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Interlocking Devices: The Good, The Bad and the Ugly

2012 June 1
This entry is part 1 of 1 in the series Guards and Guarding
  • Interlocking Devices: The Good, The Bad and the Ugly

Note: A shorter ver­sion of this arti­cle was pub­lished in the May-​​2012 edi­tion of  Manufacturing Automation Magazine.

When design­ing safe­guard­ing sys­tems for machines, one of the basic build­ing blocks is the mov­able guard. Movable guards can be doors, pan­els, gates or other phys­i­cal bar­ri­ers that can be opened with­out using tools. Every one of these guards needs to be inter­locked with the machine con­trol sys­tem so that the haz­ards cov­ered by the guards will be effec­tively con­trolled when the guard is opened.

There are a num­ber of impor­tant aspects to the design of mov­able guards. This arti­cle will focus on the selec­tion of inter­lock­ing devices that are used with mov­able guards.

The hier­ar­chy of controls

The Hierarchy of Controls as an inverted pyrimid.

Figure 1 — The Hierarchy of Controls

This arti­cle assumes that a risk assess­ment has been done as part of the design process. If you haven’t done a risk assess­ment first, start there, and then come back to this point in the process. You can find more  infor­ma­tion on risk assess­ment meth­ods in this post from 31-​​Jan-​​11. ISO 12100 [1] can also be used for guid­ance in this area.

The hier­ar­chy of con­trols describes lev­els of con­trols that a machine designer can use to con­trol the assessed risks. The hier­ar­chy is defined in [1]. Designers are required to apply every level of the hier­ar­chy in order, start­ing at the top. Each level is applied until the avail­able mea­sures are exhausted, or can­not be applied with­out destroy­ing the pur­pose of the machine, allow­ing the designer to move to the next lower level.

Engineering con­trols are sub­di­vided into a num­ber of dif­fer­ent sub-​​groups. Only mov­able guards are required to have inter­locks. There are a num­ber of sim­i­lar types of guards that can be mis­taken for mov­able guards, so let’s take a minute to look at a few impor­tant definitions.

Table 1 — Definitions

International [1] Canadian [2] USA [10]
3.27 guard phys­i­cal bar­rier, designed as part of the machine to pro­vide pro­tec­tion.NOTEA guard may act either alone, in which case it is only effec­tive when “closed” (for a mov­able guard) or “securely held in place” (for a fixed guard), or  in con­junc­tion with an inter­lock­ing device with or with­out guard lock­ing, in which case pro­tec­tion is ensured what­ever the posi­tion of the guard.NOTE 2Depending on its con­struc­tion, a guard may be described as, for exam­ple, cas­ing, shield, cover, screen, door, enclos­ing guard.NOTE 3 The terms for types of guards are defined in 3.27.1 to 3.27.6. See also 6.3.3.2 and ISO 14120 for types of guards and their requirements. Guard — a part of machin­ery specif­i­cally used to pro­vide pro­tec­tion by means of a phys­i­cal barrier. Depending on its con­struc­tion, a guard may be called a cas­ing, screen, door, enclos­ing guard, etc. 3.22 guard: A bar­rier that pre­vents expo­sure to an iden­ti­fied haz­ard.E3.22 Sometimes referred to as bar­rier guard.”
3.27.4 inter­lock­ing guard guard asso­ci­ated with an inter­lock­ing device so that, together with the con­trol sys­tem of the machine, the fol­low­ing func­tions are performed:

  • the haz­ardous machine func­tions “cov­ered” by the guard can­not oper­ate until the guard is closed,
  • if the guard is opened while haz­ardous machine func­tions are oper­at­ing, a stop com­mand is given, and
  • when the guard is closed, the haz­ardous machine func­tions “cov­ered” by the guard can oper­ate (the clo­sure of the guard does not by itself start the haz­ardous machine functions)

NOTE ISO 14119 gives detailed provisions.

Interlocked bar­rier guard — a fixed or mov­able guard attached and inter­locked in such a man­ner that the machine tool will not cycle or will not con­tinue to cycle unless the guard itself or its hinged or mov­able sec­tion encloses the haz­ardous area. 3.32 inter­locked bar­rier guard: A bar­rier, or sec­tion of a bar­rier, inter­faced with the machine con­trol sys­tem in such a man­ner as to pre­vent inad­ver­tent access to the hazard.
3.27.2 mov­able guard
guard which can be opened with­out the use of tools
Movable guard — a guard gen­er­ally con­nected by mechan­i­cal means (e.g., hinges or slides) to the machine frame or an adja­cent fixed ele­ment and that can be opened with­out the use of tools. The open­ing and clos­ing of this type of guard may be powered. 3.37 mov­able bar­rier device: A safe­guard­ing device arranged to enclose the haz­ard area before machine motion can be ini­ti­ated.E3.37 There are two types of mov­able bar­rier devices:

  • Type A, which encloses the haz­ard area dur­ing the com­plete machine cycle;
  • Type B, which encloses the haz­ard area dur­ing the haz­ardous por­tion of the machine cycle.
3.28.1 inter­lock­ing device (interlock)mechanical, elec­tri­cal or other type of device, the pur­pose of which is to pre­vent the oper­a­tion of haz­ardous machine func­tions under spec­i­fied con­di­tions (gen­er­ally as long as a guard is not closed) Interlocking device (inter­lock) — a mechan­i­cal, elec­tri­cal, or other type of device, the pur­pose of which is to pre­vent the oper­a­tion of machine ele­ments under spec­i­fied con­di­tions (usu­ally when the guard is not closed). No def­i­n­i­tion
3.27.5 inter­lock­ing guard with guard lock­ing guard asso­ci­ated with an inter­lock­ing device and a guard lock­ing device so that, together with the con­trol sys­tem of the machine, the fol­low­ing func­tions are performed:

  • the haz­ardous machine func­tions “cov­ered” by the guard can­not oper­ate until the guard is closed and locked,
  • the guard remains closed and locked until the risk due to the haz­ardous machine func­tions “cov­ered” by the guard has dis­ap­peared, and
  • when the guard is closed and locked, the haz­ardous machine func­tions “cov­ered” by the guard can oper­ate (the clo­sure and lock­ing of the guard do not by them­selves start the haz­ardous machine functions)

NOTE ISO 14119 gives detailed provisions.

Guard lock­ing device — a device that is designed to hold the guard closed and locked until the haz­ard has ceased. No def­i­n­i­tion

As you can see from the def­i­n­i­tions, mov­able guards can be opened with­out the use of tools, and are gen­er­ally fixed to the machine along one edge. Movable guards are always asso­ci­ated with an inter­lock­ing device. Guard selec­tion is cov­ered very well in ISO 14120 [11]. This stan­dard con­tains a flow­chart that is invalu­able for select­ing the appro­pri­ate style of guard for a given application.

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Though much empha­sis is placed on the cor­rect selec­tion of these inter­lock­ing devices, they rep­re­sent a very small por­tion of the hier­ar­chy. It is their wide­spread use that makes them so impor­tant when it comes to safety sys­tem design.

Electrical vs. Mechanical Interlocks

Mechanical Interlocking

Figure 2 — Mechanical Interlocking

Most mod­ern machines use elec­tri­cal inter­locks because the machine is fit­ted with an elec­tri­cal con­trol sys­tem, but it is entirely pos­si­ble to inter­lock the power to the prime movers using mechan­i­cal means. This doesn’t affect the por­tion of the hier­ar­chy involved, but it may affect the con­trol reli­a­bil­ity analy­sis that you need to do.

Mechanical Interlocks

Figure 2, from ISO 14119 [7, Fig. H.1, H.2 ], shows one exam­ple of a mechan­i­cal inter­lock.  In this case, when cam 2 is rotated into the posi­tion shown in a), the guard can­not be opened. Once the haz­ardous con­di­tion behind the guard is effec­tively con­trolled, cam 2 rotates to the posi­tion in b), and the guard can be opened.

Arrangements that use the open guard to phys­i­cally block oper­a­tion of the con­trols can also be used in this way. See Figure 3 [7, Fig. C.1, C.2].

Mechanical Interlocking using control devices

Figure 3 — Mechanical Interlocking using machine con­trol devices

Fluid Power Interlocks

Figure 4, from [7, Fig. K.2], shows an exam­ple of two fluid-​​power valves used in com­ple­men­tary mode on a sin­gle slid­ing gate.

Hydraulic interlock from ISO 14119

Figure 4 — Example of a fluid power interlock

In this exam­ple, fluid can flow from the pres­sure sup­ply (the cir­cle with the dot in it at the bot­tom of the dia­gram) through the two valves to the prime-​​mover, which could be a cylin­der, or a motor or some other device when the guard is closed (posi­tion ‘a’). There could be an addi­tional con­trol valve fol­low­ing the inter­lock that would pro­vide the nor­mal con­trol mode for the device.

When the guard is opened (posi­tion ‘b’), the two valve spools shift to the sec­ond posi­tion, the lower valve blocks the pres­sure sup­ply, and the upper valve vents the pres­sure in the cir­cuit, help­ing to pre­vent unex­pected motion from trapped energy.

If the spring in the upper valve fails, the lower spool will be dri­ven by the gate into a posi­tion that will still block the pres­sure sup­ply and vent the trapped energy in the cir­cuit.
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Categories

Comparing ANSI, CSA, and ISO Control Reliability Categories

Figure 5 — Control Reliability Categories

In Canada, CSA Z432 [2] and CSA Z434 [3] pro­vide four cat­e­gories of con­trol reli­a­bil­ity: sim­ple, sin­gle chan­nel, single-​​channel mon­i­tored and con­trol reli­able. In the U.S., the cat­e­gories are very sim­i­lar, with some dif­fer­ences in the def­i­n­i­tion for con­trol reli­able (see RIA R15.06, 1999). In the EU, there are five lev­els of con­trol reli­a­bil­ity, defined as Performance Levels (PL) given in ISO 13849–1 [4]: PL a, b, c, d and e. Underpinning these lev­els are five archi­tec­tural cat­e­gories: B, 1, 2, 3 and 4. Figure 5 shows how these archi­tec­tures line up.

To add to the con­fu­sion, IEC 62061 [5] is another inter­na­tional con­trol reli­a­bil­ity stan­dard that could be used. This stan­dard defines reli­a­bil­ity in terms of Safety Integrity Levels (SILs). These SILs do not line up exactly with the PLs in [4], but they are sim­i­lar. [5] is based on IEC 61508 [6], a well-​​respected con­trol reli­a­bil­ity stan­dard used in the process indus­tries. [5] is not well suited to appli­ca­tions involv­ing hydraulic or pneu­matic elements.

The orange arrow in Figure 5 high­lights the fact that the def­i­n­i­tion in the CSA stan­dards results in a more reli­able sys­tem than the ANSI/​RIA def­i­n­i­tion because the CSA def­i­n­i­tion requires TWO (2) sep­a­rate phys­i­cal switches on the guard to meet the require­ment, while the ANSI/​RIA def­i­n­i­tion only requires redun­dant cir­cuits, but makes no require­ment for redun­dant devices. Note that the arrow rep­re­sent­ing the ANSI/​RIA Control reli­a­bil­ity cat­e­gory falls below the ISO Category 3 arrow due to this same detail in the definition.

Note that Figure 5 does not address the ques­tion of PL’s or SIL’s and how they relate to each other. That is a topic for another article!

The North American archi­tec­tures deal pri­mar­ily with elec­tri­cal or fluid-​​power con­trols, while the EU sys­tem can accom­mo­date elec­tri­cal, fluid-​​power and mechan­i­cal systems.

From the single-​​channel-​​monitored or Category 2 level up, the sys­tems are required to have test­ing built-​​in, enabling the detec­tion of fail­ures in the sys­tem. The level of fault tol­er­ance increases as the cat­e­gory increases.

Interlocking devices

Interlocking devices are the com­po­nents that are used to cre­ate the inter­lock between the safe­guard­ing device and the machine’s power and con­trol sys­tems. Interlocking sys­tems can be purely mechan­i­cal, purely elec­tri­cal or a com­bi­na­tion of these.

Roller cam switch used as part of a complementary interlock

Photo 1 — Roller Cam Switch

Most machin­ery has an electrical/​electronic con­trol sys­tem, and these sys­tems are the most com­mon way that machine haz­ards are con­trolled. Switches and sen­sors con­nected to these sys­tems are the most com­mon types of inter­lock­ing devices.

Interlocking devices can be some­thing as sim­ple as a micro-​​switch or a reed switch, or as com­plex as a non-​​contact sen­sor with an elec­tro­mag­netic lock­ing device.

Images of inter­lock­ing devices used in this arti­cle are rep­re­sen­ta­tive of some of the types and man­u­fac­tur­ers avail­able, but should not be taken as an endorse­ment of any par­tic­u­lar make or type of device. There are lots of man­u­fac­tur­ers and unique mod­els that can fit any given appli­ca­tion, and most man­u­fac­tur­ers have sim­i­lar devices available.

Photo 1 shows a safety-​​rated, direct-​​drive roller cam switch used as half of a com­ple­men­tary switch arrange­ment on a gate inter­lock. The inte­gra­tor failed to cover the switches to pre­vent inten­tional defeat in this application.

Micro-Switch used for interlocking

Photo 2 — Micro-​​Switch used for interlocking

Photo 2 shows a ‘microswitch’ used for inter­lock­ing a machine cover panel that is nor­mally held in place with fas­ten­ers, and so is a ‘fixed guard’ as long as the fas­ten­ers require a tool to remove. Fixed guards do not require inter­locks under most cir­cum­stances. Some prod­uct fam­ily stan­dards do require inter­locks on fixed guards due to the nature of the haz­ards involved.

Microswitches are not safety-​​rated and are not rec­om­mended for use in this appli­ca­tion. They are eas­ily defeated and tend to fail to dan­ger in my experience.

Requirements for inter­lock­ing devices are pub­lished in a num­ber of stan­dards, but the key ones for indus­trial machin­ery are ISO 14119 [7], [2], and ANSI B11.0 [8]. These stan­dards define the elec­tri­cal and mechan­i­cal require­ments, and in some cases the test­ing require­ments, that devices intended for safety appli­ca­tions must meet before they can be clas­si­fied as safety com­po­nents.
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Typical plastic-bodied interlocking device

Photo 3 — Schmersal AZ15 plas­tic inter­lock switch

These devices are also inte­gral to the reli­a­bil­ity of the con­trol sys­tems into which they are inte­grated. Interlock devices, on their own, can­not meet a reli­a­bil­ity rat­ing above ISO 13849–1 Category 1, or CSA Z432-​​04 Single Channel. To under­stand this, con­sider that the def­i­n­i­tions for Category 2, 3 and 4 all require the abil­ity for the sys­tem to mon­i­tor and detect fail­ures, and in Categories 3 & 4, to pre­vent the loss of the safety func­tion. Similar require­ments exist in CSA and ANSI’s “single-​​channel-​​monitored,” and “control-​​reliable” cat­e­gories. Unless the inter­lock device has a mon­i­tor­ing sys­tem inte­grated into the device, these cat­e­gories can­not be achieved.

Guard Locking

Interlocking devices are often used in con­junc­tion with  guard lock­ing. There are a few rea­sons why a designer might want to lock a guard closed, but the most com­mon one is a lack of safety dis­tance. In some cases the guard may be locked closed to pro­tect the process rather than the oper­a­tor, or for other reasons.

Interlock Device with Guard Locking

Photo 4 — Interlocking Device with Guard Locking

Safety dis­tance is the dis­tance between the open­ing cov­ered by the mov­able guard and the haz­ard. The min­i­mum dis­tance is deter­mined using the safety dis­tance cal­cu­la­tions given in [2] and ISO 13855 [9]. This cal­cu­la­tion uses a ‘hand-​​speed con­stant’, called K, to rep­re­sent the the­o­ret­i­cal speed that the aver­age per­son can achieve when extend­ing their hand straight for­ward when stand­ing in front of the open­ing. In North America, K is usu­ally 63 inches/​second. Internationally and in the EU, there are two speeds, 2000 mm/​s, used for an approach per­pen­dic­u­lar to the plane of the guard, or 1600 mm/​second for approaches at 45 degrees or less [9]. 2000 mm/​s is used with mov­able guards, and is approx­i­mately equiv­a­lent to 79 inches/​second.

Using the stop­ping time of the machin­ery and K, the min­i­mum safety dis­tance can be calculated.

Eq. 1              Ds = K x Ts

Using Equation 1 [2], assume you have a machine that takes 250 ms to stop when the inter­lock is opened. Inserting the val­ues into the equa­tion gives you a min­i­mum safety dis­tance of:

Example 1             Ds = 63 in/​s x 0.250 s = 15.75″

Example 2             Ds = 2000 mm/​s x 0.250 = 500 mm

As you can see, the International value of K gives a more con­ser­v­a­tive value, since 500 mm is approx­i­mately 20 inches.

Note that I have not included the ‘Penetration Factor’, Dpf in this cal­cu­la­tion. This fac­tor is used with pres­ence sens­ing safe­guard­ing devices like light cur­tains, fences, mats, two-​​hand con­trols, etc. This fac­tor is not applic­a­ble to mov­able, inter­locked guards.

If you have to install the guard closer to the haz­ard than the min­i­mum safety dis­tance, lock­ing the guard closed and mon­i­tor­ing the stand-​​still of the machine allows you to ignore the safety dis­tance require­ment because the guard can­not be opened until the machin­ery is at a stand­still, or in a safe state.

Guard lock­ing devices can be mechan­i­cal, elec­tro­mag­netic, or any other type that pre­vents the guard from open­ing under con­trol of the machine.

Environment, fail­ure modes and fault exclusion

Every device has fail­ure modes. The cor­rect selec­tion of the device starts with under­stand­ing the phys­i­cal envi­ron­ment to which the device will be exposed. This means under­stand­ing the tem­per­a­ture, humid­ity, dust/​abrasives expo­sure, chem­i­cal expo­sures, and mechan­i­cal shock and vibra­tion expo­sures in the appli­ca­tion. Selecting a del­i­cate reed switch for use in a high-​​vibration, high-​​shock envi­ron­ment is a recipe for fail­ure, just as select­ing a mechan­i­cal switch in a dusty, damp, cor­ro­sive envi­ron­ment will also lead to pre­ma­ture failure.

Example of a non-contact interlocking device

Photo 5 — JOKAB EDEN Interlock System

Interlock device man­u­fac­tur­ers have a vari­ety of non-​​contact inter­lock­ing devices avail­able today that use coded RF sig­nals or RF ID tech­nolo­gies to ensure that the inter­lock can­not be defeated by sim­ple mea­sures, like tap­ing a mag­net to a reed switch. The Jokab EDEN sys­tem is one exam­ple of a sys­tem like this that also exhibits IP65 level resis­tance to mois­ture and dust. Note that sys­tems like this include a safety mon­i­tor­ing device and the sys­tem as a whole can meet Control Reliable or Category 3 /​ 4 archi­tec­tural require­ments when a sim­ple inter­lock switch could not.

The device stan­dards do pro­vide some guid­ance in mak­ing these selec­tions, but it’s pretty general.

Fault Exclusion

Fault exclu­sion is another key con­cept that needs to be under­stood. Fault exclu­sion holds that fail­ure modes that have an exceed­ingly low prob­a­bil­ity of occur­ring dur­ing the life­time of the prod­uct can be excluded from con­sid­er­a­tion. This can apply to elec­tri­cal or mechan­i­cal fail­ures. Here’s the catch: Fault exclu­sion is not per­mit­ted under any North American stan­dards at the moment. Designs based on the North American con­trol reli­a­bil­ity stan­dards can­not take advan­tage of fault exclu­sions. Designs based on the International and EU stan­dards can use fault exclu­sion, but be aware that sig­nif­i­cant doc­u­men­ta­tion sup­port­ing the exclu­sion of each fault is needed.

Defeat resis­tance

Diagram showing one method of preventing interlock defeat.

Figure 6 — Preventing Defeat

The North American stan­dards require that the devices cho­sen for safety-​​related inter­locks be defeat-​​resistant, mean­ing they can­not be eas­ily fooled with a cable-​​tie, a scrap of metal or a piece of tape.

Figure 6 [7, Fig. 10] shows a key-​​operated switch, like the Schmersal AZ15, installed with a cover that is intended to fur­ther guard against defeat. The key, some­times called a ‘tongue’, used with the switch pre­vents defeat using a flat piece of metal or a knife blade. The cover pre­vents direct access to the inter­lock­ing device itself. Use of tamper-​​resistant hard­ware will fur­ther reduce the like­li­hood that some­one can remove the key and insert it into the switch, bypass­ing the guard.

Inner-Tite tamper resistance fasteners

Photo 6 — Tamper-​​resistant fasteners

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The International and EU stan­dards do not require the devices to be inher­ently defeat resis­tant, which means that you can use “safety-​​rated” limit switches with roller-​​cam actu­a­tors, for exam­ple. However, as a designer, you are required to con­sider all rea­son­ably fore­see­able fail­ure modes, and that includes inten­tional defeat. If the inter­lock­ing devices are eas­ily acces­si­ble, then you must select defeat-​​resistant devices and install them with tamper-​​resistant hard­ware to cover these fail­ure modes.

Photo 6 shows one type of tam­per resis­tant fas­ten­ers made by Inner-​​Tite. Photo 7 [12] shows fas­ten­ers with uniquely keyed key ways made by Bryce Fastener [13], and Photo 8 shows more tra­di­tional tam­per­proof fas­ten­ers from the Tamperproof Screw Company [14]. Using fas­ten­ers like these will result in the high­est level of secu­rity in a threaded fas­tener. There are many dif­fer­ent designs avail­able from a wide vari­ety of manufacturers.

Bryce Key-Rex tamper-resistant fasteners

Photo 7 — Keyed Tamper-​​Resistant Fasteners

Tamper proof screws made by the Tamperproof Screw Company
Photo 8 — Tamper proof screws

 

Almost any inter­lock­ing device can be bypassed by a knowl­edge­able per­son using wire and the right tools. This type of defeat is not gen­er­ally con­sid­ered, as the degree of knowl­edge required is greater than that pos­sessed by “nor­mal” users.

How to select the right device

When select­ing an inter­lock­ing device, start by look­ing at the envi­ron­ment in which the device will be located. Is it dry? Is it wet (i.e., with cut­ting fluid, oil, water, etc.)? Is it abra­sive (dusty, sandy, chips, etc.)? Is it indoors or out­doors and sub­ject to wide tem­per­a­ture variations?

Is there a prod­uct stan­dard that defines the type of inter­lock you are design­ing? An exam­ple of this is the inter­lock types in ANSI B151.1 [4] for plas­tic injec­tion mould­ing machines. There may be restric­tions on the type of devices that are suit­able based on the require­ments in the standard.

Consider inte­gra­tion require­ments with the con­trols. Is the inter­lock purely mechan­i­cal? Is it inte­grated with the elec­tri­cal sys­tem? Do you require guard lock­ing capa­bil­ity? Do you require defeat resis­tance? What about device mon­i­tor­ing or annunciation?

Once you can answer these ques­tions, you will have nar­rowed down your selec­tions con­sid­er­ably. The final ques­tion is: What brand is pre­ferred? Go to your pre­ferred supplier’s cat­a­logues and make a selec­tion that fits with the answers to the pre­vi­ous questions.

The next stage is to inte­grate the device(s) into the con­trols, using whichever con­trol reli­a­bil­ity stan­dard you need to meet. That is the sub­ject for a series of articles!

References

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[1] Safety of machin­ery — General prin­ci­ples for design — Risk assess­ment and risk reduction, ISO Standard 12100, Edition 1, 2010

[2] Safeguarding of Machinery, CSA Standard Z432, 2004 (R2009)

Buy CSA Standards

[3] Industrial Robots and Robot Systems — General Safety Requirements, CSA Standard Z434, 2003 (R2008)

[4] Safety of machin­ery — Safety-​​related parts of con­trol sys­tems — Part 1: General prin­ci­ples for design, ISO Standard 13849–1, 2006

[5] Safety of machin­ery – Functional safety of safety-​​related electrical, electronic and pro­gram­ma­ble elec­tronic con­trol sys­tems, IEC Standard 62061, Edition 1, 2005

[6] Functional safety of electrical/​electronic/​programmable elec­tronic safety-​​related sys­tems (Seven Parts), IEC Standard 61508-​​X

[7] Safety of machin­ery — Interlocking devices asso­ci­ated with guards — Principles for design and selection, ISO Standard 14119, 1998

[8] American National Standard for Machines, General Safety Requirements Common to ANSI B11 Machines, ANSI Standard B11, 2008
Download ANSI standards

[9] Safety of machin­ery — Positioning of safe­guards with respect to the approach speeds of parts of the human body, ISO 13855, 2010

[10] American National Standard for Machine Tools – Performance Criteria for Safeguarding, ANSI B11.19, 2003

[11] Safety of machin­ery — Guards — General require­ments for the design and con­struc­tion of fixed and mov­able guards, ISO 14120, 2002

[12] Inner-​​Tite Corp. home page. (2012). Available: http://​www​.inner​-tite​.com/

[13] Bryce Fastener, Inc. home page. (2012). Available: http://​www​.bryce​fas​tener​.com/

[14] Tamperproof Screw Co., Inc., home page. (2013). Available: http://​www​.tam​per​proof​.com

Post By Doug Nix (95 Posts)

+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.

Website: → Compliance inSight Consulting Inc.

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  • http://machinerysafety101.com/ DougNix

    Thanks, Greg! Glad to know you found it helpful!

  • Greg Santo

    Excellent arti­cle

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