• Safety and Health Information Bulletins /
• Potential Hazards Associated with the Use of Replacement Materials for Machine Guarding

Purpose

The purpose of this Hazard Information Bulletin is:

1. To inform employers/employees that when replacing machine guarding observation windows, they must be replaced with either the original manufacturer's part or a material having at least the same impact-resistance characteristics as the original part; and
2. To advise that various materials having the same generic/ chemical name (e.g., members of the polycarbonate family) may possess different and less effective impact-resistance characteristics than the original materials used by the manufacturer.

Background

The Denver Regional Office brought to the attention of the Directorate of Technical Support a fatality caused by the installation of transparent replacement guarding material having a lower impact resistance than the manufacturer's original guard for the machine.

Description of Hazard

The fatality involved the use of the thermoplastic polymer methyl acrylate, generically known as "plexiglass," as the machine guarding window for a lathe. The fatality occurred when the bell casting on a lathe became loose while the lathe was turning and subsequently struck an employee in the head and neck as he was looking through the window. The bell casting was propelled through two, 1/2 -inch-thick plexiglass material windows. The plexiglass material windows were installed as a replacement for the manufacturer's original composite window on the machine's door frame.

The manufacturer's original observation window was made of a 1/4-inch-thick laminated glass plate with a 1/2-inch-thick polycarbonate window, separated by an approximately 1/4-inch air space. The original window was replaced with plexiglass material that had a lower impact resistance than the polycarbonate shield originally supplied by the machine manufacturer.

Technical Information

Polycarbonates represent a family of various polymers, each of which possess different impact resistance characteristics at the same thickness and surface area. Various polycarbonates include Macrolux, Lexan, Relex, Replex, Dynaglass, Exolite, Verolite, Cyrolon, and Makrolon.¹ These materials have different impact-resistance characteristics for different thicknesses and/or surface areas. It is important to note that increasing the thickness beyond a certain level does not always improve or increase the impact resistance characteristics.

Conclusions

Replacement machine guard windows must meet or exceed the manufacturer's original design specifications.

Recommendations

When replacing original equipment parts, it is recommended that employers review the specifications and ensure that the specifications of replacement materials meet or exceed the original design specifications.

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