1. Overview
Safety mechanisms are mechanical, electrical, hydraulic, pneumatic, and control-based protective devices applied to eliminate hazards or reduce risks to an acceptable level during machine operation, adjustment, cleaning, inspection, and maintenance.
The fundamental purposes of safety mechanisms are:
Preventing personnel from entering hazardous areas
Preventing trapping, entanglement, cutting, and crushing caused by rotating and moving parts
Interrupting abnormal operation
Protecting machinery against overload and collision
Preventing hazardous movement during power or pressure loss
Preventing falling of vertical axes and lifting members
Safely isolating and releasing stored energy
Isolating high-temperature, high-pressure, electrical, laser, and other hazardous sources
Rapidly stopping machinery during emergencies
Preventing unexpected restart during maintenance
Preventing contamination of products and the surrounding environment
Safety mechanisms do not refer only to devices that reduce damage after an accident has occurred. The most desirable safety design eliminates the hazard itself during the design stage.
The basic priority of machine-safety measures is:
Inherently safe design
→ Guards and protective devices
→ Detection, control, and interlocking
→ Warnings and work procedures
→ Personal protective equipment
Typical applications include:
Industrial robots
Presses and cutting machines
Machine tools
Packaging machinery
Food-processing machinery
Automatic assembly equipment
Semiconductor and display-manufacturing equipment
Lifting equipment
Material-handling systems
High-temperature and high-pressure equipment
Laser-processing equipment
Testing and research equipment
Automated storage systems
Collaborative robot systems
2. Difference Between Safety Mechanisms and Locking Devices
2.1 Locking Devices
A locking device holds a specific component or moving part in a defined position and prevents unintended movement, rotation, or release.
2.2 Safety Mechanisms
A safety mechanism may include a locking function, but it has a broader purpose of preventing hazards and preventing people from being exposed to those hazards.
Typical functions include:
Preventing access to hazardous areas
Stopping hazardous motion
Limiting hazardous speed
Isolating hazardous energy
Providing overload protection
Preventing falling
Preventing unintended operation
Monitoring operating conditions
Enabling safe recovery
2.3 Functional Relationship
Examples include:
A latch that keeps a door closed: locking device
A device that prevents the door from opening until hazardous motion has stopped: safety mechanism
A rod clamp that fixes a cylinder rod in position: locking device
Dual rod clamps that prevent a vertical axis from falling during pressure loss: safety mechanism
A normal switch that stops a motor: operational control
An emergency stop that stops all hazardous motion during a dangerous condition: safety mechanism
3. Basic Principles of Safety Design
3.1 Elimination of Hazards
Where possible, the hazard should be eliminated at the source.
Examples:
Use enclosed drives instead of exposed chains.
Eliminate sharp edges.
Remove trapping gaps.
Use low-voltage circuits.
Replace a high-temperature process with a lower-temperature process.
3.2 Reduction of Hazard Magnitude
If the hazard cannot be eliminated, reduce the force, speed, pressure, temperature, or energy.
3.3 Isolation of Hazards
Use guards, covers, barriers, and enclosed compartments to separate personnel from the hazard.
3.4 Detection of Hazards
Use sensors to detect human approach, door opening, overload, pressure loss, or abnormal position.
3.5 Interruption of Hazardous Motion
When a hazardous condition is detected, safely interrupt the power source and energy-transmission path.
3.6 Maintenance of a Safe State
After stopping, ensure that gravity, pressure, heat, or inertia cannot continue to create a hazard.
3.7 Safe Recovery
Restart should be possible only after the hazard has been removed and an intentional reset operation has been performed.
4. Basic Classification of Hazards
4.1 Mechanical Hazards
Trapping
Crushing
Shearing
Cutting
Piercing
Entanglement
Collision
Ejection
Falling
Failure of rotating components
High-pressure fluid injection
4.2 Electrical Hazards
Electric shock
Leakage current
Short circuit
Arc flash
Static electricity
Overcurrent
Overheating
4.3 Thermal Hazards
Hot surfaces
Flame
Molten material
Steam
Cold burns
Thermal radiation
4.4 Pressure Hazards
Stored hydraulic pressure
Stored pneumatic pressure
Pressure-vessel failure
Hose rupture
Vacuum collapse
Gas leakage
4.5 Radiation and Optical Hazards
Laser radiation
Ultraviolet radiation
Infrared radiation
X-rays
Intense visible light
Electromagnetic fields
4.6 Noise and Vibration
Hearing damage
Fatigue
Resonance
Component loosening
Control error
4.7 Material and Environmental Hazards
Hazardous chemicals
Dust
Mist
Explosive gas
Oxygen deficiency
Slipping
Biological contamination
5. Risk Assessment
Risk assessment must be performed before selecting safety mechanisms.
5.1 Risk-Assessment Procedure
Define the limits of the machine
→ Identify hazards
→ Analyze hazardous situations
→ Estimate risk
→ Establish risk-reduction measures
→ Evaluate residual risk
→ Validate and document
5.2 Life-Cycle Stages to Be Considered
Transport
Installation
Commissioning
Normal operation
Product changeover
Adjustment
Cleaning
Inspection
Fault recovery
Maintenance
Disposal
5.3 Reasonably Foreseeable Misuse
Not only normal use but also reasonably foreseeable misuse must be considered.
Examples:
Operating with a guard open
Bypassing a sensor
Removing a jammed workpiece by hand
Re-energizing equipment during maintenance
Applying an excessive load
Performing abnormal manual operation
6. Risk-Estimation Factors
Risk is generally evaluated by considering the following factors:
Severity of possible injury
Frequency of exposure to the hazard
Duration of exposure
Possibility of avoiding the hazard
Probability of the hazardous event
Number of people exposed
The performance of a safety mechanism is not determined only by the number of sensors or the complexity of the circuit. Reliability and fault response appropriate to the risk level must be ensured.
7. Inherently Safe Design
Inherently safe design reduces risk through the machine structure itself rather than relying on additional protective devices.
7.1 Main Methods
Limiting hazardous force and speed
Limiting range of motion
Providing safe separation distances
Eliminating trapping points
Applying rounded edges
Designing structures that do not eject fragments when they fail
Maintaining a stable center of gravity
Eliminating excessive manual force
Reducing noise and vibration
Using automatic lubrication to avoid manual access
7.2 Advantages
Lower dependence on additional safety devices
Fewer sensors and control devices
Reduced maintenance burden
Lower possibility of bypass by users
Reduced risk throughout the machine life cycle
8. Safety Distance
Sufficient safety distance must be provided so that a person cannot reach the hazardous point before the machine has stopped.
The general concept is:
Safety distance
= Human approach speed × Total stopping time
Additional allowance
Total stopping time includes:
Sensor detection time
Safety-controller processing time
Output-device response time
Valve or contactor switching time
Motor deceleration time
Actual mechanical stopping time
If stopping time increases because of wear or degradation, the required safety distance also increases.
9. Fixed Guards
9.1 Definition
A fixed guard is a protective cover secured to the machine so that it cannot be easily removed without tools.
9.2 Typical Applications
Gears
Chains
Belts
Pulleys
Couplings
Fans
Rotating shafts
Cutting blades
Hot surfaces
9.3 Advantages
Simple structure
High reliability
No electrical control required
Low maintenance
Reliable prevention of access to hazardous areas
9.4 Disadvantages
Must be removed during maintenance
Inconvenient where frequent access is required
May reduce visibility of internal conditions
May not be reinstalled correctly after removal
9.5 Design Principles
Sufficient strength
Openings that prevent entry of fingers and tools
No sharp edges
The guard itself must not create a hazard
Resistance to expected impact and ejected material
Clear reassembly after removal
10. Movable Guards
A movable guard can be opened using a hinge, slide, lift, fold, or removable-panel arrangement.
Typical forms include:
Hinged doors
Sliding doors
Lift-up covers
Folding covers
Removable panels
Where frequent access is required, movable guards should be combined with interlocking devices.
11. Interlocked Guards
11.1 Definition
An interlocked guard stops hazardous motion or prevents machine start when the guard is opened.
11.2 Basic Functions
Confirming that the guard is closed
Permitting hazardous operation
Stopping operation when the guard is opened
Preventing automatic restart after reclosing
Maintaining a safe state in the event of failure
11.3 Basic Interlocking and Guard Locking
A basic interlock detects that the guard is open but does not mechanically lock the guard.
A guard-locking device prevents the guard from opening until hazardous motion or residual energy has disappeared.
11.4 Typical Applications
Rotating equipment with long stopping times
High-temperature processes
High-pressure processes
Robot cells
Laser equipment
Presses
Automated production lines
12. Emergency-Stop Devices
12.1 Definition
An emergency-stop device is manually actuated during a hazardous condition to rapidly stop hazardous machine motion.
12.2 Basic Requirements
Easily recognizable
Easily accessible
Remains actuated until manually reset
Resetting alone must not restart the machine
Stops hazardous motion with priority
Does not replace normal stop controls
12.3 Typical Forms
Mushroom-head pushbuttons
Pull-cord switches
Foot-operated switches
Emergency-stop bars
Stop buttons on portable control devices
12.4 Caution
Emergency stop is a supplementary protective measure used after a hazardous condition has arisen. It cannot replace inherently safe design or guarding.
13. Stop Functions
13.1 Immediate Power-Removal Stop
Drive power is removed immediately and the machine stops by inertia.
Typical applications:
Simple motor drives
Equipment where residual motion is not hazardous
13.2 Controlled Stop Followed by Power Removal
The machine is decelerated under control and power is removed after stopping.
Typical applications:
High-speed axes
Servo axes
Equipment that could be damaged by abrupt stopping
13.3 Controlled Standstill
Motion is stopped while drive control remains active.
This may not be sufficient as a final safe state for maintenance or entry into a hazardous area. Separate energy isolation may be necessary.
14. Prevention of Unexpected Restart
A machine must not restart automatically merely because a safety device has been restored.
Restart should require:
All guards closed
All safety sensors normal
Emergency stop reset
Confirmation that no person remains in the hazardous area
A separate reset action
A separate start command
The reset button should preferably be located where the operator can directly observe the hazardous area.
15. Two-Hand Control Devices
15.1 Definition
A two-hand control requires the operator to actuate two separate controls simultaneously before hazardous motion can begin.
15.2 Purpose
Keeps both hands outside the hazardous area
Prevents one hand from holding a workpiece during machine operation
Prevents hand injury in presses and cutting machines
15.3 Design Elements
Simultaneity between the two buttons
Adequate distance between buttons
Arrangement that prevents operation with one hand, elbow, or another object
Hazardous motion stops when either button is released
New actuation required for every cycle
15.4 Limitation
A two-hand control protects only the operator using it. It does not prevent other people from entering the hazardous area, so additional guarding may be required.
16. Light Curtains
16.1 Definition
A light curtain is a non-contact protective device that stops hazardous motion when one or more light beams between the transmitter and receiver are interrupted.
16.2 Typical Applications
Presses
Automatic assembly machines
Robot-cell entrances
Loading and unloading stations
Packaging machinery
16.3 Advantages
Good access to the working area
Entry without a physical door
Fast detection
Suitable for repetitive operations
16.4 Disadvantages
Does not stop flying objects or heat
Blind zones may occur
Sensitive to contamination and misalignment
Requires safety-distance calculation
May be bypassed by reaching above or below
16.5 Muting
Muting temporarily suspends the protective function to allow authorized products or pallets to pass.
Sensor arrangement and logic must ensure that only the intended material-flow object, not a person, can pass during muting.
17. Laser Scanners
A safety laser scanner detects people or objects within defined planar zones.
17.1 Typical Zones
Warning zone
Speed-reduction zone
Stop zone
17.2 Applications
Automated guided vehicles
Autonomous mobile robots
Robot cells
Large open machinery
Logistics systems
17.3 Considerations
Floor reflections
Transparent objects
Dust and vapor
Sensor obstruction
Blind spots
Braking distance of moving equipment
18. Pressure-Sensitive Mats and Safety Bumpers
18.1 Pressure-Sensitive Mats
The machine stops when a person steps on the mat.
18.2 Safety Bumpers
A moving machine or door stops when its bumper contacts an object.
18.3 Safety Edges
Installed along the edge of a door, cover, or moving component to detect trapping contact.
18.4 Limitation
These devices operate after contact, so speed and impact energy must be sufficiently low.
19. Safety Sensors
Typical safety sensors include:
Guard-door switches
Non-contact safety sensors
Safety proximity sensors
Light curtains
Laser scanners
Safety mats
Safety edges
Pressure switches
Safe-speed sensors
Standstill-monitoring sensors
Rope-pull switches
Safety sensors require higher diagnostic capability and reliability than normal operating sensors.
20. Safety Control Systems
A safety control system processes safety-sensor inputs and switches off hazardous outputs.
Main components include:
Safety relays
Safety PLCs
Dual-channel inputs
Force-guided contacts
Output contactors
Safe communication
Diagnostic circuits
20.1 Basic Functions
Cross-monitoring of input signals
Detection of open and short circuits
Detection of welded contacts
Feedback monitoring of output devices
Reset monitoring
Switching outputs to a safe state during faults
21. Dual-Channel and Redundant Design
Two independent channels may be used so that failure of one sensor or circuit does not immediately eliminate the safety function.
21.1 Advantages
Tolerance of a single fault
Fault detection
Higher reliability
Continued safety function
21.2 Considerations
The two channels must not fail simultaneously because of the same cause.
Examples of common-cause failure include:
Damage to the same cable route
Loss of a common power supply
Exposure to the same contamination source
The same software error
Flooding of the same electrical enclosure
22. Force-Guided Contacts and Contactor Monitoring
Force-guided contacts use the mechanical relationship between main and auxiliary contacts to detect welded contacts.
Output-contactor monitoring confirms:
Whether the main contacts actually opened after a stop command
Whether contact welding occurred
Whether the contactor returned to a normal state before restart
23. Safe-Speed Monitoring
During access to a hazardous area, machinery may be operated at a safely limited speed instead of being completely stopped.
Typical applications:
Setup
Teaching
Jog operation
Cleaning
Precision adjustment
The following should be reviewed:
Permitted speed
Permitted torque
Permitted travel distance
Hold-to-run operation
Emergency stop
Redundant speed sensing
24. Hold-to-Run Devices
A hold-to-run device permits machine movement only while the operator continuously holds the button or switch.
Releasing the control must stop the machine immediately.
Typical applications:
Robot teaching
Mold adjustment
Slow manual feed
Maintenance work
Hold-to-run control should be combined with safe low speed, limited force, and clear visibility of the working area.
25. Enabling Devices
An enabling device generally has three positions:
Released: stop
Mid-position: operation enabled
Fully squeezed: stop
The machine stops if the operator releases the device or grips it excessively during a startled or hazardous reaction.
26. Overload-Protection Mechanisms
Overload-protection mechanisms prevent force, torque, pressure, or current from exceeding allowable values.
Typical methods include:
Torque limiters
Slip clutches
Shear pins
Relief valves
Pressure-reducing valves
Overcurrent protection
Servo torque limits
Load sensors
Mechanical stoppers
Shock absorbers
27. Torque Limiters
27.1 Definition
A torque limiter slips or disengages when transmitted torque exceeds a preset value, protecting the drive system and workpiece.
27.2 Main Types
Friction type
Ball-detent type
Ratchet type
Magnetic type
Electronically controlled type
27.3 Applications
Conveyors
Packaging machinery
Assembly equipment
Robot axes
Tool drives
Indexing devices
27.4 Design Considerations
Trip torque
Re-engagement method
Repeatability
Heat generation
Wear
High-speed rotation
Signal output
28. Shear Pins
A shear pin is a single-use safety component designed to fail intentionally when the load exceeds a predetermined value, thereby protecting major machine components.
28.1 Advantages
Simple structure
Reliable operation
Low cost
Independent mechanical protection
28.2 Disadvantages
Replacement required after operation
Fragments must be controlled
Limited trip repeatability
Premature fatigue failure may occur
A shear pin must not be replaced arbitrarily with an ordinary structural bolt.
29. Mechanical Stoppers
A mechanical stopper physically limits the permissible range of motion.
29.1 Typical Applications
Limiting cylinder stroke
Limiting robot-axis travel
Preventing slide overtravel
Limiting door opening angle
Limiting upper and lower travel of lifting axes
29.2 Design Considerations
Impact speed
Impact energy
Contact area
Stopper strength
Frame stiffness
Cushioning material
Number of repeated impacts
A stopper should preferably be used as a final protective measure rather than as a component that receives impact during every normal cycle.
30. Shock Absorbers and Impact-Absorption Devices
30.1 Main Types
Hydraulic shock absorbers
Rubber bumpers
Urethane pads
Springs
Pneumatic cushions
Crushable members
30.2 Purpose
Reduce impact force
Reduce noise
Protect structural components
Prevent product damage
Provide stopping distance
Kinetic energy is:
E_k = 1/2 mv²
Impact energy increases with the square of velocity.
31. Fall-Prevention Mechanisms
Vertical axes, lifting members, and lifting equipment require safety mechanisms to prevent gravity-driven falling.
Typical methods include:
Spring-applied brakes
Rod clamps
Ratchet-and-pawl devices
Safety nuts
Fall-prevention wedges
Mechanical locking pins
Safety gears
Counterbalance valves
Dual chains or wire ropes
31.1 Design Principles
Apply locking independently of the primary drive
Operate automatically during power or pressure loss
Use auxiliary support during manual release
Distinguish static holding from dynamic braking
Consider falling energy
Perform repeated validation tests
32. Safety Nuts
A safety nut is installed separately from the primary lifting nut and supports the load if the main nut fails.
32.1 Characteristics
Carries little or no load during normal operation
Allows monitoring of main-nut wear
Prevents falling if the main nut fails
Suitable for lifting devices and work platforms
32.2 Design Considerations
Clearance between the main nut and safety nut
Wear detection
Load capacity of the safety nut
Buckling of the screw shaft
Maintenance interval
33. Hydraulic and Pneumatic Safety Devices
33.1 Main Devices
Relief valves
Pressure-reducing valves
Check valves
Pilot-operated check valves
Counterbalance valves
Hose-burst valves
Exhaust valves
Soft-start valves
Pressure switches
Accumulator isolation valves
33.2 Main Hazards
Pressure loss
Residual pressure
Hose rupture
Sudden cylinder movement
Valve sticking
Air compressibility
Hydraulic-fluid injection
33.3 Basic Design Principles
Limit supply pressure
Indicate residual pressure
Release pressure before maintenance
Add mechanical locking for vertical loads
Shut off flow during hose failure
Prevent hazardous falling during exhaust
34. Release of Residual Pressure
Switching off power alone does not necessarily make a hydraulic or pneumatic system safe.
Residual energy may remain in:
Cylinders
Accumulators
Hoses
Piping
Valve blocks
Vacuum chambers
Springs
Before maintenance:
Isolate the energy supply
→ Isolate the pressure source
→ Release residual pressure
→ Confirm the pressure gauge
→ Mechanically support moving parts
→ Begin work
35. Electrical Safety Devices
Typical electrical safety devices include:
Circuit breakers
Fuses
Residual-current devices
Overcurrent protection
Overvoltage protection
Grounding
Insulation monitoring
Safety contactors
Main disconnect switches
Discharge circuits
35.1 Main Disconnect Device
The entire machine power supply should be clearly isolated and lockable.
35.2 Discharge Circuits
Energy stored in capacitors, inverters, and high-voltage circuits must be discharged within a safe period.
35.3 Prevention of Re-Energization
A lockable disconnect should prevent another person from restoring power during maintenance.
36. Energy Isolation and Lockout/Tagout
During maintenance, electrical, hydraulic, pneumatic, gravitational, thermal, and spring energy must all be identified.
A typical sequence is:
Stop the equipment
→ Identify all energy sources
→ Isolate energy
→ Lock the isolation devices
→ Attach warning tags
→ Remove stored energy
→ Verify zero-energy state
→ Perform maintenance
Pressing a stop button or switching off control power does not constitute complete energy isolation.
37. Thermal Safety Devices
The following protective measures may be applied to high- and low-temperature hazards:
Insulation covers
Heat shields
Temperature sensors
Overtemperature switches
Dual temperature control
Cooling-flow monitoring
Surface-temperature warnings
Automatic power shutdown
Thermal-expansion allowance
An overtemperature-protection device should preferably be independent of the normal temperature controller.
38. Pressure-Vessel and Explosion Protection
Pressure systems may use:
Safety valves
Rupture discs
Pressure switches
Pressure gauges
Explosion-relief panels
Flame arresters
Explosion vents
Pressure-resistant enclosures
Pressure relief should not discharge toward operators or walkways.
39. Protection Against Ejected Material
Where chips, broken tools, cut fragments, or products may be ejected, the following may be used:
Transparent protective windows
Metal guards
Mesh guards
Impact-resistant panels
Chip covers
Spindle covers
Coolant shields
Transparent guards must be evaluated for strength degradation caused by chemicals, impact, ultraviolet exposure, and aging.
40. Rotating-Equipment Safety
Rotating equipment presents hazards such as:
Entanglement
Centrifugal failure
Imbalance
Tool release
Protruding shafts
High-speed ejection
Typical safety mechanisms include:
Rotating-part covers
Shaft-end caps
Speed limits
Overspeed monitoring
Vibration monitoring
Balance monitoring
Door interlocks
Standstill-detection sensors
41. Safe Tool Change
Tool-changing systems require the following safety functions:
Confirmation of complete spindle stop
Tool-clamped confirmation
Tool-unclamped confirmation
Tool-presence detection
Incorrect-tool detection
Magazine-position confirmation
Emergency drop prevention
Manual recovery procedure
A tool-clamp sensor alone may not guarantee actual holding force. Drawbar position or clamping-force monitoring may also be used.
42. Safety in Collaborative Operation
Where people and robots share the same workspace, the following methods may be applied:
Safety-rated monitored stop
Hand guiding
Speed and separation monitoring
Power and force limiting
A collaborative robot does not automatically make the complete system safe.
The following must also be reviewed:
Gripper shape
Workpiece edges
Trapping between robot and fixed structures
Tool hazards
Motion speed
Collision energy
Operator position
43. Safe Manual Mode
When guards must be opened for setup or maintenance, restricted operating conditions should be applied.
Low-speed operation
Limited force
Hold-to-run control
Enabling switch
Priority control from inside the hazardous area
Automatic operation disabled
Clear mode selection
Controlled access to mode-selection keys
44. Mode-Selection Devices
Operating modes may include:
Automatic operation
Manual operation
Setup
Teaching
Maintenance
Cleaning
Mode changes should be limited to authorized personnel using a key switch or access-control system.
Selecting one mode must not automatically disable all other safety functions.
45. Warning Devices
Warning devices communicate residual hazards to users.
45.1 Visual Warnings
Beacon lights
Indicator lamps
Displays
Warning signs
Floor markings
45.2 Audible Warnings
Buzzers
Sirens
Voice announcements
45.3 Tactile Warnings
Vibration
Resistance feedback
Haptic feedback
Warnings cannot replace other protective measures. Noise, lighting conditions, and color-vision differences must be considered.
46. Safety Signs and Colors
Safety markings should clearly communicate:
Type of hazard
Possible consequence
Prohibited actions
Mandatory protective measures
Emergency-stop location
Energy-isolation location
High-temperature or high-pressure condition
Direction of rotation or movement
Markings must withstand contamination, washdown, ultraviolet exposure, and wear.
47. Failure Principles of Safety Functions
Safety mechanisms should be designed so that a failure does not create a hazardous state.
47.1 Safe State During Failure
Brake engages during power loss
Spring lock engages during pressure loss
Machine operation is inhibited during sensor wire break
Hazardous-direction motion is blocked during valve failure
Restart is prohibited when a contactor welds
47.2 Single-Fault Consideration
Failure of one component must not immediately eliminate the safety function, or the fault must be detected.
47.3 Diagnostic Coverage
Where possible, the system should detect:
Open circuits
Short circuits
Welded contacts
Sensor disagreement
Valve-response failure
Output-device failure
Communication errors
48. Common-Cause Failures
Redundant safety channels may still fail simultaneously because of the same cause.
Typical common-cause failures include:
Cutting of the same cable route
Failure of a shared power supply
Flooding
Overheating
Vibration
Software defects
Incorrect maintenance
Contamination of common piping
Countermeasures include:
Physical separation
Independent power supplies
Different operating principles
Separate cable routes
Environmental protection
Periodic validation
Diverse diagnostics
49. Prevention of Safety-Device Defeat
Operators may bypass safety devices because of:
Poor ergonomics
Reduced production speed
Frequent nuisance trips
Difficult maintenance
Poor visibility
Product jamming
Complicated recovery procedures
Preventing bypass requires designing normal work to be convenient, rather than relying only on seals or warnings.
Main methods include:
Improving workspace
Providing rapid safe recovery
Reducing nuisance trips
Improving guard visibility
Optimizing sensor position
Detecting bypass attempts
Managing user authority
50. Validation of Safety Mechanisms
Safety mechanisms must be validated by actual testing after design.
50.1 Functional Tests
Stop when a guard is opened
Emergency-stop operation
Stop when a sensor is interrupted
Prevention of unexpected restart
Confirmation of locking state
Manual recovery
50.2 Stopping-Time Measurement
Actual stopping time must be measured and used in safety-distance calculations.
50.3 Fault Tests
Sensor wire break
Welded contact
Stuck valve
Power loss
Pressure loss
Communication loss
Single-channel failure
50.4 Repeated-Life Tests
Door switches
Locking pins
Brakes
Valves
Emergency-stop devices
Safety bumpers
Light curtains
51. Maintenance and Periodic Inspection
51.1 Daily Inspection
Emergency-stop condition
Guard damage
Interlock operation
Sensor indication
Abnormal noise
Pressure and temperature
Warning signs
Cables and hoses
51.2 Periodic Inspection
Stopping time
Contactor condition
Brake clearance
Rod-clamp holding capacity
Valve leakage
Guard fastening
Safety-sensor alignment
Light-curtain detection
Manual-release devices
51.3 Replacement Items
Brake pads
Springs
Contactors
Valves
Sensors
Door switches
Relays
Hoses
Guard windows
51.4 Inspection Records
Inspection results, failures, replacements, and safety-test results should be documented.
52. Major Failure Modes
52.1 Guard Failure
Impact, corrosion, or improper modification causes loss of protective function.
52.2 Interlock Bypass
The safety function is defeated by fixing the sensor, attaching a magnet, or modifying wiring.
52.3 False Sensor Detection
Contamination, misalignment, wiring failure, or incorrect settings cause an incorrect output.
52.4 Contactor Welding
Overcurrent or repeated switching causes the contacts to weld closed, preventing power isolation.
52.5 Valve Sticking
Contamination or wear prevents fluid supply from being shut off.
52.6 Reduced Brake Performance
Friction-material wear, oil contamination, or spring fatigue reduces holding capacity.
52.7 Emergency-Stop Failure
Contact wear, wiring failure, or mechanical damage prevents operation.
52.8 Restart-Prevention Failure
The machine restarts unintentionally after a safety device is restored.
53. Safety Mechanisms and Ergonomics
Safety devices should be natural and convenient for operators to use.
Main considerations include:
Accessibility of controls
Visibility
Operating force
Operator body size
Use of gloves
Repetitive fatigue
Line of sight
Intuitive use during emergencies
Left- and right-hand operation
Maintenance access
Safety devices that are difficult to use are more likely to be bypassed.
54. Safety Mechanisms in Food Machinery
Food machinery must consider hygiene, cleanability, and contamination prevention together with operator safety.
54.1 Main Hazards
Rotating blades
Cutting blades
Conveyor-belt trapping
Sealing-jaw crushing
Hot wash water
Steam
Automatic doors
Removal of jammed products
Unexpected operation during cleaning
54.2 Design Requirements
Cleanable guards
Structures that prevent food-residue accumulation
Hygienic interlocks
Minimized exposed threads
Drainable surfaces
Separation of cleaning and production modes
Low-speed operation during cleaning
Placement of safety components outside hygienic zones
Food-compatible materials and lubricants
54.3 Caution
Complex safety sensors, hinges, and locking mechanisms should be sealed or designed for easy disassembly so that food residue does not accumulate internally.
55. Safety Mechanisms in Packaging Machinery
Packaging machinery requires the following safety functions because of high-speed repetitive movement and frequent product changeovers:
Prevention of hand access at infeed points
Prevention of trapping by sealing jaws
Shaft locking during film-roll replacement
Cutter guards
Door interlocks
Safe modes for clearing product jams
Safe-speed monitoring
Emergency-stop pull cords
Prevention of unexpected restart
56. Safety Mechanisms in Machine Tools
In machine tools, rotating parts, cutting tools, chips, coolant, and automatic tool changers are major hazards.
Typical safety mechanisms include:
Spindle-door interlocks
Rotating-part covers
Chip-impact windows
Tool-clamp monitoring
Spindle-standstill confirmation
Axis overtravel stoppers
Overload monitoring
Emergency stop
Low-speed maintenance mode
Axis brakes
57. Safety Mechanisms in Robot Cells
Robot cells may use:
Safety fences
Interlocked doors
Light curtains
Laser scanners
Safety PLCs
Safe-speed monitoring
Enabling switches for teaching
Emergency stops
Axis brakes
Personnel-presence detection
Even when the robot stops, the workpiece may fall, a pneumatic gripper may release, or a gravity-loaded axis may descend. Separate countermeasures are therefore required.
58. Safety Mechanisms in Lifting Equipment
A typical safety configuration for lifting equipment is:
Drive motor
→ Reducer
→ Lifting mechanism
→ Spring-applied brake
→ Rod clamp or safety gear
→ Upper and lower stoppers
→ Overspeed detection
Main considerations include:
Static holding load
Falling energy
Automatic braking during power loss
Dual locking
Wire-rope or chain failure
Emergency lowering
Manual rescue
Periodic braking tests
59. Application Examples
59.1 Press Safety System
Load workpiece
→ Confirm two-hand control or light curtain
→ Safety controller authorizes operation
→ Press descends
→ Monitor overload
→ Reach bottom dead center
→ Return
Main safety devices:
Fixed guards
Light curtains
Two-hand control
Emergency stop
Brake monitoring
Overload protection
Restart prevention
59.2 Robot Cell
Close door
→ Lock guard
→ Confirm absence of personnel
→ Reset
→ Permit automatic operation
Main safety devices:
Perimeter fence
Door interlock
Guard locking
Safety PLC
Laser scanner
Emergency stop
Safe-speed monitoring
59.3 Packaging Sealing Device
Feed film
→ Move sealing jaws together
→ Compress and heat
→ Hold
→ Open
Main safety devices:
Anti-trapping guards
Interlocked covers
Overtemperature protection
Pressure limitation
Safety bumpers
Emergency stop
59.4 Vertical Lift
Command upward motion
→ Release brake
→ Lift
→ Reach target position
→ Engage brake
→ Activate rod clamp
Main safety devices:
Holding brake
Rod clamp
Upper and lower limit switches
Mechanical stoppers
Overspeed detection
Chain-break detection
Emergency-lowering device
60. Analysis and Design Tools
60.1 Risk Assessment
Hazard list
Hazard scenarios
Severity
Exposure frequency
Avoidance possibility
Risk-reduction measures
60.2 CAD Review
Guard interference
Access space
Finger access
Maintenance space
Emergency-stop location
Sensor blind spots
60.3 Structural Analysis
Guard impact strength
Stopper stress
Fall-prevention pin shear
Brake housing
Ejection-resistant window
Safety-fence strength
60.4 Dynamic Analysis
Stopping distance
Braking energy
Collision energy
Falling energy
Emergency-stop load
Safety distance
60.5 Control Validation
Interlock logic
Single faults
Sensor disagreement
Welded contactor
Communication delay
Power loss
Restart prevention
61. Selection Procedure for Safety Mechanisms
61.1 Define the Limits of the Machine
Normal operation
Setup
Cleaning
Maintenance
Fault recovery
Reasonably foreseeable misuse
61.2 Identify Hazards
Motion
Force
Speed
Pressure
Temperature
Electricity
Radiation
Chemicals
Gravity
61.3 Estimate Risk
Injury severity
Exposure frequency
Possibility of avoidance
Probability of occurrence
61.4 Apply Inherent Risk Reduction
Eliminate hazardous motion
Limit speed and force
Provide safe distance
Eliminate trapping points
Improve stability
61.5 Select Protective Devices
Fixed guards
Movable guards
Interlocks
Guard locking
Light curtains
Laser scanners
Safety mats
Emergency stops
61.6 Select Overload and Fall-Prevention Devices
Torque limiters
Shear pins
Brakes
Rod clamps
Safety nuts
Mechanical stoppers
61.7 Configure Safety Control
Safety relay
Safety PLC
Dual channels
Contactor monitoring
Valve monitoring
Restart prevention
61.8 Review Environmental Conditions
Dust
Moisture
Washdown
Corrosion
Temperature
Vibration
Food contact
Explosion hazard
61.9 Validate and Test
Functional tests
Stopping-time measurement
Fault tests
Repeated-life tests
Power-loss tests
Pressure-loss tests
Drop tests
Emergency-recovery tests
62. Conclusion
Safety mechanisms are mechanical, electrical, hydraulic, pneumatic, and control-based protective measures that protect operators, equipment, products, and the environment from hazards that may occur during machine operation, adjustment, cleaning, and maintenance.
Safety design should not begin with stopping the machine after a hazardous event has occurred. Hazards should first be eliminated or reduced by limiting force and speed. Guards and protective devices should then isolate the remaining hazards, followed by sensors, safety controls, and interlocks to manage residual risk.
Fixed guards are among the simplest and most reliable means of preventing access to hazardous areas. Where frequent access is required, movable guards should be combined with interlocks or guard-locking devices. Light curtains, laser scanners, safety mats, and safety bumpers detect human approach and stop hazardous motion.
Emergency stop is a supplementary protective device that rapidly stops a machine during a hazardous condition. It cannot replace inherently safe design or guarding. Two-hand controls, hold-to-run controls, and enabling devices keep the operator’s hands and body outside hazardous areas or permit operation only under restricted conditions.
Torque limiters, shear pins, relief valves, and mechanical stoppers protect machines against overload and collision. Brakes, rod clamps, safety nuts, and ratchet devices prevent falling of vertical axes and lifting members. Electrical, hydraulic, pneumatic, gravitational, and spring energy must be fully isolated and residual energy released before maintenance.
Safety mechanisms must not be evaluated only under normal operating conditions. Open circuits, short circuits, welded contacts, stuck valves, sensor faults, power loss, pressure loss, and common-cause failures must also be considered. After a safety device has been restored, the machine must not restart automatically without a separate reset and start command.
A suitable safety-design sequence is:
Define the limits of the machine
→ Identify hazards and hazardous situations
→ Estimate risk
→ Apply inherently safe design
→ Select guards and access-prevention devices
→ Configure detection devices and interlocks
→ Design overload, fall, and pressure protection
→ Configure safety control and fault diagnostics
→ Validate stopping time and safety distance
→ Design emergency stop and restart prevention
→ Establish maintenance energy-isolation procedures
→ Perform functional, fault, and life-cycle testing
→ Indicate and document residual risks
Properly designed safety mechanisms not only reduce the probability of accidents but also improve equipment reliability, maintainability, usability, and production stability.