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Safety GuardsStandardized Safety Guard Design Guide contains design criteria and specifications used in the design of safety guards for General Mills’ Machine Design.

Table of Contents1. Introduction .......................... 1

2. Material ................................ 2

3. Perforations & Openings ..... 3

4. Perforation ........................... 5

5. Bottom Openings ................. 9

6. Fasteners .......................... 10

7. Handles ............................. 13

8. Guard Door Interlocking Devices 14

9. Light Curtains .................... 19

10. Belt / Chain Guards ........... 20

11. Shaft End Guards .............. 22

12. Product Flow Grates .......... 23

13. Revision Summaries .......... 24

1. Introduction

Consider all standards described in this document mandatory unless otherwise identified as optional or preferred. This document is to be used as a reference and guide for Machine Design Engineers and Designers. It is meant as a supplement to the General Mills Machine Guarding Workbook. It can be found at

http://gmiportal.generalmills.com/tc/eng/deps/mach/Manuals/Operations%20Machine%20Guarding%20Workbook.doc.

Although most situations will require custom guards, this guide will still provide details to help keep all machinery produced by Machine Design looking and functioning consistently.

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2. Material

Stainless Steel - SST ASTM 304 is preferred over Lexan because it has a longer service life. 16 gauge is a good starting thickness, although 14 gauge may be used if large spans must be made unsupported. If components behind the guard need to be observed, slots may be laser cut to facilitate viewing.

Lexan - Polycarbonate polymer is not preferred because it scratches easily and becomes cloudy over time. It can also crack if excessive machine vibration is experienced. If used in hot humid environments, condensation can gather and render the guard non transparent. ¼” is a good starting thickness. Lexan is only to be used when it is specifically requested by the customer due to its reduced life span.

When using Lexan, be sure to use grommets and/or flat washers to avoid cracking, which can be caused by over torqueing the fasteners, or by excessive machine vibration. Recommend McMaster Carr P/N: 9600K54

Figure 1 - Example of a grommet. McMaster Carr P/N: 9600K54

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3. Perforations & Openings

If there is an opening, or hole, the following charts should be utilized to determine if it is required to guard it. Similarly, the chart should be used for determine guard perforation size based on distance from the guard to the hazard point. Ref ANSI B11.19.

Figure 2 - ANSI Guard Opening VS Distance to Danger Graph

Distanceinches

Max. Openinginches

1/2 - 1-1/2 1/41-1/2 - 2-1/2 3/82-1/2 - 3-1/2 1/23-1/2 - 5-1/2 5/85-1/2 - 6-1/2 3/46-1/2 - 7-1/2 7/87-1/2 - 12-1/2 1-1/412-1/2 - 15-1/2 1-1/215-1/2 - 17-1/2 1-7/817-1/2 - 31-1/2 2-1/8

Figure 3 - ANSI Guard Opening VS Distance to Danger Chart

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The charts below are based on the European ISO standard of 13582 so any equipment being shipped to countries outside of the US must adhere to these values.

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Figure 4 - ISO 13852 for use in Europe.

Figure 5 - For an irregularly shaped hole, use whichever dimension “e” is largest as measured in the above figure.

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4. Perforation

When using SST, if transparency is required, slots may be laser cut into the guard. We will use two standard patterns for slotting. One is for large panels (>150 in sq), and one will be for smaller panels (<150 in sq). It is important to recognize that any time a guard opening is created, it must comply with ANSI’s guard opening to distance chart. See Guarding Concepts document.

Large Pattern - This pattern may only be used if the danger point is at least 1.5” from the guard face. The diagram below shows how the slots should be sized. The slot end centers should be 1.5” from the edge of the guard. The slots should be 0.375” wide, 3” long, and should be spaced 0.5” apart, edge to edge. The slots should be spaced 1” apart, end center to end center. The SolidWorks Linear Pattern feature should use the spacing values of 1.75” vertically and 4.0” horizontally. This pattern is 35% open.

Figure 6 - Large Slot Pattern Detail

Slots near contoured edges should be compromised in length to maintain 1.5” from the edge. Slot width and spacing should be maintained. In order to center the slot pattern within a given rectangle, the edge distance of 1.5” may be modified.

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Figure 7 - Large Slot Pattern Overview – GMI 61512

Small Pattern - This pattern may only be used if the danger point is at least 0.5” from the guard face. The pattern below shows smaller shorter slots in a tighter configuration. This pattern is 47% open. The pattern below is not centered with in the rectangle guard. The edge distance, in this case 0.75”, may be modified to center the pattern. The SolidWorks Linear Pattern feature should use the spacing values of 0.875” vertically and 1.938” horizontally.

Figure 8 - Small Slot Pattern Detail

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Figure 9 - Small Slot Pattern Overview – GMI 62674

If custom laser cutting perforation in guards is not possible or is cost prohibitive, Harrington & King provides perforated sheet stock that is also accepted. Their 1/4” x 1-1/2” Oblong Side Staggered Slot, 7/16” Centers is acceptable. This is the same pattern as the “Small Pattern” above.

Circular Pattern – This pattern may only be used if the danger point is at least 1.5” from the guard face. Another acceptable pattern by Harrington & King is the 3/8” circular pattern. The holes are staggered and ½” center to center. It is often used to make retrofit guards on existing machinery. This is not a preference on new equipment. This pattern is 51% open. The Maximo part number of that sheet is 8100235561.

This pattern may also be an attractive option in terms of reducing cost. Since laser cutting can sometimes become expensive, circular holes may be punched at a lower cost. The cost savings would be on the order of 30%.

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Figure 10 – Circular Hole Pattern – GMI xxxxx

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5. Bottom OpeningsBottom openings are commonly used on chain or belt guards were dust, debris, and lubricant has a tendency to come loose and fall to the bottom of the guard. In the past, rods have been used to serve as the perforated opening. However, the rods were an expensive method of creating openings and were difficult to clean. The rod design is now discouraged from use.

The preferred method of creating bottom openings is to utilize the slot patterns shown in Section 3. Below is a picture of an older guard with a bottom opening covered with rods. It is an example of what not to do. See Section 9 for an illustration of how to slot the bottom of belt or chain guards.

Figure 11 - Example of Rod Covered Bottom Opening – What Not to Do – GMI 62212

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6. Fasteners

Although guards can be fastened to a machine in many ways, there are three preferred methods. They are threaded fasteners, ¼ turn latches, or interlocked doors. These methods are designed to comply with OSHA regulation 1910.212. (See Reference List Item 1 on Page 21.). Unless interlocked, all guard fasteners must require tools to remove. Note: Do not drill or tap into hollow frame members!

Threaded Fasteners – Using 3/8 – 16 UNC (9/16” wrench) fasteners is the preferred method for attaching guards. 10mm is the associated metric preference. 5/16 - 18 UNC (1/2” wrench) fasteners are also acceptable in smaller guarding applications. The guard should be drilled with a 7/16 hole to allow for misalignment. The bolt holes should be between 6” and 10” apart, depending on the guard configuration.

Female Threaded Studs - Female threaded studs are the preferred type of threaded fastener, and should be stud welded to the machine frame. The studs are ¾” in overall length.

Figure 12 3/8”- 16 X ¾” Female Stud – GMI 61919

Male threaded studs – Male threaded studs may be used if a gap between the frame and the guard is not acceptable. It should be noted that standard mig weld fillets should not be used to attach the studs since it leaves a shoulder which prevents the guard from seating properly. The male studs are 1” long.

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¼ Turn Latch – These latches have the benefit of not taking as long to unfasten the guard and do not have the risk of losing fasteners, or them potentially falling into the machine or product zone. These latches made by SouthCo (www.SouthCo.com) require an tool to turn it 90 degrees, at which point the latch arm moves out of the way and the guard is removable. A Hex style head is preferred since too many objects can act as a screw driver for the slotted style heads. A good starting point is model number E3-16-20. ( http://www.southco.com/products/e3-vise-action-compression-latches/e3-16-20.html ). This model is stainless steel, medium size, adjustable for guard/panel thickness, requires an 8mm Allen wrench, and has a medium latch reach. If different parameters are required, they are available at the above web site, as are the associated CAD models.

Figure 13 – ¼ Turn Fastener - SouthCo Model E3-16-20

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Toggle Clamps – If the application requires or could benefit from a toggle clamp latching mechanism, there is a GMI designed safety locking toggle clamp latch available. It is GMI 62518. The red plastic handle covering can be removed if sanitation is a concern. The approximate cost is $600/ea.

Figure 14 DE-STA-CO Safety Locking Latch / Toggle Clamp - GMI 62518

Note that in guarding applications, typical toggle clamps do not require tools to unlatch and therefore require the additional use of an interlock to ensure that the guard is in place when the equipment is running.

Interlocks – Interlocks may be used to ensure that guards are put in place. A deeper discussion on Interlocks is included in Section 8 below.

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7. HandlesAny guard that weighs more than 25 pounds, or has any dimension larger than 48 inches should have at least one handle, preferably two. They shall be designed as shown below. There should be a minimum clearance of 2” from the bottom of the handle to the mounting surface, and a minimum interior width of 4”. They should be fillet welded on at the base.

Figure 15 Standard Handle Design – GMI 63065

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8. Guard Door Interlocking Devices

Safety Guard Switches are to be used to monitor that the safety guard is in a safe and correct position. Interlocks shall be installed if the guard can be opened without the use of a tool. Use ANSI B11.19 standards to determine how far the guard must be from the hazard. (See page Error: Reference source not found.) If the dimensions in that standard cannot be adhered to, an interlocking type safety switch must be used to hold the guard in place until the hazard (usually machine movement) is no longer present, i.e. the machine has stopped moving. Safety switches must be safety rated and have a SIL rating of 2 or 3.

For instances where a guard needs to be removed frequently, see the Risk Assessment Grid below.

Prob

abili

tyof

occ

urre

nce

of h

arm

Very Likelynearly certain to occur

No MSE.LOTO only.

No MSE. LOTO only.

PerformanceLevel D

PerformanceLevel D

Likelymay occur

No MSE.LOTO only.

PerformanceLevel D

PerformanceLevel D

PerformanceLevel C

Unlikelynot likely to occur

PerformanceLevel D

PerformanceLevel D

PerformanceLevel C

PerformanceLevel B

Remoteso unlikely as to be near zero

PerformanceLevel C

PerformanceLevel C

PerformanceLevel B

PerformanceLevel B

Figure 16 Risk Assessment Grid.

This grid applies to guards that do not require tools to remove, and therefore require an interlock. The grid above dictates the necessary performance level of that interlock, as well as the entire safety circuit.

For Catastrophic, Very Likely conditions, it is recommended as a best practice to also fasten the guards in place as a backup safety in addition to the interlock. Even though LOTO would be required in this scenario, the 2 safety features keep the operators and equipment safe in the event that someone attempts to start the machine with the guard not in place.

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Cam-Type Safety Switches Trigger bearing riding on a cam mounted on a hinged device

Precaution: The cam design must be tamper proof. The picture below is a good example of a tamper proof design.

Typical part # Allen-Bradley 802T-DPDD5 (About $200 - Limit Switch, NEMA Type 4 and 13 Oil-tight Construction, Plug-In, Safety Switch, Top Push Roller, 2-Circuit, 5-Pin DC Micro-Type Receptacle)

CAM unit is custom fit to the hinge.

Models exist in Adept titled “AB-802T-DTPD.sldprt” and “57748.sldprt”

Figure 17 Mechanical Safety Switch – GMI # AB-802T-DTPD

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Two Component Safety Switches (Non-Contacting)

Safety switch with the primary unit mounted on a stationary frame while the safety target is mounted on a movable or removable guard door.

Precaution: Will tolerate little vibration or movement of the guard doors. Any movement may cause accidental triggering of the safety device. It is generally necessary to utilize a mechanical latch to hold the guard in place.

RFID safety switches are the preferred non-contacting safety switch.

Coded magnetic safety switches are only allowed if the magnetic target is >2.5 square inches.

Typical part # 440N-Z21SS2HN9 (About $170 - SensaGuard, Large Plastic Rectangular, 2 x PNP, 0.2A, max Safety Output, 6 inch Pigtail, 8-Pin Micro (M12), Margin Indication / Magnetic Hold)

Models exist in Adept titled “Actuator, Allen-Bradley sensaguard.sldprt” and “Sensor, Allen-Bradley sensaguard.sldprt.”

o Sensor faces should be mounted 5-18mm apart for optimum performance.

o Sensors can be mounted as shown, or the top part rotated down so that the words face downward and the lower part remains as shown.

Figure 18 Two Component Safety Stitches (Non-Contacting)

For a barrel style sensor, an Allen-Bradley 18mm model is available as part # 440N-Z21S16H (About $100 - SensaGuard, 18 mm Plastic Barrel, 18 mm Plastic, 2 x PNP, 0.2A, max Safety Output, 6 inch Pigtail, 8-Pin Micro (M12))

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Tongue or Barrel Insertion Safety Switches

Safety switch with the primary unit mounted on a stationary frame while the safety target is mounted on a movable or removable guard door.

Precautions: A secondary guard door alignment method is required to assist in aligning the guard door before it comes in contact with the insertion of the safety tongue or barrel into the safety switch. If the door becomes misaligned, damage will result to the tongue or fork.

Typical part # 440K-E33078 (About $90 - Tongue Switch - Elf: Contacts (Safety and Aux): 2 Normally Closed, BBM Preference: N/A, Actuator: GD2 Metal Align. Guide w/Semi-Flex Actuator, Model Type: NA, Conduit Entry: Quick Disconnect)

Figure 19 Tongue or Barrel Insertion Safety Switch

Locking Safety Switches

Non-Contacting magnetic locking safety switches are preferred over tongue or barrel insertion locking safety switches.

Used when guard doors are to remain physically locked until the moving hazard has come to a safe and complete stop. This is a NFPA 79 Category 1 Stop requirement.

Precautions: Guard doors with rigid frames are required when used with locking safety switches. This will prevent bending and flexing of the guard panel in the event that the door is being forced open while the safety switched is still locked.

Typical part #: Pilz PENS sl-0.5p 1.1 (About $550 - Non-contact safety gate systems PSENslock combine safety gate monitoring and a non-contact magnetic interlock within one unit.)

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Figure 20 Locking Safety Switch

Below is a link to the General Machine E Stop Safety document, “CIS_Safety_E-Stop_Circuit_Design_Standard…”

http://gmiportal.generalmills.com/tc/eng/exteng/cop/xcis/Standards%20and%20Guidelines/CIS_300%20-%20Design%20Standards%20and%20Guidelines/CIS_304_Safety-E-Stop_Circuit_Design_Standard_WHQ_Rev1.pdf

OSHA Regulations – Reference List

1. Lockout Tagout - OSHA 1910.147 http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9804

2. Machinery and Machine Guarding - OSHA section 1910 Subpart O, (1910.211 – 1910.219)http://www.osha.gov/pls/oshaweb/owasrch.search_form?p_doc_type=STANDARDS&p_toc_level=1&p_keyvalue=1910

3. OSHA Issued Interpretation of 1910.212, dated 1/18/2006http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=interpretations&p_id=25296

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9. Light Curtains

Light curtains can be a convenient tool for guarding a hazard point when frequent access is required. However, one important design requirement is the distance from the hazard point to the curtain. That value is based on the time T for the hazard to be eliminated (i.e. Stop rotating, moving, etc.) and the standard hand speed constant Vc. Below is the formula for calculating safety distance.

D = T * Vc

where

D = minimum safety distance between hazard and light curtain (inches)T = stopping time of the machine (seconds)Vc = 63 inches/second = hand speed constant

For example, using this formula if the stopping time T of the machine is 1.0 seconds, the safety distance D between the curtain and the danger zone should be at least 63 inches.

Another important consideration, like guard door interlocks, is that a light curtain should not be used like an E stop. In any situation were a bodily appendage will cross the curtain plane, the machine must be locked out*.

* Note that at the time of this revision, some packaging machinery operates under the rule of “E stop + One” where if an E stop and an Interlock in combination have caused the machine to stop, a person may pass through the plane of the machine where the guard door normally resides.

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10. Belt / Chain Guards

Since belt and chain guards are used so frequently, a standard design should be used, whether or not all of the actual dimensions remain the same. The picture below shows a back plate (14 Ga) (GMI 61551) which bolts to the machine on the inside of the pulleys or sprockets through three bolt holes. That plate utilizes 1”x 2” formed flaps, 1/8” inner radius,3/8” x 16 UNC nuts welded on over predrilled holes. The back plate should be designed in SolidWorks Sheet Metal when possible in order to produce a flat pattern. The cover (GMI 63040) can be slipped over and fastened on. The holes are 3/8” from the end of the flap.

Figure 21 - Standard Belt/Chain Guard Design – GMI Assy 61550 (Slots shown on top for reference only. They should be designed on the bottom in practice.)

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If slots are required on the rounded end of the guard as shown above, refer to ANSI B11.19 and the chart on Page 4. Ensure that the belt, chain, sprocket, or pulley are at least 0.5” from the guard face. The slot pattern is the same as the Small Slot Pattern described on Page 8. Create a sketch on a plane tangent to the end of the guard. Start the pattern from the center and work to the edges. Then use a Wrap feature to wrap the “DeBoss” around the end.

Figure 22 Perforation on Curved Parts – GMI 63040

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11. Shaft End Guards

Since shaft ends stick out past a bearing so frequently in our designs, a standard design should be used, whether or not all of the actual dimensions remain the same. The picture below shows an example made out of formed 14 Ga sheet metal. This example can be found in Adept under GMI 67688.

In the case that the box shown in Figure Figure 23 – Formed shaft end guard. needs to be deeper than it is wide (such that it cannot be formed in reality), a cylindrical cover can be used instead of a formed box, as shown in Figure 24. This is a sanitary tube welded to 2 tabs, each with a standoff.

These designs will need to be adjusted for each bearing and shaft size.

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Figure 23 – Formed shaft end guard. Figure 24 – Tube shaft end guard.

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12. Product Flow Grates

Product flow grates are to be used on product in-feeds and out-feeds where there is a danger point within. For example, a hopper where operators dump raw ingredients must be guarded if there is an exposed auger or conveying device in the bottom. However, this philosophy is not practical in some applications, including cereal cutter heads and some extruder discharges. In that application the danger point is directly above the discharge and cannot be guarded due to the risk of build-up, eventually causing a plug. If space is available, a long discharge chute should be designed to prevent fingers from reaching the danger point. Otherwise extensive training and documentation must be used to mitigate that risk.

The image below represents the preferred method of designing a product flow grate. This method entails welded rods (preferably ½” OD) in a parallel pattern. The benefit is that there are no flat surfaces, and there is sufficient strength to maintain their shape. The draw back is the fabrication time and difficulty, and cost.

If spans longer than 12” are necessary, cross bars can be welded on to prevent the bars from bending apart, and thereby allowing an appendage through.

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13. Revision SummariesEnter information in these tables to track revisions to the master document.

NOTE: In the Entry column, enter your information between the dashes. The entries automatically update matching document properties.

Document Property Entry

Master Owner Zac Harder Increment revision numbers by one.

Master Revision Number 6

Master Revision Date 10/10/2014

Revision Number

Revision Date

Revision Owner

Revision Summary(Brief description of major changes)

0 5/9/2011 Zac Harder Original Development

1 6/1/2011 Zac Harder ANSI B11.19 References, Interlock Prices, Interlock P/Ns, CA OSHA Memo Removal

2 8/10/2011 Zac Harder Added Spec for Sensa Guard Mounting

3 8/15/2012 Zac Harder Add info on perforation options

4 2/25/2013 Zac Harder Added Chapter 9 – Light Curtains. Removed “Hot Surfaces” and “What to Guard” Chapters, and ANSI Charts, and added to Safety Guarding Concepts.

5 9/3/2013 Zac Harder Added Perforations and Openings chapter back in. Added Shaft End Guard chapter, and modified Product Flow Grates chapter.

6 10/10/2014 Zac Harder Added Risk Assessment Grid to Ch. 8.

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