Pneumatic Safety Fundamentals for Machine Guarding
Steve Boyette - Ross Controls
Global Industry Specialist – Safety and Press
Steve.boyette@rosscontrols.com
706-202-8991
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Pneumatic Safety Fundamentals for Machine Guarding
1. How many of you have equipment with safety
relays or safety PLCs and are using those devices to
signal valves to remove pneumatic or hydraulic
energy?
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Pneumatic Safety Fundamentals for Machine Guarding
• Is the valve you are using in the safety circuit rated
for the same category or Performance Level as the
safety relay or PLC that turns the valve on and off?
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Pneumatic Safety Fundamentals for Machine Guarding
• Do you have check valves, pilot-operated check
valves, or CC valves that can trap pressure in your
system when you turn off the electrical power?
• Do you have a method for removing that trapped
pressure before performing maintenance?
• Is the goal of your safety controls to always stop all
motion and remove all energy, no matter what?
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Pneumatic Safety Fundamentals for Machine Guarding
• Do any of your pneumatic cylinders move way too
fast when re-applying pressure to the system?
• Does your pneumatic energy isolation valve used for
loto exhaust quickly?
– Can it be locked on?
– Can the locking mechanism be easily defeated?
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Pneumatic Safety Fundamentals for Machine Guarding
There are three basic options for fluid power functions in machine safeguarding:
• Lockout
• Safety Exhaust
• Exhaust, and trap pressure to hold (stop movement)
• Reverse direction to safest position
Additionally, make sure the proper function of valve is used to achieve safety
function even when there is a fault in the system. This means that we also need to
use components that meet the required Category or Performance Level.
• Concerns:
– Trapped pressure
– Re-application of pressure – sudden movement
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Pneumatic Safety Fundamentals for Machine Guarding
There are three basic options for fluid power functions in machine safeguarding:
• Lockout
• Safety Exhaust
• Exhaust, and trap pressure to hold (stop movement)
• Reverse direction to safest position
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Pneumatic Safety Fundamentals for Machine Guarding
1. Lockout
–
–
Locks out all energy sources
Alternative methods for production
2. Machine Guarding
–
–
Production Related
Allows access to hazardous areas
3. E-Stop
–
–
Removes energy sources
After the fact but integrated with
guarding
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Lockout/Tagout
•
•
•
•
OSHA 1910.147
ANSI Z244.1 – 2003
CSA Z460 – 2005
ISO 14118 – 2000
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Pneumatic Lockout
• Requirements (OSHA & ISO)
–
–
–
–
–
A manually operated valve
Not be used for any other function
Located outside of hazardous areas
Easily identified and operated
Tamper resistant
• Best Practice (ANSI B11.0 & B155.1)
–
–
–
–
Full diameter exhaust (rapid release of stored energy)
Should only be able to be locked off
“Positive action” which would indicate only two positions (ON and OFF)
A method for the employee to verify that the energy has dissipated after initiating lock out.
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Pneumatic Lockout
• Requirements (OSHA)
– Tamper resistant
Lock placement
Nut to remove handle
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Flats for wrench
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Pneumatic Lockout
• Pneumatic issues:
– Can the isolation device be locked out directly?
– Are additional devices needed
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Pneumatic Lockout
• Best Practice (ANSI B11.0)
– Should only be able to be locked off
Locked On
Locked In-between
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Locked On/Off?
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Pneumatic Lockout
• Pneumatic issues:
– Is the air blocked and bled in one action or two?
Bleed
Block
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Pneumatic Lockout
• Best Practice (ANSI B11.0)
– Full diameter exhaust (rapid release of stored energy)
• 8 Cubic feet (60 gallons) at 100 psi
– Full exhaust = 35 seconds
– Bleed port > 11 minutes
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Pneumatic Lockout
• Valves should be:
• Accessible
• Suitable for environment
• Easily identified (unique shape)
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Pneumatic Lockout
• Best Practice
– Accessible
• Easy to reach
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Pneumatic Lockout
• Best Practice (ANSI B11.0)
– Easily identified
• Well marked
• Consistent design
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Machine Guarding Safety
1. Lockout
–
–
Locks out all energy sources
Alternative methods for production
2. Machine Guarding
–
–
Production Related
Allows access to hazardous areas
3. E-Stop
–
–
Removes energy sources
After the fact but integrated with
guarding
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Control System
The Control system does NOT END with the wire!
It includes all components involved in performing the safety function; sensors,
manual input, and mode selection elements, interlocking and decision-making
circuitry, and output elements that control machine operating devices or
mechanisms.
Mechanical Control
Devices
Input devices
Light curtains
Palm buttons
Control
System
Logic
Air & Hydraulic
Valves
Mechanical latches
Etc..
Etc..
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Control System
The problem….
Input Devices
PES
Final Elements
35%
15%
50%
The solution….
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Pneumatic Safety
• Pneumatics is part of the SRP/CS
• Risk Assessment is required
– Task based process
– Consider severity, frequency, & avoidability
– Consider foreseeable misuse & failure modes
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Pneumatic Safety
• Must consider:
– Component failure
– Loss of pressure or power
– Reapplication of energy
– Acceptable residual risk
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Pneumatic Safety
• Pneumatic risk
– Does it create motion
– Force with pneumatic energy
• 1” Bore at 100 psi = 79#
• 2” Bore at 100 psi = 314#
• Vertical or Horizontal?
– Gravity – Weight of Tooling?
– Speed – Inertia to stop?
– Tooling – Crushing? Piercing? Cutting?
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Pneumatic Safety
• What is a pneumatic hazard?
• Per EN 13736 there is a Risk of injury if:
Force > 150 N (33.8 lbf)
Weight of tooling > 15 kg (33 lbs)
• Requires redundancy &/or monitored restraint
Bore
0.50
0.75
1
2
Area
0.20
0.44
0.79
3.14
Force (60 psi) Force (80 psi) Force (100 psi)
12
16
20
27
35
44
47
63
79
188
251
314
Bore
40.00
50
63
80
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Area Force (5.5 BAR) Force (7 BAR) Force (100 BAR)
1256.64
69
88
126
1963.49
108
137
196
3117.24
171
218
312
5026.54
276
352
503
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Pneumatic Safety for Machine Guarding
There are three basic options for fluid power functions in machine safeguarding:
• Lockout
• Safety Exhaust
• Exhaust, and trap pressure to hold (stop movement)
• Reverse direction to safest position
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Safety Exhaust
0 psi
80 psi
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Safety Exhaust
80 psi
80 psi
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Safety Exhaust
Outlet Pressure is a function of:
• Downstream volume
• Inlet Pressure
• Exhaust area
• Inlet area
??? psi
80 psi
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Pneumatic Safety
Mechanical Power Press
• Flywheel
• Clutch
• Brake
Clutch
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Pneumatic Safety
Clutch Released
Brake Engaged
Exhaust
Supply
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Pneumatic Safety
Clutch Engaged
Brake Released
Exhaust
Supply
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Pneumatic Safety
• Pneumatic system
– Oil
– Water
– Debris
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Pneumatic Safety
• Valves are more likely to be sticky than fail completely
– Slows down the valve’s shift time
– Shift time directly affects overall stopping time
• Self-monitoring valves:
– Measure shifting time as a diminished performance fault
– Monitor for a complete fault
– Used on any application involving safety distances and stopping
performance
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Pneumatic Safety
Crossover or “Ghost” position
 Passes through configurations in milliseconds
 Not normally noticed
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Pneumatic Safety
Redundant – Parallel
Function:
• Valves must operate simultaneously
Closed
Opened
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Pneumatic Safety
Redundant – Parallel
Potential Hazard:
• Slow Reaction - Supply vs. exhaust & downstream volume
• Failed valve can be undetected until a second failure
occurs
Failed
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Pneumatic Safety
Redundant - Series
Function:
• Valves can operate simultaneously
• First valve can be electrically controlled “dump” valve
Closed
Opened
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Pneumatic Safety
Redundant - Series
Potential Hazard:
• Failed valve can be undetected until a second failure occurs
Failed
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Pneumatic Safety
Redundant - push down - pull trigger
Nice, but no monitoring!
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Safety Exhaust
• Remove hazardous pressure
– Add pneumatic dump valve
– Remove supply & downstream pressure to actuator valves and
actuators
– Removes positive force
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Safety Exhuast
• Valve should be control reliable:
–
–
–
–
Redundant
Monitored
Default to a safe condition
Prevent further operation
while abnormal state exists
– Requires a reset
– Third Party Certified
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Safety Exhaust
• Exhaust time dependent upon
– Volume
– Response Time
– Flow Rate
– Fault condition
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Safety Exhaust
• Think of a safety exhaust va as a Pneumatic
Safety Relay
• Intended for safety exhaust applications
• Valve can be main air supply exhaust to other
components or zones
• Monitors internal discordance
• Latches out upon a fault in the valve
• Requires reset
– Anti-tie-down
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Example – Detented Jam (Clamping)
• Clamping
– Detented valve – unclamped
– Maintains position
5
4
3
2
1
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Example – Detented Jam (Clamping)
• Clamping
– Detented valve - clamped
– Maintains clamping force
5
4
3
2
1
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Example – Detented Jam (Clamping)
• Clamping
– Detented valve - clamped
– Maintains clamping force
– Jam interferes with full 1
cylinder movement
5
4
3
2
Jam
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Example – Detented Jam (Clamping)
• Clamping
– Operator clears jam
– Cylinder is pressurized
– Sudden movement
5
4
3
2
1
Jam
Pinched
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Example – Safety Exhaust Function
• Transfer press line part removal
– Operator removes part from conveyor A
– Part is placed on rack C
– Fork trucks remove full racks, load empty racks
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Example – Safety Exhaust Function
• Hazardous situation
– Once rack C is full table D must rotate
– Operator controls 3/2 NC valve at control panel B
– Rotation creates crushing/shearing hazard
Crushing/Shearing
Hazard
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Example – Safety Exhaust Function
• Safety controls
– Electrical push button controls 3/2 NC valve
– Valve supplies air motor controlling rotation
– Safety mat E ensures no exposure to crush point
Motionless
Rotating
2
3
2
1
3
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Example – Safety Exhaust Function
• Safety mat cycle rate = every 1 minute
• Valve cycle rate = every 15 minutes
Cycles
Safety Mat
60
8
5
52
124800
Valve
900
8
5
52
8320
Input
Logic
Description Safety Mat Safety Relay
B10d
nop
124800
cycle time (sec)
hours per day
days per week
weeks per year
cycles per year
Output
Valve
20000000
8320
MTTFd
DC
Category
100
99%
4
100
99%
4
System MTTFd
System DC
System Category
PL
50
99%
1
c
High
None
24038
0%
1
PL c
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Example – Safety Exhaust Function
• Valve failure mode – Stuck in shifted position
– Rotation continued
– Safety mat & relay had no effect
Rotating
2
3
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Example – Safety Exhaust Function
• Pneumatic solution
– Dual safety exhaust valve
– Removes supply when de-energized
2
3
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TWO-HAND CONTROL EXAMPLE
Activity
Assume we have an assembly machine started by a
two-hand unit
 Operator starts by depressing both buttons
 Operator notices the part is not located properly
 Operator releases the right button
 Operator reaches into machine
• Should we expect the cylinder to stop?
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Control Circuit
Logic
Cat 3
No power to
solenoid
Cat 1
Valve stuck in solenoid
on position
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Controls
Example – Safety Exhaust Function
• Safety mat cycle rate = every 1 minute
• Valve cycle rate = every 15 minutes
Cycles
Safety Mat
60
8
5
52
124800
Valve
900
8
5
52
8320
Input
Logic
Description Safety Mat Safety Relay
B10d
nop
124800
cycle time (sec)
hours per day
days per week
weeks per year
cycles per year
Output
Valve
20000000
8320
MTTFd
DC
Category
100
99%
4
100
99%
4
System MTTFd
System DC
System Category
PL
50
99%
4
e
High
High
24038
99%
4
PL e
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Horizontal cylinder example
• 5/3 Closed Center – blocks supply and downstream air
– Drift due to rod area
– Failure to center
• Center position may be untested
– Exhaust valve – in case 5/3 fails
• Removes pressure – residual risk
– Reapplication of air can be an issue
4 2
5 1 3
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Example – Winder unloading table
• Moveable winder table
Retracted
Extended
Unloading
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Example – Winder unloading table
• Pneumatic moving table
– Three potential positions
– No consideration of failure modes
Extend
Extend
Extend
Retract
Retract
Retract
4
4
2
5
3
5
4
2
3
1
1
2
5
3
1
Maintain position
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Example – Winder unloading table
• Safety control
– Safety mat - Safety relay - Valve
Retracted
Extended
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Example – Winder unloading table
• Valve failure mode - Broken spring
– Extend function still works
Extend
Retract
4
2
5
3
1
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Example – Winder unloading table
• Valve failure mode - Broken spring
– Retract position still works
Extend
Retract
4
5
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2
3
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Example – Winder unloading table
• Valve failure mode - Broken spring
– No center position
• Always extend unless retracting
Extend
Retract
4
2
5
3
1
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Example – Winder unloading table
• Only safe mode
– Must hold button to move table back
Extend
Retract
4
2
5
3
1
What happens if someone steps
on the safety mat?
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Example – Winder unloading table
• Pneumatic solution
– Two dual valves to simulate 5/3 open center
– Exhausts cylinder when de-energized
• Allows manual movement if necessary
Retract
4
5
X
Extend
2
4
5
3
X
2
3
1
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Example – Winder unloading table
Extend
Retract
4
X2
5
4
X2
5
3
3
1
4
X
2
4
X
2
Extend
Retract
5
5
3
3
Maintain
position
1
Extend
Retract
4
5
X2
4
5
3
X2
3
1
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Pneumatic Safety for Machine Guarding
There are three basic options for fluid power functions in machine safeguarding:
• Lockout
• Safety Exhaust
• Exhaust, and trap pressure to hold (stop movement)
• Reverse direction to safest position
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Cylinder stop function
• Pilot operated check valves
– Removal of trapped pressure
• Electrical signal
• Manual signal
• Tied to lockout valve
– Reapplication of pressure?
– Leakage?
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Cylinder stop function
• Dual pilot operated check valves
– Rod area issue
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Cylinder Stop Function
• Mechanical Methods
– Hold platen or tooling
– Use spring loaded pins or cylinders
– Typically pneumatically actuated
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Example – Load Holding #6
• Rod held to maintain position
– Rod Lock
• Holds static rod – Category 1 or 2
– Rod Catcher or Brake
• Stops moving rod – Category 1 or 2
– Can use secondary rod
• Does not need to be actuator rod
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Cylinder Stop Function
• Mechanical devices are spring return but air actuated
• Not removing the air nullifies the mechanical system
• Control reliable valve ensures air removal
Hazard
Direction
Control
Reliable
Valve
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Reapplication of pressure
• Can cause sudden motion
• Soft start entire system or components
• Flow controls
– Meter in will always control flow
– Meter out only controls flow if pressure exists
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Reapplication of pressure
• System soft-start – whole circuit
– Soft-start initiated by shifting of isolation valve
– Last position of directional valve dictates direction of cylinder at
startup (soft-start)
– Could be used to start several circuit branches
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Reapplication of pressure
• System soft-start – whole circuit
• MDM2 – Modular DM2 with soft start
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Reapplication of pressure
• Point-of-use soft-start – one direction only
– Soft-start initiated by de-energizing directional
valve or by isolation valve.
– Soft-starts only when directional valve is deenergized.
– One cylinder only
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Reapplication of pressure
• Point-of-use soft-start – both directions
– Soft-start initiated by energizing directional valve.
– Soft-starts only when directional valve energized – either direction.
– One cylinder only
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Pneumatic Safety for Machine Guarding
There are three basic options for fluid power functions in machine safeguarding:
• Lockout
• Safety Exhaust
• Exhaust, and trap pressure to hold (stop movement)
• Reverse direction to safest position
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SAFETY VALVES: 5/2 CONTROL RELIABLE
5/2 CROSSMIRROR
 Dual crossflow spool valve
 Faults to known output condition
 Check valve on inlet
 Fault indicator
 Requires external monitor
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Pneumatic Safety for Machine Guarding
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