It is the employer's responsibility to furnish and ensure the use of point of operation guards of mechanical power presses. The OSHA standard covers two types of presses: full-revolution presses and part-revolution presses. (Hydraulic presses are covered under a different standard.) Last week, we looked at the guarding required for full-revolution presses. This week, let's review part-revolution presses.

Guarding of Part-Revolution Presses

Unlike a full-revolution (full-rev) press which cannot be stopped once it's tripped, a part-revolution (part-rev) press can stop at any point during the cycle. Motion is stopped through the use of friction brakes. When the control buttons are simultaneously depressed, the press cycles and runs according to:

• What is programmed into the selector switch (off, continuous, inch and single-stroke modes); and
• What the control box is programmed for (it has internal cams that are set to control items such as lubrication, light curtains, other mechanical devices, etc.).

Guarding part-rev presses can be an engineer's nightmare because of the complexity of some presses' tasks. Nevertheless, the press has to be guarded and guards must be foolproof to operators and other workers.

The Brake Monitor

When hands are in dies to place and remove parts, a brake monitor must be installed to monitor the stop-time of the press. The monitor has a manual alarm setting that is determined by maintenance (or die setup workers), so if the stopping of the press reaches a maximum stop time, it stops the press so the fault can be evaluated. This time is based upon the physical distance from the control buttons to the point of operation whereby a person can reach a distance of 36 inches in one second.

Distance Guarding

Distance may be used as a means of safeguarding workers. Safety distance timetables are set out in 29 CFR 1910.217 (c) (3) (vii) [c], which set out distances that must be exceeded based upon the press stop time.

If presence-sensing point of operation (POO) devices (light curtains) are used, that same distance must be used from the light curtain to the POO. In addition, one cannot reach over, under, around, or through a POO guard (light curtain) to access a pinch-point area, with permissible guard openings with sizes based upon distance from the guard to the POO.

Also keep in mind the risk that a worker might stand between a light curtain and the press while the press is operating. This is a violation of the standard and leaves the worker without protection from the POO.

Other Guarding Options

Other types of guards/devices include:

• A and B type gates;
• Adjustable and fixed barrier guards;
• Interlocks for barrier guards;
• Pull-out devices (pull-back); and
• Restraints.

There are many aspects to the safe operation of a mechanical power press and it all depends on the application of that press for the particular job. There are air and spring counter-balances, air filtering, hand-feeding tools, die set-up, guard fastening, scrap handling, control reliability, anti-repeated feature, brake monitoring, inspection requirements, plus others, again, depending on the application of the press.

Conclusion

The bottom line: If an operator can get at a point of operation with his fingers or hands without removing guards, then the point of operation is not properly guarded. If assistance is needed, contact the manufacturer or your local OSHA on-site consultation for assistance. Remember, it is impossible to have an accident without the presence of a hazard!

MEMBER REPLIES
Conveyor Safety Tips

Another tip I would add is to ensure that the emergency control switch (cord) is working.

Over time the cord next to the conveyor either stretches or gets broken and hence you have no emergency stop.

Barry R. Weissman, REM, CSP, CHMM, CHS-IV, CIPS

LONGER BREAKS NEEDED

To Avoid Back Injuries on the Job

By Catherine Jones

A recent study funded by the National Institute for Occupational Safety and Health (NIOSH) and conducted by Dr. William Marras and researchers from Biodynamics Laboratory at Ohio State University studied the effects on workers of lifting boxes onto conveyor belts. The study included:

• 4 new workers
• 6 experienced workers
• Boxes weighing 2, 11 and 26 pounds
• 8-hour workdays
• 30-minute lunch break
• 2 15-minute rest breaks
• Oximeters on the participants' lower backs, measuring the amount of oxygen reaching the muscles (indicating how hard the muscles worked and assessing fatigue).

The study found that while the two 15-minute breaks did help the participants' back muscles recover somewhat from the strain of lifting, the 30-minute lunch break was the most helpful, allowing oxygen levels to return to the resting level. However, as the day progressed, the workers' oxygen levels rose considerably, leading researchers to conclude that the risk of back injury is greatest towards the end of a work shift. Researchers also found that the new workers tended to tense up their muscles, preventing proper blood flow and oxygenation, more so than experienced workers.

The study suggests that workers require longer and more frequent breaks to avoid work-related back injuries, particularly later in the day. And, according to Dr. Marras, "The bottom line is that it's much more costly from a physiological standpoint for novices to do the same work as experienced people."

Source: Medline Plus, February 20, 2007

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