Abstract:
A gear drive having a third order damper operating with the antibacklash gear so as to lower the impact of the antibacklash gear at high torques to reduce gear wear while maintaining good contact with the antibacklash gear at lower torques to maintain high accuracy and bandwidth.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to anti-backlash damping of gears and more particularly to third order damping which removes backlash and improves gear life by reducing pre-load forces and eliminating backlash crossover. 
     2. Description of the Prior Art 
     Backlash, i.e., the gap between teeth of mating gears, is inherent to most gear transmission designs. It is undesirable because it causes a dead-band which produces a loss of precision in output position and velocity control. Apparatus for reducing backlash is known in the prior art and normally takes the form of a second, or anti-backlash gear adjacent the driving gear which is substantially identical to the driving gear but is off-set there from, under the force of a pre-load mechanism such as a spring so that the combined backlash gear and driving gear fill the space between the surfaces of the driven gear teeth. 
     Unfortunately, the pre-load force tends to reduce gear life by causing excess wear and material fatigue. Typically, the higher the pre-load, the shorter the gear life. Also, in cases where high control bandwidth is necessary, the pre-load force is not sufficient to filly eliminate backlash “crossover” i.e. the travel of the driving gear from one flank of the driven gear to the other flank across the backlash gap. This limits the bandwidth and results in significant transmission error. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention overcomes the problems with anti-backlash arrangements of the prior art by using a third order damper in place of the pre-load force. Third order dampers have been described in paper by Porter Davis, David Cunningham and John Harrell presented at the 35th AIAA SDM Conference, April 1994. It may also be seen in U.S. Pat. No. 5,332,070 of Lawrence P. Davis, et al., entitled “Three Parameter Viscous Damper and Isolator” which issued Jul. 26, 1994 with serial number 51,110 and is assigned to the assignee of the present invention. Applying a third order damper to an anti-backlash gear arrangement, improves the bandwidth capability, mitigates the transmission errors, and reduces the pre-load forces to increase gear life in the anti-backlash gears of the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial view of a prior art transmission with a driving gear and a driven gear; 
     FIG. 2 is partial view of a prior art anti-backlash mechanism in the transmission of FIG. 1; 
     FIG. 2 a  is a side view of FIG. 2 taken along lines  2 A— 2 A showing the pre-load force production in the anti-backlash gear; 
     FIG. 3 is a partial view of a transmission of FIG. 2 utilizing the present invention; 
     FIG. 4 is a schematic sectional view of one embodiment of the present invention; 
     FIG. 5 is a schematic top view of the FIG. 4 embodiment; and 
     FIG. 6 is a schematic exploded perspective view of the embodiment of FIGS.  4  and  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a prior art transmission  10  having a driving gear  12 , mounted on an axle  13  and with a gear tooth  14 . A single tooth is shown for convenience but it will be understood that gear  12  includes a full set of teeth shown by dashed line  16 . A driven gear  20  is shown with first and second teeth  22  and  24  and a recession  26  therebetween. While two teeth  22 , and  24  are shown, it is understood that a plurality of teeth shown by dashed line  28  extends in both directions. 
     Driving gear  12  may rotate in either direction, but in FIG. 1 it is shown rotating on axle  13  counterclockwise as shown by arrow  30 . As such, the left surface of gear tooth  14  is in contact with the left flank of recession  26  at an area  40 . It is seen that the right surface of gear tooth  14  is out of contact with the right flank of recession  26  and a gap  48  exists which produces the undesirable dead band. More particularly, when driving gear  12  rotates in the clockwise direction the tooth  14  must move across gap  48  before its right surface can contact the right surface of recession  26  and produce any motion of driven gear  20 . Furthermore, the continual collision at areas such as  40 , causes excessive wear and fatigue in the gear teeth. 
     A lessening of the backlash has been obtained in the prior art as is shown in FIGS. 2 and 2A. FIG. 2 is similar to FIG.  1  and uses the same reference numerals for those parts which are the same. In FIGS. 2 and 2A, a second driving, or anti-backlash gear  50 , best seen in FIG. 2A, is mounted contiguous to the driving gear  12  about axle  13  but not connected thereto. Anti-backlash gear  50  has a pin  52  extending from its right surface through a curved slot  54  cut through driving gear  12 . Driving gear  12  has a pin  56  extending from its right surface and a spring  58  is connected between pins  52  and  56 . Spring  58  exerts a force on driving gear  12  and anti-backlash gear  50  so as to pull anti-backlash gear in a clockwise direction and pull driving gear  12  in a counterclockwise direction. Slot  54  allows pin  52  to move under the force of spring  58  until the right surface of a tooth  62  on anti-backlash gear  50  comes into contact with the right flank of recession  26  at an area  66 . A portion of tooth  62  is seen in FIG. 2 to the right of tooth  14  of driving gear  12  and the rest of tooth  62  is shown by dashed lines  68 . 
     The problems encountered with the apparatus of FIGS. 2 and 2A is that the force exerted by the spring  58  pushes tooth  14  to the left and tooth  62  to the right by an amount that causes excessive wear and fatigue on the contact points  40  and  66 . Using a spring with less force allows the rotation of gear  12  to overcome the spring force and gears  12  and  50  move more together producing the backlash gap again. Even with a spring having enough force to hold the teeth  14  and  62  in contact at areas  40  and  66  for most driving motions, rapid turning of gears  12  and  50 , as may occur in high band width applications, where both small and large motion of output gear  20  are desired, the driving force of gear  12  may still overcome the force of spring  58  to create the backlash gap. 
     The present invention introduces a third order damper in place of the backlash spring to modify the pre-load forces in the anti-backlash gearing of the prior art. Because a third order damper, such as shown in the above mentioned paper and patent, allows a very low pre-load stiffness during low rate operation and very high stiffness during high rate operation, it will provide a relatively large force during fast maneuvers where the driving gear exerts the most force and provide a relatively small force during slower, more exact motions where the driving gear exerts only a small force. Schematically this arrangement can be seen in FIG. 3 where the apparatus of FIG. 2 is used with the same reference numerals for parts that are the same as FIG.  2 . 
     FIG. 3 is like FIG. 2 except that a dashpot  70 , in series with a spring  72 , is shown in parallel with spring  58  between the pins  52  and  56 . This combination forms a three parameter damper like that shown in the above-mentioned patent. The third order damper provided by this arrangement will produce the low pre-load stiffness at low operation rates and high pre-load stiffness at high rates as is desired. Of course, the piston type damper and springs is symbolic and other types of dampers may also be used. Furthermore, the use of springs to supply the pre-load force is only one of several ways of pre-loading the gears. However, regardless of the type of damper and of how the pre-load force is produced, the use of the third order damping will enable the transmission to achieve the desired result. 
     FIGS. 4,  5  and  6  show one embodiment of a possible structure for supplying the pre-load force and third order damping. In FIG. 4, the driving gear  12  and the antibacklash gear  50  are shown as they were in FIG. 2A with spring  58  applying pre-load force between pin  52 , connected to antibacklash gear  50 , and pin  56  connected to driving gear  12 . The dashpot/spring arrangement of FIG. 3 is shown as a pair of disks  80  and  81  mounted about shaft  13  for rotation with respect to each other, as, for example, by small ball bearings  83  between disks  80  and  81 . Disk  80  is connected to antibacklash gear  50  by a pin  85  and disk  81  is connected to driving gear  12  by a pin  86  connected to shaft  13 . A dashpot in the form of a “T” shaped bellows  88  is mounted on disk  81  and is connected to be operated on by motion of disk  80  with respect to disk  81  as will be better understood in connection with FIGS. 5 and 6. 
     In FIGS. 5 and 6, which are slightly enlarged for better clarity, disk  80  has a pair of abutments  90  and  91  extending upwardly and a plurality of ball bearings  94  around the periphery. Disk  81  has a pair of cutout portions  95  and  96 . The “T” shaped bellows arrangement  88  includes a pair of flat bellows  98  and  99  and a vertical bellows  100  all mounted on a radial portion  102  of disk  81 . Springs  104  and  106 , normally in compression, are attached to bellows  98  and  99  respectively. 
     When disks  80  and  81  are brought together, ball bearings  94  will fit into a roller bearing track (not shown) on the underside of disk  81 , and abutments  90  and  91  will extend through cut-out portions  95  and  96  to be joined at the ends with springs  104  and  106  so that upon relative motion between disks  80  and  81 , abutments  90  and  91  will push against flat bellows  98  and  99  (depending on the direction of rotation) and will cause expansion and contraction forcing fluid through a restriction (not shown) into vertical bellows  100 . The size of the restriction and the viscosity of the fluid will be set to provide the desired amount of damping in a well known manner. 
     It is seen that the damper formed by “T” shaped bellows  88  and the springs  104  and  106  is series wherewith, are arranged in parallel with the spring  58  so as to provide the desired third order damping. With high torque rotations, the damping provides high stiffness and with low torque rotations the damping provides low stiffness. The result is that the high impact of the antibacklash gear  50  against the driven gear  20  is reduced during high torque so as to provide longer life for the transmission and the full contact of the antibacklash gear  50  and the driving gear  12  are maintained for lower torques for greater accuracy and greater bandwidth. 
     It is thus seen that we have provided a long life, high bandwidth and high accuracy gear transmission by the use of a third order damper. Many changes to the specific disclosure used to describe the preferred embodiment will occur to those skilled in the art. For example, damper arrangements other than a “T” shaped bellows my be employed and activation arrangements other than using two disks, rotatable with respect to each other with abutments on one extending through slots in the other to activate the bellows may be devised. Accordingly, although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.