July 13, 2004
In-running nip points are frequent sites of injuries from machinery. Nip points exist where material enters a gradually narrowing opening, for example, pulling rolls, and the material is strong enough to pull body parts, such as fingers, hands, arms, and hair, into the pinch point.
|U.S. Department of Labor, Occupational Safety & Health Administration (OSHA)|
Dangerous nip points also occur in machinery parts not in direct contact with the material, such as near pulleys, gears, and spindles—where linear or rotary motion of the moving equipment occurs through narrowing openings between components. Frequently the machine is running too fast, or is too powerful to allow stopping before significant injury occurs.
When performing safety inspections, I see far too many cases in which personnel are exposed to dangerous in-running nip points. These situations particularly are prevalent when the material is hand-fed or personnel are working near the machinery.
It is important for everyone involved in the production process to be aware that there are many, many nip points in most production operations, and that these nip points present a high injury risk. It is equally important to be aware that the nip points become even more hazardous when the material or machine is not moving smoothly, such as when feeding problems, erratic speeds, misalignments, breakdowns, and other abnormal conditions are present. Under these circumstances, manual intervention to remedy the problem occurs frequently, and during the urgency and stress of resuming production, dangerous work methods may arise inadvertently.
Manhandling the material; using pry bars or other inappropriate tools; making adjustments on the move that should be done when the machine is stopped; reaching for dropped items; and slipping on cluttered or slippery floors all can cause body parts to enter nip points.
In-running nip points sometimes exist in unexpected places. One instance I investigated involved an operator who bent over to pick up a part that fell on the floor. Her hair was pulled in by the machine's belt drive pulley, located only a few inches above the floor. A guard covered the upper portion of the pulleys and belt. Very unfortunately for the woman, the in-running nip point was below the guard.
Because mechanics and service personnel may troubleshoot a machine while it's moving, it is very important that they respect in-running nip points. It generally is recognized that repair work may be more dangerous than production work, although some safety officials argue this should not be true or acceptable. Safety personnel always encourage safe work methods and patience when fixing problems so that haphazard work doesn't increase the injury risk. Using approved lockout/tagout procedures is appropriate when production stoppage is required for the repair.
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The Liberty Mutual Insurance Company has performed considerable laboratory work on safety issues. Many years ago the company published guideline openings for guards in front of shears and presses to protect the fingers and arms of workers feeding material into these machines. The same guideline openings apply to in-running nip points.
Recently Liberty Mutual revisited the study and slightly revised the opening guidelines. Figure 1shows the latest recommendations for a power press. A plastic measuring stick with the diameters and lengths shown in Figure 1 is available for almost instant checking for compliant guarding.
The Occupational Safety and Health Administration (OSHA) has adopted these measurements in its guard guidelines. Openings are to be guarded as specified if there is an open and obvious danger to personnel without guarding. However, other guarding methods are acceptable, such as a light curtain, provided the curtain is far enough away from the hazard so that when the light barrier is broken, the machine can be stopped before body parts can move into the nip point.
A keen awareness of actual and potentially hazardous in-running nip points and providing sufficient guarding to protect personnel are important components of an effective safety program.
Dave Withrow, a longtime member of the Fabricators & Manufacturers Association, Intl.® (FMA), is president of Withrow Industries, 206 Fox Lane, Chagrin Falls, OH, 44022, 440-338-3333, firstname.lastname@example.org. Formerly president of two large equipment-manufacturing firms, he also is chairman of two ANSI safety committees; provides forensic analysis of machines and processes; holds more than 15 domestic and foreign patents; and is a member of the FMA/CNA Safety Committee.