Torque isolation device

A torque isolation device for a force-sensitive load having a safe force limit to be imposed thereon, configured to transmit upstream rotational force within the limit to the load and transmit any force in excess thereof to a load-isolated path, is comprised of a radially displaceable driving coupling which includes a cam member having a dwell and bi-directional, outwardly oriented camming surfaces bounding same receiving a cam follower secured to a pivotal drive arm radially extensible from a biased primary transmission position, wherein the cam follower is disposed within the dwell, to an extended overload position in engagement with mechanical ground wherein the cam follower is displaced along the camming surface.

DESCRIPTION 
1. Technical Field 
The present invention relates generally to torque isolation devices and, 
more especially, to torque limiting devices of the type which will 
transmit rotational forces along a primary transmission path to a load 
until a preselected threshold value is reached, whereupon forces in excess 
of that threshold are coupled during overload conditions to a secondary 
transmission path isolated from the load to protect it against damage. The 
torque isolation device of the present invention may be operated in either 
of a self-energizing or non-self-energizing mode, and is therefore widely 
adaptable for myriad applications where a load is to be protected or 
effectively isolated from the imposition of excessive torque. A principal 
application of the torque isolation device of the present invention is in 
the context of nose wheel steering assemblies for large aircraft. 
2. Background Art 
All manner and variety of torque isolation or torque limiting devices have 
been proposed for the protection of an equally wide range of rotary 
devices. Machinery destined for diverse utilities, e.g., from 
metal-working machines to calculators, have been designed with an eye 
toward preventing damage in the event an input torque is applied in excess 
of the failure limits of load components. Such torque isolation devices 
are not only important in most such applications, in many they are 
crucial--particularly where the overall assembly is one having a high 
mechanical advantage. This is particularly true as respects various 
systems in aircraft; as it is desirable to provide sensitive control over 
sometimes considerable loads while nonetheless using lightweight 
structural components for overall system efficiency. 
Somewhat exemplary of a torque limiting device employed in aircraft is that 
disclosed in U.S. Pat. No. 4,030,578, assigned to the assignee of the 
present invention. The device described there is one which prevents the 
transmission of torque from a drive source to a torque-responsive element 
when a predetermined limit has been reached. The device utilizes plural 
axial members each including a multiplicity of balls for torque 
transmission, wherein the balls are engaged within precisely shaped 
sockets defined in each axial member. Upon the application of an excessive 
torque (i.e., one exceeding a threshold determined to be safe for the 
load), the balls cause axial displacement of one member relative to the 
other and the concomitant engagement of a plurality of discs. The latter, 
upon such engagement, effectively prevent torque transmission. While the 
torque limiter disclosed and claimed in that patent is a very effective 
one, it must be rearmed after the jamming load has disappeared in order to 
resume normal operation. For example, the input must be relaxed for the 
limiter to be reset to the operating configuration. 
Another type of protection device is disclosed in U.S. Pat. No. 2,580,298; 
there in the nature of an automatic stop for a calculating machine. Again 
the objective is to protect a sensitive part from being overdriven and 
perhaps damaged due to, e.g., jamming. The transmission includes, inter 
alia, a star wheel engaging pawl members. In the event a power-driven part 
is prevented from operating, or otherwise becomes locked, the pawls will 
yield and rock outwardly from the star wheel. During that movement, the 
pawls engage a shoe on the arm of a bail, the latter of which is pivoted 
on a tie rod. The bail also includes an arm which effectively engages a 
switch thereby de-energizing the power source. This approach is common, at 
least in a conceptual sense, to that further disclosed in U.S. Pat. No. 
2,753,029. 
Other load-limiting devices are the subjects of U.S. Pat. Nos. 1,745,738, 
2,003,115, 2,172,834 and 4,175,484. While the apparatus disclosed therein 
are designed for utilities in widely disparate applications, each includes 
some type of disconnect mechanism, usually including a ratchet and pawl 
assembly, as an overload guard. U.S. Pat. Nos. 1,548,427 and No. 2,425,736 
are also noteworthy within this context. In the '427 approach, relative 
rotational movement in excess of a predetermined amount results in a 
disconnection of the input from the output and a locking of the system by 
means of a pawl. In the '736 approach, a biased cam/cam follower (i.e., 
roller) operates to effectuate force disengagement by disconnection. 
While there has been considerable activity in the design of devies within 
the aforesaid class, a common drawback includes the complete interruption 
of force transmission upon the occurrence of excess loading. In turn, some 
type fo manual reset or other manipulation of either load or drive is 
necessary to reestablish the normal transmission configuration. Thus, in 
the event the jamming of the load is one which arises either sporadically 
or which tends to dissipate over time, there arises a certain amount of 
inconvenience by virtue of the complete disruption in transmission. In 
those situations, it would be more desirable were the device to respond 
for torque isolation as required, but only for so long as so required; 
with an automatic return to the normal operating configuration one the 
jamming (and, hence, need for protection) subsides. 
SUMMARY OF THE INVENTION 
The present invention advantageously provides a torque isolation device 
which simply yet highly efficiently protects the load against the 
application of excessive torque. The present invention is particularly 
desirable for its ability to operate in a mode which permits the 
application of torque to the load up to a limiting value and then couples 
any excess to a secondary load path independent of the principal path 
during overload conditions. This leads to the further advantage of 
permitting the application of safe torque to the load irrespective of the 
presence of excessive torque, whereby control over the load may be 
maintained under even extreme operating conditions. The device of the 
present invention is further desirable for its ability to operate in a 
non-self-energizing mode, whereby a normal operational configuration is 
established upon dissipation of the jamming force or other cause of 
excessive torque. A further advantage of the precise structure of the 
torque isolation device of the present invention is its easy adaptability 
to a self-energizing mode should that be a desirable goal. 
The foregoing and other advantages of the present invention are realized in 
a torque isolation device configured to transmit an upstream rotational 
force to a downstream load for force values less than or equal to a 
threshold or limit value and then to couple force in excess of that 
threshold to a load-isolated path during overload condition; which device 
comprises a radially displaceable driving coupling including cam means 
having an outwardly oriented cam surface engaging cam follower means 
secured to pivotal arm means extensible from a biased, primary 
transmission position with the cam follower in a dwell in the cam surface 
to an extended secondary transmission position with the cam followers 
displaced along the cam surfaces against the biasing force on those arms 
and in engagement with a mechanical ground constituting the load-isolated 
path. In the most preferred configuration, the cam members are disposed on 
the output side of the device and it operates in a non-self-energizing 
mode. However, the cam members may be disposed on the input side of the 
device and, to the extent desirable, the device may operate in a 
self-energizing mode. In either case, however, the drive and load are 
linked for force transmission up to the preestablished threshold, after 
which any excess is routed mechanically away from the load so that it does 
not experience the potentially damaging excess. When operating in the most 
preferred, non-self-energizing mode, dissipation of the cause for forces 
in excess of the limiting value results in the reestablishment of the 
normal operating configuration until and unless those causes or another 
giving rise to a force in excess of the limiting value reappear. 
A highly preferred structural embodiment of the torque isolation device of 
the present invention is comprised of input and output shaft members 
disposed for semi-independent, relative rotational displacement with 
respect to one another; first and second drive arms secured for rotation 
with the input shaft, each of which is pivotal about a fixed point at a 
first end thereof from the biased, primary transmission position to a 
radially extended overload position during which the free ends of the 
drive arms move along a radially arcuate path; cam members secured for 
rotation with the output shaft, including radially outwardly directed 
camming surfaces having a dwell position corresponding to the primary 
transmission position; cam followers secured to each of the drive arms for 
driving engagement with the cam members; and biasing means securing the 
free ends of the drive arms for urging those arms into the primary 
transmission position with a predetermined biasing force correlated to the 
limiting torque to be transmitted to the output shaft. Thus, mechanical 
engagement between input and output is coupled across the cam/cam follower 
arrangement; the engagement of which is maintained at a predetermined 
force level dictated by the constrictive biasing force in combination with 
the cam geometry. Whenever excess force over the limiting value is 
effectively experienced on the output, the cam followers are cammed 
outwardly along the cam surfaces causing radial displacement of the drive 
arms. In the most preferred configuration, the drive arms include brake 
pads which, upon radial displacment, contact a brake drum constituting a 
mechanical ground isolated from the load. Dissipation of the excess force 
over the limiting value causes a collapse of the drive arms in response to 
the biasing force and a resumption of the normal operating 
configuration--thus providing the non-self-energizing feature. 
Should the same structure be employed with the input made to the cam side 
of the coupling, a self-energizing mode is established. This 
self-energizing feature is one established within the limits of the design 
and, accordingly, under some situations collapse to the primary 
transmission configuration upon dissipation of the overload may ensue. 
The torque isolation device of the present invention is principally adapted 
for interposition within the steering system for the nose wheel of 
aircraft. Accordingly, both input and output will be in angular increments 
as opposed to full or multiple revolutions. However, upon appropriate 
design to account for inertia of the components, the torque isolation 
device of the present invention is equally well adaptable for such 
multiple-revolution applications. 
The foregoing and other advantages will become more apparent, and a fuller 
appreciation of the structure and mode of operation of the torque 
isolation device of the present invention will be gained, upon examination 
of the following detailed description of the invention taken in 
conjunction with the figures of drawing.

DETAILED DESCRIPTION 
The present invention relates, generally, to torque isolation devices and, 
more especially, to a non-self-energizing torque isolation device which 
will limit the force applied to or otherwise experienced by the load, 
permitting that force to be maintained at or below the limiting value and 
transferring any force in excess thereof to a mechanical ground having a 
load-isolated path. The torque isolation device of the present invention 
may be adapted for use in any application where the aforementioned 
features are necessary or desirable; albeit, it is particularly well 
suited for use in aircraft systems such as, e.g., nose wheel steering 
systems. Accordingly, the present invention will now be described with 
reference to certain preferred embodiments within the context aforesaid; 
although those skilled in the art will appreciate that such a description 
is meant to be exemplary only and should not be deemed limitative of the 
scope of the invention either in terms of structure or application. 
Turning to the figures of drawing, wherein like parts are identified with 
like reference numerals, a torque limiting device in accordance with the 
present invention, designated generally as 10, is comprised of an upper 
housing member 12 and a lower housing member 14 secured by fixture means 
16. An input shaft 18 is disposed through a bore 20 in the housings and is 
journalled for rotation therein. In the embodiment shown, the upper end of 
the shaft 18 is in effective communication with a power drive or prime 
mover which supplies a driving torque to the device. An output member 22 
is disposed outwardly proximate the lower circumferential periphery of the 
drive shaft 18 in a manner permitting for relative rotational displacement 
one with respect to the other, in the sense that input and output are 
semi-independent across a driving coupling as described below. In the 
preferred embodiment shown, the output member 22 is a quadrant or pulley 
havng tapered, inwardly directed grooves 24 for receipt, e.g., of control 
cables. For purposes of the present description, let it be assumed that 
the device 10 is incorporated within the steering system for the nose 
wheel of an aircraft. In such a case, the input shaft 18 will be in 
operative engagement with a drive source responsive to the pilot's tiller 
in the cockpit. In turn, cables disposed about the quadrant 22 within the 
grooves 24 will be in operative engagement with the hydraulic control 
system manipulating movement of the nose wheel. Within this environment, 
it is desirable to provide the pilot with a fairly easily moved tiller in 
order to control the wheel, supply a good mechanical advantage through the 
system, and ultimately steer a relatively heavy aircraft thus requiring 
considerable ultimate force application. And, it is equally desirable to 
employ lightweight materials which, due to the considerable mechanical 
advantage in the system, cannot be subjected to excessive forces during 
jamming. These objectives are reconciled in the torque isolation device 10 
by employing a radially displaceable driving coupling where forces in 
excess of the limiting force result in displacement of the coupling to 
transfer the excess along a load-isolated path while nonetheless 
transmitting the force up to the limit to the load. 
In the illustrated embodiment, the torque isolation device includes an 
opposed pair of driving couples, designated generally as 30, which 
collectively comprise the radially displaceable coupling aforesaid. Two 
such couples 30 are employed in this preferred structural implementation, 
but that is not a rigid requirement of the invention. The opposed pair of 
couples is found to be most practical and efficient for the designed 
utility of the torque isolation device 10 as the input and output motions 
are normally bi-directional angular increments as opposed to complete or 
multiple revolutions. The same principles might be used to good advantage 
in other applications where one driving couple would function quite 
adequately or where three or more would prove beneficial. In any of these 
events, however, each driving couple includes a cam/cam follower 
arrangement where one of the couple elements is associated with the input 
and the other with the output. Under normal operating conditions the two 
remain in mated, primary transmission configuration for force transmission 
through the device; but, once the threshold force value for which the 
device has been designed is exceeded, the couple is cammed with the 
follower riding across the camming surface to a secondary transmission 
configuration routing or bypassing the excess force to a mechanical ground 
under overload conditions. 
More specifically, and with particular reference to the figures of drawing, 
each of the couples 30 is shown to be comprised of a generally "V"-shaped 
cam 32 and a cam follower 34. The cam includes a dwell 36, corresponding 
to the apex of the "V", which receives the cam follower in the normal or 
primary transmission configuration of the device. The cam surface 38 then 
projects radially outward in either direction from the dwell; the shape, 
pitch, or other surface profile of the cam surface being designed with due 
regard for the specific limiting force to be transmitted through the 
device, as indicated in further detail below. As can best be seen with 
reference to FIG. 2, the cam 32 is secured to the output quadrant 22, in 
this case by means of a rivet 40. Accordingly, the output and cam are 
keyed for direct rotation together an force applied to the cam will be 
transmitted to the output quadrant. The riveting approach is the preferred 
one within the context of the application for torque isolation device 10 
as the output quadrant is desirably a lightweight material while the cam 
itself will most advantageously be formed from steel for improved wear 
characteristics. In other environments the cam/output could be made 
integral or individual parts could be welded or otherwise bonded into a 
unitary member. 
Looking to the input side of the device 10, the cam follower is similarly 
secured for direct rotation, but now with the input shaft 18. A rigid link 
42 is splined to the shaft 18 for rotation therewith, and supports a pair 
of opposed drive arms 44. Each of the drive arms is shown to be generally 
"L"-shaped where a first end 46 is pinned to the link 42 by a fixture pin 
48 so that the arm is free for radially arcuate displacement about the 
pivot point. The cam follower 34 associated with each arm is received in a 
channel or slot 50 therein and is pinned by fixture member 52 for rotation 
so that is may ride across the camming surface 38. The free ends 54 of the 
arms 44 are secured by a bridging spring assembly designated generally as 
56. Spring assembly 56 biases the drive arms, urging and maintaining them 
in the primary transmission configuration shown in the figures of drawing. 
The biasing spring together with the geometry of the cam surface, 
establishes the threshold torque value which can be transmitted through 
the device in this primary mode. On experiencing an excess torque, the 
biasing force will be overcome and the drive arms 44 will be cammed 
radially outward about the pivot point 48 into an overload configuration. 
The lower housing 14 includes a downwardly depending skirt 58 and guard 60 
circumferentially enveloping the drive components. The portion of skirt 58 
above the guard 60 outwardly proximate the drive arms 44 also serves as a 
mechanical ground akin to a brake drum against which the drive arms will 
bear in the overload position; in the sense that it is rigidly affixed to 
the drive side of the system through the upper housing 12 and is otherwise 
independent from the load side of the device. Each of the drive arms 
includes a brake pad 62 received in slot 50; projecting radially outward 
therefrom for engagement with the skirt 58 under overload circumstances. 
In operation, the torque isolation device of the present invention simply 
yet efficiently transfers overload torque to the mechanical ground 58, 
away from the load while continuing to transmit the force at or below the 
established threshold. That threshold is governed by the biasing force 
holding the drive arms in the primary transmission position with the cam 
follower nested in the cam dwell, along with the geometry of the cam 
surface itself. Torque applied to the input shaft 18 will cause the link 
42 to move along with the slaved drive arms 44. That force is coupled 
across the cam follower to the cam 32 and, accordingly, to the output 
quadrant 22 to which it is itself rigidly affixed. Consequently, an 
incremental angular input on drive shaft 18 results in an equal 
incremental angular output on quadrant 22. In the event of an overload or 
jamming situation effectively freezing the output quadrant, force in 
excess of that which is safe for the load to experience is routed to the 
mechanical ground 58 which constitutes an independent load isolation path. 
More specifically, as the input shaft continues to have a force applied to 
it, and once that force exceeds the limiting force, it will be transmitted 
through the drive arms 44; but now the cam 32 is stationary due to the 
jamming of the output quadrant 22. In the event, the additional force 
causes the cam followers to ride across the cam surface 38 against the 
biasing force provided by spring assembly 56. Concomitantly, the drive 
arms 44 pivot radially outward along an arcuate path about the pivot point 
at pin 48. This brings the brake pad 62 into contact with skirt 58 and the 
force in excess of the safe force is transmitted to this mechanical 
ground. If the downstream jamming dissipates, the quadrant will be freed 
for rotation and the device will collapse to its normal or primary 
transmission configuration shown in the figures of drawing. It should be 
appreciated that the time response characteristics of the instant device 
for the transition from the normal to the overload configuration upon the 
sensing of an overload input may be tailored by appropriately sizing the 
gap between the brake pad 62 and skirt 58. This may be achieved in any 
number of ways, such as simply sizing the pad to provide a desired 
temporal response, and adds to the versatility of the device in both its 
range of applications and adaptability within a specific application. 
Taking a situation within the design objective of the instant invention, 
let it be assumed that the safe force for steerage of the nose wheel of an 
aircraft is 600 pounds. Let it further be assumed that, due to the 
considerable mechanical advantage built into the system, the drive is 
capable of developing 1000 pounds of force which, if applied to the nose 
wheel steering assembly, would damage it. Let is additionally be assumed 
that the nose wheel of the aircraft is frozen to the ground in a patch of 
ice, effectively immobilized against movement. As the pilot manipulates 
the tiller and the force approaches 600 pounds, that will be transmitted 
through the isolation device 10. Should be wheel break free as the force 
increases, normal operation is maintained. However, should the wheel 
require greater than 600 pounds force to the steering assembly to break 
from the restraining ice, the continued application of force in excess of 
the design limit to, e.g., 1000 pounds, will cause radial displacement of 
the drive arm 44 so that the brake pads 62 engage the mechanical ground 58 
isolating that additional 400 pounds while the 600 pound safe load 
remains applied to the assembly. Should the ice now melt while this force 
remains applied, the wheel will respond to the primary transmission of the 
600 pound force. This also frees the output quadrant 22 and the drive arms 
collapse in response to the biasing force of the spring assembly 56 and 
resume the normal operating configuration. This is a significant advantage 
over conceptually similar torque isolation devices as the structure of the 
present invention allows for automatic rearming once the cause of the 
overload disappears; eliminating the need either for a manual reset or for 
requirements that the input be backed off to allow reestablishment of the 
primary configuration. 
It will be appreciated by those skilled in the art that the device of the 
present invention may be reversed in terms of input and output and still 
achieve good torque isolation. The couples 30 comprising the device will 
operate conceptually the same whether the force is applied to the cam 
follower and the output taken from the cam ov vice versa. Consequently, 
the input could be made through the quadrant 22 or a shaft in lieu thereof 
otherwise secured directly to the cam 32. This input will be transmitted 
across the couple to the cam follower, resulting in a drive on the arms 
44. In turn this will be transmitted through the link 42 to the shaft 18 
or other output element. When excess force is applied through the cam 32 
as the result of a jamming of the output at 18, the continued rotation of 
the input 22 will force the movement of the drive arms 44 to the overload 
configuration where the brake pad 62 engages the mechanical ground 58. 
Thus, the operation is consistent with that noted above. In many 
circumstances, the same self-collapsing feature (i.e., non-self-energizing 
feature) will be achieved; albeit, it is anticipated that conditions 
approaching the design limitation of the device may yield a 
self-energizing operation when the input and output is reversed in this 
fashion. 
Those skilled in the art will further appreciate that the principles set 
forth above can be implemented in devices operating under complete or 
multiple revolution situations as opposed to the incremential angular 
operation discussed with reference to steerage systems. Of course, balance 
to account for inertia of the components will be required, but those 
familiar with this art will have no difficulty achieving that aim. 
Accordingly, apart from the specific intended application wihtin a 
steering system, the torque isolation device of the present invention 
might equally well be employed in, e.g., a metal working machine. Should 
the load jam the tool or should there by any other overload condition, 
freezing the output will result in a coupling of the input for forces in 
excess of the limiting value to a load-isolated path thereby protecting 
the work. 
As can readily be seen from the foregoing, the torque isolation device of 
the present invention enjoys the highly desirable attribute of simplicity 
in design while nonetheless providing an extremely reliable, efficient, 
and widely adaptable apparatus which will protect loads from forces in 
excess of an established safe one. With those thoughts in mind, the 
invention has been described with reference to certain preferred 
embodiments within selected applications for utility; but those skilled in 
the art will appreciate that various substitutions, modifications, changes 
and omissions may be made without departing from the spirit thereof. 
Hence, it is intended that the scope of the present invention be limited 
solely by that of the claims granted herein.