A TOGGLE DRIVE MECHANISM

MECANISME D'EMBRAYAGE A BASCULE

Description

CA 02234350 1998-06-08
G.N. PLASTICS COMPANY LIMITED
Approved Page 1 of 1
TECHNICAL DESCRIPTION of
Revision TOGGLE DRIVE MECHANISM Date May 20/98
Described is a mechanism for applying high forces to a work piece at four
points simultaneously using a single
rotary input. The mechanism's primary use is to apply a four-point distributed
load to the platen of a pre;;s
using a single rotary actuator. An application is the drive mechanisms of
plastic thermoforming machine;s
whose presses use servomotors as prime movers. A typical high speed
thermoformer has two stations involving
presses: a forming station and a trim station. Mach of these stations will
have an upper and a lower platen
driven together by one of the mechanisms being described. Servomotors are
connected to these mechanisrr~s
through a single input shaft to produce a press that is computer controlled
for maximum speed and versatility.
The mechanism as shown in drawing GNT-00630, consists of four main knee-type
toggles positioned at the four
corners of the platen. The stationary end of each toggle is connected to the
main beam of the press and the
moving end is connected to the platen per prior art. Tooling is mounted on the
platen. The four toggles we
grouped in pairs and oriented in such a way that a shaft can join the knees of
each pair of toggles. Two
connecting drive arms join the middle of each of these knee shafts to a crank
plate located at the centre of the
pattern of four toggles. The crank plate is mounted on a vertical shaft whose
extended other end passes through
supporting bearings and couples to the output shaft of a motor or gearbox.
Drawing GNT-00631 shows the
assembly of these components.
This toggle drive provides the following features and benefits:
~ Four toggles provide a four point distributed force applied to the platen.
This reduces platen cocking
tendencies making the press less susceptible to unbalanced loads resulting
from offset tooling or othc;r
factors.
~ The crank plate/toggle combination provides for sinusoidal motion at both
ends of the stroke, minimizing
torque reflected back to the driver, i.e.: the torque required of a drive
motor is zero at both ends of the
stroke. At t:he 'closed platen' position, the toggle action minimizes the peak
torque demand on the motor
and at the 'open platen' position it drops the holding torque to zero, thus
minimizing the RMS torque
requirement. These factors reduce the size of the motor required.
~ The "toggle" action at both ends of the stroke resulting from the crank
plate/toggle combination provides for
a high motor speed-to-platen speed ratio at the beginning and end of the
stroke. This shifts the load
acceleration curve late in the stroke allowing the motor acceleration curve to
be positioned early in the
stroke by appropriate choice of velocity input profile so that the motor and
load are being accelerated ~t
different times. This effectively minimizes the peak torque by allowing a
favourable distribution of torque
demand. This feature also reduces the size of the motor required.
~ The geometry of the crank plate and its shaft are such that massive drive
components such as the motor do
not have to oscillate in unison with the toggle knee joints. This reduces the
mass of moving parts and hence
the torque and ultimately the motor size.
~ All joints in the main toggles are simple pinned type and there are no
sliding or cam type rolling joints. This
keeps the mechanism simple and allows the mechanism to deliver accelerations
to the platen that are greater
than one 'g;' without added capturing devices to keep the platen from
separating from the rest of the
mechanism.
~ One rotary input is conducive to servomotor drive.
~ Crank plate and drive arm arrangement is compact and provides equal movement
to each knee toggle.
~ A large closing force is developed at the end of the stroke due to toggle
action in the drive arms and knee
toggles.

Representative Drawing