Abstract:
A gearbox comprises a gearbox housing containing a gear train whose shafts are supported by the housing. The housing is fastened to a mounting plate at exactly three attachment points. This three point attachment reduces distortion of the housing that may cause misalignment of individual meshing gears in the gear train, leading to premature failure of those gears transmitting high torque.

Description:
BACKGROUND 
       [0001]    Certain industrial processes require the use of speed reduction gear trains to reduce the customary speed of electrical motors to a few tens of rpm or less. The output shaft gear of such a gear train is physically quite large, perhaps several feet in diameter. Such a gear train and the housing that typically holds it will be referred to as a gearbox. An input shaft provides high speed low torque power. 
         [0002]    The housing provides support for the bearings on which the gears&#39; shafts turn, and also protects the gears from damage and holds a gear lubricant such as gear grease. A typical housing comprises a case and a cover. In one preferred design the gear train includes a pair of bevel gears for allowing an output shaft whose axis is at right angles to the input shaft. 
         [0003]    The gear train has an output gear perhaps several feet in diameter that the output shaft carries. The torque that the output gear provides in these applications is often extremely large. For example, such a gear train may be used to slowly rotate a large industrial component holding materials with combined total weight of many tons. The large output torque of such a gear train produces on the teeth of both the output gear and the pinion gear driving the output gear, contact stresses that are extremely large as well. 
         [0004]    Most gears are manufactured with teeth having cycloidal profiles, which provide for rolling line contact between meshing gears. Rolling contact avoids sliding between meshing teeth surfaces so little frictional wear occurs. Line contact (as opposed to point contact) reduces the Hertzian stress (force per unit contact area) on these teeth and further increases their life. 
         [0005]    High precision machining now available for the teeth on such gears provides the opportunity for reliably creating during operation, continuous line contact between the individual teeth of the output gear and the meshing pinion gear. High precision bearings for the gear shafts further enhance the opportunity for continuous precision meshing of output gear teeth. 
         [0006]    Unfortunately, output gear teeth continue to fail, often showing damage associated with excessive Hertzian stress. Such failure is costly, in that these gearboxes are very expensive. Further, it requires the entire installation to cease operation during gearbox or gear replacement. Since these gearboxes are often specially designed for a particular installation, replacement may take a long time. Alternatively, an installation may keep a spare gearbox available, reducing downtime but adding overhead costs to the operation of the installation. 
         [0007]    Accordingly, the current state of affairs for installations using these large, high-torque gearboxes is unsatisfactory. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    Investigation suggests that the current practice of bolting high output torque gearboxes to a rigid mounting plate at four different points may distort the gearbox case. Because of the extremely high output torques involved, very strong attachment bolts are used to fasten the case to mounting points on the mounting plate. 
         [0009]    The geometry of the mounting points on the mounting plate is typically difficult to control. When these mounting plate mounting points do not accurately align with the mounting points on the gearbox and no corrective shimming occurs, bolting the gearbox case to the mounting plate at four points can potentially distort the entire case. This distortion of the case is small because the case is quite stiff compared to the mounting plate, but still causes slight misalignment of the meshing gears within the case. 
         [0010]    Such gearbox case distortion has to potential to alter the interface between the high-torque output shaft gear and the gear driving the output shaft gear. This altered interface may lead to point or even intermittent contact rather than line contact between the gear faces, causing premature damage to the gear faces, and eventual failure of the gears. 
         [0011]    This invention limits gearbox distortion arising from the common practice of using four points for mounting the gearbox on the mounting plate. The gearbox is fastened to a mounting plate in a special way. 
         [0012]    The invention comprises a gearbox of the type having a gearbox housing containing a gear train. The gear train has at least two shafts mounted for rotation therein and supported by the gearbox housing. Each shaft of the gear train carries at least one gear in mesh with a gear carried on another of the shafts. The improvement comprises on the exterior of the gearbox housing, exactly three active attachment points. Each attachment point has at least one bolt engaging the attachment point for attaching the housing to the mounting plate only at the attachment points. 
         [0013]    Each attachment point preferably comprises a flange integral with the housing and having at least one aperture such as a hole or a slot through which one of the bolts passes and is screwed into the mounting plate. 
         [0014]    The invention is most valuable for a gearbox having a reduction gear train having an output shaft turning much more slowly than an input shaft. Such a reduction gear train has an intermediate shaft carrying an intermediate gear and an output shaft having an output gear rigidly attached thereto with a preset number of teeth. The intermediate gear is in mesh with the output gear and drives the output shaft through the output gear. The intermediate gear has a fraction of the number of output gear teeth. 
         [0015]    A preferred gearbox has two flanges located closer to the input shaft than to the output shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0016]    The FIGURE shows the invention with three mounting points for the gearbox housing. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    The FIGURE shows a gearbox  10  having a housing comprising a case  15  and cover  13 . Case  15  is to be attached to a mounting plate  21  using bolts  51  that fit through holes in mounting points  18 ,  19 , and  20  into holes  46 ,  47 , and  48  of mounting plate  21 . Mounting points  18 ,  19 , and  20  preferably are all carried on a single surface of case  15 , and preferably the bottom surface of case  15  as shown. Preferably, mounting points  18 ,  19 , and  20  project slightly past the surface carrying them to avoid any interference or contact between case  15  and mounting plate  21 , other than at the individual mounting points  18 ,  19 , and  20   
         [0018]    Gearbox  10  in one version contains a gear train having an input shaft  23  and an output shaft  26 . The gear train multiplies at output shaft  26 , the torque applied to input shaft  23  by an overall ratio of 100 or more. 
         [0019]    An output gear  37  is fixed to output shaft  16  and meshes with and is driven by an intermediate gear  36 . Gears  36  and  37  are shown in phantom outline within gearbox  10 ; the intermediate gears comprising the gear train other than gear  36  are not shown. Intermediate gear  36  typically has a fraction of the number of teeth that output gear  37  has. Torque is transmitted to output gear  37  from input shaft  23  through the one or more intermediate gears in addition to gear  36 . The gear train may include bevel gears that orient the axis of the output shaft  26  to be orthogonal to the axis of input shaft  23 , which is the configuration of gearbox  10 . 
         [0020]    Bearing  43  mounts input shaft  23  and is supported by at least one wall of case  15 . A shaft  33  attached between case  15  and cover  13  carries intermediate gear  36 . 
         [0021]    Gear  36  through the gear train, applies relatively high torque to shaft  26  through output gear  37 , hence forces between the meshing tooth faces of both gears  36  and  37  are relatively high. Accordingly, any misalignment between the meshing teeth of gears  36  and  37  has the potential to create points of very high pressure on these teeth. 
         [0022]    An internal bracket  30  (typically integral with or forming a part of cover  13 ) carries the upper end of shaft  33 , and also carries an upper bearing  32  for output shaft  26 . A portion of internal bracket  30  is shown in phantom in the FIGURE. The floor of case  15  carries an inner triangular mounting plate or bracket  29  that in turn carries a second output shaft bearing  40  also having low axial and radial runout. 
         [0023]    Both mounting bracket  29  and bearing  40  are spaced from the floor or bottom of case  15  to avoid damage from contaminants that settle to the floor of case  15 . Bracket  29  is solidly mounted to the floor of case  15 , or is integral therewith, and has sufficient stiffness to distribute high thrust and torque loads across a substantial portion of the case  15 . Brackets  29  and  30  are strongly attached to case  15  and cover  13  respectively. Cover  13  is strongly attached to case  15 . Because of this, any distortion of case  15  can also distort cover  13  and brackets  29  and  30 . 
         [0024]    In one version, brackets  29  and  30  are designed to compensate for shaft  26  deflection between bearings  32  and  40  by having different in-plane stiffnesses. Thus, if gear  37  is positioned closer to bearing  32  than to bearing  40 , then bracket  30  is proportionately stiffer than bracket  29  so that the axis of shaft  26  when under maximum load is almost exactly parallel with the shaft  26  axis when shaft  26  is unloaded. 
         [0025]    To avoid premature failure of gear  37  and pinion gear  36 , shafts  33  and  26  should maintain their geometry respecting each other regardless of the torque that shaft  26  carries. The design of shaft  26  and bearings  32  and  40  allows them to contribute only a very small amount of gear  37  deflection arising from high torque transmission. This requires that shafts  33  and  26  and housing  11  have substantial stiffness. 
         [0026]    Case  15  has a mounting feature that substantially reduces distortion of housing  11  caused during attachment of housing  11  to a large, stiff base such as mounting plate  21 . In the embodiment of the FIGURE, case  15  has two spaced apart mounting points  18  and  19  near the input shaft bearing  43  and relatively distant from the axis of output shaft  26 . Mounting points  18  and  19  may be integral with case  15  as shown, with holes through which mounting bolts  51  pass. A third mounting point  20 , also integral with case  15 , is spaced from mounting points  18  and  19  and may be closer to the axis of output shaft  26 . Bolts  51  fasten case  15  at these three mounting points  18 - 20  by screwing into threaded mounting holes  46 ,  47 , and  48  on mounting plate  21 . Clamps that lock onto flanges or projections of case  15  are also possible fastener elements. 
         [0027]    Using only the three mounting points  18 ,  19 , and  20  appears to substantially reduce the distortion of housing  11  typically induced by imperfectly shimmed four-point mounting. Three point mounting, even if the individual mounting points are misaligned, distort the gear case  11  only slightly at worst. On the other hand, a fourth mounting point must be accurately shimmed with the other three mounting points to avoid such distortion. For such shimming to be effective, the installer must precisely measure the gear case  11  and the mounting plate&#39;s mating geometries. Since these gearboxes  10  are typically very heavy, mounting them properly to avoid distortion is difficult and time-consuming. 
         [0028]    The situation is similar to the performance of a three-legged stool on an uneven surface as opposed to a four-legged stool. A three-legged stool will always sit solidly on an uneven surface. Everyone is familiar with situations where a four-legged stool or table in a restaurant rocks on an uneven floor, until matchbooks or folded napkins (i.e. shims) are placed under one leg. 
         [0029]    Three-point mounting of case  15  likely solves much of the case  15  distortion problem. A further possible source for case  11  distortion may arise from the high reactive torque that shaft  26  applies to case  11 . That is, shaft  26  creates a couple tending to twist case  15 . 
         [0030]    The following may address this issue, if present. While the FIGURE shows two mounting points  18  and  19  relatively distant from output shaft  23 , alternatively two of the mounting points  18 - 20  may be placed closer to output shaft  26  and one closer to input shaft  23 . This configuration may make the overall installation more robust than that shown in the FIGURE. Placing the resistance to reactive torque applied by shaft  26  to case  15  by a single one of the mounting points  18 - 20  as far from shaft  26  as the dimensions of case  15  allow, may also further reduce distortion of case  15  arising from the high shaft  26  torque itself.