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Pressroom safety: are the terms contradictory?

A safe, comfortable, and efficient pressroom requires good equipment, straight thinking, and constant vigilance. It's something you have to do. "In a pressroom accident," says Ron Newhouse, Weldotron Corp, Piscataway, Nj, "It's no longer a case of disproving negligence on the company's part, but rather a case of proving that every conceivable safety measure was taken."

It takes common sense. For example, a light curtain will protect an operator's hands, but it won't stop parts from flying out and hitting him in the face. You may have to augment the modem electronic curtain with an old-fashioned mechanical guard.

The world's fastest control won't save a hand if the clutch and brake are too far out of adjustment to stop the ram in time. Vigilance requires periodic checking (and adjusting) of stopping time. OSHA requires it, too ! Straight thinking requires a line of sight from palm buttons to bolster-to prevent a second party from getting hurt unseen.

Mr Newhouse reminds us that older presses with full-revolution mechanical clutches still require basic pull-back units, but these protect only the operator wearing the wrist cuffs.

Even the simple hoist ring can be a hazard if it doesn't apply the load safely. Falling dies or stock can be just as dangerous as a descending ram. Get a ring that swivels, so the load won't shear off the bolt.

And don't forget the press feed. Littell, Div Allied Products Corp, Chicago, IL, provides SAR (Safety Awareness Reports) that describe the proper way to use coil-handling equipment and feeds. They tell what the warnings mean and help the user make sure that the material path and all pinch points are guarded.

"We design our equipment to take commands from the press, to be a slave. It never causes a press to stroke," says William O'Connell, chief electrical engineer of the coil-processing group. "If there's trouble, our controls will stop feed rolls, secure stock material with anti-backup rolls, and shut the motors down. We design our feed motors with armature contactors to remove all electricity in an emergency. If there's an electronic failure, it's failsafe. We use extra safety shut-off devices to take torque off the motor."

Pressotechnik Ltd, Addison, IL, speaks to the advantages of hydraulic presses. Dan Traxler points out that there's no flywheel to disengage, so it can stop instantly if a control tells it to. His firm offers the Power Package, an air/oil cylinder line with modes from 1/4-ton to 100-ton capacity. And it works on shop air-no high-pressure hydraulics external to the unit.

Monitors sign in

At the recent Metalform Show in Chicago, IL, we saw many firms displaying computerized monitoring systems for presses. Controls can check such factors as strain on press frames, stress inside the die itself, crankshaft rotation, and motor power used. The technology is becoming quite complex-and competitive ! For example, IDC Corp, Dimondale, MI, claims an exclusive form of signature analysis in its load monitors. The firm calls it a dynamic press-limit curve. It's designed to allow for the fact that presses are rated at full tonnage only at the bottom of the stroke. As the cycle goes off bottom, the rating diminishes.

Jim Black, applications engineer, told T&P that he can program a dynamic curve in the load monitors to provide early detection of an overload, even when the figure is far below the peak rating of the press. He says, "Early load detection, off bottom, lets our monitor catch problems that would otherwise cause catastrophic damage and injury to the press, die, or operator.

"We set the monitors to trip at about 15% of the total press rating. The control can stop the press immediately-if it's up to date with a partial-revolution clutch-brake."

Mr Black notes that monitors with die-signature capability work well with SPC programs, scrutinizing loads above and below control limits, watching for trends that can predict trouble before it happens. The TM-80 series, for example, overlays known parameters cycle after cycle against real-time position and load for each press stroke.

The system requires many sensors. A position encoder mounts on the press; conventional strain cells mount on the load-bearing members of the frame; and other gages serve where needed. They all feed the computerized monitor.

With such detailed monitoring, the operator can set tonnage limits far below the maximum press rating without fear of false alarms. The low setting, of course, reduces the chance of exploding dies or other calamities.

Johnson Yokogawa Corp, Dallas, TX, says it has "transcended the business of signature analysis" with its new SignatureAceT,[TM] control. The product consists of a SAM500 Statistical Process Controller[TM] and a comprehensive PLC. It performs signature-based in-process verification and closed-loop control of high-speed, discrete mechanical processes by using statistically derived process-control variables.

In other words, it provides real-time control of presses and machining operations, using the SPC data that it collects. Michael O'Brien, general manager of the Factory Automation Unit (FAU), says "Because we close the loop on SPC, we have moved quality inspection from measuring results to controlling the process statistically. In-process verification, then, is a broader term than signature analysis. It does more."

Light curtains and PSDI

Engineers have given us reliable infrared photoelectric light curtains and capacity sensors that can keep hands and bodies out of danger areas, but Fail-Safe in this equipment applies only to the electronics and possibly the control logic-not to the installer or the user. Technology won't save the three-man setup team where two men push the double set of "safety" dual palm buttons while the third is still in the path of the ram.

Wintriss Controls Group, Data Instruments Inc, Acton, MA, believes that not all light curtains are created equal. Ashok Bhide told T&P, Our units are the first US-built light curtains approved by British Standards BS 6491, Parts 1 and 2. This means we can sell our products in the rest of Europe.

"The British standard goes beyond UL standards, which deal only with electrical safety. They make sure that the light curtain is going to save somebody's life. It must have the necessary redundancy, fail-safe mechanisms, duplicate systems, etc."

Wintriss recently introduced a 48" model for area guarding. Other models, from 36" to 12", are applied to stamping operations. These Shadow V curtains offer floating beam blanking, where the operator can set the unit to relax a bit. In modern curtains, there are many infrared transmitter-receiver pairs, and if only one beam is interrupted in the floating-blank mode, the press or machine can continue to operate. A light flashes to warn that the curtain is in this mode.

This "self-defeating" mode is required for certain operations such as bending on a press brake. The part moves up and down with the bend while still in the path of the light curtain, but it won't stop the press unless it breaks more than one beam. It's hoped, of course, that your hand would break more than one beam, thus maintaining safety.

Wintriss also offers clutch-brake controls for OEM or retrofit to modernize older controls or presses. Ashok Bhide says they are OSHA and ANSI compatible, and that the new solid-state controls are easier to install and debug than older models. "We have diagnostic LED's and a built-in break monitor that checks the stop time with each cycle.

"Our workstations are ergonomically designed with palm buttons on the sides-less fatigue than top actuation. And we have touch controls for even less strain. We have self-tripping capability for PSDI wherever it's acceptable."

Last fall, Sick Optic-Electronic Inc, Eden Prairie, MN, introduced a light screen with a grid array of LED senders and receivers. Dwight S Brass, safety products engineer, says it's microprocessor based and will not have sequential failures.

"When you move into a safety product, you have to be able to predict all possible failures and take steps to ensure that none of them will result in an unsafe condition. We feel we've done it with our microprocessor-based control. It's failsafe. No matter how it fails, it will have a backup and will be able to detect that failure."

Sick uses an interesting approach. Designers employ diverse redundant microprocessors-units from two different manufacturers. Thus, the same production fault cannot occur in both, because they don't come from the same block. Engineers believe there's no chance for a hardware fault that would not stop the press.

Furthermore, the guidelines for this kind of approach say that the two (or more) people who write the code cannot talk to each other. Mr Brass told T&P, "We figuratively lock these programmers in rooms and have a third person carry messages back and forth."

Sick is ready for PSDI wherever OSHA accepts it. Its use can reduce fatigue and carpal tunnel syndrome while boosting productivity. A hand in the beam sets the system to cycle the press when the hand is removed. The machine cycles one time; completes a cycle; and sends a signal back to the light curtain to say it' s ready for the next cycle. The light curtain resets its counter and waits for breaking of the beam, which can be placing the workpiece in the die, followed by clearing the beam and starting the machine automatically.

If the operator reaches back in quickly, say to reposition the part, the curtain stops the press. It never interrupts its guard function, but at the same time, it starts the machine automatically, boosting productivity while making life more comfortable for the operator.

Clamping and gas springs

Jan Termuehlen of Optima USA Inc, Elm Grove, WI,-told T&P, "Clamping devices play a key role in ensuring operator safety in stamping. After all, these devices actually connect tooling to the moving press.

"The safety of various clamping methods has received renewed interest because of quick-die-change (QDC) programs and the need to clamp faster. Setups now require new clamping devices that are safer by design. And, even if the devices remain unchanged, the standardization necessitated by QDC programs usually enhances safety by focusing attention on any shortcomings of the present clamping procedure.

"Users now have a choice of QDC mechanical clamping units, hydraulic clamps, pneumatic clamps, hydra-mechanical clamps (hydraulically activated mechanical clamps), and new electromechanical clamps. Each has advantages and disadvantages, and Mr Newhouse believes you should understand all safety factors when you set up a new QDC die system.

Once the die is in place, is it safe itself? Many dies have internal components such as die springs. Both mechanical and gas springs have a definite service life, and should be examined and changed or repaired as needed. Associated Spring-Raymond, Barnes Group Inc, Corry, PA, recently added Raymond/ Kaller nitrogen gas springs to its line of die springs.

The new TB springs are self-contained, but with characteristics similar to manifold systems, giving users additional spring options for deep-draw and other traditional manifold-only operations. The TB springs are an upgrade of Raymond's KG line, with two to three times longer life. Model TU 7500 produces an 8.4-ton initial force.

Straight thinking demands safety, comfort, and productivity. These are mutual goals. Strict OSHA rules need not interfere with true productivity.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes directory of press-safety and control products
Author:Miller, Paul C.
Publication:Tooling & Production
Date:Jun 1, 1991
Words:1857
Previous Article:Who needs 32-bit controls?
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