Machine Safety

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Machine guarding: Category 4 versus reliability

When is a Category 4 solution ineffective in a machine guarding application? Is your machine safety risk assessment looking at the right things? Is this your accident waiting to happen? Consider this example.

November 03, 2011


JB Titus, CFSEWhen is a Category 4 (Cat. 4 or Cat 4) solution ineffective in a machine guarding application? Is your machine safety risk assessment looking at the right things? Is this your accident waiting to happen?

     Think about this one for a moment and consider this example. We’re looking at a simple shuttle table that travels between a load operator position and a combination “work” and automated transfer position. The risk assessment has identified a Category 4 hazard when the shuttle table arrives back at the operator load position. This is because of the possibility that the operator could be severely injured between the table and mechanical hard stop structure when the table automatically arrives back at the load position. There are hard guards around both sides of the shuttle table’s travel so the only danger point is at the load position.

Control Engineering machine control - robotic icon     The engineering and maintenance departments have evaluated the application and the risk assessment and determined that the hazard is a Category 4. They have also reviewed several solutions and decided on a Cat 4 plunger style limit switch because maintenance has considerable experience with this device on other applications in their plant. Additionally, they’ve decided to install this device at the work position so that the shuttle table can slow down after the table moves past the plunger style limit switch and only has three or four feet of travel left when arriving at the operator load/hazard position. When the table moves past the plunger style limit switch the plunger simply extends breaking the closed contacts signaling a red danger light for the operator and allowing the limit switch in the operator load position to sense the arrival of the table. When the load position limit switch is tripped the shuttle table safely comes to a control stop. And, everything is hard wired meeting category 4 requirements.

      This installation is fully compliant and the Cat 4 hazard is fully mitigated – correct?

      Except for one thing! The risk assessment didn’t record and account for the fact that in the work position the loaded part is sprayed with an anti corrosion material. This ambient mist requires maintenance to periodically clean all surfaces around the work position because of a buildup of the anti corrosion material. Because of this oversight in the hazard mitigation planning no one realized that the plunger style limit switch in the work position might be affected by the ambient mist as well. In fact, the plunger style limit switch at one point became stuck in the closed position after the shuttle table cleared the limit switch. Therefore, the circuit never opened for the load position limit switch to initiate a control stop of the table and the green light never switched to red to alert the operator of an immediate danger. The reliability of the plunger style limit switch was jeopardized because of the risk assessment oversight, and we can only guess at what happened to the load operator?

      This is a real simple application on the surface. But, in my opinion, this is a mistake that happens every day of the week. Does anyone have a simple solution to correct this application and which reliably mitigates the Cat 4 hazard 24/7?   

      Your comments or suggestion are always welcome so please let us know your thoughts. Submit your ideas, experiences, and challenges on this subject in the comments section below. Click on the following text if you don't see a comments box, then scroll down: Machine Guarding – Cat. 4 vs reliability.

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Contact: www.jbtitus.com for “Solutions for Machine Safety”.



For more than 30 years, J.B. Titus has advised a wide range of clients on machine functional safety solutions, including Johnson + Johnson, Siemens, General Motors, Disney, Rockwell Automation, Bridgestone Firestone, and Samsung Heavy Industries. He holds a Bachelor of Business Administration degree from Oklahoma University in industrial management and an MBA from Case Western Reserve University in marketing and finance. He is a professional member of the American Society of Safety Engineers and is OSHA-certified in machine guarding. Titus is also TUV-certified as a Functional Safety Expert and serves on several American National Standards Institute, National Fire Protection Association, and National Electrical Manufacturers Association national safety and health standards committees. Reach him at jb(at)jbtitus.com and via www.jbtitus.com.