1. Overview

A locking device is a mechanical device that holds a machine component, moving part, rotating member, door, cover, mold, or working unit in a specified position and prevents unintended movement, rotation, separation, or release under external force or vibration.

The basic functions of locking devices include:

• Locking linear-motion components in position

• Locking a rotating shaft at a specified angle

• Maintaining doors and covers in the closed position

• Preventing threaded fasteners from loosening

• Preventing tools and molds from being released

• Preventing moving components from falling

• Maintaining adjustment positions

• Preventing unintended release during operation

• Preventing hazardous motion during maintenance

• Maintaining a safe state during power or pressure loss

Locking devices are closely related to clamping mechanisms, but their purposes are different.

A clamping mechanism presses an object against a reference surface and resists process loads.
A locking device prevents an established position or engaged state from being released.

A single device may perform both clamping and locking functions. For example, an over-center toggle clamp presses a workpiece and also prevents release after the linkage passes beyond dead center.

Typical applications include:

• Doors and covers of industrial machinery

• Tool-holding devices in machine tools

• Mold- and pallet-changing systems

• Lifting and elevation equipment

• Rotary tables and indexing systems

• Slides and linear-transfer systems

• Robot axes and grippers

• Folding structures

• Transport containers

• Automotive seats and doors

• Aircraft and railway equipment

• Food and packaging machinery

• Safety supports for maintenance

When designing a locking device, not only holding force but also releasability, safety, state detection, wear, tolerances, and emergency operation must be considered.

2. Difference Between Locking and Clamping

2.1 Clamping

Clamping continuously applies a compressive force to hold a workpiece or component against a reference surface and resist external loads.

Its principal purposes are:

• Maintaining workpiece position

• Resisting process loads

• Preventing slipping

• Suppressing vibration

• Controlling deformation

2.2 Locking

Locking prevents a component or mechanism from moving or disengaging from an established position.

Its principal purposes are:

• Preventing positional displacement

• Preventing backdriving

• Preventing fastener loosening

• Preventing door opening

• Preventing falling

• Maintaining a safe condition

2.3 Functional Relationship

Clamps often rely on continuous force, while locking devices commonly use geometric engagement or mechanical interlocking.

Examples include:

• Maintaining hydraulic-cylinder pressure: clamping

• Inserting a locking pin into a cylinder rod: locking

• Pressing a tool with a screw: clamping

• Preventing screw rotation with a locknut: locking

• Pressing a door against a frame: clamping

• Preventing door opening with a latch: locking

3. Basic Requirements of Locking Devices

A locking device should provide the following performance according to its intended use.

3.1 Sufficient Holding Capacity

The lock must not release under external force, torque, vibration, or impact.

3.2 Positive Engagement

Partial or incomplete engagement must be prevented.

3.3 Reliable Release

The device must release without excessive force when a normal release command is given.

3.4 Prevention of Accidental Operation

The lock must not be released by operator error or accidental contact.

3.5 State Confirmation

The locked and released states should be visually, mechanically, or electronically verifiable.

3.6 Repeated Service Life

Wear, clearance, and holding-force variation must remain within allowable limits through repeated engagement and release cycles.

3.7 Environmental Compatibility

The device must operate under dust, moisture, washdown, corrosion, temperature variation, and vibration.

3.8 Emergency Release

The device should be manually releasable in the event of power loss, pressure loss, or failure.

4. Basic Classification of Locking Devices

4.1 Positive-Engagement Locking

Physical engagement between components prevents motion.

• Locking pins

• Indexing pins

• Latches

• Ratchet-and-pawl mechanisms

• Dog clutches

• Toothed locking devices

• Keys and splines

• Bayonet locks

4.2 Friction Locking

Contact friction prevents motion.

• Friction brakes

• Collet locks

• Taper locks

• Wedge locks

• Clamping rings

• Shaft-locking devices

• Friction-type rotary locks

4.3 Threaded Locking

Threads and fasteners are used to prevent loosening or rotation.

• Locknuts

• Double nuts

• Nylon-insert nuts

• Castle nuts and cotter pins

• Set screws

• Tab washers

• Thread-locking compounds

• Self-locking screws

4.4 Over-Center Locking

A linkage moves beyond dead center so that external force cannot easily drive it toward release.

• Toggle latches

• Over-center levers

• Link locking devices

• Folding-structure locks

4.5 Brake-Type Locking

A friction element prevents shaft or slide motion.

• Disc brakes

• Drum brakes

• Band brakes

• Spring-applied brakes

• Electromagnetic brakes

• Motor holding brakes

4.6 Fluid-Power Locking

Hydraulic or pneumatic flow is blocked to prevent actuator movement.

• Check valves

• Pilot-operated check valves

• Counterbalance valves

• Rod-lock cylinders

• Hydraulic locking valves

4.7 Electromagnetic and Electromechanical Locking

Electromagnetic force or an electrical signal operates the lock.

• Solenoid locking pins

• Electromagnetic door locks

• Electromagnetic brakes

• Magnetic locking devices

• Electro-permanent-magnet locks

5. Classification by Locking Direction

5.1 Linear-Motion Locking

Prevents motion of sliders, cylinder rods, lifting axes, or linear guides.

5.2 Rotary-Motion Locking

Prevents rotation of shafts, rotary tables, handles, or hinges.

5.3 Prevention of Axial Separation

Prevents shafts, pins, tools, or connectors from being pulled out axially.

5.4 Door and Cover Locking

Prevents opening of doors, covers, hatches, or enclosures.

5.5 Adjustment-Position Locking

Maintains the position of adjustment screws, handwheels, guides, or sensor brackets.

5.6 Fall-Prevention Locking

Prevents a lifting member or vertical axis from descending under gravity.

6. Positive Locking and Friction Locking

6.1 Positive Locking

Pins, teeth, grooves, or hooks physically engage and block motion.

Advantages:

• Holding capacity is less dependent on friction coefficient

• The engagement state is clear

• Strong resistance to vibration

• Suitable for safety functions

Disadvantages:

• Locking is possible only at predetermined positions

• Alignment is required

• Impact engagement may cause damage

• Clearance may develop

6.2 Friction Locking

Two surfaces are pressed together so that friction prevents motion.

Advantages:

• Can lock at any position

• Suitable for fine adjustment

• Relatively simple structure

• Continuous position locking is possible

Disadvantages:

• Sensitive to changes in friction coefficient

• May release under vibration or lubrication

• Large holding force requires large normal force

• Performance changes with wear

6.3 Selection Principle

Positive locking should be preferred where safety is directly involved. Friction locking is suitable where continuous adjustment is required and may be used as a supplementary method.

7. Locking Pins

7.1 Definition

A locking pin passes through aligned holes in a fixed and moving component to prevent relative motion.

7.2 Main Types

• Manually inserted pins

• Indexing pins

• Spring plungers

• Ball-lock pins

• Quick-release pins

• Taper pins

• Hydraulically actuated pins

• Pneumatically actuated pins

• Solenoid-operated pins

7.3 Advantages

• Simple structure

• Positive geometric locking

• High shear-load capacity

• Easy confirmation of locking state

• Simple maintenance

7.4 Disadvantages

• Hole alignment is required

• Impact load may act on the pin

• Clearance increases with wear

• Contamination may prevent insertion

• Release may be difficult under load

8. Load Calculation for Locking Pins

Locking pins are generally subjected to shear, bending, and bearing pressure.

8.1 Single Shear

τ = F / A

For a circular pin:

A = πd² / 4

8.2 Double Shear

τ = F / 2A

A double-shear arrangement supports a larger load than a single-shear arrangement for the same pin diameter.

8.3 Bearing Pressure

If the pin diameter is (d) and the contact thickness is (t):

p_b = F / dt

8.4 Pin Bending

If the gap between supports is large or there is clearance between the pin and hole, a bending moment acts on the pin.

8.5 Design Considerations

• Pin shear

• Pin bending

• Hole bearing pressure

• Fatigue load

• Impact load

• Pin-retention method

• Insertion and release force

• Corrosion and seizure

9. Indexing Pins and Plungers

9.1 Indexing Pins

An indexing pin repeatedly locks a rotary plate or slider at predetermined positions.

9.2 Spring Plungers

An internal spring pushes a pin or ball into a groove or hole.

9.3 Typical Applications

• Rotary indexing tables

• Adjustable guides

• Folding handles

• Slide-position locking

• Interchangeable fixtures

• Height-adjustment devices

9.4 Design Considerations

A plunger is suitable for position indication and light holding, but may not be suitable for directly supporting a large safety load.

A separate locking pin or positive stopper should be used for high loads.

10. Ball-Lock Pins

A ball-lock pin uses protruding balls near the end of the pin to prevent withdrawal.

10.1 Operating Principle

When the button is pressed, an internal rod moves and allows the balls to retract into the pin. Releasing the button causes the balls to protrude again.

10.2 Advantages

• Rapid insertion and removal

• No tools required

• Prevention of unintended pin removal

• Suitable for repeated changeovers

10.3 Disadvantages

• Internal structure is sensitive to contamination

• Local stress occurs at the balls and grooves

• Impact load must be controlled

• Periodic cleaning and lubrication are required

11. Latch Devices

11.1 Definition

A latch uses a projection, hook, groove, or catch to engage a mating component and prevent a door or cover from opening.

11.2 Main Types

• Slide-bolt latches

• Hook latches

• Cam latches

• Toggle latches

• Rotary latches

• Spring latches

• Push-to-close latches

• Rotary catches

11.3 Advantages

• Rapid locking and release

• Relatively simple structure

• Easy visual confirmation

• Easy application to doors and covers

11.4 Disadvantages

• Wear of engaging surfaces

• Sensitivity to misalignment

• Possible opening under impact

• Risk of incomplete engagement

12. Toggle Latches and Over-Center Locking

12.1 Operating Principle

When the lever and links move beyond dead center, external separating force cannot easily drive the linkage toward release.

12.2 Characteristics

• Rapid manual operation

• Pulls mating components together

• Provides a relatively consistent tensile force

• Mechanical locking in the over-center position

• Suitable for doors, molds, and containers

12.3 Design Considerations

• Over-center travel

• Hook length

• Latch tensile force

• Handle operating force

• Stopper position

• Vibration

• Locking condition after wear

12.4 Excessive Over-Center Travel

Excessive over-center travel increases release force and may increase load on links and pins.

13. Ratchet and Pawl

13.1 Definition

A ratchet wheel and pawl allow rotation in one direction while preventing reverse rotation.

13.2 Main Components

• Ratchet wheel

• Pawl

• Pawl spring

• Pivot

• Release lever

• Stopper

13.3 Typical Applications

• Winches

• Hoists

• Tension-adjustment devices

• Hand brakes

• Lifting devices

• Stepwise positioning systems

13.4 Advantages

• Stepwise position holding

• Prevention of reverse rotation

• Simple structure

• High torque capacity

13.5 Disadvantages

• Continuous arbitrary-angle locking is difficult

• Impact occurs between pawl and teeth

• Noise

• Backlash

• Fatigue due to repeated impact

14. Ratchet Load

If torque (T) acts on the ratchet and the effective tooth radius is (r), the tangential force is:

F_t = T / r

The following loads must be evaluated:

• Tooth bending

• Tooth contact stress

• Pawl shear

• Pawl bending

• Pivot-pin shear

• Impact load

• Fatigue load

If a ratchet engages suddenly while rotating, the impact load may be much higher than the static value.

15. Dog Clutches and Toothed Locks

15.1 Definition

Axially moving dogs or teeth engage mating grooves to prevent rotation or transmit torque.

15.2 Characteristics

• High torque transmission

• Positive engagement without slip

• Clear engaged state

• Suitable for locking rotating shafts

15.3 Disadvantages

• Tooth alignment is required

• Impact occurs if engaged during rotation

• Clearance may develop

• Edge wear

• Unsuitable for high-speed engagement

15.4 Typical Applications

• Gear-selection devices

• Rotary-table locking

• Tool-changing systems

• Shaft couplings

• Indexing mechanisms

16. Thread-Locking Devices

Thread-locking devices prevent fastening or adjustment screws from loosening or rotating under vibration and external load.

Common methods include:

• Double nuts

• Locknuts

• Nylon-insert nuts

• All-metal prevailing-torque nuts

• Castle nuts and cotter pins

• Tab washers

• Spring washers

• Serrated washers

• Set screws

• Thread-locking compounds

• Safety wire

17. Double Nuts

Two nuts are tightened against each other to generate preload between the threads.

17.1 Advantages

• Simple structure

• Standard nuts can be used

• Suitable for locking adjustment positions

• Reusable

17.2 Disadvantages

• Requires additional axial space

• Correct tightening procedure is required

• Operator-dependent variation

• Requires inspection in vibrating environments

18. Locknuts

A locknut resists rotation relative to the threaded shaft by friction or material deformation.

18.1 Main Types

• Nylon-insert type

• All-metal deformed-thread type

• Slotted type

• Clamping type

• Precision bearing locknuts

18.2 Selection Factors

• Temperature

• Number of reuses

• Vibration

• Tightening torque

• Chemical resistance

• Cleanliness

• Thread accuracy

Nylon-insert nuts may lose performance at high temperatures, so the allowable temperature must be checked.

19. Castle Nuts and Cotter Pins

A cotter pin is inserted through the aligned slot of a castle nut and a hole in the bolt to prevent nut rotation by positive engagement.

19.1 Advantages

• Easy visual confirmation

• Positive locking

• Strong resistance to vibration

• Suitable for safety-critical fasteners

19.2 Disadvantages

• Slot and hole alignment is required

• Fine tightening-angle adjustment is limited

• Cotter pin replacement is required

• Assembly time increases

20. Tab Washers and Locking Plates

Part of a washer or plate is bent against the nut or fixed structure to prevent rotation.

20.1 Applications

• Bearing locknuts

• Gear shafts

• Rotating assemblies

• Heavy-equipment fasteners

• Components subjected to repeated vibration

20.2 Advantages

• Positive geometric locking

• Simple structure

• Easy verification of locking state

20.3 Disadvantages

• Reuse may be limited

• Damage may occur during bending

• Assembly space is required

21. Set Screws

A set screw presses its tip against a shaft or component to prevent relative motion.

21.1 Tip Types

• Flat point

• Cup point

• Cone point

• Dog point

• Nylon tip

• Ball tip

21.2 Advantages

• Simple structure

• Small installation space

• Convenient for locking adjustment positions

• Low cost

21.3 Disadvantages

• May damage the shaft surface

• Holding force depends on friction

• May loosen under vibration

• Limited torque capacity

Keys, splines, or clamping hubs should be used together with set screws in important torque-transmission applications.

22. Thread-Locking Compounds

A liquid adhesive cures between threads and prevents loosening.

22.1 Advantages

• Load distribution over the full thread engagement

• Resistance to vibration

• Sealing effect

• Fewer additional locking components

22.2 Disadvantages

• Requires curing time

• Sensitive to surface contamination

• Increased disassembly torque

• Affected by temperature and chemicals

• Cleaning required before reassembly

22.3 Application Considerations

• Required disassembly capability

• Thread size

• Base-metal material

• Operating temperature

• Oil contamination

• Possibility of food contact

• Curing conditions

23. Wedge Locking

23.1 Definition

An inclined wedge is inserted between components to generate compressive force and prevent motion through friction and geometric action.

23.2 Self-Locking Condition

If the wedge angle is (\alpha) and the friction angle is (\phi), self-locking is generally likely when:

(\phi > \alpha)

or:

(\mu > \tan\alpha)

23.3 Advantages

• High locking force

• Compact structure

• High stiffness

• Can lock at arbitrary positions

• Can remove clearance

23.4 Disadvantages

• Sensitive to friction-coefficient variation

• Release force may be high

• Wear and seizure

• Performance changes with lubrication

A separate locking pin or stopper should be added for safety-critical functions rather than relying only on frictional self-locking.

24. Taper Locking

Conical or tapered surfaces are pressed together to lock shafts, hubs, tools, or collets.

24.1 Typical Applications

• Machine-tool holders

• Taper bushings

• Collets

• Shaft-hub connections

• Drill chucks

• Rotary tools

24.2 Advantages

• High concentricity

• Large contact area

• High torque transmission

• Low clearance

• High repeatability

24.3 Disadvantages

• Requires high surface accuracy

• Sensitive to contamination

• Excessive insertion may cause seizure

• A release mechanism is required

25. Collet Locking

A slotted collet moves along a taper and uniformly grips a shaft or tool around its circumference.

25.1 Characteristics

• High concentricity

• Uniform circumferential pressure

• Rapid engagement

• Suitable for small-diameter parts

• High repeatability

25.2 Main Design Considerations

• Collet contraction

• Taper angle

• Draw force

• Contact pressure

• Material fatigue

• Contamination

• Releasability

26. Shaft-Locking Devices

Frictional connection between a shaft and hub prevents rotation and axial movement.

26.1 Main Types

• Clamping hubs

• Split hubs

• Taper-lock bushings

• Keyless locking devices

• Shrink discs

• Clamping rings

26.2 Advantages

• Torque transmission without keyways

• Reduced shaft damage

• Good alignment

• Relatively easy assembly and disassembly

• Low backlash

26.3 Disadvantages

• Tightening torque must be controlled

• Sensitive to surface condition

• Excessive tightening may deform the hub

• Influenced by friction-coefficient variation

27. Brake-Type Locking Devices

A brake-type locking device presses friction material against a disc, drum, or shaft to prevent rotary or linear motion.

27.1 Main Types

• Disc brakes

• Drum brakes

• Band brakes

• Shoe brakes

• Electromagnetic brakes

• Spring-applied brakes

• Hydraulically released brakes

• Pneumatically released brakes

27.2 Static Holding Torque

The required holding torque is determined from the external load torque and safety factor.

T_brake ≥ S × T_load

27.3 Advantages

• Can lock at any position

• High holding torque

• Can decelerate and stop motion

• Applicable to motor shafts and lifting axes

27.4 Disadvantages

• Friction-material wear

• Heat generation

• Variation in holding torque

• Sensitive to oil and moisture

• Requires periodic inspection

28. Spring-Applied Brakes

A spring applies the brake, and electromagnetic, pneumatic, or hydraulic force releases it.

28.1 Characteristics

• Automatically engages during power loss

• Fail-safe function

• Prevents vertical-axis falling

• Suitable for motor holding

28.2 Typical Applications

• Lifting axes

• Hoists

• Elevators

• Robot joints

• Servo motors

• Automated storage systems

28.3 Design Considerations

• Static holding torque

• Emergency-stop energy

• Release voltage

• Friction-disc wear

• Manual release

• Residual torque

Holding brakes and dynamic stopping brakes have different performance requirements and must be distinguished.

29. Linear-Axis Locking Devices

These devices directly lock linear guides, cylinder rods, or linear axes.

29.1 Main Types

• Rod clamps

• Linear-guide brakes

• Wedge-type shaft locks

• Spring clamps

• Pneumatically released shaft locks

• Hydraulically released shaft locks

29.2 Applications

• Vertical slides

• Cylinder rods

• Z-axes

• Lifting platforms

• Press rams

• Maintenance-position locking

29.3 Caution

Position-holding devices and emergency-braking devices have different energy-absorption requirements.

A device designed only to hold a stationary axis may be damaged if used to stop a high-speed moving component.

30. Check-Valve Fluid Locking

30.1 Check Valves

A check valve allows flow in only one direction and limits reverse cylinder movement.

30.2 Pilot-Operated Check Valves

Flow is blocked under normal conditions and reverse flow is permitted only when pilot pressure is applied.

30.3 Advantages

• Maintains hydraulic or pneumatic cylinder position

• Relatively simple circuit

• Can restrict movement during pressure loss

• Suitable for automation

30.4 Limitations

• Internal leakage

• Loss of holding capability if a hose fails

• Fluid compressibility

• Pressure variation with temperature

• Cylinder-seal leakage

For vertical loads related to personnel safety, a mechanical lock should be added rather than relying only on a check valve.

31. Counterbalance Valves

A counterbalance valve maintains back pressure so that a vertical or overrunning load cannot move a cylinder uncontrollably.

31.1 Main Functions

• Prevention of load dropping

• Control of lowering speed

• Protection against hose failure

• Pressure retention

• Prevention of cavitation

31.2 Design Considerations

• Pressure setting

• Pilot ratio

• Load pressure

• Heat generation

• Lowering stability

• Emergency release

32. Rod-Lock Cylinders

A separate mechanical locking mechanism is integrated with the cylinder rod to hold it at any position.

32.1 Main Types

• Spring-applied, pneumatically released

• Wedge clamping

• Collet clamping

• Roller locking

• Brake-pad type

32.2 Advantages

• Maintains position during pressure loss

• Locks at any position

• Prevents vertical-axis falling

• Can be integrated with the cylinder

32.3 Disadvantages

• Sensitive to rod surface condition

• Side loading must be controlled

• Repeated wear

• Emergency-stopping capability must be confirmed

33. Solenoid Locking Devices

An electrical signal moves a plunger or pin to lock or release the mechanism.

33.1 Main Forms

• Energize-to-lock

• Energize-to-release

• Bistable solenoid

• Motor-driven latch

• Electronic door interlock

33.2 Energize-to-Release Type

The device is spring-locked without power and releases when energized.

It is suitable where the default safe state is locked.

33.3 Energize-to-Lock Type

The device is normally released and locks when energized.

It may be used where emergency escape or manual opening has priority.

33.4 Design Considerations

• Required state during power loss

• Coil heating

• Continuous-duty rating

• Plunger holding force

• Manual release

• State sensor

34. Mechanical Interlocks

A mechanical interlock prevents two or more devices from operating simultaneously in a hazardous sequence.

34.1 Application Examples

• The machine operates only when the door is closed.

• Rotation is allowed only when the locking pin is inserted.

• The press operates only when the mold is locked.

• One valve can open only when another valve is closed.

• The drive shaft is blocked when the cover is open.

34.2 Advantages

• Independent of control software

• Prevents incorrect operation

• Provides a clear relationship between states

• Can function without electrical power

34.3 Disadvantages

• Structure may become complex

• Wear and misalignment

• Possibility of forced operation

• Bypass prevention is required during maintenance

35. Guard-Door Interlock Locking

A guard-door interlock confirms that the safety door is closed and prevents it from opening until hazardous motion has stopped.

35.1 Main Functions

• Door-closed detection

• Locking-pin engagement

• Lock-state detection

• Confirmation that hazardous motion has ended

• Safe release

• Emergency escape or manual release

35.2 Difference from a Simple Door Switch

A door switch only detects the door position, whereas a guard-locking device mechanically prevents the door from opening.

35.3 Applications

• Presses

• Robot cells

• Rotating machinery

• Laser equipment

• High-temperature equipment

• Machines with long coast-down times

36. Self-Locking and Backdriving

36.1 Self-Locking

Self-locking is the ability of a mechanism to resist movement toward release under external load.

Representative self-locking mechanisms include:

• Low-lead screws

• Small-angle wedges

• Over-center toggles

• Some worm gears

36.2 Backdriving

Backdriving occurs when output load drives the input side.

Ball screws, high-efficiency gears, and some linkages may be backdriven.

36.3 Safety Considerations

Friction-based self-locking changes with:

• Lubrication

• Temperature

• Vibration

• Impact

• Wear

• Surface treatment

• Contamination

A positive locking device should be added for hazardous loads.

37. Holding-Force Calculation

37.1 Linear Holding Force

If a locking device must support a linear load (F_L), the required design holding force with safety factor (S) is:

F_lock ≥ S × F_L

37.2 Rotary Holding Torque

T_lock ≥ S × T_L

37.3 Friction Locking

If the normal force is (N) and the coefficient of friction is (\mu):

F_f = μN

For rotary friction locking, using an effective radius (r_e):

T_f = μNr_e

The number of friction interfaces must be included where applicable.

37.4 Impact Loading

Drop, sudden stopping, and ratchet engagement may generate dynamic loads much higher than static loads. Energy-based analysis is therefore required.

38. Release Force

A locking device must provide sufficient holding capacity while remaining releasable under normal conditions.

Release force is affected by:

• Load acting on the lock

• Friction

• Over-center travel

• Wedge angle

• Spring force

• Component deformation

• Corrosion

• Contamination

• Temperature

• Assembly tolerance

If a load presses directly against a locking pin, the pin may not be removable while under shear.

The load should first be removed, or an unloading mechanism should be provided.

39. Preload and Clearance

39.1 Preload

Preload is an initial force applied to a locking interface to reduce clearance and vibration.

39.2 Advantages

• Reduced clearance

• Vibration suppression

• Reduced noise

• Improved repeatability

• Reduced impact

39.3 Disadvantages

• Increased operating force

• Increased friction and wear

• Heat generation

• Component deformation

• Increased release force

Positive locks may have clearance. Springs or wedges can be added to provide preload where necessary.

40. Lock-State Detection

A locking device should communicate its actual state to the control system.

40.1 Position Sensors

Confirm that a pin, latch, or lever has reached its final position.

40.2 Proximity Sensors

Detect the position of a metallic locking component without contact.

40.3 Limit Switches

Detect the state through direct mechanical contact.

40.4 Dual-Channel Sensors

Safety functions may use two independent signals to detect failures.

40.5 Force and Pressure Detection

Hydraulic and pneumatic locks may use pressure sensors to confirm actuation.

40.6 Caution

Actuator position does not always mean that locking is complete.

A pin actuator may advance even when the pin and hole are misaligned or contamination is present. Actual insertion depth or the state of the mating component should also be verified.

41. Locking Sequence

A typical locking sequence is:

Stop the moving component
→ Confirm position
→ Stabilize the load
→ Align the locking elements
→ Activate the lock
→ Confirm complete locking
→ Remove drive force or perform the process

A typical release sequence is:

Complete the process
→ Remove external load
→ Unload the locking element
→ Release the lock
→ Confirm complete release
→ Permit movement

If drive force is removed or the process begins before locking is complete, impact may occur on pins and latches.

42. Fail-Safe Locking

A fail-safe lock changes to or remains in a safer state during loss of electrical, pneumatic, or hydraulic power.

42.1 Representative Methods

• Spring locking with electrical release

• Spring locking with pneumatic release

• Spring locking with hydraulic release

• Gravity-operated automatic locking

• Over-center mechanical locking

• Electro-permanent magnets

• Automatic fall-prevention pawls

42.2 Selection Criteria

• Which state is safer during power loss: locked or released?

• Is manual opening required for fire or emergency evacuation?

• Is there a vertical load?

• How long must the locked state be maintained?

• Is stored energy present?

43. Emergency Release

A locking device should be safely releasable during failure or power loss.

43.1 Main Methods

• Manual lever

• Manual bolt

• Cable release

• Emergency key

• Hydraulic hand pump

• Pneumatic manual valve

• Manual brake release

• Auxiliary power supply

43.2 Design Principles

• Unauthorized operation should be difficult.

• Maintenance personnel should be able to access the release using a tool or key.

• The load must not move suddenly during release.

• Normal operation must not restart automatically after emergency release.

• The released state should be indicated.

44. Overload and Incorrect-Locking Prevention

44.1 Causes of Overload

• Locking before the moving component has stopped

• Misalignment between pin and hole

• Driving the mechanism while locked

• Excessive impact

• Exceeding allowable torque

• Wear of locking components

• Foreign material

44.2 Protection Methods

• Mechanical stoppers

• Torque limiters

• Shear pins

• Force sensors

• Position interlocks

• Speed interlocks

• Lock-complete sensors

• Motor-torque limits

• Hydraulic relief valves

• Software sequences

44.3 Prevention of Incorrect Locking

The following can prevent a locking pin from advancing at an incorrect position:

• Tapered pin tip

• Guide bushing

• Position detection

• Keyed geometry

• Asymmetric groove

• Mechanical interlock

45. Material Selection

45.1 Locking Pins and Pawls

• Heat-treated alloy steel

• Carburizing steel

• Bearing steel

• Tool steel

• Stainless steel

45.2 Latches and Levers

• Carbon steel

• Alloy steel

• Cast steel

• Stainless steel

• Aluminum alloy

45.3 Friction Components

• Friction materials

• Sintered materials

• Bronze

• Reinforced polymers

• Carbon-based composites

45.4 Bushings and Guides

• Bronze

• Sintered metal

• Polymer-composite bushings

• PTFE-based materials

• Hardened steel

45.5 Selection Factors

• Shear strength

• Bending strength

• Fatigue strength

• Impact toughness

• Wear

• Corrosion

• Cleanability

• Operating temperature

• Food contact

• Cost

46. Heat Treatment and Surface Treatment

Locking components are subjected to repeated contact and impact, so both surface hardness and core toughness are required.

Typical treatments include:

• Carburizing

• Nitriding

• Induction hardening

• Quenching and tempering

• Hard chromium plating

• Electroless nickel plating

• DLC coating

• Black oxide coating

• Grinding

• Lapping

• Shot peening

Pins, pawls, ratchet teeth, and taper surfaces require high wear resistance but should not become excessively brittle.

47. Friction and Lubrication

47.1 Main Lubrication Points

• Locking pins and bushings

• Latch pivots

• Ratchet and pawl

• Wedge surfaces

• Taper surfaces

• Threads

• Brake actuating mechanisms

47.2 Benefits of Lubrication

• Reduced wear

• Reduced insertion and release force

• Corrosion prevention

• Prevention of seizure

• Improved repeatability

47.3 Caution

Lubrication of friction-based self-locking parts may reduce locking force or cause backdriving.

Unintended lubricant contamination must be prevented on brake and friction-locking surfaces.

48. Tolerances and Alignment

Locking devices may be sensitive to alignment error and dimensional tolerance.

48.1 Main Tolerance Factors

• Clearance between pin and hole

• Concentricity of shaft and bushing

• Position of latch and striker

• Tooth-pitch error

• Taper angle

• Parallelism

• Perpendicularity

• Mounting-surface position

48.2 Excessively Small Clearance

• Insertion failure

• Seizure due to thermal expansion

• Increased sensitivity to contamination

• Difficult assembly

48.3 Excessively Large Clearance

• Backlash

• Impact

• Position error

• Noise

• Reduced fatigue life

48.4 Compensation Methods

• Tapered pin tip

• Floating mount

• Spherical washer

• Guide bushing

• Spring compensation

• Adjustable striker

49. Effects of Thermal Expansion

Temperature variation changes locking clearance, insertion force, and holding capacity.

Linear expansion is:

ΔL = αLΔT

Where:

• α: Coefficient of linear expansion

• L: Reference length

• ΔT: Temperature change

The following should be reviewed in high-temperature and washdown equipment:

• Differential expansion between pin and hole

• Use of dissimilar materials

• Thermal distortion

• Lubricant viscosity

• Change in spring force

• Seal degradation

50. Vibration and Loosening

Vibration may cause release of threaded, latch, and friction-based locks.

50.1 Methods Vulnerable to Vibration

• Simple friction nuts

• Set screws

• Friction levers

• Low-preload latches

• Loose locking pins

50.2 Countermeasures

• Positive locking

• Double nuts

• Self-locking nuts

• Cotter pins

• Locking plates

• Thread-locking compounds

• Spring preload

• State detection

• Periodic torque inspection

51. Major Failure Modes

51.1 Locking-Pin Shear Failure

The pin breaks when allowable load is exceeded or impact load occurs.

51.2 Wear of Pins and Holes

Repeated impact and micro-motion increase clearance.

51.3 Incomplete Latch Engagement

Misalignment, contamination, or incorrect adjustment prevents full engagement.

51.4 Ratchet-Tooth Failure

Excessive torque, impact, or fatigue damages the teeth.

51.5 Wedge Seizure

Wear, corrosion, contamination, or excessive load prevents release.

51.6 Reduced Brake Holding Capacity

Friction-material wear, oil contamination, or spring fatigue reduces holding torque.

51.7 Thread Loosening

Caused by vibration, insufficient preload, or damage to the locking device.

51.8 False Sensor Detection

Incorrect sensor position, wiring failure, or contamination causes an incorrect signal.

52. Failure Modes and Effects

53. Maintenance and Inspection

53.1 Daily Inspection

• Locking and release operation

• Abnormal noise

• Clearance

• Pin insertion condition

• Latch engagement

• Sensor indication

• Oil and air leakage

• Corrosion

• Foreign material

• Manual-release device

53.2 Periodic Inspection

• Pin diameter and wear

• Hole deformation

• Bushing clearance

• Ratchet teeth

• Pawl spring

• Brake friction material

• Locknut condition

• Hydraulic and pneumatic leakage

• Sensor position

• Electrical wiring

53.3 Replacement Parts

• Pins

• Bushings

• Pawls

• Springs

• Friction pads

• Latch hooks

• Strikers

• Seals

• Sensors

• Locknuts

53.4 Condition-Based Maintenance

Useful data include:

• Locking time

• Release time

• Drive current

• Solenoid temperature

• Brake clearance

• Holding force

• Vibration

• Number of cycles

• Sensor error

• Fluid pressure

54. Safety Design

54.1 Main Hazards

• Falling caused by lock release

• Unintended rotation of rotating components

• Door opening

• Release of stored energy

• Flying parts caused by pin failure

• Pinching between locking components

• Load movement during emergency release

• Misinterpretation of lock state

54.2 Safety Design Principles

• Use positive locking for hazardous loads.

• Prevent a single component failure from creating a hazardous state.

• Permit operation only after lock completion is confirmed.

• Remove load before release.

• Use auxiliary support during manual release.

• Do not rely only on frictional self-locking.

• Verify strength and fatigue life of locking components.

• Apply energy-isolation procedures during maintenance.

55. Dual Locking and Redundant Design

High-risk equipment may use a second independent locking device.

Typical combinations include:

• Brake and locking pin

• Check valve and rod clamp

• Self-locking screw and safety nut

• Over-center linkage and safety pin

• Hydraulic holding and mechanical stopper

• Electronic lock and manual latch

The two devices should preferably use different operating principles so that they do not fail from the same cause.

56. Locking Devices in Food Machinery

Food machinery must consider hygiene, cleanability, and corrosion resistance together with locking performance.

56.1 Main Requirements

• Structures that prevent food-residue accumulation

• Open, cleanable construction

• Minimized deep grooves and gaps

• Minimized exposed threads

• Drainable surfaces

• Corrosion-resistant materials

• Food-grade lubricants

• Easy disassembly and assembly

• Prevention of loose-part loss

56.2 Recommended Methods

• Captive locking pins

• Hygienic latches

• Stainless-steel toggle latches

• Tool-less removable locks

• Hygienic door interlocks with sensors

56.3 Caution

Ball-lock pins, spring plungers, and complex latch interiors may retain wash fluid or food residue. Sealed or easily disassembled structures should be used in direct food-contact zones.

57. Locking Devices in Packaging Machinery

Packaging machinery requires high-speed repeated operation and frequent changeovers.

Typical applications include:

• Guide-position locking

• Sealing-jaw locking

• Cover interlocks

• Quick-release change parts

• Film-roll shaft locking

• Height-adjustment locking

• Maintenance safety pins

Requirements include:

• Rapid operation

• Repeatable positioning

• Visual confirmation of locking

• Tool-less changeover

• Resistance to loosening under vibration

• Prevention of loose-part loss

58. Locking Devices in Machine Tools

Machine tools require high stiffness, repeatability, and resistance to cutting vibration.

Typical applications include:

• Tool clamping

• Pallet locking

• Rotary-table indexing pins

• Axis brakes

• Spindle tool locks

• Guide locking

• Adjustment-screw locknuts

In machine tools, clearance and elastic deformation after locking directly affect machining accuracy. Both holding capacity and stiffness must therefore be evaluated.

59. Locking in Lifting Equipment

Failure of a locking device in a vertical axis or lifting system may cause a falling load, so the most stringent evaluation is required.

A typical configuration is:

Drive motor
→ Reducer
→ Lifting screw or chain
→ Holding brake
→ Mechanical fall-prevention device

Main considerations:

• Static holding load

• Emergency-stop energy

• Operation during power loss

• Dual locking

• Manual release

• Overspeed detection

• Structural strength

• Repeated testing

60. Locking in Robots and Automation Equipment

Locking devices are used in robots and automation systems for maintenance, tool changing, position holding, and emergency conditions.

Typical applications include:

• Robot-joint brakes

• Gripper tool locks

• Tool-changer locking pins

• Linear-axis rod clamps

• Pallet locks

• Safety-door interlocks

• Maintenance axis-locking pins

Requirements include:

• High-speed state detection

• Interlocking with the control system

• Safe state during power loss

• Low backlash

• High repeatability

• Manual recovery capability

61. Application Examples

61.1 Rotary Indexing Table

Rotate table
→ Reach target angle
→ Stop drive
→ Insert tapered locking pin
→ Confirm locking
→ Perform machining

Main considerations:

• Pin shear

• Position repeatability

• Taper contact

• Insertion impact

• Chip contamination

• Lock-complete sensor

61.2 Vertical Slide

Reach target position
→ Stop motor
→ Engage spring brake
→ Activate rod clamp
→ Hold position

Main considerations:

• Load

• Fall prevention

• Dual locking

• Release sequence

• Emergency stop

• Manual recovery

61.3 Safety Door

Close door
→ Engage latch
→ Insert locking pin
→ Detect lock state
→ Permit machine operation

Main considerations:

• Incomplete engagement

• State during power loss

• Emergency escape

• Tamper prevention

• Sensor redundancy

61.4 Pallet-Locking Device

Load pallet
→ Locate pallet
→ Engage pull stud
→ Lock with wedge or balls
→ Confirm locking force
→ Machine

Main considerations:

• Position repeatability

• Pull force

• Stiffness

• Chips and coolant

• Releasability

• Lock-force detection

61.5 Folding Structure

Deploy structure
→ Align links
→ Move over center
→ Insert auxiliary locking pin
→ Use structure

Main considerations:

• Link buckling

• Over-center travel

• Pin shear

• Operator error

• Prevention of folding

• State indication

62. Analysis and Design Tools

62.1 CAD Motion Analysis

• Lock-element alignment

• Pin insertion path

• Latch rotation range

• Interference

• Over-center position

• Release space

• Manual-access space

62.2 Structural Analysis

• Pin shear and bending

• Ratchet-tooth stress

• Pawl bending

• Latch-hook stress

• Brake-hub stress

• Lock-frame deformation

• Contact stress

62.3 Dynamic Analysis

• Engagement impact

• Emergency-stop load

• Falling energy

• Ratchet impact

• Brake energy absorption

• Vibration-induced loosening

62.4 Fluid-Circuit Analysis

• Check-valve leakage

• Counterbalance pressure

• Lock-release time

• Cylinder movement

• Hose failure

• Emergency pressure

62.5 Control Simulation

• Locking sequence

• Release sequence

• Sensor failure

• Signal delay

• Power loss

• Pressure loss

• Interlock logic

63. Selection Procedure for Locking Devices

63.1 Define the Locking Target

• Linear moving component

• Rotating shaft

• Door

• Cover

• Tool

• Mold

• Adjustment screw

• Lifting device

63.2 Define Load Conditions

• Linear load

• Torque

• Impact

• Vibration

• Gravity

• Inertial force

• Number of cycles

• Emergency-stop load

63.3 Determine the Required Locking Method

• Positive locking at fixed positions

• Friction locking at arbitrary positions

• Automatic locking

• Manual locking

• Fail-safe locking

• Dual locking

63.4 Select the Actuation Method

• Manual

• Spring

• Pneumatic

• Hydraulic

• Solenoid

• Motor

• Gravity

63.5 Review Holding Capacity and Strength

• Pin shear

• Tooth bending

• Latch tension

• Friction holding force

• Brake torque

• Frame deformation

• Safety factor

63.6 Review Alignment and Tolerances

• Insertion clearance

• Position error

• Thermal expansion

• Wear

• Contamination

• Floating structure

63.7 Review Releasability

• Normal release force

• Release under load

• Unloading method

• Emergency release

• Release during power loss

• Manual recovery

63.8 Review State Detection

• Lock complete

• Release complete

• Incomplete engagement

• Pressure

• Position

• Force

• Sensor redundancy

63.9 Review Environmental Conditions

• Temperature

• Vibration

• Dust

• Cutting fluid

• Moisture

• Washdown

• Corrosion

• Food contact

• Vacuum

• Cleanroom

63.10 Testing and Validation

• Static holding-force test

• Repeated-life test

• Impact test

• Vibration test

• Power-loss test

• Pressure-loss test

• Emergency-release test

• Corrosion test

• Contamination test

• Sensor-failure test

64. Conclusion

A locking device holds a machine component, moving part, rotating member, door, tool, or mold in a specified position and engaged state and prevents unintended movement, rotation, separation, or release caused by external force or vibration.

Locking must be distinguished from clamping. Clamping presses an object against a reference surface and resists process loads, whereas locking blocks motion so that an established state cannot be released.

Locking pins, latches, ratchets, and dog clutches provide positive locking through geometric engagement. Thread-locking devices prevent nuts and adjustment screws from loosening. Wedges, tapers, collets, and shaft-locking devices use friction and compressive force.

Brake-type locking devices can hold a shaft or moving member at any position. Check valves and rod-lock cylinders maintain the position of hydraulic and pneumatic actuators. Solenoid and electronic locking devices are advantageous for automation and state detection.

A locking device should not be evaluated by holding force alone. Alignment, clearance, wear, impact, vibration, release force, power loss, pressure loss, emergency release, and state detection must all be considered.

Particularly in equipment associated with personnel safety or vertical loads, reliable positive locking devices such as locking pins or ratchets should be used rather than relying only on friction or fluid pressure.

A suitable design and selection sequence is:

Define the locking target and direction of motion
→ Analyze normal loads, impact, and vibration
→ Select positive or friction locking
→ Select actuation and release methods
→ Calculate holding capacity and structural strength
→ Review alignment, tolerances, and thermal expansion
→ Design locking and release sequences
→ Review fail-safe and redundant locking
→ Configure state detection and interlocks
→ Design emergency release and maintenance procedures
→ Review environmental, hygienic, and corrosion requirements
→ Perform repeated-life, impact, and power-loss testing

A properly designed locking device is a critical mechanical component for ensuring machine positioning accuracy, operational stability, maintainability, and operator safety.