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Патент USA US3076341

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Feb. 5, 1963
Filed NOV. 6, 1959
United States Patent
Patented Feb. 5., 1963
‘ the piston on the ?uid increases. The effect of this, of
Robert S. Strirnel, Penllyn, Pa, assignor to Tinius Olsen
Testing Machine Company, Willow Grove, Pa., a cor
poration of Pennsylvania
Filed Nov. 6, 1959, Ser. No. 851,355
4 ‘Claims. (Cl. 73-438)
course, is to increase the ?uid pressure in the cylinder
and such increase being sensed and measured by the
indicating means causes the same to be in error.
The principal object of this invention is to overcome
. the foregoing and for this purpose the invention contem
plates providing for the controlling of the indicator
means by mechanism which is operative as a function
This invention relates to hydraulic type testing ma
of the position of the piston along the axis of the cylin
chines for stressing a specimen in compression, tension 10 der so as to provide continuous compensation for change
in piston buoyancy.v
and the like and, in particular, the invention relates to
apparatus to compensate the stress indicating means of
The principle of the invention will be understood by
the machine for changes in piston tare weight.
those skilled in- the art from the following description and
drawings wherein:
Hydraulic type testing machines usually include a ?xed
cylinder within which is a piston and by supplying ?uid 15 ' FIGURE 1 is a diagrammatic view showing a typical
to the cylinder chamber the piston is moved so that a
piston and cylinder and stress indicator arrangements
specimen appropriately supported in the machine will be
for use in a testing machine and having the invention
incorporated therein;
stressed in compression, tension, ?exure and the like.
Commonly known arrangements for indicating stress are
FIGURE 2 is a diagrammatic, isometric view of a
operated on the principle that the ?uid pressure in the 20 typical piston and cylinder arrangement and showing
cylinder is directly proportional to the stress applied to
the specimen. Thus, such arrangements are directly
mechanism which is responsive to the change in the
position of the piston along the axis of the cylinder;
coupled to the cylinder to sense the ?uid pressure .and
FIGURE 3 is a graph generally illustrating the rela
tionship between the pressure in the cylinder dile to the
indicate its value, the indication ordinarily being in
25 effective weight of the piston and the position of the'
piston along the axis of the cylinder.
' The piston and other machine components connected
In FIGURE 1 the piston is indicated by the numeral
to the same, of course, have weight and this Weight acting
on the ?uid in the cylinder constitutes a means for de
1 and the cylinder indicated by the numeral 2. The
cylinder has a loading chamber 3 which is supplied with
veloping fluid pressure. For example, inv certain hy
draulic type testing machines with which I am familiar, 30 ?uid from a pump 4 via a control mechanism 5. As
?uid is forced into the loading chamber the piston is
this weight may develop ?uid pressures in theorder of
moved up and when the loading chamber is connected
7 to 20 psi, depending upon the size of the'machine.
This pressure is additive to the pressure developed as a
to exhaust, the weight of the piston causes the same to
result of the piston reacting against the specimen.- Since‘
move down. The foregoing elements are all of standard
neither the ?uid pressure in the loading chamber nor the 35 construction.
indicating system coupled thereto can discriminate as
The stress measuring and indicating mechanism is
identi?ed by the numeral 6 and is of the type which
between causes of fluid pressure, the weight of‘ the piston
senses or measures the ?uid pressure in the loading cham
constitutes a source of error. At very high loads or
ber 3 and provides a visual indication of this pressure
stress on a specimen, the error due to the piston weight
may be small as compared to the pressure due to the 40 by' means of the indicator comprising the pointer 11
operating over the scale 12. The scale 12 is usually
reaction against the specimen and, therefore, may’ be
calibrated in terms of pounds. As will be understood
ignored. However, at smaller loads the error due to
by those in the art, an indicating system of the kind
the weight of the piston may be a substantial part or
mentioned may take several different forms and is usu~
even more than the pressure due to the reaction against
the specimen. it will be apparent that to eliminate this 45 ally of .the servo mechanism type and may be arranged
source of error is a highly desirable objective.
in the form of a null balance system or in the form of a
Regarding the above, I have discovered that the effec
tive weight of the piston changes with change in position
of the piston along the cylinder axis and that, therefore,
bridge circuit. The particular system disclosed herein
it is impossible to eliminate the error by way of simple 50
is of the null balance type such‘as shown in my Patent
In certain instances the indicating system
may be a mechanical type.
_ _
For example, if the piston is
A plurality of ?uid operated devices such as Bourdon
moved to the start position of a test, the weight of the
tubes 13, 14 and 15 are interconnected to a valve 16
calibration procedure.
which is arranged to selectively interconnect any one of
the Bourdon tubes with the line 10 and hence with the
moved during‘the stressing of the specimen, the effective 55 loading chamber 3. The Bourdon tubes provide for op
eration over several ranges of pressures, ‘for example,
weight will change and the initial compensation will no
longer be true.
the tube 13 may be used in'the low pressure area, the
tube 14 in the intermediate area, and the tube 15 in the
According to my discovery the change in effective
high pressure area. These tubes are respectively con
weight of the piston is due to changes in buoyancy, the
buoyancy force being proportional to the weight of the 60 nected to control elements 20, 21 and 22. These ele
ments are conventional diiferential transformers, the cores,
?uid displaced by the piston. For example, when the
of which are respectively connected to the Bourdon tubes.
piston is slightly spaced from the bottom of the cylinder
As a tube is moved under the in?uence of the change
the buoyancy force is proportional to the weight of a
volume of ?uid equivalent to the volume of the piston 65 in pressure in the loading chamber 3, the core is moved
up (or down) so as to generate an electrical signal which
within the cylinder and when the piston is moved, say
is proportional to the fluid pressure in the chamber. The
half-way through its total possible movement, the buoy
elements 20, 21 and 22 are interconnected to a switching
ancy force is proportional to the weight of volume of
mechanism 23 which is mechanically coupled to the valve
?uid equal to the remaining volume of the piston within
16 and is arranged to condition a control element for
the cylinder. The buoyancy effect decreases as the piston
operation in accordance with the Bourdon tube con
moves out of the cylinder and, therefore, the eifective
nected to the loading chamber. The switch 23 is con
weight of the piston increases. Therefore, the force of
nected to a balancing control element 24 which is adapted
piston may be taken into account by setting the indicating
mechanism for zero reading. However, as the piston is
to produce a signal which is in phase opposition to the
ways in the same place under no load conditions. Dis
signalof the particular control element selected for oper
placement of the Bourdontubewould, of.course,.de.velop
ation. The output or resultant signal is used to drive a
servo motor 25 which is connected to drive the pointer
a signal and the zeroing control elements provide a means
for blocking this out. For reasons of simplicity I have
not included the zeroing control elements herein.
I claim:
1., In a testing machine having a piston and cylinder
for stressing a test specimen together with motor driven
means for indicating the stress,- apparatus to compensate
11. The operation of the balancing-element;24 iscon
trolled by feedback from the motor and'this- is, accom
plished by means of the mechanical system including the
gears 26, cam 30 andv lever 3‘1_which, being pivotedv at;
32, is’ moved-in accordance with the" rotation of the
cam 30. The _lever-_3_1 is_coupled_stotthe core of the bal 10 for changein pistonbuoyancy with change in position
ancing element'24 and, thereby controlsthe developed of the piston along the aXis of the cylinder comprising:
signal. The circuit operates on the principle that when
a circuit to develop an output signal to operate the
a signal from 1 one of? the‘control‘elernents drives; the
motor,v the-circuit having aecontrol element for develop
motor lithe same. will be turned so as-tomove the
ing. a signal1 proportional to the ?uid pressure in the cyl
arm 31 to develop a signal in the balancing-element 24 15 inder, a: balancing element for developing a signal to.
which is in oppositionrto theisignal'fromvthegcontrol
balance, the signal of the;control element and a com
element. Whsm the resultantsignatiszero, themotor.
stops turning. Since the motor is connected tosdrive-the;
pensating‘elementfor‘developing a signal proportional
tqtheposition of the piston along the cylinder axis to
pointer 11, the pointer will indieate,thelpressurein cham
modulate one of said signals, the output signal of the cir-»
ber 3 or theload applied to the-specimen.
20 cuit being the algebraic sum of three said signals; and
AS- mentioned herelqfqrei the Weightof the-'pistonal
meansconnectedlbetween the piston and the compensat
causes a corresponding pressure to be developed in the
ing element for operating the element in accordance with
loading chamber Sand ‘this pressure varies with‘th'eposi
tion of the piston alongtythe axis-ofthelcylindert- This is
explained fol-lowing.
the positionof the piston along the axis ofthe cylinder.
2. A construction in accordance with claim 1 wherein
In the- graph of FIGURE_ ,3, the; 25 said, elements all are differential transformers and last
dotted line‘33 representsjthebuoyancy effect and thefull
said ‘means-includes a nut, a screw operating in said nut
line 34 represents _~the_ weight- otjthe piston or the force
and» connected with said balancing element and linkage
which it exerts, oaths ?uidt when the piston isvspaced.
slightly from the bottom oi thescylinder, fonexarnple at
connected betweenthe screw and. the piston constructed
X distance in FIGURE}, the buoyancy-e?ect is-highest
and the e?ective weight’ of the piston or the force which
it exerts ‘on the ?uid -is lowest; As thepiston moves onl
wardly 0t. the cylinder. say; to. the position Y, the.
buoyancy effect is smallerand the forcewhich the piston
exerts on the ?uidis greater.‘ Thus, at, position X jthe
to rotate the screw with motion of the piston.
3. In a; testing machine having a piston and cylinder
for stressing a test specimen together with indicator,
means for indicating the stress: a circuit means for driv
inglthe indicator vhaving a’ control element operating to
develop av signal which isa function of the ?uid pres~v
35 sure-in the cylinder and-a compensating element operat-, in the loading-chamber dueytoithe piston issmailer
ing to develop a signal which is afunction of the posi
than the p.s.i. at position _Y,
tion- of the piston in the cylinder and which, changes the
effect of the ?rst said signal to provide a compensation
My Preferred. manner to; compensate for the fore80<
ing is to include in the motor drive circuit acompensat
ing control ‘element/35whichmay' be arranged to. con
tinuously subtract from the control element signal as
in the circuit forechange in piston buoyancy with change
thepistonymoves out of-thecylindor or, alternatively, to
in piston position along the axis of the cylinder; and,
means connected between said piston and said compensat
ing element to operate the element in accordance with
add to the signal ‘of the balancing control element Has
the position of the piston in the cylinder.
the piston moves outoflthe-cylinder. In other words, the
4. In a testing machine having a piston and cylinder;
compensating control element _35 develops a, signal which 45 for stressing a test specimen: mechanism for measuring
corrects oncompensates for the increase in pressure due
and indicating the stress comprising an indicator, a ?uid
to the movement of the piston, out ofthe cylinder.
operated device connected with said cylinder, a drive cir
The dotted lines 36 indicate mechanism interconnected
cuit for the indicator having a control element connected
between the compensating'element and. the piston to op
with saiddevice and operating to develop a signal which
crate the element as afunctionvof piston position. A 50 is a function of the ?uid pressure in the cylinder, a bal
typical mechanism. for this purpose is illustrated in FIG
ancing element and means connected to the indicator and
URE 2. The compensating element ;35 has its core stem
operating the balancing element to develop a signal to
40 connected to a micrometer screw. 41 which operates
balance the signal of thecontrol element; and means con
in the nut 42 ?xedly held in position. The'screw 41 is
nected between said piston and said mechanism and oper
adapted to be rotated by the linkage comprising the arm 55 ating as-a function of the position of the piston along the
43 ?xedly secured to the screw andthe arm 44,.one end
axis of the cylinder to modulate ?rst said signal and con
of which is pivotally connected to the, arm 43v and the
trol the operation ofthe mechanism so that the stress
other end pivotallyconnected to the piston. It will be
indication of the mechanism is compensated for the ef
apparent that .as they piston is moved vup, the. arm‘ 43 is
fect of change. ofpiston buoyancy with change in pis
rotated clockwise which causes arotation of the‘screw. 60 ton position alongvthe axis of the cylinder.
and thescrew being?xed in the nut,i_is moved axially-to
move the core of the compensating element 35.
It, will be recalled that the circuit of ;my Patent 2,808,
721 includes a ‘plurality of vzeroing control elements»
which are selectively connectiblein the circuit. and. arev 65
usedtor zeroing orotrimmingthe circuit atthe start
of, an operations Suchcontrolis desirable in the event,
for, example, that the tip of the Bourdon tubeis not al
References Cited in the ?le of this patent
Dinzl- _, ____________ __ Aug. 17, 1937
Macgeorge __________ _.. Sept. 9, 1952
Lauler et a1. __________ __ Oct. 16, 1956
Strimel ______________ __ Oct. 8, 1957
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