{"id":243,"date":"2023-06-11T23:17:38","date_gmt":"2023-06-11T23:17:38","guid":{"rendered":"https:\/\/safetyengineeringresources.com\/?page_id=243"},"modified":"2023-07-24T17:23:27","modified_gmt":"2023-07-24T17:23:27","slug":"forensic-clues-issue-79-limit-switches-and-guard-locking","status":"publish","type":"page","link":"https:\/\/safetyengineeringresources.com\/index.php\/forensic-clues-issue-79-limit-switches-and-guard-locking\/","title":{"rendered":"Forensic Clues Issue 79: Limit Switches and Guard Locking"},"content":{"rendered":"\n\n\n\n\n

<\/h3>

Limit Switches and Guard Locking<\/span><\/h3>\n\n\n

First published Winter of 2021.<\/span><\/p>\n\n\n\n

Thomasnet.com\ndefines limit switches as switches \u201cused to automatically detect or sense the\npresence of an object or to monitor and indicate whether the movement limits of\nthat object have been exceeded. The original use for limit switches, as implied\nby their name, was to define the limit or endpoint over which an object could\ntravel before being stopped. It was at this point that the switch was engaged\nto control the limit of travel.\u201d<\/o:p><\/span><\/p>

Limit\nswitches have many applications, including common household applications such\nas a food<\/span>  <\/span>processor that requires the\nlid to be on and the bowl locked on, automobiles requiring the transmission be\nplaced in Park to start, coffee bean grinders with interlocks on the lid or\ncollection compartment, an oven that turns off when the door is opened, a\nmicrowave that will not operate without the door closed,<\/span>  <\/span>and certain automobiles that<\/span>  <\/span>wouldn\u2019t start without the seatbelt\nfastened.<\/span>  <\/span>Limit switches are an integral\npart of machine safeguarding because they can determine when a guard has been\nremoved or reinstalled.<\/span> <\/span><\/p>

 Sometimes, defendants\nraise the issue of state of the art. If it isn\u2019t invented, it is not state of\nthe art. If it is available somewhere on the commercial market, it is state of\nthe art. If the objective can be achieved with combinations of components and\nthis information is known, it is state of the art. Interlocks on guards are\nancient technology. No one can make a claim that guarding and interlocking\nguards is something new. It is not.<\/span>   <\/span>The\npatent shown on this page details an early form of interlocking from 1913.<\/span>  <\/span>When a hazard cannot be eliminated through\nthe design of a machine, the hazard must be guarded.<\/span> <\/span><\/p>

 Careful consideration\nneeds to be given to proper design and access to the system, since a guarding\nsystem that is difficult to use or access is more likely to be bypassed or\nremoved, placing the operator or maintenance personnel in danger.<\/span><\/p>\n\n\n

<\/o:p><\/span><\/p>\n


<\/span><\/em><\/strong><\/p>\n\n\n\n

Guarding must prevent\ndangerous motion during access to the hazard, which would be accomplished by\nusing interlock mechanisms described in this section.  To be effective, an interlocking guard must\nsatisfy three requirements:<\/o:p><\/span><\/p>

\u00b7<\/span> It must guard the dangerous part\nbefore the machine can be operated.<\/o:p><\/span><\/p>

\u00b7<\/span> It must stay closed until the\ndangerous part is at rest.<\/o:p><\/span><\/p>

\u00b7<\/span> It must prevent operation of the\nmachine if the interlocking device fails.<\/o:p><\/span><\/p>

 <\/span>These guards must be\ninterlocked with the power source of the hazard to ensure that the power will\nbe switched off whenever the guard mechanism is not properly in place.<\/span>  <\/span>The power source itself, or control of the\npower source, is routed through an interlock switch on the guard itself.<\/span>  <\/span>When the switch detects movement of the\nguard, it will isolate the power supply to the hazard, and machine movement\nwill be prevented.<\/span>  <\/span>The switches must\nalso be \u201cfail-safe\u201d, such that if they fail, the power source will be safely\nisolated, preventing machine operation. The effectiveness of an interlock\nswitch depends on its ability to withstand attempts to defeat the\nmechanism.<\/span>  <\/span>Unfortunately, it is not\nuncommon to encounter interlocks that have been removed, taped down, or\notherwise tampered with by maintenance personnel or operators who require frequent\naccess to machine parts behind the guarding.<\/span> \n<\/span>Depending on the risk involved, switches may be designed to resist\nimpulsive tampering, or they may be designed to be virtually impossible to\ndefeat.<\/span><\/p>\n\n\n\n\n

If a guard is the best\npossible solution to protect the user it is imperative that the guard be\ndesigned in such a way as to remain in place if at all possible.  This requires the manufacturer to insure that\nthe guard is    designed, manufactured,\nand installed in such a way as to insure permanent reliability.  If a guard is made to be easily removable for\ncleaning, maintenance, or other functions, it must be interlocked to    prevent hazardous machinery from being\noperated with the guard removed.  Suited to machines with run-down\ncharacteristics, interlocks can be can be designed to prevent opening a guard\nuntil it is safe to do so, either through mechanical time delay switches or\nthrough a control circuit which locks the guard shut.   <\/o:p><\/span><\/p>\n\n\n\n

Time Delay Bolt Switch<\/h6>\n\n\n

The time delay bolt switch\nshown here is an example of a mechanical delay system.  While there are higher tech designs that\nensure the machine is stopped before allowing the guard to be opened, this\nhistorical method of guard locking can still have applications today.  The first few turns of the operating knob\nopens the contacts and shuts down the machine, but the guard locking bolt is\nnot fully withdrawn until the knob is turned many more revolutions, allowing a\nmachine run-down time of up to 20 seconds. \nThis interlock must be designed such that it is only possible to extend\nthe locking bolt when the guard is fully closed.<\/o:p><\/span><\/p>\n\n\n\n

Allen Bradley Guard Locking Interlock<\/h6>\n\n\n

<\/span><\/p>

This Allen-Bradley modern guard locking safety interlock found on <\/span>rockwellautomation.com <\/span><\/a>is an example of a safety interlock that can be integrated with a PLC (Programable Logic Controller) which is essentially a computer that controls machine function including safeguards, that can ensure a machine is fully stopped prior to allowing machine access.  The latch is a key that is part of the guard that inserts into the switch, and the switch will keep it locked preventing machine access, until the programmed criteria are met.  Even with such advanced technology, there can be design failures, including errors in     programming of the PLC, or miswiring of the safety device and power sources that allow access even with machine motion still occurring.  <\/o:p><\/span><\/p>

\n<\/p>

 <\/span><\/p><\/span>

<\/o:p><\/span><\/p>\n\n\n

How We Can Help<\/o:p><\/span><\/em><\/strong><\/p>\n

At MASE, we are licensed professional engineers, with experience in the design and analysis of safety systems    including machinery that requires safety interlocks to comply with OSHA and industry standards as well as protect the health and welfare of the machine user. <\/span>  Call us at  (855) 627-6273 or email us at info@mase.pro\n<\/u>to discuss your accident case.<\/span><\/p>\n\n\n

Contact MASE today at (855) 627-6273 <\/h2>\n\n\n\n\n\n\n\n","protected":false},"excerpt":{"rendered":"","protected":false},"author":1,"featured_media":61,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"pagelayer_contact_templates":[],"_pagelayer_content":"","footnotes":""},"class_list":["post-243","page","type-page","status-publish","has-post-thumbnail","hentry"],"yoast_head":"\nForensic Clues Issue 79: Limit Switches and Guard Locking - Mechanical and Safety Engineering Expert Witnesses<\/title>\n<meta name=\"description\" content=\"Limit switches detect motion, and guard locking can ensure machine guards stay in the safe position until all motion is stopped.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/safetyengineeringresources.com\/index.php\/forensic-clues-issue-79-limit-switches-and-guard-locking\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Forensic Clues Issue 79: Limit Switches and Guard Locking - 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