Spring Terminology: A Glossary of Spring Design and Manufacturing Terms
This glossary covers the key terms used in spring design, spring specification and spring manufacturing. Whether you are writing a spring drawing for the first time, interpreting a supplier quotation or checking a spring calculation, these definitions will help you communicate clearly and accurately with our technical team.
If a term you need is not listed here, call our technical team on 01623 455500. We are happy to advise on any aspect of spring design and specification.
Spring Dimensions and Geometry
Free Length
The overall length of a compression or extension spring when not under any applied load. For compression springs, this is the length from which deflection is measured. For extension springs, the free length includes the body coils and the end hooks or loops.
Wire Diameter
The diameter of the wire from which the spring is coiled. Wire diameter is one of the primary determinants of spring stiffness and load capacity, along with coil diameter and number of active coils. Ashfield Springs manufactures from wire diameters of 0.1mm to 25.4mm.
Outer Diameter (OD)
The overall outside diameter of the spring coil. Relevant when a spring must fit inside a bore or housing. The outer diameter equals the mean coil diameter plus one wire diameter.
Inner Diameter (ID)
The inside diameter of the spring coil. Relevant when a spring must fit over a rod or shaft. The inner diameter equals the mean coil diameter minus one wire diameter.
Mean Coil Diameter (D)
The average diameter of the spring, measured to the centreline of the wire. Equal to (OD + ID) / 2. Used in spring rate and stress calculations.
Pitch
The distance between adjacent coils of a compression spring, measured from the centreline of one coil to the centreline of the next. Constant pitch means all coils are evenly spaced; variable pitch means the spacing changes along the length of the spring. Variable pitch can be used to produce a progressive (non-linear) spring rate.
Solid Height
The length of a compression spring when it is fully compressed so that all coils are in contact. Also called solid length. The difference between free length and solid height defines the maximum possible deflection. Springs should not normally be compressed to solid height in service.
Active Coils
The coils of a spring that deflect under load and contribute to the spring rate. For a compression spring with closed and ground ends, the end coils are inactive (they do not deflect); the active coil count is the total coils minus the inactive end coils.
Inactive Coils
The end coils of a compression spring that do not deflect under load. For closed and ground ends, typically the final coil at each end is inactive. Also called dead coils.
Slenderness Ratio
The ratio of a compression spring's free length to its mean coil diameter (L/D). Springs with a high slenderness ratio (greater than approximately 4) are susceptible to buckling under load and may require guidance over a rod or within a bore.
Spring Rate and Load
Spring Rate (k)
The rate of change of load with deflection, typically expressed as Newtons per millimetre (N/mm) or pounds per inch (lb/in). A spring with a rate of 10 N/mm will require 10 N of force for every 1mm of deflection. Also known as stiffness or the spring constant.
Deflection (f)
The change in length of a spring under an applied load. For a compression spring, deflection is the reduction in length from the free length. For an extension spring, deflection is the increase in length from the free length.
Load
The force applied to a spring. Typically specified at one or more defined lengths: for example, the load at a fitted (assembled) length and the load at a working length.
Initial Tension
Applicable to extension springs only. The initial tension is the force that must be overcome before the coils of a close-wound extension spring begin to separate. It is a result of the spring being wound with the coils in contact. Initial tension is not always desirable and can be controlled during manufacture.
Spring Index (C)
The ratio of mean coil diameter to wire diameter (D/d). Springs with a low index (less than 4) are difficult to manufacture and subject to high surface stresses; springs with a very high index (above 12–15) may be prone to tangling and handling difficulties. A spring index between 4 and 12 is generally preferred.
End Configurations
Closed Ends
A compression spring in which the pitch of the end coils is reduced until they touch the adjacent coil. Closed ends provide a bearing surface and reduce the tendency for the spring to buckle. Can be supplied ground or unground.
Closed and Ground Ends
Closed ends that have been ground flat and perpendicular to the spring axis. This provides the most accurate seating and is required when precise load measurement or good axial alignment is important. Ashfield Springs operates multiple CNC downfeed grinders to achieve length tolerances of ±0.25mm on springs up to 8mm wire diameter.
Open Ends
A compression spring in which the end coils maintain the same pitch as the body coils - no reduction in pitch at the ends. Less common than closed ends; used where the application does not require a flat bearing surface.
Machine Loop
The standard end type for an extension spring, formed automatically during coiling. A rounded loop formed in the last coil, aligned with the spring axis. This is sometimes called a German hook or loop.
Extended Loop
An extension spring end in which the loop extends beyond the last body coil, increasing the hook-to-hook working length.
Double Loop
An extension spring end with two loops for increased attachment area or strength. Specified where single machine loops are not adequate for the load.
Cross Centre Loop
An extension spring end in which the loop is offset 90 degrees from the machine loop orientation, allowing attachment in a different plane.
Leg (Torsion Spring)
The straight tangential section at each end of a torsion spring that connects to the adjacent component. Leg length, angle and orientation are key specification parameters for torsion springs.
Material and Condition
Hard Drawn Wire
Carbon steel spring wire drawn to its final diameter in the cold-drawn condition. The most common wire grade for general-purpose springs. Suitable for static and moderate cyclic applications at ambient temperatures.
Oil Tempered Wire
Carbon steel spring wire that has been heat treated (oil tempered and drawn) after patenting, giving higher tensile strength and improved fatigue performance compared to hard drawn wire. Preferred for more demanding cyclic applications.
Chrome Vanadium (CrV)
An alloy steel spring wire containing chromium and vanadium, offering higher strength and better fatigue resistance than carbon steel grades. Used in demanding cyclic applications and elevated temperature service.
Chrome Silicon (CrSi)
An alloy steel spring wire with higher chromium and silicon content, offering the highest strength and fatigue life of the common alloy spring steels. Used in valve springs and other high-stress, high-cycle applications.
Stainless Steel (302 / 304)
Austenitic stainless steel spring wire. Provides good corrosion resistance in most environments. 304 is broadly equivalent to 302 and is sometimes preferred for its wider availability. Not suitable for applications with high chloride exposure.
Stainless Steel 316
Austenitic stainless steel with added molybdenum, providing superior resistance to chloride corrosion. The preferred grade for food processing, washdown, marine and medical-adjacent applications. Specified by Ashfield Springs as standard for hygiene-critical applications.
Manufacturing Processes and Treatments
Shot Peening
A surface treatment process in which the spring is blasted with steel or glass shot (small beads). The impact induces compressive residual stresses in the surface layer of the wire, opposing the tensile stresses generated during spring operation. Shot peening significantly improves fatigue life and is specified for high-cycle, high-stress applications including valve springs and safety-critical assemblies.
Bead Peening
Similar in principle to shot peening but using spherical glass or ceramic beads rather than steel shot. Produces a smoother surface finish and is often preferred for springs in hygiene or aesthetic applications where surface condition is important.
Vibratory Deburring (Barrelling)
A mass-finishing process in which springs are tumbled with abrasive media in a vibratory bowl or drum. Removes burrs, sharp edges and surface contamination, improving surface condition and consistency. Often specified alongside plating or coating operations to improve adhesion and appearance.
Stress Relieving
A low-temperature heat treatment applied after coiling to reduce residual stresses introduced during the coiling process. Stress relieving improves the dimensional stability of the spring and reduces the risk of relaxation in service. Distinct from full heat treatment or tempering.
Presetting (Set Removal)
A process in which a compression spring is compressed to solid height one or more times after manufacture. This introduces beneficial residual stresses and permanently removes a small amount of set from the spring, resulting in a more stable free length in service. Springs that will be used at high stress levels are often preset to reduce relaxation in service.
Hydrogen Embrittlement Relief
Electroplated springs (particularly zinc-plated springs) can absorb hydrogen during the plating process, making the wire brittle and susceptible to cracking under load. A low-temperature bake treatment (stress relieve after plating) drives out the absorbed hydrogen and restores toughness. Ashfield Springs applies this treatment as standard to all zinc-plated springs.
Zinc Plating
An electroplated zinc coating providing corrosion protection for carbon steel springs. The most common finish for general-purpose springs. Typically supplied with a clear passivate or yellow passivate finish over the zinc. Ashfield Springs' stock range is manufactured from carbon steel with a bright zinc and clear passivate finish.
Delta Tone / Delta Seal
Water-based zinc flake coating systems that provide enhanced corrosion protection without the risk of hydrogen embrittlement associated with electroplating. Delta Tone is the base coat; Delta Seal is an additional topcoat that further improves corrosion resistance and provides a more uniform appearance. Widely specified for springs in automotive and outdoor applications.
Spring Types
Compression Spring
A helical spring designed to resist compressive forces along its axis. The most common spring type, used in an enormous range of applications. Can be cylindrical, conical, barrel-shaped, hourglass-shaped or tapered. Ashfield Springs manufactures compression springs from 0.1mm to 8.00mm wire diameter.
Extension Spring (Tension Spring)
A close-wound helical spring designed to resist tensile forces: that is, to resist being pulled apart. Extension springs typically have hooks or loops at each end for attachment. They often incorporate initial tension, the pre-load that must be overcome before the coils begin to separate.
Torsion Spring
A helical spring that works by twisting about its axis, applying a torque to attached components. Torsion springs are specified by wire diameter, coil diameter, number of coils, leg length and leg angle. Common applications include door handles, latch mechanisms, hinges and switches.
Wire Form
A bespoke metal component produced by bending, forming and shaping wire into a functional shape. Wire forms are not primarily springs. Instead they perform structural, retaining, contact or guiding functions in assemblies. Ashfield Springs manufactures wire forms from 0.1mm to 10mm wire diameter.
Hose Protection Spring
An open-coiled compression spring fitted around a hydraulic or pneumatic hose at its end fitting to prevent kinking, abrasion and fatigue failure. Also known as a hose guard spring. Ashfield Springs is the only UK spring manufacturer with BFPA membership and manufactures hose protection springs to customer specification.
Die Spring
A heavy-duty rectangular wire compression spring manufactured to ISO 10243, designed for use in press tool and die applications requiring high loads in compact installed heights. Colour-coded by load rate (yellow, blue, red, green).
For technical advice on spring design, material selection or specification, contact our team on 01623 455500 or email sales@ashfield-springs.com. We offer free technical guidance to all customers and prospective customers.
Request a sample or quotation: Call 01623 455500 or email sales@ashfield-springs.com
Spring Dimensions and Geometry
Free Length
The overall length of a compression or extension spring when not under any applied load. For compression springs, this is the length from which deflection is measured. For extension springs, the free length includes the body coils and the end hooks or loops.
Wire Diameter
The diameter of the wire from which the spring is coiled. Wire diameter is one of the primary determinants of spring stiffness and load capacity, along with coil diameter and number of active coils. Ashfield Springs manufactures from wire diameters of 0.1mm to 25.4mm.
Outer Diameter (OD)
The overall outside diameter of the spring coil. Relevant when a spring must fit inside a bore or housing. The outer diameter equals the mean coil diameter plus one wire diameter.
Inner Diameter (ID)
The inside diameter of the spring coil. Relevant when a spring must fit over a rod or shaft. The inner diameter equals the mean coil diameter minus one wire diameter.
Mean Coil Diameter (D)
The average diameter of the spring, measured to the centreline of the wire. Equal to (OD + ID) / 2. Used in spring rate and stress calculations.
Pitch
The distance between adjacent coils of a compression spring, measured from the centreline of one coil to the centreline of the next. Constant pitch means all coils are evenly spaced; variable pitch means the spacing changes along the length of the spring. Variable pitch can be used to produce a progressive (non-linear) spring rate.
Solid Height
The length of a compression spring when it is fully compressed so that all coils are in contact. Also called solid length. The difference between free length and solid height defines the maximum possible deflection. Springs should not normally be compressed to solid height in service.
Active Coils
The coils of a spring that deflect under load and contribute to the spring rate. For a compression spring with closed and ground ends, the end coils are inactive (they do not deflect); the active coil count is the total coils minus the inactive end coils.
Inactive Coils
The end coils of a compression spring that do not deflect under load. For closed and ground ends, typically the final coil at each end is inactive. Also called dead coils.
Slenderness Ratio
The ratio of a compression spring's free length to its mean coil diameter (L/D). Springs with a high slenderness ratio (greater than approximately 4) are susceptible to buckling under load and may require guidance over a rod or within a bore.
Spring Rate and Load
Spring Rate (k)
The rate of change of load with deflection, typically expressed as Newtons per millimetre (N/mm) or pounds per inch (lb/in). A spring with a rate of 10 N/mm will require 10 N of force for every 1mm of deflection. Also known as stiffness or the spring constant.
Deflection (f)
The change in length of a spring under an applied load. For a compression spring, deflection is the reduction in length from the free length. For an extension spring, deflection is the increase in length from the free length.
Load
The force applied to a spring. Typically specified at one or more defined lengths: for example, the load at a fitted (assembled) length and the load at a working length.
Initial Tension
Applicable to extension springs only. The initial tension is the force that must be overcome before the coils of a close-wound extension spring begin to separate. It is a result of the spring being wound with the coils in contact. Initial tension is not always desirable and can be controlled during manufacture.
Spring Index (C)
The ratio of mean coil diameter to wire diameter (D/d). Springs with a low index (less than 4) are difficult to manufacture and subject to high surface stresses; springs with a very high index (above 12–15) may be prone to tangling and handling difficulties. A spring index between 4 and 12 is generally preferred.
End Configurations
Closed Ends
A compression spring in which the pitch of the end coils is reduced until they touch the adjacent coil. Closed ends provide a bearing surface and reduce the tendency for the spring to buckle. Can be supplied ground or unground.
Closed and Ground Ends
Closed ends that have been ground flat and perpendicular to the spring axis. This provides the most accurate seating and is required when precise load measurement or good axial alignment is important. Ashfield Springs operates multiple CNC downfeed grinders to achieve length tolerances of ±0.25mm on springs up to 8mm wire diameter.
Open Ends
A compression spring in which the end coils maintain the same pitch as the body coils - no reduction in pitch at the ends. Less common than closed ends; used where the application does not require a flat bearing surface.
Machine Loop
The standard end type for an extension spring, formed automatically during coiling. A rounded loop formed in the last coil, aligned with the spring axis. This is sometimes called a German hook or loop.
Extended Loop
An extension spring end in which the loop extends beyond the last body coil, increasing the hook-to-hook working length.
Double Loop
An extension spring end with two loops for increased attachment area or strength. Specified where single machine loops are not adequate for the load.
Cross Centre Loop
An extension spring end in which the loop is offset 90 degrees from the machine loop orientation, allowing attachment in a different plane.
Leg (Torsion Spring)
The straight tangential section at each end of a torsion spring that connects to the adjacent component. Leg length, angle and orientation are key specification parameters for torsion springs.
Material and Condition
Hard Drawn Wire
Carbon steel spring wire drawn to its final diameter in the cold-drawn condition. The most common wire grade for general-purpose springs. Suitable for static and moderate cyclic applications at ambient temperatures.
Oil Tempered Wire
Carbon steel spring wire that has been heat treated (oil tempered and drawn) after patenting, giving higher tensile strength and improved fatigue performance compared to hard drawn wire. Preferred for more demanding cyclic applications.
Chrome Vanadium (CrV)
An alloy steel spring wire containing chromium and vanadium, offering higher strength and better fatigue resistance than carbon steel grades. Used in demanding cyclic applications and elevated temperature service.
Chrome Silicon (CrSi)
An alloy steel spring wire with higher chromium and silicon content, offering the highest strength and fatigue life of the common alloy spring steels. Used in valve springs and other high-stress, high-cycle applications.
Stainless Steel (302 / 304)
Austenitic stainless steel spring wire. Provides good corrosion resistance in most environments. 304 is broadly equivalent to 302 and is sometimes preferred for its wider availability. Not suitable for applications with high chloride exposure.
Stainless Steel 316
Austenitic stainless steel with added molybdenum, providing superior resistance to chloride corrosion. The preferred grade for food processing, washdown, marine and medical-adjacent applications. Specified by Ashfield Springs as standard for hygiene-critical applications.
Manufacturing Processes and Treatments
Shot Peening
A surface treatment process in which the spring is blasted with steel or glass shot (small beads). The impact induces compressive residual stresses in the surface layer of the wire, opposing the tensile stresses generated during spring operation. Shot peening significantly improves fatigue life and is specified for high-cycle, high-stress applications including valve springs and safety-critical assemblies.
Bead Peening
Similar in principle to shot peening but using spherical glass or ceramic beads rather than steel shot. Produces a smoother surface finish and is often preferred for springs in hygiene or aesthetic applications where surface condition is important.
Vibratory Deburring (Barrelling)
A mass-finishing process in which springs are tumbled with abrasive media in a vibratory bowl or drum. Removes burrs, sharp edges and surface contamination, improving surface condition and consistency. Often specified alongside plating or coating operations to improve adhesion and appearance.
Stress Relieving
A low-temperature heat treatment applied after coiling to reduce residual stresses introduced during the coiling process. Stress relieving improves the dimensional stability of the spring and reduces the risk of relaxation in service. Distinct from full heat treatment or tempering.
Presetting (Set Removal)
A process in which a compression spring is compressed to solid height one or more times after manufacture. This introduces beneficial residual stresses and permanently removes a small amount of set from the spring, resulting in a more stable free length in service. Springs that will be used at high stress levels are often preset to reduce relaxation in service.
Hydrogen Embrittlement Relief
Electroplated springs (particularly zinc-plated springs) can absorb hydrogen during the plating process, making the wire brittle and susceptible to cracking under load. A low-temperature bake treatment (stress relieve after plating) drives out the absorbed hydrogen and restores toughness. Ashfield Springs applies this treatment as standard to all zinc-plated springs.
Zinc Plating
An electroplated zinc coating providing corrosion protection for carbon steel springs. The most common finish for general-purpose springs. Typically supplied with a clear passivate or yellow passivate finish over the zinc. Ashfield Springs' stock range is manufactured from carbon steel with a bright zinc and clear passivate finish.
Delta Tone / Delta Seal
Water-based zinc flake coating systems that provide enhanced corrosion protection without the risk of hydrogen embrittlement associated with electroplating. Delta Tone is the base coat; Delta Seal is an additional topcoat that further improves corrosion resistance and provides a more uniform appearance. Widely specified for springs in automotive and outdoor applications.
Spring Types
Compression Spring
A helical spring designed to resist compressive forces along its axis. The most common spring type, used in an enormous range of applications. Can be cylindrical, conical, barrel-shaped, hourglass-shaped or tapered. Ashfield Springs manufactures compression springs from 0.1mm to 8.00mm wire diameter.
Extension Spring (Tension Spring)
A close-wound helical spring designed to resist tensile forces: that is, to resist being pulled apart. Extension springs typically have hooks or loops at each end for attachment. They often incorporate initial tension, the pre-load that must be overcome before the coils begin to separate.
Torsion Spring
A helical spring that works by twisting about its axis, applying a torque to attached components. Torsion springs are specified by wire diameter, coil diameter, number of coils, leg length and leg angle. Common applications include door handles, latch mechanisms, hinges and switches.
Wire Form
A bespoke metal component produced by bending, forming and shaping wire into a functional shape. Wire forms are not primarily springs. Instead they perform structural, retaining, contact or guiding functions in assemblies. Ashfield Springs manufactures wire forms from 0.1mm to 10mm wire diameter.
Hose Protection Spring
An open-coiled compression spring fitted around a hydraulic or pneumatic hose at its end fitting to prevent kinking, abrasion and fatigue failure. Also known as a hose guard spring. Ashfield Springs is the only UK spring manufacturer with BFPA membership and manufactures hose protection springs to customer specification.
Die Spring
A heavy-duty rectangular wire compression spring manufactured to ISO 10243, designed for use in press tool and die applications requiring high loads in compact installed heights. Colour-coded by load rate (yellow, blue, red, green).
For technical advice on spring design, material selection or specification, contact our team on 01623 455500 or email sales@ashfield-springs.com. We offer free technical guidance to all customers and prospective customers.
Request a sample or quotation: Call 01623 455500 or email sales@ashfield-springs.com