Machinery Safety 101

The Top Five errors I see machine builders make on a depress­ingly reg­u­lar basis:

1) Poor or Absent Risk Assessment

Risk assess­ments are fun­da­men­tal to safe machine design and lia­bil­ity lim­i­ta­tion, and are required by law in the EU. They are a included in all of the mod­ern North American machin­ery safety stan­dards as well.

Machine builders fre­quently have trou­ble with the risk assess­ment process, usu­ally because they fail to under­stand the process or because they fail to devote enough resources to get­ting it done.

If risk assess­ment is built into your design process, it becomes the norm for how you do busi­ness. Time and resources will auto­mat­i­cally be devoted to the process, and since it’s part of how you do things it will become rel­a­tively pain­less. Where peo­ple go wrong is in mak­ing it a ‘big deal’ one-​​time event. Also get­ting it done early in the design process and iter­ated as the design pro­gresses means that you have time to react to the find­ings, and you can com­plete any nec­es­sary changes at more cost-​​effective points in the design and build process. The worst time to do risk assess­ment is at the point where the machine is on the shop floor ready to start pro­duc­tion. Costs for mod­i­fi­ca­tion are then expo­nen­tially higher than dur­ing design and construction.

Poorly done, risk assess­ments become a lia­bil­ity defense lawyer’s worst night­mare and a plaintiff’s lawyer’s dream. Shortchanging the risk assess­ment process ensures that you will lose, either now or later.

Fight this prob­lem by: learn­ing how to con­duct a risk assess­ment, using qual­ity risk assess­ment soft­ware tools, and build­ing risk assess­ment into your stan­dard design process/​practice in your organization.

2) Failure to be Aware of Regulations & Use Design Standards

This one is a mys­tery to me.

Every mar­ket has prod­uct safety leg­is­la­tion, sup­ported by reg­u­la­tions. Granted, the scope and qual­ity of these reg­u­la­tions varies widely, but if you want to sell a prod­uct in a mar­ket, it doesn’t take a lot of effort to find out what reg­u­la­tions may apply.

Design stan­dards have been in exis­tence for a long time. Most pur­chase orders, at least for cus­tom machin­ery, con­tain lists of stan­dards that the equip­ment is required to meet at Factory Acceptance Testing (FAT).

Why machine builders fail to grasp that using these stan­dards can actu­ally give them a com­pet­i­tive edge, as well as help­ing them to meet reg­u­la­tory require­ments, I don’t know. If you do, please either com­ment on this story or send me an email. I’d love to hear your thoughts on this!

Fight this prob­lem by: Doing some research. Understand the mar­ket envi­ron­ment in which you sell your prod­ucts. If you aren’t sure how to do this, use a con­sul­tant to assist you. Buy the stan­dards, espe­cially if your client calls them out in their spec­i­fi­ca­tions. Read and apply them to your designs.

One great resource for infor­ma­tion on reg­u­la­tory envi­ron­ments and stan­dards appli­ca­tions is the IEEE Product Safety Engineering Society and the EMC-​​PSTC Listserv that they maintain.

3) Fixed Guard Design

Fixed guard­ing design is dri­ven by at least two fac­tors, a) pre­vent­ing peo­ple from access­ing haz­ards, and b) allow­ing raw mate­ri­als and prod­ucts into and out of the machinery.

Designers fre­quently go wrong by select­ing a fixed guard where a mov­able guard is nec­es­sary to per­mit fre­quent access (say more than once per shift). This is some­times done in an effort to avoid hav­ing to add inter­locks to the con­trol sys­tems. Frequently the guard will be removed and replaced a cou­ple of times, and then the screws will be left off, and even­tu­ally the guard itself will be left off, leav­ing the user with an unguarded haz­ard.

The other com­mon fault with fixed guards relates to the sec­ond fac­tor I men­tioned — get­ting raw mate­ri­als and prod­ucts in an out of the machine. There are lim­its on the size of open­ings that can be left in guards, depen­dent on the dis­tance from the open­ing to the haz­ards behind the guard and the size of the open­ing itself. Often the only fac­tor con­sid­ered is the size of the item that needs to enter or exit the machinery.

Both of these faults often occur because the guard­ing is not designed, but is allowed to hap­pen dur­ing machine build. The size and shape of the guards is then often dri­ven by con­ve­nience in fab­ri­ca­tion rather than by thought­ful design and appli­ca­tion of the min­i­mum code requirements.

Fight this prob­lem by: Designing the guards on your prod­uct rather than allow­ing them to hap­pen, based on the out­come of the risk assess­ment and the lim­its defined in the stan­dards. Tables for guard open­ings and safety dis­tances are avail­able in North American, EU and International standards.

4) Movable Guard Interlocking

Movable guards them­selves are usu­ally rea­son­ably well done. Note that I am not talk­ing about self adjust­ing guards like those found on a table saw for instance. I am talk­ing about guard doors, gates, and covers.

The prob­lem usu­ally comes with the design of the inter­lock that is required to go with the mov­able guard. The first part of the prob­lem goes back to my #1 mis­take: Risk Assessment. No risk assess­ment means that you can­not rea­son­ably hope to get the reli­a­bil­ity require­ments right for the inter­lock­ing sys­tem. Next, there are small but sig­nif­i­cant dif­fer­ences in how the Canadian, US, EU and International stan­dards han­dle con­trol reli­a­bil­ity, and the biggest dif­fer­ences occur in the higher reli­a­bil­ity classifications.

In the USA, the stan­dards speak of con­trol reli­able cir­cuits (see ANSI RIA R15.06–1999, 4.5.5). This require­ment is writ­ten in such a way that a sin­gle inter­lock­ing device, installed with dual chan­nel elec­tri­cal cir­cuits and suit­ably selected com­po­nents will meet the require­ments. No sin­gle ELECTRICAL com­po­nent fail­ure will lead to the loss of the safety func­tion, but a sin­gle mechan­i­cal fault could.

In Canada, the machin­ery and robot­ics stan­dards speak of con­trol reli­able sys­tems (see CSA Z432, 8.2.5), not cir­cuits as in the US stan­dards. This require­ment is writ­ten in such a way that TWO electro­mechan­i­cal inter­lock­ing devices are required, one in each elec­tri­cal chan­nel of the inter­lock­ing sys­tem. This per­mits the sys­tem to detect mechan­i­cal fail­ures such as bro­ken or miss­ing keys, and if dif­fer­ent types of inter­lock­ing devices are cho­sen, may also per­mit detec­tion of efforts to bypass the inter­lock. Most sin­gle mechan­i­cal faults and elec­tri­cal faults will be detected.

In the EU and Internationally, con­trol reli­a­bil­ity is much more highly devel­oped. Here, the appli­ca­tion of ISO 13849, IEC 62061 or IEC 61508 have taken con­trol reli­a­bil­ity to higher lev­els than any­thing seen to date in North America. Under these stan­dards, the required Performance Level (PL_r) or Safety Integrity Level (SIL) must be known. This is based on the out­come of, you guessed it, the Risk Assessment. No risk assess­ment, or a poor risk assess­ment, dooms the designer to likely fail­ure. Significant skill is required to han­dle the analy­sis and design of safety related parts of con­trol sys­tems under these standards.

Fight this prob­lem by: Getting the train­ing you need to prop­erly apply these stan­dards and then using them in your designs.

5) Safety Distances

Safety dis­tances crop up any­where you don’t have a phys­i­cal bar­rier keep­ing the user away from the haz­ard. Whether its an open­ing in a fixed guard, a mov­able guard like a guard door or gate, or a presence-​​sensing safe­guard­ing device like a light cur­tain, safety dis­tances have to be con­sid­ered in the machine design. The eas­ier it is for the user to come in con­tact with the haz­ard, the more safety dis­tance matters.

Stopping per­for­mance of the machin­ery must be tested to val­i­date the safety dis­tances used. Failure to get the safety dis­tance right means that your guards will give your users a false sense of secu­rity, and will expose them to injury. This will also expose your com­pany to sig­nif­i­cant lia­bil­ity when some­one gets hurt, because they will. Its only a mat­ter of time.

Fight this prob­lem by: Testing safe­guard­ing devices.

6) Validation

OK, so this list should really be SIX things. Just con­sider this to be a bonus for read­ing this far!

Designs, and par­tic­u­larly safety crit­i­cal designs, must be tested. Let me say it again:

Safety Critical Designs MUST Be Tested.

Whatever the­ory you are work­ing under, whether it’s North American, European, International or some­thing else, you can­not afford miss­ing the val­i­da­tion step. Without val­i­da­tion you have no evi­dence that your sys­tem worked at all, let alone if it worked correctly.

Fight this prob­lem by: TESTING YOUR DESIGNS.

A wise man once said: “If you think safety is expen­sive, try hav­ing an acci­dent.” The gen­tle­man was involved in inves­ti­gat­ing the crash of a Sikorsky S-​​92 heli­copter off the coast of Newfoundland. 17 peo­ple died as a result of the fail­ure of two tita­nium studs that held an oil fil­ter onto the main gear­box, and the fact that the heli­copter failed the ‘1/​2-​​hour gear­box run-​​dry test’ that is required for all new heli­copter designs. This was a clear case of fail­ure in the risk assess­ment process com­pli­cated by fail­ure in the test process.

Watch the CBC doc­u­men­tary “Cougar 491″. This is def­i­nitely worth the time. If you are located out­side Canada, you will have a prob­lem with this link. Unfortunately, CBC does not stream it’s video out­side Canada. Sorry.

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://​www​.com​pli​an​cein​sight​.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug’s work includes teach­ing machin­ery risk assess­ment tech­niques pri­vately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as pro­vid­ing tech­ni­cal ser­vices and train­ing pro­grams to clients related to risk assess­ment, indus­trial machin­ery safety, safety-​​related con­trol sys­tem inte­gra­tion and reli­a­bil­ity, laser safety and reg­u­la­tory conformity.

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