[0001] The present invention relates to mechanical presses, and in particular to a speed
control and indication system for such presses.
[0002] One well-known type of mechanical press comprises a frame having a slide reciprocably
guided therein with at least one crankshaft rotatable in the crown portion of the
frame and connected to the slide by a connection so that when the crankshaft rotates,
the slide is caused to reciprocate. In order to provide sufficient mechanical rotational
inertia to the drive mechanism for the slide, a press of this type includes a massive
flywheel which is driven by an electric motor through a clutch mechanism. A variable
speed coupling, such as an eddy current coupling is provided between the motor and
flywheel so that the running speed of the flywheel can be varied by varying a speed
control voltage. The flywheel rotates continuously when the motor is energised, but
the rotary motion is not coupled to the crankshaft as long as the clutch is de-energised.
When the clutch is energised, however, the rotary motion of the flywheel is coupled-to
the crankshaft, which generally runs at the same speed as the flywheel, although a
gearing arrangement may be used to provide a drive ratio which is greater or less
than 1:1. A tachometer generator is mechanically coupled to the flywheel to provide
to a speed meter a voltage indicative of the speed of the flywheel.
[0003] Presses which utilise a massive flywheel for the storage of kinetic energy have a
characteristic speed slow-down during engagement of the clutch because of the energy
required to start the rotary and reciprocating parts in motion. As will be appreciated,
the drive mechanism for a mechanical press is quite massive and there is a considerable
amount of inertia which must be overcome. This results in a press speed which is lower
than its set running speed, and it often requires several strokes before the running
speed can be attained. Accordingly, the speed-time relationship of the press is at
its normal running level with the clutch de-energised and the flywheel up to speed,
then drops momentarily for several cycles of the press as the mechanical inertia of
the drive mechanism is overcome, and then attains its normal running speed.
[0004] In some tooling applications, it is important that the press attain running speed
on the first stroke after engagement of the clutch thereby making it necessary to
prevent the drop in press speed discussed above. Since the amount of speed drop is
proportional to the rotational energy of the press parts which must be accelerated
to running speed from a static condition, and since this energy is taken from the
flywheel, the slow down effect can be eliminated by increasing the flywheel speed
by a proportional amount. After the inertia is overcome, the press speed must be maintained
at its normal running level.
[0005] According to the present invention a mechanical press of the type described is characterised
by first means activated only when the clutch is not energised for modifying the voltage
supplied to the meter to cause the meter to indicate a pre-set normal running speed
of the press even though the flywheel is running at a speed higher than the said normal
running speed, and second means activated only when the clutch is energised and which,
when de-activated, allows the speed control voltage to assume a value to cause the
flywheel to be driven at higher than its said normal running speed but when activated
modifies the speed control voltage so that the flywheel is driven at the said normal
running speed.
[0006] The press speed control and indication system of the present invention thus enables
the press to be started and operated on its first stroke at its predetermined running
speed by providing a control voltage to the speed control circuit that causes the
flywheel to be rotated at a speed greater than its normal running speed when the clutch
is de-energised. When the clutch is energised, the press will immediately drop to
its normal running speed so that the proper speed will be realised on the first stroke
of the slide. In order to enable the operator to set the press speed at the normal
level, the control voltage to the speed meter is modified so that it indicates a speed
lower than the actual speed of the flywheel during the time that the clutch is de-energised.
[0007] When the clutch is energised and the inertia of the press drive mechanism is being
overcome, the actual flywheel speed will drop to its normal running speed, and at
this time, the speed control voltage is modified so that the flywheel speed stays
at the normal running speed, rather than increasing to the over speed condition as
it otherwise would once the drive mechanism is accelerated. In order that the meter
not read a speed which is now too low, the control voltage from the flywheel tachometer
generator is no longer modified so that the meter reads the actual flywheel speed,
which is now rotating at its predetermined running speed.
[0008] Preferably the first and second modifying means are variable resistors or potentiometers
connected in parallel with respective relay contacts which are always in opposite
states, that is to say when one is open, the other is closed, and vice versa.
[0009] The method of operating a press according to the present invention so as to control
the speed of the flywheel and indicate to the press operator the speed thereof is
thus characterised by causing the flywheel to run at a speed higher than a pre-set
normal running speed when the clutch is de-energised and then run at the said pre-set
normal running speed when the clutch is energised and causing the meter to indicate
the flywheel speed as being the said pre-set normal running speed both when the clutch
is de-energised and the flywheel is running at a higher speed, and when the clutch
is energised and the flywheel is actually running at the said pre-set normal running
speed.
[0010] An example of mechanical press in accordance with the invention will now be described
with reference to the accompanying drawings, in which:-
Figure 1 is a diagrammatic view of the press; and
Figure 2 is a schematic diagram of a press speed control and indication circuit for
the press of Figure 1.
[0011] The speed control and indication system shown in Figure 2 is intended to be incorporated
with a mechanical press 2 (Figure 1) which may be of the conventional variety, and
includes a reciprocating slide 3 mechanically driven by a drive assembly 4, such as
a crankshaft and connection assembly. The crankshaft 5 is connected to a massive flywheel
6, which in turn is belt driven by an electric motor 7 through a variable speed drive
mechanism 8. For example, the speed at which motor output shaft is driven may be controlled
by an eddy current coupling. As is conventional, the flywheel 6 is rotated continuously
by the motor 7 at a speed selected by the press operator, and the crankshaft 5 is
rotated only when a clutch 9 is energised thereby mechanically coupling the rotational
energy of the flywheel 6 to the crankshaft 5, which in turn reciprocates the slide
through the connections.
[0012] Turning now to Figure 2, the assembly of motor, clutch and flywheel indicated at
10 is controlled by a conventional speed control 12, whereby the flywheel 6 rotates
continuously at the pre-set speed. A control voltage for speed control 12 is derived
from potentiometer 14 and connected to input 16 by line 18; terminal 17 and line 19
carry a bias voltage.
[0013] With the motor 7 and flywheel 6 running and the clutch 9 de-energised, relay contacts
20 are closed by a flywheel coil 11 thereby shorting out a variable resistor or potentiometer
22 connected in series with the potentiometer 14 and applying to input 16 the appropriate
voltage to cause speed control 12 to operate the motor 7 and flywheel 6 at an over-speed
condition. In this particular arrangement, a higher than normal voltage will be applied
to input 16 over line 18 with potentiometer 22 shorted out.
[0014] Tachometer generator 24 which is mechanically coupled to the flywheel 6, produces
an output voltage on line 26 which is proportional to the actual speed of flywheel
6 which, when the flywheel 6 is running but the clutch 9 is not yet energised, will
be higher than the normal operating speed of the press 2. When the press operator
is about to operate the press 2, this would normally necessitate setting the press
speed higher than the eventual running speed so that when the press slows down as
the clutch 9 is energised, the first stroke of the press will be at the proper running
speed. This involves either estimating the amount of over-speed necessary to cause
the press to operate at its normal running speed on the first stroke or requires the
operator to refer to a table for the proper conversion. According to the present invention,
however, press speed meter 28 is caused to read the desired running speed of the press
at all times, whether clutch 9 is energised or not. This is accomplished by variable
resistor or potentiometer 30 connected in parallel with normally open contacts 32
between the output 26 of tachometer generator 24 and the input 34 of speed meter 28.
Contacts 32 are open when clutch 9 is de-energised and function to decrease the voltage
at input 34 so that meter 28 will read low. Thus, even though flywheel 6 may be rotating
at its over-speed level, potentiometer 30 causes meter 28 to indicate to the operator
that the press is set properly for an eventual press running speed at the desired
level.
[0015] Potentiometer 22 is adjustable for the amount of inertial slow-down associated with
the press 2, and potentiometer 14 is set for the proper over-speed level necessary
to compensate for this inertial slow-down. In operation, before clutch 9 is energised,
potentiometer 22 is shorted out and the high voltage on line 18 causes flywheel 6
to be rotated in an over-speed condition. Since contacts 32 are open, potentiometer
30 causes meter 28 to indicate the pre-set speed that the potentiometer 14 is set
for. When clutch 9 is engaged, however, relay contacts 20 are opened and relay contacts
32 are closed. This results in placing potentiometer 22 in series with line 18 so
that a lower voltage is applied at input 16 thereby causing speed control 12 to run
flywheel 6 at a lower speed, which speed is the normal running speed of the press
that is desired for that particular operation. Since contacts 32 are now closed, potentiometer
30 is shorted out and meter 28 is controlled by the actual voltage developed on the
output 26 of tachometer generator 24, which voltage corresponds to the actual running
speed of flywheel 6. By synchronising the opening of contacts 20 with the energisation
of clutch 9, as soon as the press speed is slowed down, the lower voltage input on
line 18 causes press speed control 21 to maintain the speed of flywheel 6 at the slow-down
level from that point on, rather than permitting the system to speed up again to its
over speed condition. Assuming that the system is operating properly, meter 12 will
always indicate the pre-set running speed of the press, regardless of whether flywheel
6 is rotating at its over speed or normal running speed. This makes it easy for the
operator to see and adjust the actual speed of the press so that the proper continuous
speed can be maintained once clutch 9 has been energised.
1. A mechanical press having a reciprocable slide driven by a mechanical drive train,
a motor drivingly connected to a flywheel for continuously rotating the flywheel,
a clutch for coupling the rotation of the flywheel to the mechanical drive train when
the clutch is energised, a speed control for providing a variable speed control voltage
to the motor to control the speed of the flywheel, and a tachometer generator mechanically
coupled to the flywheel to provide to a speed meter a voltage indicative of the speed
of the flywheel characterised by first means 30, 32 activated only when the clutch
is not energised for modifying the voltage supplied to the meter 28 to cause the meter
to indicate a pre-set normal running speed of the press even though the flywheel is
running at a speed higher than the said normal running speed, and second means 20,
22 activated only when the clutch is energised and which, when de-activated, allows
the speed control voltage to assume a value to cause the flywheel to be driven at
higher than its said normal running speed but when activated modifies the speed control
voltage so that the flywheel is driven at the said normal running speed.
2. A press according to claim 1 characterised in that the first'and second modifying
means are variable resistors or potentiometers 22, 30 connected in parallel with respective
relay contacts 20, 32 which are always in opposite states.
3. A method of operating a press according to claim 1 so as to control the speed of
the flywheel and indicate to the press operator the speed thereof characterised by
causing the flywheel to run at a speed higher than a pre-set normal running speed
when the clutch is de-energised and then run at the said pre-set normal running speed
when the clutch is energised, and causing the meter to indicate the flywheel speed
as being the said pre-set normal running speed both when the clutch is de-energised
and the flywheel is running at a higher speed, and when the clutch is energised and
the flywheel is actually running at the said pre-set normal running speed.