CA2234350A1 - A toggle drive mechanism - Google Patents

Description

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.

Claims