Abstract:
A rotary actuator with improved damping and stiffness is disclosed. The rotary actuator includes one or more bearing plates that form sliding-surface bearings to provide the desired preload to the rotary actuator. One embodiment of the invention includes a multi-piece bearing plate that makes repair or replacement of the bearing material in the sliding-surface bearings easy to perform. Another embodiment for applications where heat dissipation is critical includes thermal barriers on either side of the sliding-element bearings, with resilient members between the thermal barrier and a bearing ring used to supply the appropriate preload to the output shaft. The invention is particularly suited to precision applications, such as the drive unit for the swivel mechanism on the spindle head of certain types of five-axis milling machines.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to rotary actuators, and in particular to rotary actuators designed for precision applications such as machine tools.  
           [0002]    A rotary actuator may be defined as a mechanism for translating or controlling rotary motion. One important class of rotary actuators is speed reducers (sometimes also referred to as gear reducers). The function of a speed reducer is to receive rotational energy in the form of a particular torque and rotational speed at its input, and translate that rotational energy into a higher torque and lower rotational speed at its output. This is generally accomplished by means of a gearing mechanism within the speed reducer.  
           [0003]    Speed reducers are a common element in the drive system for high-torque, low-speed applications. In general, electric motors that provide a relatively low-torque, high-speed output are significantly less expensive than high-torque, low-speed motors. Many applications, however, require a motor with a high-torque output with a low rotational speed. Thus speed reducers are often coupled with a low-torque electric motor in the drive system for such applications as a less expensive alternative to a high-torque, low-speed motor.  
           [0004]    A typical speed reducer comprises an input shaft, an output shaft, some arrangement of gears whereby rotational energy is transferred from the input shaft to the output shaft, one or more bearings, and a drive housing. Most speed reducers use mechanical gearing mechanisms, but other arrangements are possible. Speed reducers may be found in many sizes and configurations, including elbow, offset, and straight-line varieties. Virtually all speed reducers, however, use low-friction internal bearings. These bearings are most often of the rolling-element variety, such as ball bearings or roller-type bearings. Other possible types of low-friction bearings might include hydrostatic bearings. Speed reducers that incorporate the output bearings within their housing are commonly known as bearing reducers.  
           [0005]    Rolling-element and hydrostatic bearings provide relatively low friction, and thus these bearings are ideal for applications where the minimization of energy loss is critical. Most low-friction bearing reducers do not, however, provide sufficient damping and stiffness for precision applications where the driven component is subject to large dynamic forces. Damping may be generally defined as the ability of a component to dissipate rather than transmit energy. Stiffness may be defined as the ability of a component to maintain its position under load. Low-friction bearings have poor damping and stiffness characteristics simply as a function of their design. The ball elements of ball bearings only contact the bearing surface at a point on the sphere of each element, and thus there is only a tiny contact area between the ball elements and the bearing surface. This small contact area results in poor damping qualities. The rollers of roller bearings have a greater surface contact area with the bearing surface, since each of the rollers in such bearings contact the bearing surface along a line down the side of each of the cylindrical or frustoconical roller elements. As a result, roller bearings have better damping than ball bearings. Sliding-element bearings have damping qualities that are superior to rolling-element bearings because the sliding element contacts the bearing surface along a plane. The size of this contact plane, which is determined by the area of the bearing surface and the sliding element, may be varied to control the amount of damping provided by the sliding-element bearing.  
           [0006]    As noted above, damping is particularly important for precision applications where the driven component is subject to large dynamic forces. Precision applications are those in which the accuracy and controllability of the speed reducer output characteristics must meet strict tolerances. One precision application where high accuracy and controllability is required, as well as superior stiffness and damping qualities, is milling machines. In particular, the drive system for the spindle head of a five-axis milling machine requires very high accuracy of motion and very high stability under dynamic loads. Unlike traditional three-axis machines, some five-axis machines include the capability for the spindle head to swivel. The swivel of the spindle head must be controlled with great accuracy in order to maintain the tolerance of parts produced using the milling machine. Any error in the positioning of the spindle head along its swivel path will be magnified at the tip of the machining tool as the spindle head moves through an arc. The degree to which this error is magnified is a function of the length of the attached tool.  
           [0007]    Due to inaccuracies in the manufacturing of the gear assemblies and clearances necessary for smooth, efficient operation, most rotary actuators have some rotational deflection in the gear assembly when a load is applied to the output shaft. This rotational deflection, or lack of torsional stiffness, is sometimes called soft wind-up, hysteresis, backlash, or lost motion. Typical sources of this problem in bearing reducers include the misalignment of components; manufacturing errors or poor tolerances; and friction between moving parts. Generally, there is a relatively large difference between the coefficients of static and dynamic friction for these devices.  
           [0008]    While there are a few high-precision bearing reducers available on the market today, they are generally designed in the same manner as general-use speed reducers, except that very tight tolerances are enforced on the manufacture and fitting of their critical components. The requirement of very tight tolerances significantly increases the cost of these devices, and thereby offsets the cost advantage of using a speed reducer with a low-torque electric motor for precision applications.  
           [0009]    Certain precision applications subject to large dynamic forces, such as the drive system for the swivel motion of a spindle head on a milling machine, require that a “preload” be applied to the drive system. A preload mechanism can be thought of as an energy absorption device that isolates the rest of the machine from large, temporary force spikes; this effect is commonly referred to as damping. Because these forces are only nominally predictable and are subject to rapid changes, failure to preload the drive system could cause the dynamic forces to exceed the performance specifications of the machine, and thereby damage the machine, damage the workpiece, or endanger the machine operator. Furthermore, the deflection and static load characteristics for many types of speed reducers are not linear, especially at or near the no-load condition. When such devices first come under load, they deflect more per unit load than they do after some minimum force is achieved. Preloading is also useful in “taking up” this softness around the no-load condition.  
           [0010]    Prior art bearing reducers as described above do not include a means for preloading, and therefore cannot be used without additional drive system elements in applications where dynamic force spikes are a concern. In fact, preloading would be detrimental to bearing reducers for many common applications, since preloading would increase the friction inherent in the device, and therefore would increase the energy losses in the drive system as well as increase the heat generated during use. For precision applications such as the swivel drive system for a spindle head on a milling machine, energy losses and heat generation are a less important concern than sufficient stiffness and damping. Heat generation in such applications will be limited by the relatively slow rotational speed of the spindle head, and the energy loss is less important than precisely controlling the position of the spindle head during milling.  
           [0011]    The prior art does include a number of mechanical means for achieving preloading of rotary actuators. For example, torsion springs or air or hydraulic cylinders could be used to resist unwanted deflections and to absorb energy. Such devices as these, however, do not add significant bearing load capacity to the drive and tend to be bulky or unwieldy in many applications. Also, it is more difficult to arrange this type of preload into a “normally on” configuration, which is important in many applications for safety reasons, such as the stated application of a swivel drive mechanism for the spindle head of a milling machine.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention comprises an inexpensive bearing reducer produced with relatively low tolerances that is nevertheless suited for high-precision applications. The invention enhances the stiffness and damping characteristics of the gear assembly and bearings of a rotary actuator by adding one or more sliding element bearings in a plane parallel to the plane of rotation of the rotary actuator. The sliding element bearing is sized to provide a torsionally-acting friction force equal to the desired torsional preload. This invention may also be utilized to supply any desired axial preload. Further, by using at least two auxiliary bearings, any desired torsional preload and any desired axial preload may be achieved simultaneously. Also, by using at least two auxiliary bearings, it is possible to achieve both of these objectives without altering the preload of the bearings within the speed reducer housing, which otherwise might alter their useful service life.  
           [0013]    Because the invention includes the use of auxiliary bearings that are relatively stiff compared to the internal bearings of the device, the auxiliary bearings are more prone to wear. In some embodiments, the invention comprises means to easily access and service, or simply replace, the stiffer bearing element or elements. Also, in applications where thermal control is critical, the invention may include insulating barriers between the auxiliary bearings and the other reducer elements, with springs or other resilient means used to provide the appropriate preload force.  
           [0014]    It is therefore an object of the present invention to provide for a speed reducer with improved axial and torsional stiffness.  
           [0015]    It is a further object of the present invention to provide for a speed reducer wherein axial and torsional stiffness may be increased to any desired level based on the amount of preload built into a set of auxiliary bearings.  
           [0016]    It is also an object of the present invention to provide for a speed reducer wherein axial and torsional stiffness may be improved without altering the preload on the rolling-element bearings by using auxiliary bearings.  
           [0017]    It is also an object of the present invention to provide for a speed reducer with auxiliary bearings in which the damping of the system is increased through the friction forces and viscous shear forces acting over the large surface area of the auxiliary bearings.  
           [0018]    It is also an object of the present invention to provide for a speed reducer in which the trueness of circularity of the output shaft rotation is dominated by the stiffer auxiliary bearings and thus becomes generally a function of the quality of those bearings rather than the quality of the interior bearings.  
           [0019]    It is also an object of the present invention to provide for a speed reducer useful for precision applications that may be manufactured easily and at a low cost.  
           [0020]    It is also an object of the present invention to provide for a speed reducer with enhanced service life by the use of easily serviced auxiliary bearings.  
           [0021]    It is also an object of the present invention to provide for a speed reducer whereby heat transfer to other parts of the machine is inhibited and the preload on the bearings is maintained constant using preload springs and thermal barriers.  
           [0022]    It is also an object of the present invention to negate the need for counterbalance systems or brake motors in some applications where gravity would otherwise cause the actuator to spontaneously move when power was shut off.  
           [0023]    These and other features, objects and advantages of the present invention will become better understood from a consideration of the following detailed description of the preferred embodiments and appended claims in conjunction with the drawings as described following.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0024]    [0024]FIG. 1. is a cut-away elevational view of a preferred embodiment of the present invention in which the bearing plates are mounted to the input and output shafts.  
         [0025]    [0025]FIG. 2 is a cut-away elevational view of a preferred embodiment of the present invention in which the bearing plates are mounted to the housing.  
         [0026]    [0026]FIG. 3 is a cut-away elevational detail view of a preferred embodiment of the present invention with a multi-piece bearing plate.  
         [0027]    [0027]FIG. 4 is a plan view of a multi-piece bearing plate according to a preferred embodiment of the present invention.  
         [0028]    [0028]FIG. 5. is a cut-away elevational detail view of a preferred embodiment of the present invention that may be used in situations where thermal control is critical. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    With reference to FIG. 1, a first preferred embodiment of the present invention may be described. Bearing reducer  10  comprises input shaft  12 , output shaft  14 , and gearing mechanism  16  that links input shaft  12  to output shaft  14 . Gearing mechanism  16  functions to lower the speed and increase the torque applied to output shaft  14  relative to input shaft  12 . Many types of gearing mechanism  16  are known in the art and may be used with the invention. Input shaft  12  and output shaft  14  ride on rolling-element bearings  18  within housing  20 . Numerous types of suitable rolling-element bearings  18  are known in the art. Rolling-element bearings  18  are characterized by a relatively low internal friction, such that very little energy is lost in the transfer of rotational energy to input shaft  12 , through gearing mechanism  16 , and then into output shaft  14 . Rolling-element bearings  18  do not, however, provide a sufficient degree of stiffness at output shaft  14  for some applications.  
         [0030]    In the first preferred embodiment of FIG. 1, output bearing plate  22  is mounted to the housing of bearing reducer  10  at output shaft  14  with bolts  26 . Likewise, input bearing plate  24  is mounted to output shaft  14  with bolts  26 . Each of output bearing plate  22  and input bearing plate  24  are firmly fitted against output shaft  14  with an annular surface  32  on the inside surface of each of output bearing plate  22  and input bearing plate  24  extending beyond the circumference of output shaft  14 . In the case where output shaft shoulder  36  extends beyond the end of housing  20 , as shown in FIG. 1, output bearing plate  22  may include a lip  30  that extends inward such that the plate forms a cup around output shaft shoulder  36 , with annular surface  32  on output bearing plate  22  appearing on the face of lip  30 .  
         [0031]    Bearing material  28  is adhered to annular surface  32  on each of output bearing plate  22  and input bearing plate  24 , such that bearing material  28  presses against bearing surface  34  at each end of housing  20 . Preferably, one of several suitable commercially available materials made of polymer or polymer-based composites are used for bearing material  28 . In particular, the preferred polymer for bearing material  28  is polytetrafluoroethylene (PTFE). PTFE is unique in that its coefficients of static and dynamic friction are nearly equal. Some commons trade names for such materials are Garlock, which is manufactured by Garlock Sealing Technologies of Palmyra, New York, and Turcite, which is manufactured by Busack+Shambam, Inc. of Abindgon, United Kingdom. These materials are cut to the desired shape and adhered to output bearing plate  22  and input bearing plate  24  to form bearing material  28 . The adhesive used to attaching bearing material  28  to annular surface  32  is preferably Scotch-Weld, which is manufactured by the 3M Corporation of St. Paul, Minn., but other suitable adhesives may also be used. Bearing surface  34  is ground or milled to a bearing-quality finish.  
         [0032]    In alternative embodiments, injectable bearing materials may be used in place of sheet polymer-based materials for bearing material  28 . Preferably, injectable materials sold under the trade names Moglice and Diamante, both manufactured by Diamante Metallplastic Gmbh of Mönchengladbach, Germany, can be used, but other similar materials are available that may be substituted. Only one of the bearing surface  34  and annular surface  32  must be machined to bearing quality when injectable materials are used for bearing material  28 .  
         [0033]    Bearing material  28  may be ground such that the desired preload is achieved at output shaft  14  when output bearing plate  22  and input bearing plate  24  are firmly seated with respect to output shaft  14 . The preload is generated from the frictional forces between bearing material  28  and bearing surface  34 . The pressure with which bearing material  28  is pressed against bearing surface  34  will then determine the preload. Tightening or loosening of bolts  26  will increase or decrease, respectively, the preload. Other fastening devices or mechanisms may alternatively be used to apply pressure between bearing material  28  and bearing surface  34  such that the desired preload is achieved.  
         [0034]    The preload can also be varied by controlling the size of annular surface  32  to which bearing material  28  is applied and the size of bearing surface  34 . The torsional preload force at bearing material  28  is proportional to the normal force (that is, the axial preload) of output shaft  14  and the effective radius at which the normal force is applied. Thus, for a given desired axial preload, the radius of annular surface  32  must be sized to deliver the desired torsional preload, while taking into account the coefficient of friction of bearing material  28 . The normal force is determined by the amount of “interference” between bearing material  28  and bearing surface  34 . Interference is measured as the combined amount that bearing material  28  compresses and output bearing plate  22  deflects when bolts  26  are tightened that connect output bearing plate  22  to output shaft  14 . The manufacturers of materials that may be used for bearing material  28  commonly furnish specifications as to the compressibility of such material, including the force required to compress a given area of such material by a given distance.  
         [0035]    The function of bearing reducer  10  as shown in FIG. 1 may be described as follows. As rotational energy is applied at input shaft  12 , the friction between bearing material  28  and bearing surface  34  at each of output bearing plate  22  and input bearing plate  24  serves to increase the stiffness of bearing reducer  10  at output shaft  14 . Since the sliding-element bearings formed by the interference between bearing material  28  and annular surface  32  are much stiffer than rolling-element bearings  18 , the sliding-element bearings force the trueness of circularity of output shaft  14  rotation to be a function of the flatness of annular surface  32  and bearing surface  28  with respect to each other. Thus deviations in output shaft  14  rotation arising from manufacturing errors and misalignment of components are minimized. Also, since the sliding-element bearings formed in the preferred embodiment contain one continuous flat surface, high surface quality (that is, flatness to a high degree of accuracy) is easily and cost-effectively achieved with commercially available milling and grinding equipment.  
         [0036]    In other alternative embodiments, only one sliding element bearing may be used, such that only one of output bearing plate  22  and input bearing plate  24  is present. The use of two sliding-element bearings, however, has the additional advantage of providing preload at output shaft  14  without increasing the preload on rolling-element bearings  18 . By precisely balancing the friction between bearing material  28  and bearing surface  34  at each end of output shaft  14 , the desired preload at output shaft  14  can be achieved without applying any additional preload upon rolling-element bearings  18 . Since an additional preload on rolling-element bearings  18  may reduce their service life, the use of sliding-element bearings at each end of output shaft  14  may increase the service life of bearing reducer  10 . This becomes especially important since rolling-element bearings  18  are located deep within housing  20 , and therefore service to or replacement of rolling-element bearings  18  would be relatively time-consuming and expensive. It should also be noted that alternative embodiments of the present invention may comprise more than two sliding-element bearings.  
         [0037]    In further alternative embodiments, annular surface  32  can be so sized and placed such that bearing material  28  slides against any surface on housing  20 . Annular surface  32  could be located on a lip extending from housing  20 , or a separate part attached to, and thereby incorporated into, housing  20 . In addition, the placement of bearing material  28  and bearing surface  34  may be reversed, such that bearing material  28  is adhered to housing  20  or other components instead of annular surface  32  on output bearing plate  22  or input bearing plate  24 , and bearing surface  34  appears on output bearing plate  22  or input bearing plate  24 .  
         [0038]    With reference now to FIG. 2, a second preferred embodiment of the present invention may be described. This second preferred embodiment is generally similar to the embodiment of FIG. 1, except that in this embodiment output bearing plate  22  and input bearing plate  24  are attached to opposite ends of housing  20  with bolts  26 . Input shaft  12  and output shaft  14  extend through input bearing plate  24  and output bearing plate  22 , respectively, with output bearing plate  22  making no contact with output shaft  14  where it passes through output bearing plate  22 . Bearing material  28  is mounted on the interior surface of output bearing plate  22  and input bearing plate  24 , such that it contacts output shaft shoulder  36  at the output end of bearing reducer  10 , and it contacts the end of output shaft  14  at the input end of bearing reducer  10 . Annular surface  32  in this embodiment is located radially inward from its location in the embodiment of FIG. 1, such that bolts  26  may pass through output bearing plate  22  and input bearing plate  24  into drive housing  10 . Bearing surface  34  appears opposite annular surface  32  and bearing material  28  on the input end of output shaft  14  and the outside surface of lip  30  of output shaft  14 . Thus in this embodiment, output bearing plate  22  and input bearing plate  24  do not turn with output shaft  14 , but are instead stationary with respect to housing  10 .  
         [0039]    As with the other embodiments described herein, bearing material  28  is ground such that the desired preload is achieved at output shaft  14  when output bearing plate  22  and input bearing plate  24  are firmed seated. In the case of the embodiment of FIG. 2, output bearing plate  22  and input bearing plate  24  are seated against the ends of housing  20 . As rotational energy is applied at input shaft  12 , the friction between bearing material  28  and bearing surface  34  on output shaft  14  at each of output bearing plate  22  and input bearing plate  24  serves to increase the stiffness of bearing reducer  10  at output shaft  14 . Also as with other embodiments, since the sliding-element bearings formed in the preferred embodiment by bearing material  28  and bearing surface  34  contain one continuous flat surface at bearing surface  34 , high surface quality (that is, flatness to a high degree of accuracy) is easily and cost-effectively achieved with commercially available milling and grinding equipment.  
         [0040]    In other alternative embodiments based on the embodiment of FIG. 2, only one sliding element bearing may be used, such that only one of output bearing plate  22  and input bearing plate  24  is present. The use of two sliding-element bearings, however, has the additional advantage as described with respect to the embodiment of FIG. 1 of providing preload at output shaft  14  without increasing the preload on rolling-element bearings  18 . In another alternative embodiment, the invention also comprises a combination of the designs of FIG. 1 and FIG. 2, such that one of output bearing plate  22  and input bearing plate  24  is attached to output shaft  14  with bolts  26 , while the other is attached to housing  10  or to a machine to which housing  10  is mounted.  
         [0041]    Referring now to FIGS. 3 and 4, a modification of the embodiment of the invention shown in FIG. 1 is disclosed in which bearing material  28  may be easily and quickly replaced, thereby making bearing material  28  a wear element and increasing the service life of bearing reducer  10 . The loads being driven by typical bearing reducers during use are large and bulky, and require considerable time and effort to disconnect from the bearing reducer for the replacement of a bearing. In addition, the time required to reconnect the load to the bearing reducer, including the time to reset the alignment of the overall drive system, makes such an operation costly to an operator whose machine must be down while this replacement operation occurs.  
         [0042]    In the present invention, since the bulk of the bearing loads are carried by the sliding-element bearings formed by bearing material  28  and bearing surface  34 , these bearings will wear more quickly than rolling-element bearings  18 . To make bearing material  28  easily replaceable without disconnecting bearing reducer  10  from the drive system of which it is a part, the bearing plate or plates can be formed by bearing plate halves  38  as shown in FIG. 4. Each bearing plate half  38  is connected to output shaft  14  using bolts  26 . Each bearing plate half  38  can be easily removed by simply removing the appropriate bolts  26 , without disconnecting output shaft  14  from load  40 , as illustrated in FIG. 3. Bearing material  28 , which is adhered to annular surface  32  on each bearing plate half  38 , can then be easily replaced, and bearing plate half  38  can be reattached to output shaft  14  with bolts  26 .  
         [0043]    Numerous alternative embodiments of the invention may be constructed using bearing plates that are easily removable. The invention is not limited to removable bearing plates comprising two bearing plate halves  38 , but also comprises bearing plates of any number of pieces. In addition, the easily removable bearing plate can be attached at either the input or output end of bearing reducer  10 , and can be one, some, or all of the bearing plates used in bearing reducer  10 .  
         [0044]    A further embodiment of the invention, which is a modification of the embodiment shown in FIG. 1, may now be described with reference to FIG. 5. In some applications, particularly where there is a higher degree of relative motion between bearing material  28  and bearing surface  34 , a considerable amount of heat may be generated through friction at the interface between bearing material  28  and bearing surface  34 . This heat may lead to damage or reduced life for those components of bearing reducer  10  directly exposed to the heat, or machine components directly adjacent to the interface of bearing material  28  and bearing surface  34 . Due to the use of a bearing plate, however, thermal barriers  42  may be installed to reduce this problem. As shown in FIG. 5, bearing surface  34  is formed of a ring that is insulated from housing  20  by annular-shaped thermal barrier  42 . Thermal barriers  42  may be formed of any suitable materials with appropriate heat-insulating qualities as are known in the art. Bearing surface  34  in this embodiment may be formed of any suitably hard material, such as steel, that may be machined to a bearing-quality surface. Opposite bearing surface  34  is bearing material ring  44 , which may also be formed of steel or the like. Bearing material  28  is adhered to the inside surface of bearing material ring  44 . Adjacent to the outside surface of bearing material ring  44  is spring washer  46 . The outside edge of spring washer  46  rests against thermal barrier  42 , which is attached to annular surface  32 .  
         [0045]    When output bearing plate  22  is attached to output shaft  14  and firmly seated with bolts  26 , spring washer  46  provides a force that presses bearing material  28  against bearing surface  34 , thereby providing the preload that increases the stiffness of bearing  10 . In addition to the means of controlling the amount of the preload as described above with respect to the embodiment of FIG. 1, the preload may also be varied by adjusting the tension of spring washer  46 . Elements of bearing reducer  10  and surrounding machine components are protected from heat generated due to the friction between bearing material  28  and bearing surface  34  by thermal barriers  42 .  
         [0046]    The present invention has been described with reference to certain preferred and alternative embodiments that are intended to be exemplary only and not limiting to the full scope of the present invention as set forth in the appended claims.