Abstract:
A universal joint includes a first joint fork having a joint axis and a second joint fork having a joint axis, the joint axes being borne by plain bearings and the joint forks each being formed by a base part and two fork elements. Each fork element has a closable bore, and the joint axes intersect between the bores, whereby a bearing prestress unit allows the joint fork to function without play about the joint axis thereof so that the universal joint can be positioned precisely.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a universal joint. 
         [0002]    As a rule, such universal joints have a roller or plain bearing. However, it is disadvantageous that production-imposed tolerances can lead to distortions or that there can be too much play, which has a negative effect on the operational reliability of the bearing and on the accuracy of the universal joint and the positioning thereof. 
         [0003]    Known from DE 44 39 998 A1 is a universal joint that, for connecting two joint forks, has a cross link assembly made of four journals, two journals being located on a common axis. Each axis is borne by means of a roller bearing on a bearing bush and has a spring element made of a high-strength plastic. A tolerance imposed by the assembly is supposed to be compensated using the spring element. However, the spring element also enables the journal in the bearing bush to yield so that the universal joint can no longer be positioned accurately. 
       SUMMARY OF THE INVENTION 
       [0004]    In contrast, the inventive universal joint has the advantage that a bearing prestress unit allows the joint forks to function without play about the joint axis thereof, thus making it possible to accurately position the universal joint. In addition, it is possible to reduce the stip slick effect by using newly developed materials. 
         [0005]    The inventive universal joint can in particular be used with robots that are equipped with rod kinematics and that have a platform (Stuart platform) and that move a tool or other article that is arranged on the platform by means of an article holder in three dimensions corresponding to a prespecified program. Such automated units are used not only in inaccessible or dangerous spaces, but primarily also to replace manual human actions. The use of a plain bearing is advantageous because roller bearings that are used in general for such types of technologies have much lower bearing capacity compared to plain bearings, which provide a large bearing surface. 
         [0006]    In accordance with one advantageous embodiment of the invention, the bearing prestress unit is formed at least by one spring element that acts on the joint axis. 
         [0007]    In accordance with one advantageous embodiment of the invention in this regard, at least one of the bores is closed by a pressure plate that acts on the spring element. The inventive universal joint can be adjusted using the pressure plate. 
         [0008]    In accordance with one advantageous embodiment of the invention in this regard, a spacer ring is arranged between the pressure plate and the base part. The inventive universal joint can be adjusted using this spacer ring. 
         [0009]    In accordance with one additional advantageous embodiment of the invention, the spring element is a spring collar. 
         [0010]    In accordance with one additional advantageous embodiment of the invention, a positioning block is arranged between the bores for positioning the joint axes. 
         [0011]    In accordance with one advantageous embodiment of the invention in this regard, the joint axes are borne by a plain bearing in the positioning block. 
         [0012]    In accordance with one additional advantageous embodiment of the invention, the joint axes are formed by one pin and two sleeves arranged perpendicular thereto. 
         [0013]    In accordance with one advantageous embodiment of the invention in this regard, the joint axes are affixed to one another. 
         [0014]    In accordance with one advantageous embodiment of the invention in this regard, this affixing is accomplished using a screw that can be inserted through the sleeves and a bore in the pin. This results in a cross pin. 
         [0015]    In accordance with one additional advantageous embodiment of the invention, the base part has at least one fastening means on the side facing away from the fork elements. 
         [0016]    Additional advantages and advantageous embodiments of the invention can be found in the following description, drawings, and claims. 
         [0017]    Exemplary embodiments of the invention are depicted in the drawings and are described in greater detail in the following. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]      FIG. 1  is an exploded depiction of an inventive universal joint; 
           [0019]      FIG. 2  is a top view of an inventive universal joint; 
           [0020]      FIG. 3  is a side view of an inventive universal joint; 
           [0021]      FIG. 4  is a side view of an inventive universal joint; 
           [0022]      FIG. 5  is a view of an inventive universal joint in accordance with the section A-A from  FIG. 3 ; 
           [0023]      FIG. 6  is a view of an inventive universal joint in accordance with the section B-B from  FIG. 3 ; 
           [0024]      FIG. 7  is a detailed perspective view of a cross pin and a view of the cross pin in accordance with the section A-A from  FIG. 3 ; 
           [0025]      FIG. 8  is a detailed perspective view of a cross pin and a view of the cross pin in accordance with the section A-A from  FIG. 3 , in a modified form; 
           [0026]      FIG. 9  is a detailed perspective view of a cross pin and a view of the cross pin in accordance with the section A-A from  FIG. 3 , in another modified form; 
           [0027]      FIG. 10  is a perspective view of a positioning block and sectional drawings of the positioning block; 
           [0028]      FIG. 11  is a perspective view and a view of a joint fork with installed positioning block; 
           [0029]      FIG. 12  is a view of an inventive universal joint in accordance with the section B-B from  FIG. 3  and a detailed view of a first variant of the installed placement of the positioning block; 
           [0030]      FIG. 13  is a view of an inventive universal joint in accordance with the section B-B from  FIG. 3  and a detailed view of a second variant of the installed placement of the positioning block; 
           [0031]      FIG. 14  is a view of an inventive universal joint in accordance with the section B-B from  FIG. 3  and a detailed view of a third variant of the installed placement of the positioning block; 
           [0032]      FIG. 15  is a view of an inventive universal joint in accordance with the section B-B from  FIG. 3  and a detailed view of a radial bearing of the pin in the joint eyes; 
           [0033]      FIG. 16  is a variant of the embodiment of the bearing ring (spring collar  15 ) in sectional drawings; and, 
           [0034]      FIG. 17  is a view of an inventive universal joint  1  in accordance with the section B-B from  FIG. 3  and a detailed view of a modified radial bearing. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]      FIG. 1  is an exploded depiction of an inventive universal joint  1 . It comprises a first joint fork  2 , which constitutes a base part  3  and two fork elements  4 , and a second joint fork  5 , which constitutes a base part  6  and two fork elements  7 . The base part  3  has a threaded insert  8  and acts for instance as a connecting part for a Stuart plate (not shown). It can have a hard-coated surface plain bearing and ground interior sides and is a bearing element for the entire bearing. The base part  6  has a flange  9  and acts for instance as an interface to different materials e.g. a carbon tube (not shown). If there is wear or if the inventive universal joint  1  is destroyed, it is possible to exchange it easily using the base parts  3  and  6 . The fork elements  4  and  7  each have bores  10  in their joint axes. The joint axis of the first joint fork  2  is formed by a pin  11  that is borne in the bores  10  of the first joint fork  2  by bearing rings  12  (plain bearings). These bearing rings  12  have an exterior geometry that is ground conically on two sides (exterior ring). The interior diameter is reduced or prestressed by a cone effect. 
         [0036]    The bores  10  are each closed by a pressure plate  13  and are connected to the fork element  4  by means of screws  14  (cylinder screws, torx, hex socket screws, or the like), the pressure plates  13  exerting pressure on the pins  11  via the spring collars  15 . The pressure plates  13  are intended to transmit the bearing prestress to the bearing rings  12 . At the same time, the pressure plates  13  contain an anti-rotation element for the plain bearing. With the pressure plate  13 , the exterior ring of the spring collar  15  forms the bearing prestress unit and the bearing housing. 
         [0037]    Arranged perpendicular to the pin  11  are two sleeves  16  that are screwed to the pin  11  by means of a screw  17 , for instance a fitted screw machined on its head, and that form the joint axis of the second joint fork  5 . The joint axis of the second joint fork  5  is also borne by bearing rings  12  (plain bearings). The bores  10  are each closed by a pressure plate  13  and are connected to the fork unit  7  by means of screws (cylinder screws, torx, hex socket screws, or the like), the pressure plates  13  exerting pressure on the sleeves  16  via spring collars  15 . Spacer rings  18  can be added or removed between the pressure plates  13  and the fork elements  4  and  7  for additional bearing prestress. 
         [0038]    Screwing the sleeves  16  to the pin  11  forms a cross pin that has a very compact construction. The cross pin is easy to assemble and can also be disassembled without being destroyed. 
         [0039]    Arranged centrally between the fork elements  4  and  7  at the point of intersection for the joint axes is a positioning block  19  that is produced from the same material as the bearing rings  12 . The positioning block  19  forms the point of intersection for the universal joint axes with highly precise production and transmits the forces in the radial and axial directions and positions the axes relative to one another. 
         [0040]      FIG. 2  depicts a top view of an assembled inventive universal joint  1  on the base part  6  with the threaded insert  8 . 
         [0041]      FIG. 3  and  FIG. 4  each depict a side view of an inventive universal joint  1 . The pressure plates  13  are affixed to the fork elements  4  and  7  by means of screws  14 . 
         [0042]      FIG. 5  depicts a view of an inventive universal joint  1  in accordance with section A-A from  FIG. 3  and  FIG. 6  depicts a view of an inventive universal joint  1  in accordance with the section B-B from  FIG. 3 . The sleeves  16  are secured to the pin  11  using the screw  17 . The positioning block  19  is arranged at the point of intersection for the two joint axes. 
         [0043]      FIG. 7  is a detailed perspective view of a cross pin  20  and a view of the cross pin  20  in accordance with the section A-A from  FIG. 3 . The cross pin  20  is the core piece of the inventive universal joint  1 . All forces are transmitted in it. Ideally the axes of the inventive universal joint  1  intersect one another precisely in the center. However, in practice this is not possible due to production tolerances. Therefore, normally the axis cross in a universal joint is produced from a solid body using the manufacturing methods that are consequently necessary and also cost intensive. Since the precise overlap of the axes in the center is not determined by the geometries of the individual parts of the cross pin  20 , the individual parts can be produced with relatively rough tolerances. The cross pin  20  used in the inventive universal joint  1  is preferably cylindrical on all sides on all and is distinguished in particular by the following improvements:
       1. Construction in parts, therefore significantly higher flexibility in terms of assembly;   2. Individual parts with surfaces that are geometrically simple to produce;   3. Great variety in terms of materials;   4. Great variety in terms of coatings;   5. Great positioning accuracy for individual axes not required; and   6. Wear parts can be exchanged individually.         
         [0050]    Coating of the plain bearing and roller bearing surfaces is particularly important, especially for the individual parts of the universal joint pin  20  [possibly typographical error for Kreuzbolzen=cross pin] described herein. Appropriate oxide coating, nitride coating, or the like is existential [sic] for the wear resistance of the bearing unit. All combinations of PVD/CVD coatings or resin methods, as well as TIN, TIALN, CR/C, TICN, hard chrome, etc. are advantageous for the coatings. 
         [0051]    High quality tool steel has proved to be an ideal material for producing the individual cross pin parts. The annealing temperature for these steels is greater than the temperature used during PVD coating. Thus the substrate properties are not affected or are affected only to a negligible degree. However, the use of totally ceramic materials, plastics, or other materials would also be conceivable. 
         [0052]    The cross pin  20  depicted in  FIG. 7  is produced as a through cylindrical body. A transverse bore  21 , required and appropriately produced for the screw  17  (fitted axis screw), is located in the center. The outlet areas for the transverse bore  21  have corresponding recessed surfaces  22  for the contact surface of the sleeves  16  (joint sleeves). The sleeves  16  are screwed on by means of the screw  17  (standard part). In the guide or overlap areas of the fitted pin, the sleeves  16  are produced with correspondingly narrow interior tolerances. Thus a lower bending torque is transmitted to the threading of the fitted axis pin. 
         [0053]      FIG. 8  is a detailed perspective view of a cross pin  20  and a view of the cross pin  20  in accordance with the section A-A from  FIG. 3 , in a modified form. It shows the structure of the cross pin  20 , which can be taken apart, with offset pin  11  or sleeve. In this solution the bearing diameter can be varied without greatly affecting the basic stiffness of the cross pin  20 . Moreover, the cross pin  20  can also be screwed together without a fitted axis screw. A thread  23  is added to the sleeve  16 . The bolt  24  is screwed into this thread  23  through the pin  11 . The advantage of this arrangement is that the pin connection is forced geometrically to the axis center and thus to the desired point of intersection. 
         [0054]      FIG. 9  is a detailed perspective view of a cross pin  20  and a view of the cross pin  20  in accordance with the section A-A from  FIG. 3 , in another modified form. The contact surfaces (recessed surfaces  22 ) are conical. Both of the parts to be joined are thus forced into the position described in the foregoing. 
         [0055]      FIG. 10  is a perspective view of a positioning block  19  and sectional drawings of the positioning block  19 . The positioning block  19 , the material of which is preferably AL (surface hard-coated, anodized, or the like), plastic, ceramic, steel, or the like, represents the critical component between the two joint forks  2  and  5  and the cross pin  20  ( FIG. 11 ). By means of its very precise cross bore  30 , the positioning block  19  brings the individual parts of the cross pin  20  into the precisely fitting position. Moreover, its lateral positioning surfaces determine the spacing of the two joint forks  2  and  5  to the axis intersection. The lateral running surfaces (slide surfaces  25 ) can be finished directly together or can be designed in the form of a plain bearing bush, spring retainer, or axial roller bearing. The systems described in the following can be combined as needed. 
         [0056]      FIG. 12  is a view of an inventive universal joint  1  in accordance with the section B-B from  FIG. 3  and a detailed view of a first variant of the installed position of the positioning block  19 . The positioning block  19  preferably comprises plastics with a high teflon/graphite content. The contact surfaces on the joint forks are hard-coated so that the forks are free of wear (surface coating especially for high-strength AL alloys). The lateral slide surfaces are preferably made from a solid piece. The advantage of this variant is comprised in simple assembly, large surface-area support and associated lower surface pressure on joint forks  2  and  5 , and exact positioning of the joint forks  2  and  5  and of the cross pin  19 . 
         [0057]      FIG. 13  is a view of an inventive universal joint  1  in accordance with the section B-B from  FIG. 3  and a detailed view of a second variant of the installed placement of the positioning block  19 . In this, the opportunity is used to design the lateral positioning surfaces in the positioning block  19  as spring retainers  26  to the joint forks  2  and  5 . The positioning block  19  is preferably made of an AL alloy with appropriate surface coating. The spring retainer  26 , assembled here, preferably comprises a plastic compound containing teflon. However, it would be possible to use other materials if the slide elements were designed accordingly. The advantage of this variant is comprised in a balance of axial play and production tolerances, axial vibration damping, lower surface pressure by the plain bearing, in a cost-effective wear part, in an equal thermal expansion coefficient for the joint fork/positioning block combination, and in simple assembly. 
         [0058]      FIG. 14  is a view of an inventive universal joint  1  in accordance with the section B-B from  FIG. 3  and a detailed view of a third variant of the installed placement of the positioning block  19 . In this case, the bores in the positioning block  19  are designed such that an additional plain bearing bush  27  made of plastic, metal, coating materials, sintered materials, or other coated materials can be pressed in. The bush (plain bearing bush  27 ) transmits the movement or the forces from the forks  2  and  5  to the positioning block  19  or to the cross pin  20 . Thus the bearing bush (plain bearing bush  27 ) acts both as axial bearing and as radial bearing. The advantage of this variant is comprised in a balance of axial play or production tolerances, axial vibration damping, lower surface pressure by the plain bearing, in a cost-effective wear part, in an equal thermal expansion coefficient for the joint fork/positioning block combination, and in simple assembly. 
         [0059]      FIG. 15  is a view of an inventive universal joint  1  in accordance with the section B-B from  FIG. 3  and a detailed view of a radial bearing of the pins in the joint eyes. This is a plain bearing that has no play, is maintenance-free, and is also somewhat self-adjusting. The combination with conventional roller bearings, in particular small needle bearings, is further possible in this case. 
         [0060]    The pressure plate  13 , spacer ring (leveling element)  18 , conical ring  12 , and bearing ring (spring collar  15 ) represent core elements for the radial bearing. 
         [0061]    Screwing the pressure plates  13  to the joint forks  2  and  5  changes the axial spacing of the entire bearing unit. Using the spacer rings  18  it is possible to adjust the bearing prestress and to compensate the wear that occurs with adjustments. The bearing ring (spring collar  15 ) is clamped between the pressure plate  13  and the conical ring  12  situated in the joint forks  2  and  5 . Using the surfaces positioned conically against the components, the interior pressure gauge diameter of the bearing ring is tapered until play in the bearing is no longer possible. 
         [0062]    This structure attains the following improvements:
       1. Freedom from play;   2. No pressure on surfaces from plain bearing;   3. Very high forces can be transmitted statically;   4. High forces can be transmitted dynamically;   5. Wear compensation possible;   6. Low friction due to optimized material pairings;   7. Vibration damping;   8. High positioning accuracy over extended period;   9. Low mass due to light structural materials; and   10. Cost-effective wear parts.       
 
         [0073]      FIG. 16  is sectional depictions of an embodiment variant of the bearing ring (spring collar  15 ). Since the adjustment when the bearing ring is closed is relatively minor and functions reliably only with relatively elastic materials, it is possible to embody the bearing ring with a slit. The slit substantially increases adjustability since it permits use as a self-adjusting bearing element. Preferably materials such as plastics containing teflon/graphite, ceramic, steel, or coated materials are used. 
         [0074]      FIG. 17  is a view of an inventive universal joint  1  in accordance with the section B-B from  FIG. 3  and a detailed view of a modified radial bearing that is self-adjusting. In order to provide the freedom from play of the inventive universal joint  1  over a prolonged period, the bearing unit is equipped with corresponding spring elements. In this arrangement the play or the wear that occurs is compensated by the interposed spring elements. The spring elements can be embodied as slitted steel spring retainers  29 . However, it is also moreover possible to use other spring elements, for instance coil springs, elastomers, or liquid or gas pressures. 
         [0075]    The different variants and detailed solutions of the inventive universal joint  1  provide play-free bearing for parallel kinematic applications. The inventive universal joint  1  thereby advantageously satisfied the following requirements:
       1. Freedom from play;   2. Freedom from maintenance;   3. Long service life under industrial use conditions;   4. Precise implementation of initiated movement;   5. Avoidance of the stip slick effect;   6. Minimum moved masses;   7. Low interior friction;   8. Balanced stiffness/damping behavior;   9. Cost factors; and   10. Simple assembly.       
 
         [0086]    All of the features depicted in the description, in the following claims, and in the drawing can be essential to the invention, both individually and in any desired combination. 
       LEGENDS 
       [0000]    
       
         Universal joint 
         First joint fork 
         Base part 
         Fork element 
         Second joint fork 
         Base part 
         Fork element 
         Threaded insert 
         Flange 
         Bore 
         Pin 
         Bearing ring 
         Pressure plate 
         Screw 
         Spring collar 
         Sleeve 
         Screw 
         Spacer ring 
         Positioning block 
         Cross pin 
         Transverse bore 
         Recessed surface 
         Thread 
         Bolt 
         Slide surface 
         Spring retainer 
         Plain bearing bush 
         Slit 
         Steel spring retainer 
         Cross bore