IN THE LOOP: CATCHER IN THE RYE

Some power presses come equipped with safety equipment that can operate in only one or a few positions, such as interference rods or linkage mechanisms. In this type of system, the upper platen must travel the full stroke even if a fraction of the stroke would be sufficient for production reasons. Total safety cannot be guaranteed in an application such as this.

If, for example, an emergency stop or a power failure occurs in the middle of the full stroke, the latching device cannot be activated. With multiple-ratchet-position devices, whether the ratchet will properly engage and withstand the forces of the dropping mass is questionable.

Now a new device from Advanced Machine & Engineering (Rockford, IL) called a safety catcher can eliminate such dangers by securing platens under suspended load on stamping presses.

This control technology, manufactured by Sitema (Karlsruhe, Germany), stops and locks a dropping platen via a wedge mechanism located on a smooth rod that can be placed in any position and that is activated by the weight of the dropping mass. Because the safety catcher locks the mass to a smooth round bar, infinite position locking becomes an advantage.

This safety system can be used not only on hydraulic presses, but on any vertical-axis device of a machine tool, or on portals, material handling systems, platforms in theaters or amusement park rides where dropping masses must be stopped for safety reasons.

The safety catcher achieves a high degree of safety in tandem with a productivity increase by working as follows:

1) The load is secured by stopping the dropping mass using the weight as a means to wedge the safety catcher against the acting hydraulic cylinder rod or a separate catcher rod. The locking capability is proportioned to the dropping mass; the heavier the dropping mass, the stronger the grip.

2) The locking position functions for the entire stroke of the machine to guarantee a safety stop at any stroke position. This eliminates productivity loss as a result of positioning the platen to a certain point to interlock the safety device.

3) Depending on the device type, the locking system is held open by either hydraulic or pneumatic pressure. Whenever the pressure drops, the safety catcher is set to automatically secure any dropping load.

4) The full locking force is created only by the load. The unit will lock only when the safety catcher is in locking position (no hydraulic or pneumatic pressure is present) and the load is moving downward (such as when the acting hydraulic cylinder of the press is leaking).

5) Stopping the movement of the dropping mass is not an issue with proper safety catcher selection. Mass deceleration generally is 1-3 γ.

6) The proximity switches signal "load secured" or "unlocked" and can be used for the machine control.

This safety system is certified by the TÜV Rheinland Group in Germany to comply with the European DIN EN 693 Standard as a lift secure device, along with other certifications.

HOW IT WORKS
(Note: Please reference "How it Works" illustration)

STEP 1. UNLOCKED
The locking unit is unclamped by pneumatic or hydraulic pressure applied to the annular piston. A well-defined clearance insures that the shaft can move without friction.

STEP 2. SECURE LOAD
The Safety Catcher becomes effective as soon as pressure is released from the plungers. The force of the springs then causes the clamping jaws to contact the shaft firmly, thus securing the load.

STEP 3. CLAMPING
The clamping force, however, is not built up unless the shaft has been moved by the load. Using the self-intensifying static friction at the shaft, the clamping jaws are drawn into the final clamping position at their mechanical stops.

STEP 4. LOAD
If the load is increased further, the shaft remains in its position until the static holding force is reached.

STEP 5. OVERLOAD
As soon as this limit is exceeded, the Clamping Device breaks down the rod movement, dissipating the kinetic energy of the falling mass.

STEP 6. RELEASE
The clamping is released by an upward movement of the shaft and by simultaneously applying pressure to the clamping device.

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Advanced Machine & Engineering, 2500 Latham Street, Rockford, IL 61103, 815-962-6076, www.ame.com.

Sitema, Im Mittelfeld 10, 76135 Karlsruhe, Germany, (049) 0721/98 66 10, Fax: (049) 0721/98 66 111, www.sitema.de.