shown locking system comprises pushing protrusions  38  located in pushing cavities  37 . The pushing protrusions slide against the locking element  8  and push the tongue  30  into a tongue groove  20 . The locking element is preferably located vertically below the cooperating locking surfaces  23 , 33  of the tongue  30  and the tongue groove  20 .

Summary:
Floor panels are shown, which are provided with a vertical locking systems on short edges including a displaceable tongue that is displaced in one direction into a tongue groove during vertical displacement of two panels.