Means to minimize the backlash of meshing gears

A precision power gear train assembly particularly adaptable for use with vehicles having a 360.degree. rotatable turret mounted thereon. The assembly embodies novel spring and stop means to minimize backlash and to limit gear tooth disengagement of the enmeshed output pinion and ring gears of such an assembly. More specifically a pinion-ring gear assembly mounts a gear box to oscillate about an axis parallel to the pinion axis of rotation such that the tooth pressure between pinion and ring gear biases the teeth apart, increasing backlash. A preloaded spring biases the gear teeth into tight mesh to eliminate backlash until the tooth pressure force exceeds a predetermined maximum. Thereafter, further increases in the tooth pressure force separates the teeth by compression of the spring beyond its preloaded compression. A stop limits tooth separation.

BACKGROUND AND SUMMARY OF PRIOR ART 
Prior art devices designed to cope with backlash approach the problem by 
means of springs disposed to put a torsional bias on a gear. Such devices 
are unsatisfactory in that they are too expensive, unreliable from the 
accuracy point of view, and too expensive to maintain. Fire control 
systems require a high degree of accuracy in order to come at all close to 
a first-round hit capability, because the accuracy of enemy fire and the 
speed and fluidity of movement in modern war are such that one chance at 
the target may be all the gunner will get. 
SUMMARY OF THE INVENTION 
The invention mounts a drive motor on a support for drive of an output 
pinion, the support pivoting about an axis which enables backlash to vary 
from zero to a maximum which is substantially less than total separation 
of the meshing gears. Spring pressure biases the output pinion into 
maximum engagement with its compatible meshing gear, toward zero backlash 
mesh, and prevents backlash until the tooth pressure load attains a 
predetermined maximum. Tooth pressure loads greater than said maximum push 
the teeth toward disengagement against the spring bias, and a positive 
stop is engaged by the pivoting support before total separation of the 
teeth of the meshing gears takes place. So that the tooth pressure load 
which biases the gear teeth apart may be independent of the direction of 
turret rotation, the line connecting the axes of rotation of the drive 
pinion and its meshing ring gear is perpendicular to the line connecting 
the support pivot axis with the pinion axis of rotation. 
It is accordingly an object of the invention to provide a backlash 
eliminator which provides precise control of turrent position during 
target tracking with zero backlash in the gears of the turret drive, but 
which permits very rapid slewing of the turret for initial target pick-up 
while limiting gear tooth separation to something less than total 
disengagement.

DETAILED DESCRIPTION OF STRUCTURE 
The turret drive disclosed and claimed herein was invented and developed 
for a combat vehicle adapted to launch missiles from a turret having 
360.degree. traverse such as described in copending application Ser. No. 
189,980 filed Oct. 20, 1980. For precise fire control, the turret drive 
should be as free as possible from backlash. It will be understood by 
those skilled in the art that a precision drive like that disclosed and 
claimed in this application can be used in many other applications 
requiring a zero backlash gear train. 
In this application, I disclose (FIG. 4) a combat vehicle 2 which is 
conventionally exoskeletal, having a body hull 3 or armor plate which 
forms the basic relatively stationary supporting structure of the vehicle, 
propelled by a track 4 upon which a turret 6 is mounted to rotate about a 
given body axis X--X. The turret carries one or more weapons, such as 
those shown at 8 and 10. Turret 6 is mounted for 360.degree. traverse 
about said body axis X, FIG. 3, and X--X, FIG. 4. A base plate 11 is in 
the form of an annulus, is an integral part of the turret, and as such 
rotates with the turret relative to the relatively stationary body hull. 
Secured to the hull below base plate 11 is a ring gear 12, here shown 
fragmentarily in FIG. 1; the ring gear appears more fully in FIG. 3 of the 
above-identified (Ser. No. 189,980) copending patent application. An 
output gear in the form of a drive pinion 14 meshes with ring gear 12 and 
is connected to be driven by a motor 16. As the referenced copending 
application indicates, a preferred mode of the invention has been 
constructed in which motor 16 is an electric motor. 
As indicated above the output gear of the power train is pinion 14, and the 
gear it "drives" is ring gear 12 which in conventional terms is the 
compatible gear of the power user. These conventional terms, output gear 
and compatible gear of a power user, are the terminology used in the 
claims. From one conventional point of view, pinion 14, the output gear, 
is seen as "driving" ring gear 12, the compatible gear. 
Motor 16 is connected to drive the output gear, pinion 14, through 
conventional power train means for transmitting torque shown schematically 
at 18 in the referenced application and here shown without details as 
housed in a gear box 20. Thus motor 16 drives pinion 14 through a 
conventional power train. However, because the vehicle hull, and thus the 
ring gear 12 attached thereto, are relatively stationary in comparison to 
the turret rotatably mounted thereon, the net effect of the drive force 
applied at the output pinion gear 14 is to drive the turret 6 within its 
aforesaid 360.degree. traverse path about axis X--X. 
For the purpose of describing this invention, gear box 20 (the "housing"), 
will be referred to as a power train support to indicate that it is the 
structural supporting base for the power train assembly. That power train 
support is mounted on base plate 11 for oscillation about an axis B--B as 
seen in FIG. 1 and as shown at B in FIG. 2. The axis of rotation of pinion 
gear 14 is shown at C--C in FIG. 1 and at C in each of FIGS. 2 and 3. Axes 
B--B and C--C are conventionally parallel to the turret axis, X in FIG. 3, 
which, also conventionally, passes through the center of ring gear 12. 
It will be understood by those skilled in the art that the point of tooth 
contact, as seen in a plane perpendicular to axis C--C, between gears 12 
and 14 will be at the point of tangency of the pitch circles of both gears 
and that the point of tooth contact is also the pressure point at which 
one gear drives the other. As gear designers know, that pressure point 
lies in a plane tangent to both tooth surfaces. Conventionally, that 
tangent plane makes an angle with the tangent to the pitch circle which is 
141/2.degree. or 20.degree.--the two most common angles. Consequently, the 
force concentrated at that pressure point has a radial component which has 
a tendency to push the gears out of mesh. 
It is desirable that the disengaging force, which tends to unmesh the 
gears, be independent of the direction of turret rotation. Attention is 
therefore now invited to the line of centers E--E, FIGS. 2 and 3, 
connecting axes B--B and C--C; and likewise to the line of centers H--H, 
also FIGS. 2 and 3, connecting axes C--C and X. So that the tooth 
disengaging force may be independent of the direction of turret rotation, 
the referenced two lines of centers should intersect at right angles, at 
the point C shown in each of FIGS. 2 and 3. 
Referring now particularly to FIG. 3, I show resilient means for biasing 
the two gears 12 and 14 toward a tighter tooth engagement, and thus toward 
minimization and preferably complete elimination of backlash. Toward that 
end, power train support 20 is biased clockwise as seen in FIG. 3 to force 
pinion 14 into more firm engagement with its compatible ring gear 12. A 
fixed bracket 22 has a horizontal member 24 which is affixed to base plate 
11 by means of threaded fasteners 26. Bracket 22 also has an upright 
member 28 which has threaded engagement with threaded rod or screw members 
30 and 32. One end of threaded member 30 has secured to it a spring 34 
adapted to engage a swinging bracket 36 which is fastened to the power 
train base or support 20 by other threaded members 38. 
The gear tooth contact pressure biases gears 12 and 14 apart which tends to 
oscillate power train support or base 20 counterclockwise about axis B--B. 
If that pressure is great enough, it can if not restrained, force the two 
gears out of mesh. To prevent total disengagement of the teeth, I have 
provided aforesaid threaded rod member 32 which serves, at its end 40, as 
a stop to limit separation of the gear teeth to a distance which is safely 
less than enough to disengage the gears. Jam nuts 42 and 44 on threaded 
members 30 and 32, respectively, are adapted to engage upright member 28 
to lock their threaded members against accidental turning. 
It is noted that zero backlash is in this application not a constant 
requirement. When moving into combat, the gunner is scanning the field for 
a target; eventually he finds a target and may need to quickly operate the 
fire control means, i.e., he may need to slew the turret quickly to bring 
the target into the field of view of the tracking scope. At the time of 
slewing the turret, zero backlash is of no use and may even be a handicap. 
Threaded member 30 is adjusted to put spring 34 under such a preload that 
it can be compressed enough to allow sufficient backlash for ready slewing 
of the turret to rapidly bring the target into the field of view of the 
sighting device. The close proximity of end 40 to swinging bracket 36 
keeps gears 12 and 14 from greater disengagement than may be desirable in 
terms of tooth wear, shock loads on the teeth, and the like, as will be 
understood by competent gear designers. 
It is noted that motor 16 is equipped with a conventional brake which is 
operable directly at the will of the operator by means of an elongated 
lever 46, here shown as a handle. When lever 46 is alined with line of 
centers E--E, as in the drawings, the brake is in its "off" position. When 
lever 46 is at right angles to line E--E, the motor brake is applied. 
Operator-operable means 48 are provided for ready nonpowered traverse of 
turret 6. Thus, a crank having a radial element 50 and an actuator 52 are 
provided on a crankshaft 54 for direct operation (i.e., not powered by 
motor 16) by the gunner in case motor 16 is disabled. Preferably, means 48 
are conventionally so designed that motor 16 will be disengaged from 
turret drive with actuator 52 in the position shown in FIG. 1, and 
crankshaft 54 will be disengaged from turret drive when actuator 52 is 
folded, on hinge 56, through 90.degree. as to lie against radial element 
50. 
The operator-operable brake controlled by lever 46 will normally be "off" 
for shipping or for operator operation of turret traverse by means 48. The 
mechanisms for the brake controlled by lever 46 and turret traverse by 
means 48 form no part of this invention and thus are not detailed here. 
OPERATION 
As the vehicle heads into combat, crew members are on the alert for 
possible targets, such as emplaced artillery, enemy tanks, and the like. 
When a target is initially sighted, there is a good probability that it 
will not lie in the field of view of the sighting device, in which case 
the gunner must as quickly as possible slew the turret around to bring the 
target within said field of view. Powered maximum traverse, or rapid 
traverse by the means 48, may put enough pressure on spring 34 to exceed 
the preset maximum for zero backlash, and if so, the teeth of gears 12 and 
14 will separate somewhat. However, total disengagement of the gears will 
not occur because of the limit imposed by swinging bracket 36 engaging 
stop 40 on the end of threaded member 32. 
During target tracking by the gunner before the missile is fired, precise 
control of weapon traverse is accomplished by the zero backlash achieved 
by spring 34 in contact with swinging bracket 36, and the gunner keeps the 
weapon on target before firing, by accurate control of turret traverse. 
It is noted that, in the fire control device actually constructed, after 
all adjustments were made, spring 34 was compressed approximately 3/8 of 
an inch. At that setting of the spring, there was no separation of the 
gear teeth (i.e., backlash) until approximately 20% to 33% of the 
available torque was reached. 
With the invention herein disclosed and claimed, an AGMA (American Gear 
Manufacturers' Association) Grade 5 pinion-ring gear assembly, costing 
about $500, can be used to achieve the necessary precision. With 
conventional gear designs, an AGMA Grade 15 pinion-ring gear assembly 
costing $40,000 would be required to achieve the needed precision. 
I wish it to be understood that I do not desire to be limited to the exact 
details of construction shown and described, for obvious modifications 
will occur to a person skilled in the art.