Note: Descriptions are shown in the official language in which they were submitted.
136
SPEED-RESPONSIVE REVF.RSING HYDRAULIC
DRIVE FOR ROTARY SHREDDER
BACKGROUND OF THE INVENTION
This invention relates generally to shear-type
shredders and, more particularly, t~ automatically
reversible hydraulic drive arrangements for such
shredders.
Hydraulically driven, shear-type shredders are
disclosed in U.S. Pat. No. 3,868,062 to Cunningham, et
al. and U.S. Pat. No. 4,034,918 to Culbertson, et al.
Prior to the drive arrangements disclosed in those pat-
ents, a shear-type shredder was typically driven by an
electric motor through a high speed-reduction gear
train~ Any jamming condition occurring in the shredder
was transmitted directly to the motor through the year
train. The motor was provided with electric current-
sensin~ and motor-reversing circuitry to detect a
ialnming condition in the shredder and reverse the elec-
tric motor briefly to clear the jam. The arrangement
was satisfactory for small shredders having a maximum
rating o~ approximately 20-30 horsepower. By compari-
son, the high torques required by larger shredders,
coupled with frequent jamming and reversing sequences,
oEten caused the electric motors to overheat and burn
~5 o~t.
Accordingly, it was proposed that such shred-
der~ be driven hydraulically by interposing a hydraulic
pump, motor, and fluid circuit with pressure relief
valves between the shredder mechanism and the electric
motor. The electric motor would then drive the
hydraulic pump. Persons involved in shear-type shredder
design believed that this arrangement would effectively
isolate the electric motor from excessive tor~ue loads
due to jamming conditions in the shredder, and thereby
prevent burnout. The earliest hydraulic shredder drive
~2~L~9~
designs employed hydraulic sequencing valves in their
hydraulic circuits which both detected jamming condi-
tions upon an increase in hydraulic pressure and briefly
actuated a flow-reversin~ valve in the circuit to
5 reverse the hydraulic motor and thereby clear the jam-
ming condition. This design operated erratically due to
both variations in fluid viscosity with temperature and
resultant difficulties in determining a consistent
reversal pressure threshold.
To correct these problems, as well as others,
the aforementioned patents proposed drive arrangements
which continued to both sense jamming conditions and
actuate a flow reversal means in the hydraulic circuit,
but did so with electrical means rather than with
hydraulic means. More specifically, those designs
employed hydraulic pressure-actuated electric switches,
electrically operated pneumatic timers and control
relays, and electric reversal solenoids. By electri-
cally signaling overpressures in the hydraulic circuit
and electrically reversing the hydraulic shredder motor,
the reversal cycle was no longer subject to hydraulic
fluid temperature and viscosity variations.
These electric-hydraulic reversing circuits,
however, introduced several new problems. One problem
was the initiation of unintended reversals when the
~hredder jammed momentarily on tough or excess material
and then cut through the material. Another problem was
frequent f~ilure of hydraulic pressure-actuated switches.
Both problems are characterized by momentary
3Q pressure spiking in the hydraulic circuit. Due to the
relative incompressibility of the fluid, a momentary
jamming condition in the shredder causes the pressure in
the hydraulic circuit to rise very quickly. When the
shredder mechanism breaks through the materiai being
35 shredded, hydraulic pressure suddenly decreases. This
momentary rise and fall in hydraulic pressure forms a
pressure spike. Such a momentary jamming condition
i2~ ?3Ei
often causes pressure spikes of sufficient magnitude to
actuate the hydraulic pressure switch and thereby
initiate a reversing cycle. Even though a true jamminy
condition had not occurred and reversal subsequently
proves unnecessary, the reversing sequence, once
initiated, would continue until completion.
Each reversal cycle is about one to three
seconds duration. In shredding tough materials, such as
truck tires or sheet aluminum, true jamming conditions
can occur up to several times a minute but usually occur
less often. However, momentary jamming conditions occur
more frequently, typically a half dozen or more times a
minute. Under these conditions, a significant portion
of available shredding time can be lost.
This problem is especially significant in very
large, for example, 300-600 horsepower shear-type
shredders, not only because of the greater inefficiency
of unnecessary reversals, but because such large
machines are also more prone to pressure spikes. Small
shredder drives use high speed electric or hydraulic
motors with reduction gear trains which provide suffi-
cient angular momentum to help cut through tough
material and thereby help overcome momentary jamming
conditions without initiating unintended reversals.
~5 ~lowever, very large shredders use high torque, low speed
radial piston motors with little or no speed reduction
gearing. ~Ience, they rely much less on angular momentum
to assist in overcoming momentary iamming conditions.
Minimal angular momentum enables the large shredders to
reverse quickly without damage to the drive arrangement,
but it makes such machines more prone to pressure spik-
ing and, therefore, unnecessary actuation of reversal.
One proposed solution to this problem employs a
second timer in the electrical reversing control circuit
between the pressure switch and the reversal actuation
and timing circuitry. This timer is started when the
pressure switch is actuated by either a momentary or a
36
-- 4
true jamming condition. Upon completion of its timing
interval, about one-half second, this timer starts the
reversal cycle if the pressure switch is still actuated,
indicating a true jamming condition. If the pressure
switch is no longer actuated, indicating a momentary
jamming condition which has been relieved, the reversal
cycle is not started and the shredder continues shred-
ding uninterrupted.
While this approach reduces the amount of
unnecessary reversing, it does not prevent overuse of
the pressure switches, ~hich causes them to wear out
sooner than desired. It has, therefore, been proposed
to modify the hydraulic fluid circuit to include fluid
accumulators and flow constrictors to filter out
pressure spikes due to momentary jamming conditions.
Some improvement in operation was noted, but
not enough to enable elimination of the second timer or
to prevent premature failure of the pressure switch. In
addition, the second timer and added hydraulic com-
2Q ponents are expensive and unduly increase the complexityof the drive arrangement. It would be preferable to
avoid such complexity because of the dirty environment
in which such shredders are used and the difficulty of
maintaining and adjusting both the hydraulic and elec-
trical control circuits by servicemen without special
training. It would also be desirable to avoid relying
on failure-prone components, such as pressure-actuated
electrical switches.
Accordingly, there remains a need for an
improved automatically reversible hydraulic drive
arrangement for shear-type shredders.
SUMMARY OF THE INVENTION
A primary object of the invention is to
reliably actuate reversal of hydraulically driven shear-
type shredders when a true jamming condition occurs but
not otherwise.
~L99t36
A second object is to sense jamming conditions
in the shredder without reliance on a fluid pressure-
actuated electrical switch.
A third object is to simplify the hydraulic
fluid circuitry in hydraulic drive arrangements for such
shredders.
Another important object is to sninimize the
cost and complexity of such circuitry and, accordingly,
the skill level re~uired to maintain drive arrangements
as aforementioned.
The invention meets the foregoing objects by
removing the function of sensing a jamming condition
from the hydraulic fluid circuit altogether while
continuing to effect reversal within the hydraulic
circuit. This jam-sensing function is instead accom-
plished by measuring the speed of a mechanical drive
element of the shredder, such as the rotating output
shat of the electric motor or other prime mover driving
the hydraulic pump, the output shaft of the h~draulic
motor, or any other rotational element of the shredder
drive train between the hydraulic motor and the cutter
shafts, or even the cutter shafts themselves~
Hydraulic pressure switches respond instan-
taneously to fluid pressure changes and, therefore, to
p~essure spikes due to momentary jamming conditions. In
contrast, ~easuring speed changesr such as variations in
rotation o~ a shaft during fixed time intervals,
averages out brief speed changes occurring during such
time intervals. This capability enables the jam sensing
3~ m~ans to disregard instantaneous speed changes, includ-
ing many due to momentary jamming conditions, while
remaining fully responsive to true jamming conditions.
The electric motor-driven hydraulic pump is
operatively connected to the cutter shafts of the
shredder through the hydraulic fluid circuit for the
hydraulic motor. There~ore, an increase in load on the
shredder correspondingly decreases the speed of rotation
936
-- 6
of the cutter shafts and of all connected rotational
components of the drive train all the way back to the
electric motor. A true jamming condition is typically
characterized by a cessation of rotation of the cutter
and hydraulic motor shafts and a slowing of the electric
motor shaft~ A momentary jamming condition briefly
stops or slows the cutter and hydraulic motor shafts
but, because of the filtering effects of the hydraulic
fluid circuit, including a pressure relief valve, and
the momentum of the electric motor, does not usually
perceptibly slow the electric motor shaft.
In accordance with these principles, the inven-
tion is a hydraulic drive arrangement for a shear-type
shredder, which includes an electrically operable
reversing control means comprising a jam sensing means
operatively connected externally of the hydraulic fluid
circuit to a mechanical element of the shredder drive
for sensing jamming conditions in the shredder, and
actuation means for actuating a flow-reversing means ~or
reversing the flow in the fluid circuit. Reversal of
the direction of fluid flow reverses the shredder and
thereby clears the jamming condition. The drive
arrangement preferably includes a discriminator means
for damping or filtering out the effects of momentary
jamming conditions before they can cause the jam sensing
means to detect a jamming condition and transmit a jam
sigl~al to the actuation means. This function can be
provided electrically, in the jam sensing means, or
mechanically, in fluid circuit and pumping means by
positioning the jam sensing means, for example, at the
output shaft of the electric motor.
The jam sensing means preferably comprises a
rotation sensor means and means responsive thereto for
measuring rotational speed or revolutions per minute
(RPM). The rotation sensor means can be located proxi-
mate to the drive train between the electric motor
driving the hydraulic fluid pumping means or to the
9~36
-- 7
drive train between the reversible hydraulic motor means
and the cutter shafts. The jam sensing means
continually monitors shaft rotation and provi~es a cor-
responding signal to the measuring means or RPM
monitor. Upon detection of cessation or slowing oE
rotation below a predetermined speed, the measuring
means produces a jam signal to the actuation means. The
measuring means monitors shaft position over intervals
of time and thereby provides the aforementioned filter-
ing ~unction.
The actuation means provides electrical controlsignals including a timed reversing signal to the flow-
reversing means, which can be solenoid operated~ It can
also include a time delay means actuated by the ~am
sensing means to delay briefly production of a reversing
signal by the actuator means until a true jamming condi-
tion is confirmed. The reversing signal actuates the
flow-reversing means whenever the jam sensing means
output signal indicates a continued slowing of shaft
rotation below a minimu~ threshold speed after a pre-
determined time delay has expired. The actuation means
can further include lockout means responsive to the
reversing signal for temporarily blocking operation of
the jam sensing means or generation of a forward shred-
2~ ~ling si~nal, or both, during shredding.
The Eoregoing and other objects, features, and
a(lvantaqes oE the present invention will become more
apparent from the following detailed description, which
proceeds with reference to the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a functional block diagram of a
hydraulically-driven, shear-type shredder incorporating
the present invention.
Fig. 2 is a fluid circuit diagram of the
hydraulic drive portion of Fig. 1.
~2l~136~
Fig. 3 is an enlarged vertical sectional view
taken along lines 3--3 of Fig. 1 showing the rotation
sensing apparatus located at the hydraulic motor output
shaft.
Fig. 4 is an electrical circuit diagram of a
first example of the electrical reversing control por-
tion of Figs. 1 and 2.
Fig. 5 is an electrical circuit diagram of a
second example o~ the electrical reversing control por-
tion of Figs. 1 and 2.
Fig. 6 is a logic timing diagram associated
with the electrical control circuit of Fig. 5.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Overall Arrangement
In general, the overall structure o~ a typical
shear-type shredder incorporating the present invention
is like that of U.S. Pat. No. 4,034,918 and Canadian
Patent No. 1,lQ4,984. Figs. 2 and 4 hereof correspond
rou~hly to the left side of Fig. 5 and to Fig. 6 o~ such
patent, respectively, with the differences forming the
present invention described below. This invention can
also be readily adapted to a shredder using the hydrau-
lic circuit of Fig. 4 of U.S. Pat. No. 4tO34,918 or to
the shredder drive arrangement disclosed in U.S. Pat.
No. 3,868,0~2. However, the following description dis-
closes the presently preferred and best modes of the
invention.
Referring to Fig. 1, the shear-type shredding
mechanism 5 is driven by a reversible hydraulic drive
means 6 through a gear drive train 7 arranged to
counterrotate cutter shaf~s 8 of the shredder at differ-
ent speeds, for example, 40 and 60 RPM. The hydraulic
drive means includes a hydraulic pump 10 which pumps
fluid through a fluid circuit 12 to a reversible, high-
torque, low-speed hydraulic motor 14. A flow-reversing
means 15 is positioned in circuit 12 for reversing fluid
9~36
flow to reverse the shredder. An electric motor 16
contin~ously drives pump 10 in one rotational direction
during operation. An electrical reversing control means
17 controls flow-reversing means 15 during operation.
It includes a rotation sensor 18 positioned as shown to
monitor the speed of shaft rotation of either hydraulic
motor 14 or electric motor 16. Sensor 18 can alter-
natively be positioned at any other rotating element oE
the drive arrangement, such as one of the gears in gear
train 7 or one of cutter shafts 8. The control means
senses a slowing of shaft rotation, discriminates
between reductions in shaft speed due to momentary and
true jamming conditions in the shredder mechanism and
responds solely to the latter condition to actuate the
flow-reversing means, as described in further detail
hereinafter.
Hydraulic Drive Arrangement
Referring to Fig. 2, hydraulic pump 10 is a
fixed displacement pump which draws hydraulic pressure
fluid from a tank 19 and pumps it through hydraulic
circuit 12. Circuit 12 includes a fluid supply line 20
leading to what is normally the intake side of the
hydraulic ~otor 14 and a fluid return line 22 from such
motor to a return line 24 leading to tank 19. The flow-
2~ reversing ~eans comprises a three-position, open-center,
spring-centered four-way valve 26, actuable by a forward
solenoid 28 to deliver fluid via line 20 to drive
hydraulic motor 14 in the forward direction and by a
reverse solenoid 29 to deliver fluid via line 22 and
~Q thereby reverse the fluid flow and, hence, the direction
of motor 1~. As those skilled in the art will under-
stand, valve 26 is shown symbolically. It is preferably
a master-slave or pilot-operated valve with a choke
block or adjustable orifice for controlling the speed at
35 which the slave valve is shifted.
Hydraulic motor circuit 12 also includes a
pressure gauge 30 and a high pressure relief valve 31 to
~2~9~36
-- 10 --
bleed fluid from the high pressure line 20 of the fluid
circuit into tank 19 whenever the hydraulic circuit
pressure exceeds a predetermined upper limit. That
limit is set at a pressure, for example, 2800 p.s.i.,
somewhat above the pressures prevailing durin~ shred-
ding, around 2500 p.s.i. This setting is below the
setting conventionally used to protect the fluid circuit
elements of the shredder, for example, about 3200
p.s.i. So set, the relief valve can aid in discrimi-
nating between ~rue and momentary jamming conditions byrelieving and thereby attenuating fluid pressure spikes
due to the latter condition. Sensing changes in rota-
tion speed at the electric motor shaft takes advantage
of this capability.
Electric reversing control means 17 is opera-
tively coupled to the rotational output shaft of either
hydraulic motor 14 or electric motor 16, or any other
rotational element of the shredder drive train, to
detect a slowing of rotation due to changes in load on
the cutter shafts. During shredding, such changes are
transmitted through gear train 7 and the shaft of
hyd aulic motor 14. These changes in load are trans-
mitted further through the hydraulic motor, fluid cir-
cuit and pump to electric motor 16 for detection at the
~5 electric motor shaft. Control means 17 transmits
electrical control current signals through either
control line 32 or line 33 to alternatively energize
either valve solenoid 28 or solenoid 29 for maintaining
the flow-reversing valve 26 in either a forward or
reverse position to operate the hydraulic motor 14 in
either a "forward" direction for shredding or "reverse"
direction for clearing a jam ~y driving the inter-
connected cutter shafts ~ in corresponding directions.
Control means 17 automatically actuates a reversal only
when the slowing or cessation of shaft rotation indi-
cates a true stoppage or jamming condition in the shred-
ding mechanism, as next explained.
~2~
Rotation Sensor and RPM Monitor
Referring ~o Figs. 1 and 3, rotation sensor 18
typically includes an annular disk 34 securely mounted
to the periphery of the shaft of either hydraulic motor
14, electric motor 16 or any other rotatable element of
the shredder drive train. On a face of disk 34 is a
plurality (eight are shown in Fig. 3) of circular mag-
netic elements 36 which are angularly spaced equi-
distantly and positioned along equal radii near the
outer edge of the disk. Aligned with and spaced from
this annul~r array of disks is a stationary rotation
sensor probe 38 secured to the shredder structure. As
the shaft rotates, magnetic elements 36 produce a chang-
ing magnetic field proportional to the speed of ro~ation
of the disk relative to the probe. Probe 38 includes a
magnetic pickup coil that is responsive to each element
36 to produce an electric current pulse each time a
maqnetic element passes by. Thus, sensor probe 38 pro-
duces an output pulse train consisting of eight pulses
per shaft revolution. Since the magnetic elements are
uniformly spaced about the periphery of the disk, a
given speed of shaft rotation corresponds to a single
pulse train frequency with a constant duty factor. A
chan~e in shaEt speed causes a proportional change in
2S pulse train frequency.
The selection of the shaft to be monitored must
coordinated with the speed measuring capability of
the rotation sensor. The electric motor shaft rotates
typically at 1800 RPM and therefore requires a rotation
30 sensor having sufficient bandwidth to track a minimum of
10,000 pulses per minute for an eight-element disk 34.
The hydraulic motor shafts, on the other hand, rotate
typically at 60 RPM or less, requiring a lower bandwidth
rotation sensor. Increasing the number of magnetic
35 elements increases the sensitivity of the rotation
sensor so that very slow hydraulic motor or cutter shaf~
speeds can be detected i~ a shorter period of time.
~2~993~
- 12 -
Fewer magnetic elements can be used on the high speed
shaft of the electric motor. As fur-~her described be-
low, the shaft of electric motor 16 rotates in only one
direction. Therefore, monitoring this shaft would eli-
minate the need for certain components in the controlcircui~ry described in Example 1 below.
~ he pulse train signal produced by rotation
sensor 18 is applied to an RPM monitor 52, the part of
the reversing control means 17 which measures the speed
of shaft rotation.
~ esponsive to the RPM monitor is an electrical
discriminator element 51, including an adjustable time-
interval based measuring circuit, which detec-ts sub-
normal shaft speeds persisting longer than a preset time
~5 interval. Discriminator element 51 thereby detects true
ja~nin~ conditions but not most momentary jamming condi-
tions. When average shaft speed drops below a preset
threshold during such time interval, element 51 triggers
a reversal actuator 53 which, in turn, actua~es the flow
~0 reversal means 15 and controls the reversal cycle. The
foregoing functional elements of the jam sensing means
may be provided by discrete components or by components
which combine functions, as will be further described
hereinafter.
Electrical Control Circuit
~n electrical circuit energizes the three-phase
electric drive motor 16 and controls the various func-
tions of the shredder. The portion (not shown) of the
circui-try dedicated to energi~ing drive motor 16 is con-
3~ verltional ~Iree-phase motor circuitry. Power is provi-
ded from the three-phase circuit to the control circuit
of Figs. 4 and 5 through a transformer as shown in
United States patent No. 4,034,91~ issued to Culbertson
et al. in Fig. 6. The control portion of the circuit
(Figs. 4 and 5) of the present invention is next des-
cribed in two examples.
~2~36
- 13 -
The circuit of the first example, shown in FigO
4, uses discrete components including a pneumatic timer,
contact relays, and a iam sensing device which includes
the rotation sensor 18 and measuring circuitry which
combines the functions of RPM monitor 52 and discri-
minator 51. Such a device is the Model RlOOSP Rotector
Speed Switch manufactured by Electro-Sensors~ Inc., of
Minneapolis, Minnesota. This device includes a
resistor-capacitor (R-C) circuit which filters the
signal from sensor 18 to produce a voltage level which
varies with average rotation speed. This level is
applied to a silicon-controlled rectifier which controls
an output relay switch 50 which is activated upon detec-
tion of a stoppage or slowing of a monitored shaft. The
resistor in the R-C circuit is adjustable to set the
speed threshold at which switch 50 is activated. The
values of the capacitors can be altered to set the time
interval over which the signal from sensor 18 is av-
eraged to avoid activating switch 50 whenever the shaft
rotation briefly slows during shredding. Switch 50
triggers a timer-relay circuit, further described here-
inafter, which provides the reversal actua~or function.
The second example, shown in Fig. 5, comprises
a digital logic circuit designed to receive output
pul~es fcom the rotation sensor 18 and carry out the
~unctions of all three elements 51, 52, 53. Using
integrated digital logic circuitry enables the jam-
sensing and reversing control functions to be
accomplished reliably and with much less costly
3Q components than in the first example.
Example 1
With reference to Fig. 4, the electrical
control circuit of this example includes a 120 VAC power
source (not shown) applied to electrical conductors 40
35 and 42. This voltage is derived from the supply voltage
source ~not shown) of electric motor 16. The control
circuit includes numerous subcircuits of conventional
9~3G
- 14 -
design, two of which are shown: pump motor power-on
subcircuit 44 (line A) and shredder drive power-on sub-
circuit 46 (line C~. Other conventional circuits (not
shown), for performing "housekeeping" functions, are
disclosed in my copending application and are incor-
porated by reference hereinO
The control circuit also includes a start up
delay subcircuit 48 (line D) and a reversing control
subcircuit (lines E, F, G, H, I, and J). The latter
subcircuit includes RPM monitor switch contacts 50 (line
F~ of RPM monitor 52 (line E), reversal time delay means
conductor 54 (line G), reverse valve solenoid conductor
56 (line H), and forward valve solenoid conductor 58
(line J).
P~mp motor control subcircuit 44 (line A)
includes a momentary start switch 60 ~or starting
electric motor 16. Depressing this switch energizes a
pump motor starter 62 and closes contacts 64 (line B).
Contacts 64 electrically connect starter 62 to the 120
VAC applied to conductors 40 and 42, thereby sustaining
hydraulic pump motor 16 operation after momentary switch
60 returns to its normal position. Electric motor 16
runs in one direction and continues until motor stop
swi~ch 66 is depressed.
momentary start switch 68 (line C) enables
the shredder drive. Depressing switch 68 energi~es
relay 70, thereby closing contacts 72 in subcircuit 48
(line D) to maintain the 120 VAC control voltage to
shredder drive subcircuit 48 until momentary stop switch
30 7~ is depress~d.
Subcircuit 48 includes a start-up delay timer
76. Whenever the shredder drive is enabled, delay timer
76 is activated to open contacts 78 in subcircuit 8~
(line E) to disable RPM monitor 52 for the delay time
35 interval preset therein. That time interval allows the
cutter shafts to reach a speed of rotation above that
set in RP~I monitor 52 as a minimum threshold which would
3~
correspond to a jamming condition and thereby trigger a
reversal. By "locking out" RPM monitor 52, delay timer
76 effectively disables the reversing control means
during start-up of the shredder. When the time interval
of timer 76 expires, contacts 78 re-close, thereby re-
enabling RPM monitor 52.
During normal shredding operation, the follow-
ing conditions exist. Relay contacts 78 (line E) remain
closed to enable RPM monitor 52. Rotation sensor 18
detects shaft rotation and produces a pulse train signal
having a repetition rate corresponding to shaft speed.
RPM monitor 52 receives the signal and measures the
shaft speed.
RPM monitor 52 includes an electrical switch
lS (not shown) which automatically trips whenever the
measured shaft speed drops below a predetermined minimum
speed threshold. This switch closes contacts 50 (line
F) to commence a reversal cycle in the reversing sub-
circuits F, G, H, I, and J, as will be hereinafter
described.
As long as contacts 50 remain open, delay timer
82 (line F) and reversal timer 84 (line G) are deacti-
vated. With reversal timer 84 deactivated, normally
open contacts 86 (line H) remain open, keeping reverse
~5 valve solenoid 29 in a de-energized state, and normally
clos~d contacts 88 (line J) remain closed, energizing
~orward valve solenoid 28 to sustain shredding.
Upon a substantial slowing of cutter shafts 8,
a corresponding reduction in shaft speed is transmitted
to the shafts of both hydraulic motor 14 and electric
motor 16. Rotation sensor lB located at either shaft
detects the slowing of rotation and produces a corres-
pondingly lower frequency pulse train signal. Receiving
this signal, RPM monitor 52 measures the shaft speed and
closes contacts 50 (line F) upon detection of a shaft
speed below the predetermined threshold. Closure of
contacts 50 applies 120 VAC power to delay ti~er 82.
1~9~3~;
- 16 -
Delay timer 82 delays the response o~ the
reversing subcircuit to the closure of RPM monitor
contacts 50 for a predetermined length oE time, for
example, O.S second, from the detection of a substantial
slowing of rotation. It thus functions as a
discriminator means for determining whether the slowing
of shaft rotation was caused by a momentary or true
jamming condition. If, during the delay interval to
timer 82, the shaft speed has increased above the mini-
mum threshold, thereby indicating that normal shreddinghas resumed followin~ a stall or momentary interruption,
RPM monitor contacts 50 re-open. Whenever this occurs,
delay timer 82 simply "times out" without affecting the
reversing subcircuit or shredding operation.
Whenever RPM monitor contacts 50 remain closed
following the expiration of the time interval in delay
timer 82, the ~amming condition persists and the timer
initiates reversal sequence by closing switch contact 90
(line G), which is in series with pneumatic delay rever-
sal timer 84, upon detection of a jamming condition
within the shredder. Closure of switch contact 90
energizes reversal timer 84 (line G), which is prefer-
abl~ a relay device having two sets of complementary
acting contacts 86 and 88. Contacts 86 are normally
opén and are included in subcircuit 56 while contacts B8
are normally closed and are included in subcircuit 58.
Reversal timer 84, therefore, is operatively connected
to both reverse valve solenoid 29 in subcircuit 56 and
forward valve solenoid 28 in subcircuit 58. As long as
switch contact 90 remains open, contacts 88 remain
closed, forward valve solenoid 28 is energized and
reverse valve solenoid 29 is de-energized. The sole-
noids thus hold valve 26 in a forward flow position for
running motor 14 in a forward direction for shredding.
35 When reversal timer 84 is activated by the closure of
switch contact 90, forward valve solenoid 28 is de-
energized and reverse valve solenoid 29 is energized for
3~
- 17 -
the predetermined length of time preset into reversal
timer 84, for example, 2-3 seconds. The flowreversing
valve is thus shifted to a reverse flow position for
that period of time and then automatically returned to
the forward flow position to reverse briefly the
shredder and thereby clear the jamming condition.
During reversal, relay 92 (line I) is ener-
gized, thereby opening normally closed contacts 94 (line
D) to disa~le start-up delay timer 76, which in turn
disables RP~I monitor 52 (line E) by opening contacts
78. With RP~ monitor 52 disabled during reversal, RPM
monitor contacts 50 open to withdraw electric power from
the pneumatic reversal timer 84. Having been previously
enabled, however, a pneumatic timer allows the reversing
sequence to proceed irrespective of whether electric
power is applied to the device. By "locking out" RPM
monitor 52 while timer 84 remains operationalr relay 92
effectively disables only the jam sensing means during
reversal of the shredder.
Relay 92 can be eliminated if the shaft of
electric motor 16 is monitored. This is so because
electric motor 16 rotates only in one direction at all
times, thereby eliminating a need for locking out RPM
monitor 52 during a reversal. Delay timer 82 and its
2~ contacts ~0 can also be eliminated, so that reversal
timer 84 is controlled directly by contacts 50. The
hydraulic drive 6r including relief valve 31r and the
momentum of the electric motor7 dampen the effects of
most momentary jams so that they do not slow the el-
3~ ectric motor shaft as much as a true jamming conditiondoes. In that case the hydraulic drive and electric
motor themselves function as a discriminator means,
rendering the delay time 82 unnecessary. As discussed
previously, monitoring the electric motor shaft necessi-
tates the use of a wideband rotation sensor.
Operation of Automatic Reversing Controls
In operationr electric motor 16 drives pump 10
~9~36
- 18 -
continuously in one direction to deliver pressure fluid
through circuit 12 to valve 26. At start-up, valve 26
is sprinq-centered to its neutral position and the fluid
passes through the open center of the valve back to tank
13 via line 24. The shredder drive is actuated by
pushing button 68 (line C), causing the normally open
relay contact 72 (line D) of relay 70 to close. Closing
contact 72 applies 120 VAC through normally closed relay
contact 94 to actuate delay timer 76. After the delay
preset in timer 76 has expired, such timer closes relay
contact 78 to energize forward solenoid 28 in subcircuit
58 (line J), thereby shifting valve 26 to its forward
position. Reverse valve solenoid 29 in subcircuit 56
remains de-energized because the reversal time delay
contacts 86 (line H) remain open. ~ligh pressure
hydraulic 1uid is thus directed through line 20 to
hydraulic motor 14 to drive the cutter shafts in their
forward directions for shredding material.
Material is then fed into the shredder for
shredding in a shearing action between coacting cutter
discs mounted on the counterrotating shafts 8. The
material resists the tor~ue of the cutter shafts. This
load resistance causes the fluid pressure in line 20 of
the hydraulic circuit to rise, for example, to an
2~ operating pressure of about 2500 psi.
Intermittently, as the cutters encounter
tougher or greater amo~nts of material, resistance
increases, slowing or stopping rotation of the cutter
shaEts and causing fluid pressure to rise to the setting
of relief valve 31 (Fig. 2). If the cutters then break
through the material, the pressure drops, forming pres-
sure spikes. If a true jamming condition occurs, the
fluid pressure increases to the setting of the relief
valve and remains at a plateau.
3~ In the aforementioned Culbertson, et al. and
Cunningham, et al. designs, any pressure spikes exceed-
ing a threshold, for example, 3200 psi, would actuate an
~2~36
electrical pressure switch in the ~luid circuit,
initiating a reversal cycle even though a true jamming
condition had not occurred. However, they do not do so
in the present invention. Rather than a pressure switch
in the fluid circuit, the present inv~ntion utilizes the
aforementioned rotation sensor 18 proximate the shaft o~
either the hydraulic motor or the electric motor, apart
from the fluid circuit. The relay contacts 50 of RPM
monitor 52 are set to trip at a shaft speed threshold,
just below the shaft speed characterizing a true jamming
condition.
When the RPM monitor circuitry in device 52
detects a shaft speed below the predetermined threshold
level corresponding to a true jamming condition, it
transmits a signal to actuate relay switch means 29. As
~ntioned above, electrical reversing control means 17
may include delay timer 82, which delays the closure of
switch contact 90 until a first time delay intervalr for
example, 0.5 seconds, has elapsed. Whenever the stop-
page or low shaft speed persists beyond the first timedelay interval, switch contact 90 closes to actuate the
flow-reversing circuit. This delay action technique
serves as an electrical discriminator ~or distinguishing
spurious responses by RPM monitor 52 to reduced shaft
speed and momentary interruptions in shredding caused by
th~ introduction oE especially difficult to shred
objects, from a jamming condition requiring reversal of
the cutting mechanismO
Closing switch contact 90 activates the
reversing time delay relay 84 in subcircuit 54, to
actuate flow reversal in hydraulic circuit 12 in Fig.
1. Energi~ing time delay relay 84 simultaneously opens
relay contacts 88 in subcircuit 58, thereby de-
energizing forward solenoid valve 28, and closes relay
contacts 86 in subcircuit 56, thereby energizing reverse
valve solenoid 29. Solenoid 29 shiEts flow-reversing
valve 26 to the reverse position for reversing the fluid
3~;
- 20 -
flow to motor 14, thereby reversing such motor. Rever-
sal of motor 14 reverses the counterrotation oE the
cutter shaEts, disgorging material upwar~ly from between
such shafts to relieve the jamming condition. Once the
monitored shaft speed returns to a value sufficient to
deactivate RPM monitor 52, thereby causin~ relay con-
tacts 50 to reopen, time delay relay 84 is de-energized
but continues timing.
After a predetermined time period determined by
the time delay setting of relay 84, relay contacts 86
reopen and relay contacts 88 reclose, thereby de-
energizing reverse valve solenoid 29 and re-energizing
forward valve solenoid 28~ Valve 26 again directs high
pressure flui~ fr~m pump 10 through line 20 o~ hydraulic
circuit 12 to cause drive motor 14 to resume rotating in
the forward direction to drive the cutter shafts in
their shredding directions.
If, after reversal, the speed of shaft rotation
as detected by RPM monitor 52 again decreases below the
threshold level, the foregoing reversal cycle is
repeated and continues so long as the true jamming
conditions persist~
If the rotation sensor is positioned at the
electric motor shaft, abrupt changes in cutter sha~t
speed are attenuated, as they are transmitted through
the hyclraulic drive, by the action oE the relief valve
31 bleeding fluid back to tank 19. The high rotational
momentum and electrical inductance of the electric motor
further dampens such changes. As a result, momentary
~0 jamming conditions seldom slow the electric motor shaft
su~ficiently to initiate a reversal cycl~.
The fluid cir~uit, drive train, and RPM monitor
thereby cooperate to filter o~t momentary jamming condi-
tions. Hydraulic pressure switches and fluid accumu-
lators become unnecessary~ Delay timer 82 can beomitted as wellO The relief valve can be set to lower
pressures than in prior systems, without interfering
~2~3~
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with reversal. On the contrary, doing so improves the
spike filtering abilit~ of the hydraulic circuit. ~s an
added benefit, it reduces the peak pressures in the
hydraulic fluid circuit, reducing the risks of seal
failures and hydraulic component damage.
Example 2
~ he electrical control circuit of this example
includes integrated circuit digital logic components
interconnected to accomplish the same reversing control
functions described in E~ample 1. The use of digital
lo~ic components, however, costs substantially less,
reduces electric power consumption, and provides both a
compact and lightweight control module. The arrangement
and operation of this circuit is best understood by
reference to both Figs. 5 and 6 throughout the Eollowing
description. In this e~ample, the shaft of hydraulic
motor 14 is the monitored shaft.
The circuit of Example 2 includes electric
motor starting circuitry and a 120 VAC power source (not
shown) as described in Example 1 for use in producing a
DC supply voltage suitable for the particular integrated
circuit logic family chosen. The CMOS family of integ-
rated circuits, which possess superior noise immunity
properties, is considered best suited for application to
this invention. In that case, a ~15 VDC power supply
(not shown) is used to power the control circuit. The
circuit oE Example 2 also uses a motor pump-on sub-
circuit identical with that described in Example 1 and
may include the "housekeeping" subcircuits referenced
3(~ therein.
Referring to Fig. 5, the reversing control
subcircuit includes a retriggerable monostable multi-
vibrator 96, which controls a buffer amplifier (not
shown) on outpu~ line 132 to drive forward solenoid 28;
timer 98, which controls a second buffer amplifier (not
shown) on output line 140 to drive reverse solenoid 29;
and J-K flip-flop 100, which produces a ~'kick start"
~LZ~36
- 22 -
pulse to re-start shredder operation a~ter a reversal
sequence has been completed.
In operation, electric motor 16 is started in
the manner described in Example 1. When the logic
5 device supply voltage (+V) is initially applied, the
subcircuit comprising resistor 102 in series with
capacitor 104 produces a short negative-going pulse
(having duration equal to the product o~ the values of
the resistance times the capacitance) at their juncture
103, which is connected to one input of each AND gate
106, 108, and 110. These gates in turn transmit this
pulse to the CL inputs 112, 114, 116 of monostable
multivibrator 96, timer 98, and flip-flop 100, respec-
tively. This pulse is generated only at logic device
power-on to produce a logic 0 state (i.e. "clear") at
the Q output of each o~ logic devices 96, 98, 100. ~t
this point, neither solenoid 28, 29 is energized so the
shredder drive is off. ~he starting logic state of each
device is shown at the left most end o~ lines D, E, and
G of the timing diagram of Fig. 6.
The shredder drive is enabled by depressing
momentary start switch 118. Connected to switch 118 are
cross-coupled NAND gates 120, 122 to eliminate switch
bounce. The output of NAND gate 122 is coupled to clock
pulse input 124 of monostable multivibrator 9~ through
QR gate 126 to initiate shredder operation as will be
described below. Also applied to inputs of OR gate 126,
and therefore coupled to input 124 of multivibrator 96,
are outputs 125 and 144 of rotation sensor 18 and flip-
flop 100, respectively.
Monostable multivibrator 96 is retriggerableand produces a logic 1 state output pulse having a dura-
tion Tl at output 132 (Q output) upon the occurrence
o~ the leading edge of each pulse applied at its input
124. The pulse duration is set by variable resistor 128
and capacitor 130, for example, to 0.5 second, and may
be adjusted by changing the value of resistor 128.
1~9~3~
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Therefore, successive pulses occurring within a time
interval less than 0.5 second continually retrigger
multivibrator ~6 and sustain a logic 1 state at output
132.
During normal shredding, rotation sensor 18
produces a stream of pulses to retrigger multivibrator
96. Its output 132 controls the operation oE forward
valve solenoid 28 to hold the flow-reversing valve 26 in
the forward position.
Shredder operation commences when monostable
multivibrator 96 receives the shredder drive pulse (Fig.
6, line A) activated by switch 118 at input 124. In
response to this pulse, output 132 of multivibrator 96
switches to a logic 1 state to energize forward valve
solenoid 28 and start cutter shaft rotationO Upon the
start of shaft rotation, pulses produced by rotation
sensor 18 delivered to the input 124 of multivibrator 36
through OR gate 126 retrigger the device to sustain a
shredding operation. To successfully sustain shredding
immediately after start-up, a pulse from rotation sensor
18 must trigger the input to monostable multivibrator 96
before the 0.5 second period (Tl) expires. For a
rotation detector having 8 magnetic elements, this
corresponds to a minimum re~uired shaft rotation of 45
~5 within 0.5 second after start-up. A 0.5 second interval
is suE~icient to s~art a shredder under a no load condi-
tion within the cutter shafts. Where both pump 10 and
motor 14 are fixed displacement devices, the motor shaft
attains operating speed more quickly than if a bidirec-
3~ tional pump is used, so interval Tl can be shortenedslightly, if desired.
With reference to Fig. 6, line B shows a
representative se~uence of events occurring during
shredding. These events are shown to correspond wi~h
the state of output 132 of monostable multivibrator 96,
the timing diagram of which is repeated for clarity in
line D. As shown in line C~ the pulses produced by
~Z~3~
- 2~ -
rotation sensor 18 are more closely spaced (i.e.,
increasing in frequency) as the monitored shaft reaches
operating speed. Thus, during the SHRED 1 interval of
line B, forward valve solenoid 28 remains energized
(line D) while reverse valve solenoid 29 is de-energized
(line E) as described below.
With reference to Fig. 5, reverse valve sole-
noid 29 is controlled by timer 98 which, when triggered,
produces an output pulse having a 2-3 second duration
T2 set by resistor 134 and capacitor 136. This time
interval controls the duration of reversal of the cutter
shafts when a true jam exists. ~imer 98 is triggered by
the leading edge o~ a pulse produced at output 138 (Q
output) of monostable multivibrator 96, which thereby
signals a jam condition. Until such event occurs, out-
put 140 (Q output) of timer 98 remains at logic 0 so
that reverse solenoid 29 remains deenergized during
normal shredding.
With reference to Fig. 6, line B, a STALL
(momentary jam) condition exists whenever the shaft
rotation slows substantially or stops temporarily.
Under these conditions, monostable multivibrator 96
serves as the discriminator by continuing to enable
~orward shredde~ operation as long as two consecutive
rot~tion sensor pulses occur within 0.5 second of each
other, i.e. t C Tl. As shown in Fig. 6, lines B, D,
and E, the shredding operation proceeds without a
reversal operation.
Whenever two consecutive pulses do not occur
~0 within 0.5 seconds, i.e. t ~ Tl, corresponding to a
stoppage or shaft rotation of less than 45 during that
time, a jamming condition is considered to exist, as
shown in line B.
With reference to Figs. 5 and 6~ immediately
upon expiration of the 0.5 second delay, the following
events occur. The monostable multivibrator 96 output
3~
- 25 -
132 returns to logic 0, thereby de-energizing forward
valve solenoid 28; and its output 138 (Q output)
produces a leading edge to trigger timer 98, which pro-
duces a pulse T2 of 2-3 second duration, thereby ener-
gizing reverse valve solenoid 29. In Fig. 6, time linesD and E show these transitions as corresponding to the
REVERSE 1 event of line B.
During reversal, shaft rotation detected by
rotation sensor 18 causes a signal to be applied to
input 124 of monostable multivibrator 96. To prevent
production of a signal at output 132 which would drive
simultaneously forward valve solenoid 28 during reversal,
output 142 (Q output) of timer 98 is connected through
AND gate 106 to the CL input of monostable multivibrator
96. The output 142 forces multivibrator 96 to remain
cleared and thereby locks out the trigger pulse pre-
sented at its input 124. Upon completion of reversal,
this clear signal is removed.
At the completion of reversal, the operation of
flip-flop 100 becomes important. Flip-flop 100 is
triggered by a positive-going edge from output 142 of
timer 98 and is wired to produce a logic state 1 at its
output 144 (Q output) only upon such an event. Such
occ~lrs only after the reversal time delay T2 bas
2~ ~?xpired. Upon completion of a reversing sequence, flip-
flop 100 produces a logic 1 signal at its output 144
which passes through OR gate 126 and triggers monostable
multivibrator 96. This signal, shown in Fig. 6, line G,
serves as a means to "kick start" (i.e. restart) forward
3~ shredder rotation rotation after reversal. Output 144 of
flip-flop 100 is fed back to its CL input 116 through OR
gate 126, differentiator circuit 146, and A~D gate 110.
This arrangement provides a narrow pulse to multivi~ra-
tor 96 so as not to inhibit its response to rotation
35 sensor 18 upon resumption of shredding.
36
- 26 -
Circuit 1~6 includes in~erter 148, capacitor
15Q, resistor 152, and diode 154 to form a buffered
differentiator circuit having an output pulse of
sufficient width only to clear flip-flop 100 and thereby
avoid inhibiting the operation of monostable multi-
vibrator 96. Circuit 146 also clears flip-flop 100
after each rotation sensor pulse is detected (Fig. 6,
line F). Continual clearing of flip-flop 100 simplifies
the design but is not otherwise necessary for proper
circuit operation.
Continuing along line B in Fig. 6, after the
REVERSE 1 event, a jamming condition persists. The
circuit in this example responds by attemptin~ to resume
forward shredding for Tl = 0.5 second. If no appre-
ciable shaft rotation is detected by rotation sensor 18,a second reversal sequence (REVERSE 2 in Fig. 6, line B)
takes place in the manner described hereinabove.
Following the second reversal sequence, a second "kick
start" pulse (Fig. 6, line G) is applied to input 124 of
monostable multivibrator 96 and normal shredding resumes
(SERED 3 in Fig. 6, line B).
A shredder drive stop switch 160, connected
through cross-coupled NAND gates 156 and 158 to elimi-
nate switch bounce, clears both monostable multivibrator
~5 96 and timer 98, thereby de-energiæin~ forward valve
solenoid 2B and reverse valve solenoid 29, respec-
tively. The circuit of Fig. 6 can alternatively ~e used
to monitor rotation speed at the shaft of the electric
pump motor 16. Because its shaft rotates faster than
3~ the shaft of motor 14, a conventional divider circuit
(not shownl would be added in conductor 125 to reduce
the frequency from the rotation sensor in proportion to
the relative speeds of such motors. This result can be
obtained in part by reducin~ the number of elements 36
in the sensor 18 (Fig. 3). Also, time interval Tl can
be adjusted to properly discriminate between true and
momentary jamming conditions as sensed at motor 1~.
~'2~9~3~
- ~7 -
Having described and illustrated the principles
of my invention in a preferred embodiment, it should be
apparent that it may be modified in arrangement and
detail without departing from such principles~ I claim
all modifications coming within the scope and spirit of
the following claims.
~()
