U.S. patent number 3,768,495 [Application Number 05/152,166] was granted by the patent office on 1973-10-30 for crutch with adjustable handgrip.
Invention is credited to Alfred A. Smith.
United States Patent |
3,768,495 |
Smith |
October 30, 1973 |
CRUTCH WITH ADJUSTABLE HANDGRIP
Abstract
The metal tubular shaft of a single-shaft crutch is locally
reinforced by a slidable sleeve of substantial length that embraces
the shaft and the sleeve carries a laterally extending handgrip.
The sleeve is adjustable along the shaft to vary the distance of
the handgrip from the upper end of the shaft. The length of the
sleeve is not less than the distance from the axis of the shaft to
the center of the applied load on the handgrip and therefore the
lateral force transmitted to the shaft at each of the two ends of
the sleeve is not more than the applied load.
Inventors: |
Smith; Alfred A. (North
Hollywood, CA) |
Family
ID: |
22541762 |
Appl.
No.: |
05/152,166 |
Filed: |
June 11, 1971 |
Current U.S.
Class: |
135/72;
135/69 |
Current CPC
Class: |
A61H
3/02 (20130101) |
Current International
Class: |
A61H
3/00 (20060101); A61H 3/02 (20060101); A61h
003/02 () |
Field of
Search: |
;135/52,51,50,49 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
675,275 |
|
Jul 1952 |
|
GB |
|
242,287 |
|
Dec 1962 |
|
AU |
|
118,446 |
|
Aug 1918 |
|
GB |
|
Primary Examiner: Taylor; Dennis L.
Claims
I claim:
1. A crutch including
a tubular shaft comprising
a first end,
a second end,
a substantially straight portion extending along a first axis from
said first end to said second end, and
a bent portion, integral with and extending from said second end
and angularly related to said first axis, having
a distal end,
tubular extension means telescopically mounted on said shaft at
said first end and having
means for contacting and supporting the crutch relative to a
surface being traversed by a user of the crutch,
means for releasably fixing the telescopic relationship between
said shaft and said extension means,
first support means mounted on said distal end of said bent portion
and defining a second axis passing through said first support means
and said contacting and supporting means,
second support means mounted on said tubular shaft and so oriented
relative thereto as to intersect both said first and second axes,
and
means for adjustably mounting said second support means on said
shaft comprising
tubular means telescopically mounted on said shaft and having
an upper end,
a lower end, and
a substantially straight portion extending between said upper and
lower ends, said tubular means being substantially longer than the
moment arm about said first axis of a force applied to said second
support means,
means for fixing said second support means to said mounting means,
and
means for releasably fastening said mounting means to said shaft
for adjustment of the position of the former along the latter.
2. The crutch of claim 1 including
means fixed at said upper and lower ends of said tubular means for
reducing play between said tubular means and said shaft.
3. The crutch of claim 1 wherein
said first support means comprises
armrest means which fits into an armpit of the user of the
crutch.
4. The crutch of claim 1 wherein
said first support means comprises
cuff means which at least partially encircle a portion of the arm
of a user of the crutch.
5. The crutch of claim 1 wherein said second axis is closer to the
body of a user of the crutch, at the intersection thereof with said
second support means, than is said first axis.
6. The crutch of claim 1 wherein the moment arm about said first
axis of a force exerted upon said second support means, by a user
of the crutch is substantially smaller than the moment arm along
said first axis of the resultant force couple exerted by said
tubular means on said shaft.
7. A crutch comprising
a tubular shaft having a predetermined wall thickness,
a handgrip for attachment to said shaft to support a user of the
crutch, and
means for attaching said handgrip to said shaft comprising
sleeve means, concentrically mounted on said shaft, to which said
handgrip is fixedly attached, said sleeve means having
a length, relative to the moment arm of said handgrip about the
axis of said shaft, sufficiently greater than said moment arm that
damage to said shaft wall, by the force couple exerted on said
shaft wall at the opposite ends of said sleeve as a result of a
force imposed upon said handgrip, is prevented by a reduction of
the size of the forces of the force couple relative to the size of
the force exerted on said handgrip.
8. The crutch of claim 7 including
means for adjusting the position of said sleeve along the axis of
said shaft.
Description
BACKGROUND OF THE INVENTION
This invention relates to a single-shaft tubular crutch of the type
disclosed in the Wood U.S. Pat. No. 2,736,330. Such a crutch has a
handgrip mounted on the shaft at an intermediate level to bear a
substantial portion of the weight of the user and the crutch is
adjustable in overall length as well as adjustable both with
respect to the distance of the lower end of the shaft from the
handgrip and the distance of the upper end of the shaft from the
handgrip.
In order to mount the handgrip rigidly on the shaft, the handgrip
of the above crutch is made integral with a collar that embraces
the shaft. The axial dimension of the collar is relatively short,
being comparable to the thickness of the handgrip. To permit
adjustment of the handgrip longitudinally of the shaft, the collar
is slidingly mounted on the shaft and is releasable tightened
against the shaft.
The short axial dimension of the collar is important in the
transmission to the shaft of forces created by the weight imposed
by the user on the handgrip. For the purpose of calculation it may
be assumed that the weight is applied to the midpoint of the length
of the handgrip so that the length of the moment arm that receives
the load is the distance from the midpoint of the handgrip to the
axis of the shaft.
The load that is transmitted to the handgrip by the user is
transmitted by the collar to the shaft as two opposite lateral
forces against the shaft at the two opposite ends respectively of
the collar. In the crutch disclosed in the above mentioned patent,
the collar on which the handgrip is mounted is so short that the
unavoidable consequence is that the collar actually multiplies the
force it receives from the handgrip. This fact increases the
possibility that the shaft will be marred if not permanently
deformed and it also increases the possibility that troublesome
play or looseness will develop between the collar and the shaft to
make it frequently necessary to tighten the collar against the
shaft.
It would be highly desirable for the short collar of the Wood
crutch to be easily and quickly adjustable along the tubular shaft
of the crutch to vary the distance of the handgrip from the upper
arm rest. For example, the collar could be provided in a well known
manner with a manually retractable spring-pressed locking pin that
is capable of selectively engaging a series of radial locking bores
in the wall of the tubular shaft. Since the collar must be slidable
for adjustment, however, such a construction would require that the
collar have an appreciable degree of looseness when in locked
position, whereas any perceptible looseness when the collar is
locked would be intolerable. The force-multiplying effect caused by
the relatively short axial dimension of the collar would magnify
the backlash at the collar in response to the periodic application
of load to the handgrip, the load being reversed each time the
clutch is lifted by the handgrip for a forward stride. The noise
created by repeated metal-to-metal impacts at the two ends of the
collar would not be acceptable and the perceptible looseness of the
handgrip would tend to undermine the confidence of the user in the
reliability of the crutch.
With reference to looseness or play between the handgrip and the
crutch shaft, it is to be noted that a collar that is short enough
to multiply the force that is transmitted to the shaft also has a
motion-multiplying effect in the reverse direction in that
clearance of a given magnitude between the two ends of the collar
and the shaft permits corresponding backlash which is multiplied in
the backlash movement of the handgrip when the load on the handgrip
is periodically reversed. Thus, a relatively short collar not only
increases the forces that tend to deform the crutch shaft but also
increases the play of the handgrip relative to the shaft.
To meet this problem of making the collar adjustable along the
shaft without looseness of the handgrip when the crutch is in use,
the handgrip of the crutch disclosed in the Wood patent is made
rigidly unitary with the collar and a set screw inside the handgrip
on the axis thereof is releasably tightened against the shaft to
prevent backlash. Unfortunately, the set screw must be severly
tightened against the shaft in a damaging manner. Sever tightening
is necessary to eliminate looseness of the handgrip and sever
tightening is also necessary because the set screw actually carries
the weight that is imposed on the handgrip by the user and
therefore must be tight enough to prevent slippage.
The unloosening and subsequent tightening of the set screw that is
involved in changing the elevation of the handgrip is not only time
consuming but also requires considerable strength and effort on the
part of the user. There is a definite need, therefore, for a crutch
construction wherein the handgrip is quickly and conveniently
locked at selected positions of adjustment by means that makes
positive engagement with the shaft instead of frictional engagement
and wherein the problem of backlash is solved in a satisfactory
manner.
SUMMARY OF THE INVENTION
A primary object of the invention is to solve the problem of making
the handgrip quickly and conveniently adjustable by means that
makes positive engagement with the shaft and to solve the related
problem of avoiding marring and deforming the shaft, together with
the related problem of avoiding disturbing play between the
handgrip and the shaft.
This primary object is obtained by two structural provisions that
distinguish the invention from the type of crutch that is disclosed
in the Wood patent. The first distinction is that the handgrip is
fixedly mounted on a reenforcement sleeve which is at least equal
to the length of the moment arm of the handgrip as measured from
the axis of the shaft to the midpoint of the length of the
handgrip. Making the adjustable reinforcement sleeve of this minmum
length both avoids multiplying the force that is applied to the
handgrip and avoids the reverse motion-multiplying effect of the
collar on the handgrip. Preferably the length of the sleeve
substantially exceeds the length of the moment arm to actually
reduce the force that is transmitted to the shaft by the
sleeve.
The second distinction is the provision of two annular spacers
between the crutch shaft and the respective two ends of the sleeve.
The two annular spacers are dimensioned radially to take up
substantially all of the annular clearance between the shaft and
the sleeve. One feature of the preferred practice of the invention
is that the two annular spacers are made of yieldable plastic
material for a number of purposes: first, to compensate for
tolerances in the outside diameter of the shaft and the inside
diameter of the sleeve, second, to provide yielding action between
the shaft and the sleeve that desirably cushions the loads that are
applied to the handgrip, and third, to avoid any snap action
between the sleeve and the shaft when the forces that are applied
to the handgrip are repeatedly reversed as the handgrip is employed
to bear a portion of the user's weight and alternately to lift the
crutch for a forward stride.
A further feature of the invention in this respect is the concept
of employing annular spacers of a plastic material that has a low
coefficient of friction with respect to the surface of the crutch
shaft. Using annular spacers made of nylon, for example, makes it
extremely easy to slide the sleeve along the crutch shaft from one
position of adjustment to another.
Further objects of the invention relate to the use of a slidable
external sleeve to reinforce a tubular shaft of a crutch. Providing
a relatively long external sleeve in the described manner not only
reduces the two forces that are transmitted to the shaft from the
handgrip and not only increases the spacing longitudinally of the
shaft between the two forces, but also reinforces the shaft itself
along the substantial length of the sleeve.
Another object of one practice of the invention is to reduce the
total mass of metal in the shaft by taking advantage of the
reinforcement effect of a relatively long external sleeve. If a
shaft were used without the benefit of a relatively long slidable
reinforcement sleeve, it would be necessary to make the shaft of a
given weight to keep the flexing of the shaft by the users weight
within a given degree of deflection. With the sleeve locally
reinforcing the shaft, however, the mass of material in the shaft
may be substantially reduced to reduce the overall weight and also
the overall cost of the crutch without exceeding the given degree
of deflection. The weight of the crutch as a whole is reduced by
reducing the mass of material of the shaft by a given amount and by
employing an effective reinforcement sleeve that weighs less than
the given amount. Or the cost of the crutch may be reduced by
reducing the cost of the shaft by a given amount and by using a
reinforcement sleeve that costs less than the given amount. Or both
the cost and the weight may be reduced in this manner.
Viewed from another aspect, a further object of the invention is to
improve on a crutch construction wherein the tubular shaft of the
crutch has a conventional adjustable handgrip and the tubular shaft
is reinforced by a fixed inner steel tube. The improvement consists
in transferring the steel reinforcement tube from inside the
tubular shaft to the outside of the tubular shaft, placing the
handgrip on the external reinforcement tube and providing
adjustable latch means to secure the external reinforcement tube at
selected positions along the shaft. With this change in the
organization of the structure of the crutch, a special advantage is
that the reinforcement tube now has the new capability of shifting
longitudinally with changes in adjustment in the handgrip so that
the spatial relationship between the reinforcement tube and the
hand-grip remains constant throughout the range of adjustment of
the position of the handgrip.
A still further object of one practice of the invention is to
provide an especially strong crutch with a plurality of overlapping
reinforcement tubes and with one of the reinforcement tubes at a
constant spatial relationship to the adjustable handgrip. In this
practice of the invention, the handgrip is mounted on an external
slidable reinforcement tube and throughout the range of adjustment
of the external reinforcement tube it overlaps two spaced internal
reinforcement tubes. The upper of the internal reinforcement tubes
may be fixed in the upper end of the crutch shaft to overlap the
upper end of the external reinforcement tube throughout the range
of longitudinal adjustment of the external reinforcement tube. The
lower internal reinforcement tube may be telescoped into the lower
end of the crutch shaft in an adjustable manner to serve as means
to change the overall length of the crutch and this lower
reinforcement tube may extend upward into the range of adjustment
of the external reinforcement tube without interferring with the
adjustability of the external reinforcement tube.
In another practice of the invention the slidable sleeve on which
the handgrip is mounted extends well below the handgrip to
reinforce a substantial portion of the lower half of the shaft in
the region where the shaft is stressed to the maximum by the load
that is imposed on the handgrip. In such a construction the
slidable sleeve may extend a substantial distance above the
handgrip and therefore be exceptionally long or, if desired, the
handgrip may be on the upper end of a sleeve of lesser length that
extends the same distance below the handgrip.
The features and advantages of the invention may be understood from
the following detailed description and the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view of an axillary crutch embodying
the invention;
FIG. 2 is a rear elevational view of the crutch;
FIG. 3 is an enlarged sectional view of the upper end of the crutch
as seen along the lines 3--3 of FIG. 2;
FIG. 4 is a section of the handgrip structure taken along the line
4--4 of FIG. 1;
FIG. 5 is a fragmentary section along the line 5--5 of FIG. 2
showing the means for adjustably anchoring the sleeve on the
shaft;
FIG. 6 is a fragmentary longitudinal sectional view taken along the
line 6--6 of FIG. 2 showing the adjustable extension at the lower
end of the shaft;
FIG. 7 is a sectional view along the line 7--7 of FIG. 2 showing
the lower end of the shaft;
FIG. 8 is a side elevational view of a forearm crutch embodying a
second practice of the invention;
FIG. 9 is a perspective view of the cuff at the upper end of the
crutch;
FIG. 10 is a side elevational view of a forearm crutch embodying a
third practice of the invention;
FIG. 11 is an enlarged perspective view of the cuff shown in FIG.
10;
FIG. 12 is an enlarged sectional view showing a form of a silencer
that may be employed at the two ends of a reinforcement sleeve,
FIG. 13 is an enlarged sectional view of another form of silencer
that may be employed;
FIG. 14 is a view similar to FIG. 1 indicating the construction of
an exceptionally strong crutch for use by an exceptionally heavy
person;
FIG. 15 is a fragmentary sectional view of the handgrip in FIG.
1;
FIG. 16 is a diagrammatic view showing the forces involved in the
application of a load to a handgrip that is mounted on an
exceptionally short slidable external sleeve;
FIG. 17 is a similar view showing the forces involved when the
length of the sleeve is equal to the moment arm of the force that
is applied to the handgrip;
FIG. 18 is a similar view showing the forces involved when the
length of the sleeve is 25 percent greater than the moment arm;
and
FIG. 19 is a similar view showing the forces that are involved when
the length of the sleeve is increased to 2.25 times the length of
the moment arm.
PREFERRED EMBODIMENTS OF THE INVENTION
FIGS. 1 to 7 show the construction of a single shaft axillary
crutch having an armrest 20 at the upper end of the crutch and a
handgrip 22 at an intermediate level. The single shaft 24 of the
crutch is tubular and for adjustment in length is provided with a
tubular extension 25 at the lower end which telescopes into the
shaft and which is provided in the usual manner with a rubber tip
or cushion 26 of well known construction.
As shown in section in FIG. 6, the tubular extension 25 may be
provided in its interior with a U-shaped leaf spring 28, one arm 30
of the leaf spring being permanently bonded to the inner wall of
the extension with the free arm of the leaf spring carrying a
round-nosed locking element 32. The locking element 32 extends
through a corresponding radial aperture in the wall of the tubular
extension 25 and extends through any one of a row of similar
apertures 34 in the shaft 24. Thus to change the effective length
of the shaft 24 it is merely necessary to depress the locking
element 32 manually to permit the tubular extension 25 to be
shifted as desired for engagement of the locking element with a
different aperture 34.
As taught by the disclosure of the Wood patent, awkwardness and
discomfort on the part of the user is avoided by forming the upper
end of the shaft with a curve 35 such that a straight line from the
middle of the armrest 20 to the bottom end of the crutch intersects
the handgrip 22. Such a line indicated at 36 in FIG. 1 may be
termed the support axis of the crutch. The inclination of the shaft
24 from the line 36 may be approximately 5.degree. and the line 36
is assumed to be a vertical line when the user's weight is balanced
on the crutch. The handgrip 22 is perpendicular to line 36 and
therefore is at an angle of approximately 95.degree. relative to
the shaft 24. It is apparent that the crutch is reversible about
the axis 36 and therefore the handgrip 22 may project either
forwardly or rearwardly, the handgrip being perpendicular to the
axis in each instance.
In accord with the teaching of the invention, the handgrip 22 is
rigidly mounted on a relatively long sleeve 40 that slidably
embraces the shaft 24 and is adjustable along the length of the
shaft. In the construction shown, the handgrip 22 is rigidly
mounted on a collar 42 and the collar, in turn is rigidly mounted
on the sleeve 40. As shown in FIG. 4, the collar 42 is formed with
an integral radially extending shank 44 which forms the core of the
handgrip 22, the core being surrounded by an elastomer casing 45
having an end wall 46. The casing is secured on the core 44 by
means of a screw 48 and a cooperating washer 50.
A feature of the invention is that the annular space between the
sleeve 40 and the surrounding collar 42 is filled with a suitable
adhesive cement that is bonded both to the outer surface of the
shaft and to the inner surface of the collar. For this purpose the
collar may have one or more radial bores 52 into which a suitable
material such as epoxy may be introduced in liquid form. When the
epoxy cures it serves in effect to make the core 44 of the handgrip
22 rigidly integral with the sleeve 40.
The sleeve 40 may be adjustable along the length of the shaft 24 in
the same manner as the lower extension 25 of the shaft. Thus FIG. 5
shows a U-shaped leaf spring 54 inside the shaft 24 with one arm 55
of the leaf spring permanently bonded to the interior of the shaft
and with the free arm of the leaf spring carrying a round-nosed
locking element 56. The locking element 56 may selectively engage a
series of longitudinally spaced apertures 58 in the sleeve 40.
To make the crutch structurally tight and to prevent any rattling
of the sleeve 40 on the shaft 24, suitable annular spacers may be
employed to provide snug fit at the opposite ends of the sleeve.
For this purpose, FIG. 5 shows how a bushing 60 of suitable
resiliently deformable plastic or elastomer may fixedly telescope
over each of the two ends of the sleeve with the bushing snuggly
but slidingly embracing the shaft 24.
Other types of annular spacers that may be employed at the opposite
ends of the sleeve 40 are shown in FIGS. 12 and 13. In FIG. 12 a
resiliently deformable plastic or elastomer bushing 62 is similar
to the bushing 60 except for the fact that the bushing has an inner
circumferential skirt 64 that is tapered in longitudinal cross
section to wedge into the annular space between the shaft 24 and
the sleeve 40.
In the construction shown in FIG. 13, each end of a sleeve 40a
screw-threadedly carries a collar 65 that confines an annular
spacer 66 which is made of a suitable resiliently deformable
plastic or elastomer. The collar 65 may be of split construction
and may be made in two separate halves. One of the circumferential
edges of the annular spacer 66 is formed with a bevel 68 and the
corresponding end of the sleeve 40a is formed with a chamfer 70 to
mate with the bevel 68. In like manner the other circumferential
edge of the annular spacer 66 is formed with a bevel 72 and the
inner circumference of the collar 65 is formed with an inner
circumferential conical shoulder 74 that mates with the bevel 72.
It is apparent that tightening the collar 65 on the end of the
sleeve 40a not only causes the annular spacer 66 to wedge into the
end of the sleeve but also causes the annular spacer to make
wedging engagement with the outer end of the collar 65.
The sleeve 40 is at least as long as the distance from the
longitudinal midpoint of the handgrip 22 to the axis of the shaft
24 and the handgrip is spaced a substantial distance from the upper
end of the sleeve. In this first embodiment of the invention, the
length of the sleeve 40 is approximately five times the length of
the moment arm measured from the midpoint of the handgrip.
It is apparent that if the handgrip 22 were adjustably mounted on
the shaft 24 by means of a short slidable sleeve in the form of the
collar 42, the sleeve 40 being omitted, the moment arm along the
handgrip would greatly exceed the length of the collar instead of
the length of the collar greatly exceeding the length of the moment
arm. Thus, with the handgrip rigidly united with the relatively
long sleeve 40 the force-multiplying effect is reversed with
consequent drastic reduction of the magnitude of the two opposite
forces that are transmitted by the sleeve to the shaft 24 laterally
thereof at the opposite ends respectively of the sleeve and, in
addition, the space between the two opposite forces measured
longitudinally of the shaft is greatly increased. At the same time
the slidable sleeve serves as a reinforcement for the shaft and
this reinforcement advantageously shifts with adjustment in the
level of the handgrip.
It is apparent from the foregoing that if a given shaft is flexed a
given amount by a given load applied to a handgrip that is attached
to the shaft solely by a relatively short collar, introducing the
relatively long sleeve 40 between the collar and the shaft
reinforces the shaft so effectively as to greatly reduce the degree
to which the shaft is flexed by the given load. Therefore, the
invention makes it possible to use a relatively light shaft without
the shaft being flexed to an unacceptable degree by the weight
imposed on the handgrip.
The second embodiment of the invention shown in FIGS. 8 and 9 is a
forearm crutch having a tubular shaft 78 with the usual upper
U-shaped cuff 80 and the usual handgrip 82. The U-shaped cuff 80
straddles the upper end of the shaft 78 and is anchored by a bolt
86 that extends through apertures 88 in the cuff and through a
diametrical bore (not shown) of the shaft 78, the bolt being
provided with a washer 90 and a thumb nut 92.
In the previously described manner, the lower end of the shaft 78
is provided with an adjustable extension 94 that is provided with
the usual rubber tip or cushion 95 and a locking element 56 inside
the extension 94 is adapted to selectively engage a series of
longitudinally spaced apertures 96 in the shaft 78.
The shaft 78 is formed with two opposite bends 96 and 98 which
divide the shaft into an upper end portion and a lower end portion
and these two opposite end portions of the shaft are at a slight
angle relative to each other. The angular relationship of the two
opposite end portions of the shaft 78 is such that the support axis
that is indicated by the broken line 100 from the middle of the
cuff 80 to the lower end of the shaft passes through the handgrip
82. In this instance, however, the handgrip 82 is perpendicular to
the upper portion of the shaft instead of being perpendicular to
the support axis and the crutch is not reversible.
In the previously described manner, the handgrip 82 is rigidly
mounted on a sleeve 102 by means of a collar 104 and the sleeve
embraces the upper end portion of the shaft in a slidable manner
for adjustment of the position of the handgrip 82. The sleeve 102
is provided with silencers or annular spacers 105 which may be
similar to the annular spacer 60 shown in FIG. 5, or may be similar
to the annular spacer 62 shown in FIG. 12, or may be of the
construction shown in FIG. 13. The sleeve 102 is adjustable
longitudinally of the shaft in the previously described manner, a
manually retractable locking element 56 releasably interconnecting
the sleeve and the shaft.
The sleeve 102 is somewhat shorter than the sleeve 40 in the first
embodiment but the length of the sleeve is more than three times
the distance from the midpoint of the handgrip 82 to the axis of
the shaft 78.
The third embodiment of the invention shown in FIGS. 10 and 11 is a
forearm crutch that has a straight shaft 110 to make the
construction relatively simple and to reduce the cost of the
crutch. The bottom end of the shaft 110 is provided with the usual
adjustable extension 112 and rubber tip 114 and the length of the
crutch may be changed in the previously described manner by
depressing a previously described locking element 32.
The handgrip 115 of the crutch is perpendicular to the shaft 110
and in the previously described manner is mounted on a slidable
sleeve 116 by means of a collar 118. The shaft 110 has the usual
locking element 56 that may be manipulated for changing the
position of the sleeve longitudinally of the shaft.
To make it possible for the support axis represented by the line
120 to intersect the handgrip 115 at a desired distance from the
shaft 110, the cuff 122 at the upper end of the shaft is spaced
from the shaft by a greater distance than the spacing between the
cuff 80 and the shaft 84 in FIG. 8. For this purpose the cuff may
be mounted on the upper end of the shaft 110 by means of a casting
123 as shown in FIG. 11. In FIG. 11 the cuff 122 is mounted by
rivets 124 to the back of a U-shaped bracket 125 and the bracket in
turn is mounted on a wing 126 of the casting 123 by means of a bolt
127.
FIG. 14 indicates the construction of an exceptionally strong
crutch to be used by an exceptionally heavy person. The crutch is
similar to the first embodiment of the invention shown in FIG. 1,
as indicated by the use of corresponding numerals to indicate
corresponding parts. The external slidable steel sleeve 40a is
exceptionally long and throughout its range of adjustment is in
constant overlapping relationship with an internal steel
reinforcement tube 130 inside the upper half of the shaft 24a and
is in constant overlapping relationship with a lower reinforcement
tube which is the extension 25a that is adjustably telescoped into
the interior of the shaft 24a. It is to be noted that the lower end
132 of the upper internal reinforcement tube 130 extends well into
the upper half of the sleeve 40a and the upper end 134 of the lower
extension tube 25a extends well into the interior of the lower end
of the sleeve 40a. Since the slidable external tube 40a overlaps
both of the internal tubes 130 and 25a throughout its range of
adjustment, the two internal reinforcement tubes and the external
tube overlap at all times for continuous reinforcement of the shaft
24a from the lower end of the shaft to a point well above the
slidable sleeve 40a.
In this last embodiment of the invention the handgrip 22a is
mounted on the shaft 24a in the manner shown in FIG. 15. In FIG. 15
a steel shank 135 is rigidly fixed to the sleeve 40a by welding 136
instead of being welded to a short collar that embraces the sleeve.
A casing 138 telescopes over the shank 135 and overhangs the outer
end of the shank in the same manner as heretofore described with
reference to FIG. 4. The casing 138 is secured to the shank in the
usual manner by a screw 140.
FIGS. 16 - 19 are diagrams that make clear the significance of
mounting a handgrip 142 on a slidable sleeve that is at least as
long as the moment arm that is applied to the handgrip by a load
imposed on the handgrip. In FIG. 16 the handgrip 142 is mounted on
a relatively short slidable sleeve 145 that is equipped with the
usual annular spacers at its opposite ends. The weight of the user
of the crutch applies a downward force F at the midpoint of the
handgrip 142 and the length of the resultant moment arm measured to
the axis of the shaft 145 is "a." The downward force F creates a
couple comprising a lateral force F.sub.1 at the upper end of the
sleeve 144 and an opposite force F.sub.2 at the lower end of the
sleeve. With the length of the sleeve 144 equal to only half of the
moment arm "a" the force received by the handgrip is multiplied
when it is transmitted to the shaft 145. Thus if the force F is 10
pounds, each of the forces F.sub.1 and F.sub.2 is twice as much or
20 pounds.
In FIG. 17 the handgrip 142 is mounted on a slidable sleeve 146 the
length of which is equal to the length of the moment arm "a." If
the force F is 10 pounds the two transmitted forces F.sub.1 and
F.sub.2 are each 10 pounds.
In FIG. 18 the handgrip 142 is mounted on a sleeve 148 that is 25
percent longer than the moment arm "a." The applied force of 10
pounds is transmitted to the shaft 145 as two opposite forces of
only 8 pounds each.
In FIG. 19 the handgrip 142 is mounted on a slidable sleeve 150 the
length of which is 2.25a. Here the applied force of 10 pounds is
reduced to two transmitted forces of only 4.45 pounds each.
It is apparent that in progressively increasing the length of the
slidable sleeve from the short length in FIG. 16 to the relatively
long length in FIG. 19, what may be termed a "reversal" point is
reached, the length of the sleeve at the reversal point being the
length of the moment arm "a" as shown in FIG. 17. When the length
of the slidable sleeve is less than "a" the force that is applied
to the handgrip is multiplied by the slidable sleeve but when the
length of the slidable sleeve is greater than "a" the applied force
is reduced by the slidable collar. Thus FIG. 17 illustrates the
reversal point where the length of the sleeve is equal to the
length of the moment arm. FIGS. 18 and 19 show that increasing the
length of the slidable sleeve greatly decreases the two forces that
are transmitted by the slidable collar to the shaft and in addition
increases the spacing along the shaft between the two applied
forces.
My description in specific detail of the selected embodiments of
the invention will suggest various changes, substitutions and other
departures from my disclosure within the spirit and scope of the
appended claims.
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