Note: Descriptions are shown in the official language in which they were submitted.
123L5B60
.
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for eliminating
or minimizing backlash in the power transmission gear trains
of electromechanical servo mechanisms such as are used in
the aerospace industry for the precise positioning of Nero-
dynamic flight surfaces, trim tabs, air and fuel controls
and related structures. Backlash in precision servo-mech-
anises adversely affects position accuracy, system stability,
frequency response and overall static and dynamic stiffness.
Conventional methods for reducing backlash in such power
transmission gear trains include establishing rigid toter-
fences on gear specifications, gear housing center distances
and support bearing specifications. Such methods are in
particular a costly approach to the problem, and at best
a considerable amount of backlash is still required to act
commodity tolerance accumulations and temperature variations.
Another method of conventional design uses adjustable
center distance devices which usually consist of eccentric
bearing mounts that are rotated at assembly and locked in
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position when the but compromise between backlash and
free operation is struck. with these devices, however,
reliability and efficiency are low and assembly time is
high.
Still another conventional method is gear lapping at
assembly in which gears are specified slightly oversized
and reduced to a nearly perfect mesh by running in their
housings with an abrasive compound applied to the teeth.
This method produces a smooth running gear train with very
little backlash, but such a method is quite labor-intensive,
requires stringent process control measures to assure
complete removal of the abrasive compound, and necessitates
maintenance of matched sets. Further, elaborate tooling
is usually required. Also. at elevated temperatures soys-
them performance can be appreciably degraded as the gears
expand to an interference condition.
In the case of conventional anti backlash gears two gears
of the same size and type are mounted coccal, one fix-
Ed to the driving end and the other free to rotate about it,
I the two gears being in close proximity to one another. The
gears are coupled through a helical extension (compression
or torsion) spring mounted in slots provided in the radial
faces of the gears. Upon installation the gears are rotated
with respect to one another, thereby extending or compress-
in the spring and producing a torque sufficient to main-
lain a zero backlash mesh with the mating gear. These gears
find wide acceptance in instrument applications where the
torques involved are negligible. Their use in power transmission,
however. is not feasible, because the forces required
necessitate untenable large springs. Aside from the
awkward packaging consideration ! the susceptibility of
such springs to harsh vibration environments. particularly
in airborne equipment, prohibits the use of conventional
anti backlash design for power transmission.
SUMMARY OF THE INVENTION
The present invention has for its main purpose the
elimination of backlash in gear-train assemblies
generally. such as to enhance communication between the
output and the driving components in such systems. By
means of the invention, a simple and reliable apparatus
is provided, whereby lost motion between the Sears in
a gear train is effectively eliminated, thus rendering
the systemic which such gears are used more stable 7
more sensitive to command inputs, more responsive and
more accurately adjustable than conventional systems
The anti backlash device according to the present
invention is an axially-loaded, power transmission type.
Specification a gear and shaft member is provided with
a helical device by which it is coupled in a spaced-
apart relationship to a hubbed~ear member via its helical
counterpart. An axial force acting upon the tubbed gear
it provided by a spring means, for example, a disc spring,
when compressed by a loading device, such as a load nut and
washer assembly affixed to the shaft of the tubbed gear.
Through the action of the helical device this axial force
is converted to torque which then maintains a zero back-
lash mesh with the mating gear throughout, or through any
part of, the load spectrum of the system.
According to the principles of the invention it it posy
sidle to use spring loaded anti backlash gearing for the
purpose of power transmission. The design according to the
invention capitalizes on the space effectiveness of the disc
spring configuration in particular and thereby converts its
high linear force via the helical device to the desired
torque in a compact and feasible design package The present
invention thus represents a radical departure from trade-
tonal approaches to backlash reduction in both concept and
construction and functions in a simple, effective manner
that is less expensive and more reliable than traditional
methods which are never 100% effective.
The invention will be better understood as well as fur-
then objects and advantages thereof become more apparent
from the ensuing detailed description taken in conjunction
with the drawing.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 shows a partial cross-sectional elevation of
the device according to the invention; and
Figure 2 shows a partial cross-sectional elevation of
the same device.
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DETAILED DESCRIPTION
Referring to the drawing there is shown a gear 1 affix-
Ed to a shaft member 2. Axially spaced from the gear 1 is
a tubbed gear 3 which surrounds the shaft 2 by means of the
helical device. to be more fully described below. Urging
the gear 3 towards the gear 1 along the axis thereof is a
disc spring 4 which is journal Ed on the hub of the gear 3,
as shown. A load washer 5 rotatable mounted on the threaded
sleeve member is provided for affixing the disc spring 4
on the hub of the gear 3 so that the disc spring can press
against the face thereof. A load nut 7 is threaded on the
sleeve 6 and positioned along the sleeve at the point
which produces the desired axial force against the tubbed
gear 3. Surrounding the shaft 2 within the tubbed portion
of the gear 3 is a helical device 8,9 of suitable design
which can be in the form of an inclined plane coiled about
the axis of the shaft 2 on that portion extending into the
hub of the gear 3. Thus a helical groove 8 on the shaft 2
finds its counterpart groove 9 within the inner circumfer-
20 once of the hub of gear 3. A jack screw, ball screw hot-
teal groove and key, or any similar helical device can be
used in accordance with the principles of the invention so
long as the helix angle is greater than the angle of friction
of the materials used. As shown in the drawing the axial
force F produced by the disc spring 4 is converted by the
helical device 8~9 into a torque T around the shaft 2, thus pro-
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voiding the desired anti backlash force between the rotating gears.
As already indicated, an important criterion in
the operation of the invention is that the helix angle be
greater than the friction angle of the materials employed.
It is also important, in the configuration shown in
Figures 1 and 2, that a gap exist between the two gears 1
and 3 in order to sustain the axial force on the helical
device. If, for example the gap is closed the force
is shunted through the adjacent gear faces and no relative
torque therefore is imparted. The width of the zap can
be determined by the lead of the helical device the
size of the gears and the amount of backlash to be
eliminated, as well as the effects of tolerance awoke-
mutation.
The proper axial force F exhibited by the disc
spring combination is determined by the size and type of
the helical device employed, the helix angle, the Coffey-
client of friction of the materials involved and the torque
I levels required to eliminate backlash. Any suitable dish
spring or disc spring combination can be devised for use
in the invention to achieve any desired level of torque.
The assembly according to the invention can be
installed in the unloaded condition, that is: with the
load nut 7 and washer 5 loosely assembled to the threaded
sleeve 6 and subsequently adjusted into a loaded condition
by ordinary hand tools. If, however access is limited
the assembly can be loaded prior to installation by lock-
in the gears 103 together by means of a bolt or pin
assembled through adjacent holes in the radial faces of
the gears. The load nut can be then preset to a desired
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setting for effecting the required axial force E' via
the disc spring combination and, after installation
of the assembly, the pin, or bolt, is removed, allowing
the assembly to spring into mesh with the associated
mating gears, not -shown.
The advantages ox the anti backlash assembly
according to the invention over conventional backlash
reduction methods are:
1) Low cost, since low ALMA class gears
can be used
2) Ample center distance tolerances can be
specified, thus reducing cost
3) Special bearings are not required
4) Matched sews, selective assembly, and gear
lapping are unnecessary
5) Absolutely Nero backlash can be achieved
at all operating loads
6) Environmental temperature variations do nut
affect performance
I . 7) No special assembly techniques or tooling
are required
8) Essentially infinite adjustability of the anti-
backlash torque can be provided within the
design load range
9) Reloaded bearings are not necessary
I
10) The invention is adaptable to any type of
gearing
11) Gear and bearing wear are automatically
compensated, and backlash remains at zero
throughout the life of the system
The foregoing relates to a preferred embodiment of
the invention it being understood that other embodiments
and variants thereof are possible within the spirit and
scope of the invention, the latter being defined by the
appended claims.
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SUPPLEMENTARY DISCLOSURE
In the principal disclosure, the disc spring
is disposed outside the gears. As an alternative, it may
be interposed between the gears, without a load nut and
washer. In this case, loading is accomplished via external
tooling and the device is locked in position until assembled
with its mating gear. Through the action of the helical
device, the axial force is converted to torque, which then
maintains a zero backlash mesh with the mating gear through-
out, or through any part of, the load spectrum of the system.
Figure 3 is a cross-sectional view of the embodiment
in accordance with the supplementary disclosure.
Referring to Figure 3, disc spring 4 is mounted
between gears 1 and 3, so that load washer 5 and load nut 7 are
not necessary. Similarly mounted is the anti friction bearing
device 10.
In the configuration shown in Fig. 3, no gap between
gears is required, because the disc spring, being interposed
between the gears, forces them apart, so that the alone-
mentioned shunting of the axial force cannot occur. It is important in this configuration, however, to maintain a gap
between the tubbed gear 3 and the adjacent end of sleeve 6
where supporting bearings abut when the whole is assembled
in the gear train housings. If this gap is lucid whereby
the tubbed gear 3 bears upon the supporting bearing, the axial
force is shunted through the bearing to the housing and no
relative torque is imparted to the gears.
I L should also be noted thaw loading before
assembly is required.
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