Are Machine Safeguarding Standards Outdated?


By Joe Lazzara, President & CEO, Scientific Technologies Inc.

Much of the safety criteria for today’s machine safeguarding standards were based on body measurement data taken 50 to 60 years ago. The global workforce has changed dramatically since the post-World War II period – do machine guarding standards need revision to reflect the workforce of 2001?

For the requirement of machinery safety standards, anthropometric data such as arm length, distance between finger joints, finger diameters, length of the fingers and of the hand are used in the calculation of safety distances, the openings of guards, reach lengths, stride lengths, etc. Anthropometric data consists of static measurements, as for example the length of the finger or thumb, and dynamic measurement, as the length of a stride or the speed of motion of a limb. This data can be used to derive safety criteria and safeguarding formulas.

For example, anthropometric data is employed in the formulas for determining the safe mounting distance of a safety light curtain from the area of hazardous machine operation, or pinch point.

Another example of safety criteria using anthropometric measurements involves the relationship among the size of arms, hands and fingers, the size of openings in a physical guard, and the distance from a point of operation hazard determining where such a guard may be mounted.

Typically, the anthropometric measurements are presented in percentiles, with the typical data range from the 1st percentile to the 99th percentile. The 50th percentile is the median of the data. For example, in a survey conducted on U.S. civilian males in 1960-1962, the waist girth was found to range from 26.8 inches for the 1st percentile measurement to 46.6 inches for the 99th percentile. This means that 99 percent of the population would have a waist measurement of 46.6 inches or less. Conversely, 99 percent have a waist larger than 26.8 inches. The 50th percentile (median) data is 34.6 inches. So you can see how this data might be used, for example, by a clothing manufacturer trying to determine a production forecast. How many size 48-inch pants would you make?

New data (Vaillancourt, D. and Stover, S., "Review of Machine-Guarding Recommendations", Applied Ergonomics, Vol. 25, No.2, pp. 141-145, 1995) indicates that the recommended maximum width opening for the elbow joint be decreased from 54 mm to 49 mm. The suggested maximum allowable opening size was reduced to no greater than 132 mm from 152 mm, and was a result of the head breadth measurement of the 1st percentile female. The one increase, from 22 mm to 32 mm was at the point where the thumb joins the hand, known as the thenar eminence, an area of the hand where the thickness is reasonably compressible.

Since mechanical machine guards, such as plastic, wire or metal, depend on a physical barrier to prevent contact with the pinch point, the Vaillancourt and Snook report implied the allowable openings in such guards to be one millimeter less than the anatomical thickness. Therefore, of interest in machine guard design, the opening guarding the width of the second finger joint, called the proximal interphalangeal (PIP) joint, for the 1st percentile female was found to be 12 mm, which is smaller than the allowable machine guard opening. Thus the PIP finger joint should be decreased from 12mm to 11 mm for physical machine guards. However, optical machine guards such as safety light curtains depend on the object detection capability of the device, not a physical barrier. As a result a 12mm guard opening is suitable where optical machine guards are employed.

A safety light curtain with a detection capability of 12 mm would detect a 99th percentile female finger at the PIP joint. But a mechanical guard with a 13 mm opening or a light curtain with a 14 mm detection capability, consistent with EN 999, may not. The report also suggests a smaller number of machine guard gap changes as you move further from the point of operation hazard.

With revised anthropometric data supporting a reduction in selected allowable dimensions of mechanical machine guards, STI recently introduced the MicroSafe MC4700-12 Series of safety light curtains.

They feature a minimum object detection capability of 12 mm, and are third party certified as a Type 4 device to the rigorous requirements of IEC 61496-1,-2, the international standard on active opto-electric protective devices.

Two variables involved in the mounting distance calculations and specific to the opto-electric safeguarding device selected are minimum object detection capability and the response time. In addition to its 12 mm object detection capability, the MicroSafe MC4700-12 also has response times as fast as 8 milliseconds. The product is designed to be used in applications where it is beneficial to reduce the minimum distance from the danger zone, consistent with the distance calculation formulas and appropriate standards.

In the meantime, be safe out there!


Lazzara on Safeguarding is a regular column written for Safetyonline.com and Plantautomation.com. It also is published in Plant Safety & Maintenance magazine, Vertacross.com, and the Precision Metalforming Association's member website.

Joseph J. Lazzara is President and CEO of Scientific Technologies, Inc ("STI"), the largest provider of automation safeguarding solutions in North America (www.sti.com).

Joe began his career with Hewlett Packard in 1973 where he had responsibility for safety and environmental issues for one of HP's largest divisions. After several other positions at HP in environmental, process and engineering management, Lazzara joined Scientific Technologies, Inc. (STI) in 1981 as Vice President and became President in 1989 and then
President and CEO in 1993.

He has a Bachelor of Environmental Engineering degree from Purdue University and a Masters in Business Administration degree from Santa Clara University.

Joe is the Chairman of the Safety, Health and Environmental Committee for the Association of Manufacturing Technology (AMT). He is also a member of the Board of Directors of the American Electronics Association (AEA), the nation's largest high-tech trade association.