1. Overview
Elastic and return mechanisms are mechanical systems that deform under an external load, store energy, and release the stored energy when the load is removed to return a component or mechanism to its original or designated position.
Elastic and return functions are implemented through mechanical elements such as:
Compression coil springs
Extension coil springs
Torsion springs
Leaf springs
Disc springs
Wave springs
Constant-force springs
Gas springs
Elastomers
Rubber bushings
Flexure hinges
Torsion bars
Pneumatic or hydraulic energy-storage elements
Gravity-return mechanisms
Counterweights
Magnetic return mechanisms
The principal functions of elastic and return mechanisms include:
Returning moving components to their original position
Maintaining contact force
Absorbing assembly tolerances
Reducing shock and vibration
Providing positioning preload
Eliminating clearance and backlash
Compensating for load variation
Preventing crushing of products or components
Allowing displacement during overload
Storing and releasing energy
Returning equipment to a safe position during power loss
Supporting repeated opening, closing, and actuation
Typical applications include:
Pushbuttons and switches
Valve-return devices
Doors and covers
Clamps and latches
Robot grippers
Automotive suspension systems
Presses and molds
Packaging machinery
Food-processing machinery
Machine tools
Electrical contacts
Precision-positioning mechanisms
Shock-absorption devices
Safety brakes
Pneumatic and hydraulic systems
Elastic mechanisms are structurally simple, but improper design may result in fatigue failure, permanent deformation, resonance, buckling, excessive contact force, or unreliable return motion.
2. Basic Concepts of Elasticity
2.1 Elasticity
Elasticity is the property of a material or component to return to its original shape after the external force causing deformation is removed.
2.2 Elastic Deformation
Elastic deformation is deformation from which the material can recover after the load is removed.
2.3 Plastic Deformation
Plastic deformation is permanent deformation that remains after the material has exceeded its elastic limit and the load has been removed.
Elastic mechanisms must be designed so that plastic deformation does not occur within the normal operating range.
2.4 Restoring Force
Restoring force is the force generated by a deformed elastic element as it attempts to return to its original state.
For a linear elastic element:
F = kx
Where:
F: Restoring force
k: Spring rate
x: Displacement
3. Principal Functions of Elastic and Return Mechanisms
3.1 Position Return
Returns a button, valve, lever, clamp, or slider to its initial position.
3.2 Contact-Force Maintenance
Maintains a consistent contact force despite dimensional variation and wear.
3.3 Preload
Applies an initial load to bearings, gears, ball screws, guides, and electrical contacts.
3.4 Shock Absorption
Converts impact energy generated over a short period into elastic deformation energy.
3.5 Vibration Isolation
Reduces vibration transmitted from the environment to the machine or from the machine to its surroundings.
3.6 Tolerance Compensation
Absorbs dimensional variation, assembly error, thermal expansion, and wear through elastic deformation.
3.7 Safety Return
Uses spring force to move a device to a safe position during loss of electrical or fluid power.
4. Relationship Between Load and Displacement
4.1 Linear Springs
A linear spring has a proportional relationship between load and displacement.
F = kx
Its constant stiffness makes analysis and control relatively straightforward.
4.2 Nonlinear Springs
A nonlinear spring does not have a proportional relationship between load and displacement.
Representative types include:
Conical coil springs
Variable-pitch springs
Disc springs
Rubber springs
Cam-and-spring mechanisms
Link-and-spring mechanisms
4.3 Progressive Stiffness
Progressive stiffness increases as displacement increases.
It is useful for shock absorption and prevention of excessive displacement.
4.4 Regressive Stiffness
With regressive stiffness, the incremental increase in force becomes smaller as displacement increases.
It can be used to maintain a relatively constant operating force over a specified range.
4.5 Negative Stiffness
Negative stiffness occurs when force decreases as displacement increases within a certain range.
It is used in snap-action, bistable, and rapid-switching mechanisms.
5. Spring Rate
Spring rate is the force required to produce a unit displacement.
k = F / x
Typical units include:
N/mm
N/m
N·mm/rad
N·m/rad
A high spring rate produces a large force with a small displacement. A low spring rate permits a larger displacement with a softer response.
5.1 Factors in Selecting Spring Rate
Required restoring force
Permissible stroke
Workpiece deformation
Shock-energy absorption
Response speed
Installation space
Natural frequency
Repeated service life
Manufacturing tolerances
6. Series and Parallel Spring Combinations
6.1 Series Connection
When springs are connected in series, total stiffness decreases.
1 / k_total = 1 / k₁ + 1 / k₂ + ··· + 1 / kₙ
If two identical springs are connected in series, the total stiffness is half the stiffness of each spring.
6.2 Parallel Connection
When springs are connected in parallel, total stiffness increases.
k_total = k₁ + k₂ + ··· + kₙ
6.3 Applications
Series combinations are used when a long stroke and low stiffness are required.
Parallel combinations are used when a large restoring force is required within limited space.
7. Elastic Energy
The elastic energy stored in a linear spring is:
U = 1/2 kx²
or:
U = 1/2 Fx
Where:
U: Stored energy
k: Spring rate
x: Displacement
F: Final load
If displacement is doubled, the stored energy becomes four times greater.
If an elastic mechanism is suddenly released, the stored energy may be discharged rapidly and create impact or ejection hazards.
8. Preload
8.1 Definition
Preload is the initial load applied to an elastic element before the mechanism begins normal operation.
8.2 Purposes
Eliminate initial clearance
Maintain contact
Reduce vibration
Prevent separation
Eliminate an unloaded section at the beginning of motion
Improve return reliability
Stabilize electrical contact
8.3 Force of a Preloaded Spring
If the initial compression is (x_0) and the additional displacement is (x):
F = k(x₀ + x)
8.4 Caution
Excessive preload may cause:
Increased operating force
Increased friction and wear
Workpiece deformation
Reduced fatigue life
Heat generation
Reduced efficiency
9. Compression Coil Springs
9.1 Definition
A compression coil spring carries an axial compressive load and returns toward its original length after compression.
9.2 Main Shapes
Cylindrical
Conical
Barrel-shaped
Hourglass-shaped
Variable-pitch
Double-coil
9.3 Typical Applications
Button return
Valve return
Clamp preload
Molds
Suspension
Shock absorption
Contact-force maintenance
9.4 Advantages
Simple structure
Wide availability of standard products
Long stroke capability
Relatively easy stiffness design
Suitable for repeated operation
9.5 Disadvantages
May buckle under compression
Noise from coil contact
Torsional stress in the wire
Possible surge during high-speed operation
Requires installation space
10. Stiffness of Compression Coil Springs
The stiffness of a compression coil spring made from round wire can be approximated by:
k = Gd⁴ / 8D³n
Where:
G: Shear modulus
d: Wire diameter
D: Mean coil diameter
n: Number of active coils
Stiffness is proportional to the fourth power of wire diameter and inversely proportional to the third power of mean coil diameter.
A small change in wire diameter therefore has a large effect on stiffness.
11. Spring Index
The spring index is the ratio of mean coil diameter to wire diameter.
C = D / d
If the spring index is too small, the following problems may occur:
Difficult manufacturing
High stress concentration
Forming cracks
Reduced fatigue life
If the spring index is too large, the following problems may occur:
Increased buckling risk
Shape instability
Large installation space
Deformation under external loading
12. Solid Height of Compression Springs
The length of a spring when all coils are in contact is called the solid height.
The design stroke should include sufficient clearance so that the spring is not repeatedly compressed fully to solid height.
Repeated solid compression may cause:
Impact loading
Surface damage
Permanent set
Reduced fatigue life
Damage to spring seats
13. Buckling of Compression Springs
A long, slender compression spring may deflect sideways under compression.
Buckling risk is influenced by:
Free length
Coil diameter
End-support conditions
Parallelism of spring seats
Presence of guides
Eccentric loading
13.1 Methods for Preventing Buckling
Use a guide rod.
Use a guide tube.
Reduce spring length.
Increase spring diameter.
Align both spring seats.
Use several shorter springs.
Use barrel-shaped or conical springs.
Appropriate clearance between the guide and spring is required to accommodate thermal expansion, deformation, and dimensional tolerances.
14. Extension Coil Springs
14.1 Definition
An extension coil spring carries an axial tensile load and pulls connected components toward each other through hooks or loops at both ends.
14.2 Main Characteristics
May have initial tension
Restoring force increases with extension
Relatively easy to install
Suitable for tensile-direction return
14.3 Typical Applications
Door return
Lever return
Link mechanisms
Brake return
Cover retention
Belt-tension adjustment
14.4 Main Failure Locations
Stress concentration in the hooks and loops may be greater than in the spring body.
Fatigue analysis of hook geometry and connecting features is important.
15. Initial Tension of Extension Springs
An extension spring may have initial tension even when its coils are closed.
Its force can be expressed as:
F = F_i + kx
Where:
(F_i): Initial tension
k: Spring rate
x: Extension
Initial tension is useful for eliminating clearance and maintaining tension, but unexpected force may occur during assembly or release.
16. Torsion Springs
16.1 Definition
A torsion spring deforms rotationally about its coil axis and generates restoring torque.
16.2 Basic Relationship
T = k_θ θ
Where:
T: Restoring torque
(k_θ): Torsional spring rate
θ: Rotation angle
16.3 Typical Applications
Hinges
Door return
Lever return
Clips
Latches
Switches
Tongs and grippers
16.4 Design Considerations
Coil winding direction
Operating rotation direction
Leg length
Support-shaft diameter
Maximum rotation angle
Friction
Change in coil diameter
Stress in the legs
A torsion spring is generally more stable when operated in the direction that further winds the coil.
17. Leaf Springs
17.1 Definition
A leaf spring uses the bending elasticity of a thin, elongated strip to generate restoring force.
17.2 Main Types
Single leaf spring
Laminated leaf spring
Cantilever leaf spring
Curved leaf spring
Snap leaf spring
Electrical-contact leaf spring
17.3 Advantages
Spring and structural functions can be integrated
Low installation height
Suitable for electrical contacts
High flexibility in a specified direction
High geometric design freedom
17.4 Disadvantages
Stress concentration at the fixed end
Limited large-displacement capability
Sensitivity to surface defects
Repeated bending fatigue
Vulnerability to torsion and lateral loading
18. Cantilever Leaf Springs
For a simple cantilever leaf spring of length (L) with an end load (F), displacement is:
δ = FL³ / 3EI
Where:
E: Elastic modulus
I: Second moment of area
The stiffness is:
k = 3EI / L³
As length increases, stiffness decreases in proportion to the cube of length.
19. Disc Springs
19.1 Definition
A disc spring is a conical washer-shaped spring that produces a large axial force within a small axial space.
19.2 Characteristics
High force over a small displacement
High axial stiffness
Can be combined in series or parallel
Can provide nonlinear load characteristics
Suitable for maintaining preload
19.3 Typical Applications
Bearing preload
Bolt-force retention
Clutches
Brakes
Molds
Safety valves
Tool clamping
19.4 Combination Methods
Stacking disc springs in the same direction creates a parallel arrangement and increases load capacity.
Alternating their direction creates a series arrangement and increases stroke.
20. Wave Springs
20.1 Definition
A wave spring uses a circular ring or wire formed into a wave shape to provide axial elasticity.
20.2 Advantages
Small axial space
Low solid height
Suitable for bearing preload
Relatively uniform load
Potential for weight reduction
20.3 Applications
Bearings
Seals
Connectors
Valves
Compact mechanical devices
Electrical and electronic equipment
21. Constant-Force Springs
21.1 Definition
A constant-force spring produces a relatively constant force over a wide displacement range.
21.2 Main Types
Wound constant-force springs
Negator springs
Constant-torque springs
Tape springs
21.3 Typical Applications
Cable retractors
Height-adjustment devices
Windows and doors
Medical equipment
Display arms
Product-ejection devices
21.4 Advantages
Long stroke
Relatively constant restoring force
Compact installation
Suitable for counterbalancing
21.5 Disadvantages
Fatigue at the winding section
Hazard from sharp edges
Sensitivity to alignment
Risk of separation or twisting
22. Torsion Bars
22.1 Definition
A torsion bar uses the torsional elasticity of a shaft or rod to generate restoring torque.
22.2 Basic Relationship
The angle of twist of a circular shaft is:
θ = TL / GJ
The torsional stiffness is:
k_θ = GJ / L
Where:
T: Torque
L: Effective length
G: Shear modulus
J: Polar moment of inertia
22.3 Applications
Automotive suspension
Hinges
Rotary return systems
Balancing mechanisms
Torque sensors
Shock-absorption systems
23. Gas Springs
23.1 Definition
A gas spring uses compressed gas pressure inside a sealed cylinder to generate axial force.
23.2 Main Components
Cylinder
Piston
Piston rod
Seals
Gas
Oil
End fittings
23.3 Advantages
High force
Long stroke
Relatively gradual force variation
Can incorporate damping
Assists opening and closing of doors and covers
23.4 Disadvantages
Force changes with temperature
Seal wear and gas leakage
Force reduction over service life
Replacement is more common than repair
Sensitive to piston-rod damage
23.5 Applications
Machine covers
Automotive hoods
Equipment doors
Height-adjustment devices
Medical equipment
Chairs
Inspection panels
24. Force Characteristics of Gas Springs
Gas-spring force is primarily determined by internal pressure and piston-rod cross-sectional area.
F ≈ P A_r
Where:
P: Internal gas pressure
(A_r): Piston-rod cross-sectional area
Actual force also includes seal friction and temperature effects.
As gas temperature rises, internal pressure and output force may increase.
25. Elastomer Springs
25.1 Definition
An elastomer spring uses polymeric elastic materials such as rubber, urethane, or silicone to support loads and generate restoring force.
25.2 Advantages
Provides both elasticity and damping
Good shock absorption
Noise reduction
Can be produced in many shapes
Prevents metal-to-metal contact
Corrosion-resistant materials are available
25.3 Disadvantages
Nonlinear behavior
Temperature dependence
Creep
Stress relaxation
Chemical sensitivity
Aging and hardening
Difficult analysis
25.4 Applications
Vibration-isolation mounts
Bumpers
Stoppers
Bushings
Couplings
Seals
Product-protection pads
Compliant food-contact components
26. Compression and Shear Use of Rubber
Rubber elements can be used in compression or shear.
26.1 Compression Use
High load capacity
Relatively high stiffness
Suitable for bumpers and stoppers
26.2 Shear Use
Lower stiffness
Allows larger displacement
Suitable for vibration isolation
26.3 Caution
Rubber expands laterally under compression. If fully confined, it may exhibit much higher stiffness than expected.
27. Flexure Mechanisms
27.1 Definition
A flexure mechanism uses elastic deformation of material to produce limited motion without pins, bearings, or sliders.
27.2 Main Types
Flexure hinges
Cross-spring pivots
Parallel leaf-spring guides
Notch hinges
Diaphragm flexures
Blade flexures
27.3 Advantages
No backlash
No lubrication required
Very low friction
High repeatability
Low particle generation
Can be manufactured as a monolithic component
27.4 Disadvantages
Limited range of motion
Stress concentration
Limited fatigue life
Sensitivity to load direction
Performance variation caused by manufacturing error
Failure under excessive displacement
27.5 Applications
Precision positioning
Optical equipment
Semiconductor equipment
Sensors
Fine-adjustment mechanisms
Medical equipment
Vacuum equipment
28. Flexure Hinges
A flexure hinge provides rotational motion through bending of a locally thinned section.
28.1 Main Design Elements
Minimum thickness
Notch radius
Hinge width
Material
Rotation angle
Axial load
Fatigue life
Stress concentration
28.2 Advantages
Highly repeatable center of rotation
No backlash
Reduced part count
No lubrication required
28.3 Caution
Maximum stress increases rapidly as the rotation angle becomes larger.
29. Diaphragms and Bellows
29.1 Diaphragms
A diaphragm transfers pressure and displacement through elastic deformation of a thin plate or membrane.
Typical applications:
Pressure sensors
Valves
Pumps
Actuators
Pressure regulators
29.2 Bellows
A bellows is a corrugated cylindrical structure that expands and contracts axially.
Typical applications:
Vacuum equipment
Shaft seals
Thermal-expansion compensation
Pressure sensors
Precision actuators
29.3 Main Design Considerations
Pressure
Axial displacement
Repeated service life
Buckling
Welded joints
Corrosion
Temperature
Leak tightness
30. Gravity-Return Mechanisms
30.1 Definition
A gravity-return mechanism uses the weight of a component or a counterweight to return the system to its original position.
30.2 Advantages
No separate spring required
Simple structure
Low risk of fatigue failure
Long stroke possible
30.3 Disadvantages
Orientation dependent
Increased moving mass
Sensitive to acceleration changes
Return force varies with position
Limited high-speed capability
30.4 Applications
Flaps
Gravity chutes
Check doors
Valves
Safety covers
Self-returning levers
31. Counterweights
A counterweight balances the weight of a moving component and reduces required drive and return forces.
The balancing condition can be expressed through moment equilibrium:
W₁L₁ = W₂L₂
Counterweights are used in elevators, doors, covers, robot arms, and large manual mechanisms.
31.1 Caution
Counterweight separation
Wire-rope or chain failure
Collision
Guide wear
Maintenance access
Fall prevention
32. Magnetic Return Mechanisms
Permanent magnets or electromagnets can generate restoring or holding force through attraction or repulsion.
32.1 Advantages
Non-contact force transmission
Reduced friction and wear
Sealed construction possible
Suitable for clean environments
32.2 Disadvantages
Nonlinear force characteristics
Sensitive to air gap
Magnetic strength changes with temperature
Influence of external magnetic fields
Attraction of metallic dust
32.3 Applications
Magnetic latches
Non-contact return devices
Sensor mechanisms
Precision positioning
Vacuum equipment
33. Pneumatic Return Mechanisms
A pneumatic return mechanism uses a pneumatic cylinder or air spring to generate return force.
33.1 Methods
Spring-return single-acting cylinder
Pneumatic-pressure return
Air cushioning
Air spring
Pneumatic balancing circuit
33.2 Advantages
Adjustable force
Long stroke
Easy automation
Can incorporate cushioning
33.3 Disadvantages
Compressibility of air
Dependence on supply pressure
Leakage
Temperature effects
Possible return failure during pressure loss
34. Hydraulic Accumulator Return Mechanisms
Hydraulic pressure energy stored in an accumulator or circuit can be used for return motion or emergency actuation.
34.1 Applications
Emergency return
Pressure maintenance
Shock absorption
Pulsation reduction
Brake actuation
Valve return
34.2 Caution
A hydraulic accumulator may retain substantial energy even after electrical power is switched off.
Pressure must be released safely before maintenance.
35. Snap-Action Mechanisms
35.1 Definition
A snap-action mechanism rapidly transitions from one stable state to another when an elastic element passes a critical point.
35.2 Characteristics
Rapid state transition
Clear tactile click
Reduced contact chatter
Fast output motion from a small input
Two stable states can be created
35.3 Applications
Electrical switches
Snap domes
Latches
Safety covers
Temperature switches
Bimetal devices
35.4 Caution
Hysteresis may occur because the critical switching load and return load differ.
36. Bistable Elastic Mechanisms
A bistable mechanism maintains either of two stable positions without continuous energy input.
Representative methods include:
Snap plates
Over-center springs
Buckled beams
Magnet-and-spring combinations
Bistable flexure mechanisms
36.1 Advantages
No energy required to maintain either state
Clear position switching
Compact design possible
36.2 Disadvantages
Switching impact
Nonlinear analysis required
Sensitivity to fatigue and buckling
Strong influence of assembly tolerances
37. Arrangement of Return Mechanisms
The position and direction of spring force strongly affect mechanism force, torque, and stability.
37.1 Inline Arrangement
The spring acts along the direction of motion.
Advantages:
Simple load path
Low lateral loading
Easy calculation
37.2 Eccentric Arrangement
Spring force acts at a distance from the rotational axis and generates restoring torque.
T = Fr
Where:
F: Spring force
r: Distance from the rotational axis to the force line of action
37.3 Combination with Linkages
Spring displacement and output torque vary nonlinearly with linkage angle.
37.4 Combination with Cams
A cam profile can be used to create a desired force–displacement relationship.
38. Combination of Springs and Levers
Combining a lever and spring can produce a large output displacement from a small spring displacement, or convert a large spring force into a smaller output force.
Torque equilibrium about the pivot is:
F_s L_s = F_o L_o
Where:
(F_s): Spring force
(L_s): Spring moment arm
(F_o): Output force
(L_o): Output moment arm
As lever angle changes, the effective moment arms also change, so the actual output may be nonlinear.
39. Adjusting Restoring Force with Cams
A cam-and-spring arrangement can control restoring force according to position.
Typical objectives include:
Constant operating force
Low force at the beginning and high force near the end
Dead-center passage
Over-center locking
Soft closing
Rapid return
Cam profiles must be designed together with contact stress, friction, roller size, and manufacturing tolerances.
40. Return Speed and Dynamic Behavior
A larger restoring force does not always provide better return performance.
Return speed depends on:
Spring rate
Moving mass
Friction
Damping
Initial displacement
Air resistance
Impact stoppers
Fluid resistance
The natural frequency of a mass–spring system can be approximated as:
f_n = 1 / 2π × √(k / m)
Where:
(f_n): Natural frequency
k: Stiffness
m: Effective mass
Resonance may occur when the operating frequency approaches the natural frequency.
41. Damping
Damping dissipates mechanical energy as heat or another form of energy and reduces vibration and repeated motion.
41.1 Main Damping Methods
Viscous damping
Friction damping
Internal damping of rubber
Pneumatic damping
Hydraulic damping
Eddy-current damping
41.2 Relationship Between Springs and Dampers
A spring stores and returns energy, while a damper dissipates energy.
Springs and dampers are often used together to provide fast and stable return.
41.3 Excessive Damping
If damping is too high, return motion may become slow or incomplete.
42. Spring Surge and Resonance
When a coil spring operates repeatedly at high speed, elastic waves may travel through the coils and produce spring surge.
Spring surge may cause:
Spring vibration
Unstable force
Noise
Coil collision
Fatigue failure
42.1 Prevention Methods
Change the operating frequency.
Reduce spring mass.
Use variable pitch.
Add damping.
Apply internal or external guides.
Change the spring geometry.
43. Friction and Return Failure
If friction exceeds restoring force, the mechanism may fail to return to its original position.
The general return condition is:
F_return > F_friction + F_external
Main sources of friction include:
Guide misalignment
Seal friction
Bushing friction
Contamination
Corrosion
Insufficient lubrication
Side loading
Assembly deformation
The minimum restoring force should be sufficiently greater than the maximum expected friction force.
44. Hysteresis
Hysteresis is the difference in force at the same displacement during loading and unloading.
Causes include:
Internal material damping
Friction
Viscoelasticity of rubber
Linkage clearance
Sliding at contact surfaces
Variation in magnetic force
Hysteresis can help reduce shock and vibration, but it creates error in precise force control and position return.
45. Creep and Stress Relaxation
45.1 Creep
Creep is the gradual increase in deformation under a constant load over time.
45.2 Stress Relaxation
Stress relaxation is the gradual reduction of restoring force under constant deformation.
45.3 Materials Strongly Affected
Rubber
Urethane
Plastics
Metal springs operating at high temperature
45.4 Countermeasures
Reduce operating temperature.
Reduce initial deformation.
Select an appropriate material.
Provide preload allowance.
Periodically verify force.
Establish replacement intervals.
46. Spring Fatigue
Because springs experience repeated loading, fatigue strength may be more important than static strength.
46.1 Factors Affecting Fatigue
Stress amplitude
Mean stress
Number of cycles
Surface roughness
Corrosion
Scratches
Heat treatment
Residual stress
Operating temperature
Coil contact
46.2 Methods for Improving Fatigue Life
Reduce maximum stress.
Reduce stress range.
Apply shot peening.
Improve surface finish.
Use appropriate heat treatment.
Prevent corrosion.
Apply presetting.
Prevent coil collision.
Improve hook geometry.
47. Presetting and Scragging
Presetting or scragging is the process of compressing a spring beyond its normal operating load to intentionally create a small initial permanent deformation.
47.1 Purposes
Reduce dimensional change during use
Stabilize load characteristics
Improve allowable stress
Eliminate initial seating deformation
Excessive presetting may reduce effective stroke and fatigue life.
48. Spring Materials
48.1 Carbon-Steel Spring Wire
General mechanical use
Economical
High strength
Requires surface protection in corrosive environments
48.2 Music Wire
High tensile strength
Excellent fatigue performance
Suitable for precision small springs
48.3 Oil-Tempered Wire
Suitable for impact and repeated loading
Used for medium and large springs
48.4 Stainless Spring Steel
Corrosion resistance
Food machinery
Medical equipment
Wet environments
48.5 Heat-Resistant Alloys
High-temperature environments
Engines
Exhaust systems
Heat-treatment equipment
48.6 Copper Alloys
Electrical conductivity
Nonmagnetic properties
Electrical contacts
Corrosion resistance
49. Surface Treatment and Corrosion Protection
Typical surface treatments include:
Zinc plating
Nickel plating
Phosphate coating
Black oxide
Electrodeposition coating
Powder coating
Passivation
Organic coating
Shot peening
Surface treatment may improve corrosion resistance, but effects on hydrogen embrittlement, dimensions, and fatigue performance must be considered.
50. Spring Ends and Seats
The ends of compression springs should transfer load uniformly.
Common end types include:
Open ends
Open and ground ends
Closed ends
Closed and ground ends
Closed and ground ends are used when flatness is important.
Spring-seat design should consider:
Centering
Flatness
Wear
Rotation
Lubrication
Retention
51. Spring Guides
Spring guides prevent buckling, separation, and lateral motion.
51.1 Internal Guides
A guide rod passes through the spring.
51.2 External Guides
A guide tube surrounds the spring.
51.3 Design Considerations
Maximum outside diameter and minimum inside diameter
Thermal expansion
Coil deformation
Contamination
Friction
Lubrication
Assembly
Excessive friction between the guide and spring may cause unstable return force.
52. Redundant and Dual Springs
Dual springs can be used when function must be maintained after failure of one spring.
Representative arrangements include:
Concentric dual springs
Parallel multiple springs
Main and auxiliary springs
Spring and gravity-return combination
Spring and brake combination
Concentric dual springs can use opposite winding directions to reduce interference if one spring breaks.
53. Overtravel Protection
Mechanical limiting devices should prevent springs or flexures from deforming beyond their allowable range.
Typical methods include:
Stoppers
Guide shoulders
Limiting pins
Cables
Auxiliary links
Bumpers
Overload sensors
An overtravel device should preferably remain inactive during normal operation and engage only under abnormal conditions.
54. Fail-Safe Design for Elastic Elements
If a spring fails, fragments may be ejected or the mechanism may move suddenly.
Typical countermeasures include:
Protective covers
Safety rods inside springs
Retention cables
Dual springs
Failure-detection sensors
Motion-range limitations
Stored-energy release procedures
Scheduled replacement
Extension springs require particular caution because both connected components may move rapidly if the spring or hook fails.
55. Sensors and Condition Monitoring
The condition of elastic and return mechanisms can be monitored using:
Position sensors
Displacement sensors
Force sensors
Torque sensors
Strain gauges
Proximity sensors
Spring-failure sensors
Vibration sensors
Camera inspection
55.1 Monitoring Items
Return completion
Maximum displacement
Preload
Restoring force
Cycle time
Permanent set
Fatigue cracking
Return delay
Monitoring only the final return position may not reveal reduced restoring force. Force or displacement data may also be required.
56. Safety Design
56.1 Main Hazards
Sudden release of stored energy
Spring failure and ejection
Finger trapping
Extension-spring hook separation
Gas-spring failure
High-pressure gas leakage
Flexure fracture
Collision of returning components
Sudden movement during maintenance
56.2 Protective Measures
Protective covers
Stored-energy release procedures
Stoppers
Retention devices
Safety pins
Maintenance supports
Failure monitoring
Warning labels
Replacement-interval management
56.3 Maintenance Precautions
Preload and stored energy must be removed before disassembling elastic components.
Pressurized gas springs and accumulators must not be cut or drilled.
57. Elastic and Return Mechanisms in Food Machinery
Food machinery must consider hygiene, cleanability, corrosion resistance, and loose-part prevention together with elastic performance.
57.1 Main Requirements
Structures that prevent food-residue accumulation
Minimized exposed coils and deep gaps
Open, cleanable construction
Corrosion-resistant materials
Food-grade lubricants
Prevention of loose fragments after failure
Resistance to wash chemicals
Controlled tool-less disassembly
57.2 Recommended Methods
Stainless-steel springs
Sealed gas springs
Hygienic elastomers
Captive spring structures
Protective tubes preventing external contamination
Monolithic flexures
57.3 Caution
The inside of coil springs and hook connections may be difficult to clean. In direct food-contact zones, they should be sealed or located outside the hygienic area.
58. Elastic and Return Mechanisms in Packaging Machinery
Packaging machinery requires high-speed repeated operation and flexibility for product changeovers.
Typical applications include:
Sealing-jaw return
Cutter return
Film-tension arms
Guide preload
Product stoppers
Quick-change latches
Doors and covers
Sensor brackets
Main requirements include:
Rapid return
High cycle life
Low inertia
Return-complete detection
Prevention of product damage
Resistance to contamination and dust
59. Elastic and Return Mechanisms in Machine Tools
Machine tools require high stiffness, accuracy, fatigue life, and resistance to coolant.
Typical applications include:
Disc springs for tool clamping
Bearing preload
Ball-screw preload
Cover return
Chip-cover tensioning
Tool-changer return
Sensor contact mechanisms
Guide-clearance elimination
Tool-clamping springs experience high repeated loads. The overall clamping-force reduction should be monitored in addition to individual spring failure.
60. Applications in Robots and Automation Equipment
Typical applications include:
Gripper-finger return
Cable tensioners
Tool-changer locking
Collision-compliance mechanisms
Floating sensor mounts
Safety bumpers
Joint balancing
Emergency return
Design considerations include:
Repeated service life
Return time
Robot acceleration
Cable interference
Product drop after failure
Position sensing
Maintainability
61. Application Examples
61.1 Spring-Return Pneumatic Cylinder
Supply compressed air
→ Cylinder extends
→ Spring compresses
→ Air pressure is exhausted
→ Spring returns the cylinder
Main considerations:
Supply pressure
Spring force
Seal friction
Return speed
Safe position during pressure loss
Spring fatigue
61.2 Gas Spring for a Machine Cover
Open cover
→ Gas spring extends
→ Opening torque is assisted
→ Close cover
→ Gas is compressed
Main considerations:
Cover weight
Hinge location
Gas-spring installation angle
Opening torque
Initial closing force
Temperature variation
Service life
61.3 Disc Springs for Tool Clamping
Disc springs are preloaded
→ Drawbar is tensioned
→ Tool is clamped
→ Hydraulic or pneumatic pressure releases the tool
→ Pressure is removed and the tool is reclamped
Main considerations:
Clamping force
Disc-spring arrangement
Fatigue life
Spring failure
Unclamping stroke
Force monitoring
61.4 Precision Flexure Guide
Actuator input
→ Flexure deformation
→ Parallel displacement
→ Input removed
→ Return to original position
Main considerations:
Maximum stress
Range of motion
Parasitic motion
Repeatability
Fatigue
Thermal deformation
61.5 Film-Tension Arm
Film tension changes
→ Arm rotates
→ Spring deforms
→ Tension is compensated
→ Feed speed is stabilized
Main considerations:
Tension range
Arm inertia
Restoring torque
Damping
Film damage
Position sensor
62. Analysis and Design Tools
62.1 Static Analysis
Spring force
Displacement
Preload
Maximum stress
Contact pressure
Buckling
62.2 Fatigue Analysis
Mean stress
Stress amplitude
Number of cycles
Stress concentration
Surface condition
Corrosion effects
62.3 Dynamic Analysis
Natural frequency
Resonance
Return time
Impact velocity
Spring surge
Damping
62.4 Nonlinear Analysis
Disc springs
Rubber
Flexures
Large deformation
Contact
Snap-through
62.5 Motion Analysis
Displacement relationship between linkage and spring
Cam profile
Change in force application point
Restoring torque
Interference
Stopper contact
63. Selection Procedure for Elastic and Return Mechanisms
63.1 Define the Required Function
Return to the original position
Maintain contact force
Apply preload
Absorb shock
Isolate vibration
Compensate for tolerance
Provide safety return
Store energy
63.2 Define Loads and Motion
Linear force
Torque
Stroke
Rotation angle
Operating speed
Number of cycles
Impact
Installation orientation
63.3 Determine the Required Force Characteristics
Linear
Nonlinear
Progressive stiffness
Constant force
Snap action
Bistable behavior
63.4 Select the Elastic Element
Compression spring
Extension spring
Torsion spring
Leaf spring
Disc spring
Wave spring
Gas spring
Elastomer
Flexure
Torsion bar
63.5 Review Installation Space
Free length
Solid height
Outside and inside diameters
Guides
Motion path
Assembly space
Replacement space
63.6 Review Strength and Service Life
Maximum stress
Elastic limit
Fatigue
Buckling
Creep
Stress relaxation
Corrosion
Temperature
63.7 Review Dynamic Performance
Return time
Natural frequency
Resonance
Spring surge
Impact
Damping
Noise
63.8 Review Safety
Stored energy
Failure and ejection
Separation
Overtravel
Energy removal during maintenance
Redundancy
Failure monitoring
63.9 Review Environmental Conditions
Temperature
Humidity
Washdown
Chemicals
Corrosion
Vacuum
Food contact
Cleanroom
63.10 Testing and Validation
Load–displacement test
Restoring-force test
Repeated-life test
Permanent-set test
Temperature test
Corrosion test
Vibration test
Failure test
Return-time measurement
64. Conclusion
Elastic and return mechanisms store energy when deformed by an external force and use the stored energy to return a component or mechanism to its original or designated position after the force is removed.
Compression, extension, and torsion coil springs are the most common return elements. Leaf springs and flexures are advantageous when structural and elastic functions must be integrated. Disc and wave springs provide preload in restricted axial space, while constant-force springs generate a relatively uniform force over a long stroke.
Gas springs provide high force and long stroke and are suitable for assisting the opening and closing of covers and doors. Elastomers provide both elasticity and damping and are effective in reducing vibration and shock. Gravity, counterweights, and magnetic forces can provide return functions without conventional metal springs.
Elastic mechanisms must not be selected solely by restoring force. Stiffness, displacement, preload, stored energy, stress, fatigue, buckling, resonance, damping, creep, temperature, and friction must all be considered. In repeatedly operating mechanisms, stress range and fatigue life may be more important than maximum static stress.
If restoring force is lower than friction, return may be incomplete. If restoring force is excessive, operating force, impact, wear, and workpiece deformation increase. For rapid return, moving mass and damping must be considered together with spring stiffness.
Springs, gas springs, and accumulators may contain substantial stored energy. Preload and pressure must be safely removed before disassembly or maintenance. Covers, stoppers, and retention devices should be used to prevent fragments and sudden movement during failure.
A suitable design and selection sequence is:
Define the required function and safe position
→ Analyze load, torque, stroke, and rotation angle
→ Determine the required force–displacement characteristic
→ Select the elastic element and return method
→ Determine preload and installation space
→ Review stiffness, stress, and buckling
→ Review fatigue, creep, and temperature effects
→ Review moving mass, resonance, and damping
→ Design guides, stoppers, and retention devices
→ Review failure and stored-energy hazards
→ Review environmental, hygienic, and corrosion conditions
→ Perform load–displacement, return-time, and life-cycle tests
A properly designed elastic and return mechanism improves repeatability, response, shock absorption, stability, safety, and maintainability.