TWELVE WAYS TO PUT SPRINGS TO WORK
Variable-rate arrangements, roller positioning,
space saving, and other ingenious ways
to get the most from springs.
177
This setup provides a variable rate with a sudden change
from a light load to a heavy load by limiting the low-rate
extension with a spring.
This mechanism provides a three-step rate change at prede-
termined positions. The lighter springs will always compress
first, regardless of their position.
This differential-rate linkage sets the actuator
stroke under light tension at the start, then
allows a gradual transition to heavier tension.
This compressing mechanism has a dual rate for double-
action compacting. In one direction pressure is high, but in
the reverse direction pressure is low.
Roller positioning by a tightly wound
spring on the shaft is provided by this
assembly. The roller will slide under
excess end thrust.
A short extension of the spring for a long
movement of the slide keeps the tension
change between maximum and minimum low.
Sclater Chapter 6 5/3/01 12:24 PM Page 177
178
Increased tension for the same movement is
gained by providing a movable spring mount
and gearing it to the other movable lever.
This pin grip is a spring that holds a pin by friction
against end movement or rotation, but lets the pin be
repositioned without tools.
A close-wound spring is attached to
a hopper, and it will not buckle when it
is used as a movable feed-duct for
nongranular material.
Toggle action here ensures that the gear-
shift lever will not inadvertently be thrown
past its neutral position.
Tension varies at a different rate when
the brake-applying lever reaches the posi-
tion shown. The rate is reduced when the
tilting lever tilts.
The spring wheel helps to distribute deflection
over more coils that if the spring rested on the cor-
ner. The result is less fatigue and longer life.
Sclater Chapter 6 5/3/01 12:24 PM Page 178
OVERRIDING SPRING MECHANISMS FOR
LOW-TORQUE DRIVES
Fig. 1 Unidirectional override. The take-off lever of this mechanism can rotate nearly
360°. Its movement is limited only by one stop pin. In one direction, motion of the driving
shaft is also impeded by the stop pin. But in the reverse direction the driving shaft is
capable or rotating approximately 270° past the stop pin. In operation, as the driving
shaft is turned clockwise, motion is transmitted through the bracket to the take-off lever.
The spring holds the bracket against the drive pin. When the take-off lever has traveled
the desired limit, it strikes the adjustable stop pin. However, the drive pin can continue
its rotation by moving the bracket away from the drive pin and winding up the spring. An
overriding mechanism is essential in instruments employing powerful driving elements,
such as bimetallic elements, to prevent damage in the overrange regions.
Fig. 2 Two-directional override. This mechanism is similar to that described under
Fig. 1, except that two stop pins limit the travel of the take-off lever. Also, the incoming
motion can override the outgoing motion in either direction. With this device, only a
small part of the total rotation of the driving shaft need be transmitted to the take-off
lever, and this small part can be anywhere in the range. The motion of the deriving shaft
is transmitted through the lower bracket to the lower drive pin, which is held against the
bracket by the spring. In turn, the lower drive pin transfers the motion through the upper
bracket to the upper drive pin. A second spring holds this pin against the upper drive
bracket. Because the upper drive pin is attached to the take-off lever, any rotation of the
drive shaft is transmitted to the lever, provided it is not against either stop A or B. When
the driving shaft turns in a counterclockwise direction, the take-off lever finally strikes
against the adjustable stop
A. The upper bracket then moves away from the upper drive
pin, and the upper spring starts to wind up. When the driving shaft is rotated in a clock-
wise direction, the take-off lever hits adjustable stop
B, and the lower bracket moves
away from the lower drive pin, winding up the other spring. Although the principal appli-
cations for overriding spring arrangements are in instrumentation, it is feasible to apply
these devices in the drives of heavy-duty machines by strengthening the springs and
other load-bearing members.
Overriding spring mechanisms are widely
used in the design of instruments and controls.
All of the arrangements illustrated allow an
incoming motion to override the outgoing
motion whose limit has been reached. In an
instrument, for example, the spring mechanism
can be placed between the sensing and
indicating elements to provide overrange
protection. The dial pointer is driven positively
up to its limit before it stops while the input
shaft is free to continue its travel. Six of the
mechanisms described here are for rotary
motion of varying amounts. The last is for
small linear movements.
Fig. 3 Two-directional, limited-travel override. This mecha-
nism performs the same function as that shown in Fig. 2, except
that the maximum override in either direction is limited to about
40°. By contrast, the unit shown in Fig. 2 is capable of 270°
movement. This device is suited for applications where most of
the incoming motion is to be used, and only a small amount of
travel past the stops in either direction is required. As the arbor is
rotated, the motion is transmitted through the arbor lever to the
bracket The arbor lever and the bracket are held in contact by
spring B. The motion of the bracket is then transmitted to the
take-off lever in a similar manner, with spring A holding the take-
off lever until the lever engages either stops A or B. When the
arbor is rotated in a counterclockwise direction, the take-off lever
eventually comes up against the stop B. If the arbor lever contin-
ues to drive the bracket, spring A will be put in tension.
179
Sclater Chapter 6 5/3/01 12:24 PM Page 179
Fig. 4 Unidirectional, 90° override. This is a single
overriding unit that allows a maximum travel of 90°
past its stop. The unit, as shown, is arranged for
overtravel in a clockwise direction, but it can also be
made for a counterclockwise override. The arbor
lever, which is secured to the arbor, transmits the
rotation of the arbor to the take-off lever. The spring
holds the drive pin against the arbor lever until the
take-off lever hits the adjustable stop. Then, if the
arbor lever continues to rotate, the spring will be
placed in tension. In the counterclockwise direction,
the drive pin is in direct contact with the arbor lever
so that no overriding is possible.
Fig. 5 Two-directional, 90° override. This double-overriding mechanism allows a
maximum overtravel of 90° in either direction. As the arbor turns, the motion is carried
from the bracket to the arbor lever, then to the take-off lever. Both the bracket and the
take-off lever are held against the arbor lever by spring A and B. When the arbor is
rotated counterclockwise, the takeoff lever hits stop A. The arbor lever is held station-
ary in contact with the take-off lever. The bracket, which is fastened to the arbor,
rotates away from the arbor lever, putting spring A in tension. When the arbor is
rotated n a clockwise direction, the take-off lever comes against stop B, and the
bracket picks up the arbor lever, putting spring B in tension.
Fig. 6 Unidirectional, 90° override. This mech-
anism operates exactly the same as that shown in
Fig. 4. However, it is equipped with a flat spiral
spring in place of the helical coil spring used in
the previous version. The advantage of the flat
spiral spring is that it allows for a greater override
and minimizes the space required. The spring
holds the take-off lever in contact with the arbor
lever. When the take-off lever comes in contact
with the stop, the arbor lever can continue to
rotate and the arbor winds up the spring.
Fig. 7 Two-directional override, linear motion. The previous mechanisms were over-
rides for rotary motion. The device in Fig. 7 is primarily a double override for small linear
travel, although it could be used on rotary motion. When a force is applied to the input lever,
which pivots about point C, the motion is transmitted directly to the take-off lever through the
two pivot posts, A and B. The take-off lever is held against these posts by the spring. When
the travel causes the take-off lever to hit the adjustable stop A, the take-off lever revolves
about pivot post A, pulling away from pivot post B, and putting additional tension in the
spring. When the force is diminished, the input lever moves in the opposite direction until the
take-off lever contacts the stop B. This causes the take-off lever to rotate about pivot post B,
and pivot post A is moved away from the take-off lever.
180
Sclater Chapter 6 5/3/01 12:24 PM Page 180
SPRING MOTORS AND TYPICAL ASSOCIATED
MECHANISMS
Many applications of spring motors in clocks, motion picture
cameras, game machines, and other mechanisms offer practical
ideas for adaptation to any mechanism that is intended to operate
for an appreciable length of time. While spring motors are usu-
ally limited to comparatively small power application where
other sources of power are unavailable or impracticable, they
might also be useful for intermittent operation requiring compar-
atively high torque or high speed, using a low-power electric
motor or other means for building up energy.
181
Sclater Chapter 6 5/3/01 12:24 PM Page 181
The accompanying patented spring motor designs show vari-
ous methods for the transmission and control of spring-motor
power. Flat-coil springs, confined in drums, are most widely used
because they are compact, produce torque directly, and permit
long angular displacement. Gear trains and feedback mecha-
nisms reduce excess power drain so that power can be applied for
a longer time. Governors are commonly used to regulate speed.
182
Sclater Chapter 6 5/3/01 12:24 PM Page 182
FLEXURES ACCURATELY SUPPORT PIVOTING
MECHANISMS AND INSTRUMENTS
Flexures, often bypassed by various
rolling bearing, have been making steady
progress—often getting the nod for
applications in space and industry where
their many assets outweigh the fact that
they cannot give the full rotation that
bearings offer.
Flexures, or flexible suspensions as
they are usually called, lie between the
worlds of rolling bearings—such as the
ball and roller bearings—and of sliding
bearings—which include sleeve and
hydrostatic bearings. Neither rolling nor
sliding, flexures simply cross-suspend a
part and flex to allow the necessary
movement.
There are many applications for parts
of components that must reciprocate or
oscillate, so flexure are becoming more
readily available as the off-the-shelf part
with precise characteristics.
Flexures for space. Flexures have
been selected over bearings in space
applications because they do not wear
out, have simpler lubrication require-
ments, and are less subject to backlash.
One aerospace flexure—scarcely
more than 2 in. high—was used for a key
task on the Apollo Applications Program
(AAP), in which Apollo spacecraft and
hardware were employed for scientific
research. The flexures’ job was to keep a
5000-lb telescope pointed at the sun with
unprecedented accuracy so that solar
phenomena could be viewed.
The flexure pivot selected contained
thin connecting beams that had flexing
action so they performed like a combina-
tion spring and bearing.
Unlike a true bearing, however, it had no
rubbing surfaces. Unloaded, or with a small
load, a flexure pivot acts as a positive—or
center-seeking—spring; loaded above a
certain amount, it acts as a negative spring.
A consequence of this duality is that
in space, the AAP telescope always
returned to a central position, while dur-
ing ground testing it drifted away from
center. The Lockheed design took advan-
tage of this phenomenon of flexure piv-
ots: By attaching a balancing weight to
the telescope during ground tests,
Lockheed closely simulated the dynamic
conditions of space.
Potential of flexures. Lockheed
adapted flexure pivots to other situations
as well. In one case, a flexure was used
for a gimbal mount in a submarine.
Another operated a safety shutter to pro-
tect delicate sensors in a satellite.
Realizing the potential of flexure piv-
ots, Bendix Corp. (Utica, N.Y.) devel-
oped an improved type of bearing flex-
ure, commonly known as “flexure
pivot.” It was designed to be compliant
around one axis and rigid around the
cross axes. The flexure pivots have the
same kind of flat, crossed springs as the
rectangular kind, but they were designed
as a simple package that could be easily
183
A frictionless flexure pivot, which resembles a bearing, is made
of flat, angular crossed springs that support rotating sleeves in a
variety of structural designs.
A universal joint has flexure pivots so there is no need for
lubrication. There is also a two-directional pivot made with inte-
gral housing.
A pressure transducer with a flexure pivot can oscillate 30º to
translate the movements of bellows expansion and contraction into
electrical signals.
A balance scale substitutes flexure pivots in place of a knife edge,
which can be affected by dirt, dust, and sometimes even by the
lubricants themselves.
Sclater Chapter 6 5/3/01 12:24 PM Page 183
installed and integrated into a design (see
photo). The compactness of the flexure
pivot make it suitable to replace ordinary
bearings in many oscillating applications
(see drawings).
The Bendix units were built around
three elements: flexures, a core or inner
housing, and an outer housing or mount-
ing case. They permit angular deflections
of 7
1
⁄2°, 15°, or 30°.
The cantilever type (see drawing) can
support an overhung load. There is also a
double-ended kind that supports central
loads. The width of each cross member of
the outer flexure is equal to one-half that
of the inner flexure, so that when assem-
bled at 90° from each other, the total flex-
ure width in each plane is the same.
184
The Apollo telescope-mount cluster (top
left) had flexures for tilting an X-ray tele-
scope. The platform (top right) is tilted with-
out break-away torque. The photo above
shows typical range of flexure sizes.
Key point. The heart of any flexure
pivot is the flexure itself.
A key factor in applying a flexure is
the torsional-spring constant of the
assembly—in other words, the resisting
restoring torque per angle of twist, which
can be predicted from the following
equation:
where
K = spring constant, in lb/deg
N = number of flexures of width b
E
= modulus of elasticity, lb/in.
2
b = flexure width, in.
t = flexure thickness, in.
L = flexure length, in.
C = summation of constants result-
ing from variations in tolerances and
flexure shape.
Flat Springs Serve as a
Frictionless Pivot
A flexible mount, suspended by a series
of flat vertical springs that converge
spoke-like from a hub, is capable of piv-
KC
NEbt
L
=
3
12
An assembly of flat springs gives accu-
rate, smooth pivoting with no starting friction.
oting through small angles without any
friction. The device, developed by C. O.
Highman of Ball Bros. Research Corp.
under contract to Marshall Space Flight
Center, Huntsville, Ala., is also free of
any hysteresis when rotated (it will
return exactly to its position before being
pivoted). Moreover, its rotation is
smooth and linearly proportional to
torque.
The pivot mount, which in a true
sense acts as a pivot bearing without
need for any lubrication, was developed
with the aim of improving the pointing
accuracies of telescopes, radar antennas,
and laser ranging systems. It has other
interesting potential applications, how-
ever. When the pivot mount is supported
by springs that have different thermal
expansion coefficients, for example, heat
applied to one spring segment produces
an angular rotation independent of exter-
nal drive.
Flexing springs. The steel pivot mount
is supported by beryllium-copper springs
attached to the outer frame. Stops limit
the thrust load. The flexure spring con-
stant is about 4 ft-lb/radian.
The flexible pivot mount can be made
in tiny sizes, and it can be driven by a dc
torque motor or a mechanical linkage. In
general, the mount can be used in any
application requiring small rotary motion
with zero chatter.
Sclater Chapter 6 5/3/01 12:24 PM Page 184
A pair of opposed, taut, flexible bands in
combination with a leadscrew provides
an extremely accurate technique for con-
verting rotary motion in one plane to
rotary motion in another plane. Normally,
a worm-gear set would be employed for
such motion. The technique, however,
developed by Kenneth G. Johnson of
Jet Propulsion Laboratory, Pasadena,
California, under a NASA sponsored
project, provided repeatable, precise posi-
tioning within two seconds of an arc for a
star tracker mechanism (drawing, photo).
Crossed bands. In the mechanism, a
precision-finished leadscrew and a fitted
mating nut member produce linear trans-
latory motion. This motion is then trans-
formed to a rotary movement of a pivotal
platform member. The transformation was
achieved by coupling the nut member and
the platform member through a pair of
crossed flexible phosphor-bronze bands.
The precision leadscrew is journaled
at its ends in the two supports.
With the bands drawn taut, the lead-
screw is rotated to translate the nut mem-
ber. The platform member will be drawn
about its pivot without any lost motion or
play. Because the nut member is accu-
rately fitted to the leadscrew, and because
precision-ground leadscrews have a mini-
mum of lead error, the uniform linear
translation produced by rotation of the
lead screw resulted in a uniform angular
rotation of the platform member.
Points on the radial periphery of the
sector are governed by the relationship S
=
R
Θ
, which means that rotation is
directly proportional to distance as meas-
ured at the circumference. The nut that
translates on the leadscrew was directly
related to the rotary input because the
leadscrew was accurately ground and
lapped. Also, 360° of rotation of the lead-
screw translates the saddle nut a distance
of one thread pitch. This translation
result in rotation of the sector through an
angle equal to
S/R.
The relationship is true at any point
within the operating rang of the instru-
ment, provided that
R remains constant.
Two other necessary conditions for
maintaining relationship are that the sad-
dle nut be constrained against rotation,
and that there be a zero gap between sec-
tor and saddle nut.
Pivots with a Twist
A multipin flexure-type pivot, developed
by Smiths Industries in England, com-
bined high radial and axial stiffness with
the inherent advantages of a cross-spring
pivot—which it is.
The pivot provides non-sliding, non-
rolling radial and axial support without
the need for lubrication. The design com-
bines high radial and axial stiffness with
a relatively low and controlled angular
stiffness. Considerable attention was
given to solving the practical problems
of mounting the pivot in a precise and
controlled way.
The finished pivot is substantially free
from residual mechanical stress to
achieve stability in service. Maraging
steel is used throughout the assembly to
avoid any differential expansion due to
material mismatch. The blades of the
flexure pivot are free from residual braze
t o avoid any bimetallic movements when
the temperature of the pivot changes.
The comparatively open construction
of the pivot made it less susceptible to
jamming caused by any loose particles.
Furthermore, the simple geometric
arrangement of the support pins and flex-
ure blade allowed blade anchor points to
be defined with greater accuracy. The
precision ground integral mounting
flanges simplified installation.
Advantages, according to its designer,
include frictionless, stictionless and neg-
ligible hysteresis characteristics. The
bearing is radiation-resistant and can be
used in high vacuum conditions or in
environments where there is dirt and
contamination.
185
TAUT BANDS AND LEADSCREW
PROVIDE ACCURATE ROTARY MOTION
Flexible bands substitute for a worm gear in a precisely repeatable rotary
mechanism used as a star tracker. The tracker instrumentation, mounted on the
platform, is rotated by an input motion to the leadscrew.
A flexure pivot boasts high mechanical stability for use in pre-
cision instruments.
Sclater Chapter 6 5/3/01 12:24 PM Page 185
the vertical rate); therefore the spring is
quite stable laterally when used for
industrial vibration isolation. It can be
filled manually or kept inflated to a con-
stant height if is connected to factory air
186
AIR SPRING MECHANISMS
Linear force link: A one- or two-
convolution air spring drives the guide rod.
The rod is returned by gravity, opposing
force, metal spring or, at times, internal stiff-
ness of an air spring.
Clamp: A jaw is normally held open by a
metal spring. Actuation of the air spring
then closes the clamp. The amount of open-
ing in the jaws of the clamp can be up to
30° of arc.
Direct-acting press: One-, two-, or three-
convolution air springs are assembled
singly or in gangs. They are naturally stable
when used in groups. Gravity returns the
platform to its starting position.
Rotary shaft actuator: The activator shifts
the shaft longitudinally while the shaft is
rotating. Air springs with one, two, or three
convolutions can be used. A standard rotat-
ing-air fitting is required.
Reciprocating linear force link: It recipro-
cates with one-, two-, or three-convolution
air springs in a back-to-back arrangement.
Two- and three-convolution springs might
need guides for their force rods.
stroke and a relatively high spring rate.
Its natural frequency is about 150 cpm
without auxiliary volume for most sizes,
and as high as 240 cpm for the smallest
size. Lateral stiffness is high (about half
Rotary force link: A pivoted plate can be
driven by a one-convolution or two-
convolution spring to 30° of rotation. The
limitation on the angle is based on permissi-
ble spring misalignment.
EIGHT WAYS TO ACTUATE MECHANISMS WITH AIR SPRINGS
POPULAR TYPES OF AIR SPRINGS
Air is an ideal load-carrying medium. It
is highly elastic, its spring rate can be
easily varied, and it is not subject to per-
manent set.
Air springs are elastic devices that
employ compressed air as the spring ele-
ment. They maintain a soft ride and a
constant vehicle height under varying
load. In industrial applications they con-
trol vibration (isolate or amplify it) and
actuate linkages to provide either rotary
or linear movement. Three kinds of air
springs (bellows, rolling sleeve, and
rolling diaphragm) are illustrated.
Bellows Type
A single-convolution spring looks like a
tire lying on its side. It has a limited
Sclater Chapter 6 5/3/01 12:24 PM Page 186
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