March 9, 2010
A load catching device has been developed that prevents catastrophic press crashes and allows operators to optimize the stroke for any size part.
Most modern hydraulic and pneumatic presses have OSHA-mandated protection systems in place to help ensure operator safety. Guards, interlocks, electrosensitive and optoelectronic devices, emergency-stop devices, and other redundant systems have helped make presses safer in recent years.
But when it comes to safeguarding the presses and dies themselves from expensive, catastrophic damage, standards are not as strict.
For most American press operators, a ratchet bar, locking bolt, or latch is all that's standing between them and a catastrophic crash if hydraulic or pneumatic pressure is lost suddenly or the lifting mechanism breaks.
When functioning properly, a ratchet system—usually running the length of the press stroke—can stop the fall of the ram and prevent a catastrophic crash. A spring latch automatically extends to engage the teeth of the ratchet before a crash can occur.
Unfortunately, the ratchet is a wear part; after hundreds or thousands of press cycles, it can begin to exhibit signs of wear that are difficult to detect visually or audibly. The spring latch makes contact with the teeth on the upstroke every time the ram is raised for the next part, and this can cause the ratchet and spring latch to wear to the point at which a fall cannot be prevented.
In addition, locking bolts and latches often operate only at the top of the stroke, and ratchet bars at fixed-interval positions.
Consequently, the ram often must be retracted to its full stroke position before each part, despite the fact that the part requires only a short opening stroke. This can add nonproductive time to the cycle.
A gravity fall load-catching device, compliant with DIN EN 693 and CSA Z142-02 standards, has been developed that prevents catastrophic press crashes and allows operators to optimize the stroke for any part size (see Figure 1). If hydraulic or pneumatic system pressure fails, or if a rope, chain, belt, or toothed drive breaks, the device prevents the load from crashing down at any position of the descent.
A cylinder rod is mounted to the top of the platen and extends through the press crown and the device's housing (see Figure 2). The housing is secured to the machine crown/frame and surrounds the rod. Wedge-shaped clamping jaws inside the housing are held with hydraulic or pneumatic pressure to keep the wedges in position so that the rod can move freely.
When hydraulic or pneumatic pressure is lost or released, springs position the clamping jaws to contact the rod. Any downward movement from this point creates a self-intensifying clamping action that dissipates the kinetic energy of the falling mass and secures the load within a few millimeters.
For returning the press to normal operation, hydraulic or pneumatic pressure is restored in conjunction with the equivalent reverse movement of the rod, and the clamping wedges are released.
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