Tag: interlock

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

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.NOTE 1 A 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

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].

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.

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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Electrical Interlocks

By far the major­ity of inter­locks used on machinery are elec­tric­al. Electrical inter­locks offer ease of install­a­tion, flex­ib­il­ity in selec­tion of inter­lock­ing devices, and com­plex­ity from simple to extremely com­plex. The archi­tec­tur­al cat­egor­ies cov­er any tech­no­logy, wheth­er it is mech­an­ic­al, flu­id­ic, or elec­tric­al, so let’s have a look at archi­tec­tures first.

Architecture 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.

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.

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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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.

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, or 1600 mm/​s. 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 International approach, if the value of D_s is great­er than 500 mm when cal­cu­lated using K = 2 000, then [9] per­mits the cal­cu­la­tion to be done using K = 1 600 instead.

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

Eq. 1              D_s = K x T_s

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             D_s = 63 in/​s x 0.250 s = 15.75 inches

Example 2             D_s = 2000 mm/​s x 0.250 s = 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’, D_pf 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.

Also import­ant to con­sider is the amount the guard can be opened before activ­at­ing the inter­lock. This will depend on many factors, but for sim­pli­city, con­sider a hinged gate on an access point. If the guard uses two hinge-​pin style switches, you may be able to open the gate a few inches before the switches rotate enough to detect the open­ing of the guard. In order to determ­ine the open­ing size, you would slowly open the gate just to the point where the inter­lock is tripped, and then meas­ure the width of the open­ing. Using the tables found in [2], [3], [10], or ISO 13857 [12], you can then determ­ine how far the guard must be from the haz­ards behind it. If that dis­tance is great­er than what is avail­able, you could remove one hinge-​pin switch, and replace it with anoth­er type moun­ted on the post oppos­ite the hinges. This could be a keyed inter­lock like Photo 3, or a non-​contact device like Photo 5. This would reduce the open­ing width at the point of detec­tion, and thereby reduce the safety dis­tance behind the guard. But what if that is still not good enough?

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. The guard lock­ing device is only released when the machine has been made safe.

There are many types of safety-​rated stand-​still mon­it­or­ing devices avail­able now, and many variable-​frequency drives and servo drive sys­tems are avail­able with safety-​rated stand-​still mon­it­or­ing.

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.

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

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.

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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 [13]. Photo 7 shows fasten­ers with uniquely keyed key ways made by Bryce Fastener [14], and Photo 8 shows more tra­di­tion­al tamper­proof fasten­ers from the Tamperproof Screw Company [15]. 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.

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)

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[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
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[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] Safety of machinery – Safety dis­tances to pre­vent haz­ard zones being reached by upper and lower limbs, ISO 13857. 2008.

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

[14] Bryce Fastener, Inc. home page. (2012). Available: http://​www​.bryce​fasten​er​.com/

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