Compression springs are everywhere — inside valves, machine tool fixtures, door latches, industrial equipment, and workshop jigs. Most of the time you don't notice them until one fails and you need a replacement. That is when the selection process matters, and it is more specific than many people expect. The wrong free length, wire diameter, or spring rate will either make the spring useless or over-stress it into early failure.
This guide covers the four main types, the four end configurations, the six key dimensions you need to measure, how spring rate works, which material to choose, and when an assortment kit is smarter than specifying an individual spring. If you need to identify or replace a compression spring, start here.
For an overview of all spring types — extension, torsion, gas struts, leaf, Belleville disc and constant force springs — see our Types of Springs Guide. This guide focuses specifically on compression springs.
How Compression Springs Work
A compression spring is a helical coil of metal wire designed to resist compressive axial force. When a load is applied along the spring axis, the coils deflect — moving closer together — and the spring stores that energy elastically. Remove the load and the spring returns to its original length, releasing the stored energy as a push force.
The key characteristic is that the coils are open (spaced apart) in their free, unloaded state. This distinguishes compression springs from extension springs, where coils are tightly wound together, and torsion springs, which resist rotational force rather than axial compression.
The relationship between force and deflection is linear for most compression springs operating within their working range — apply twice the force, get twice the deflection. This linearity is expressed as the spring rate (also called spring constant or stiffness), measured in Newtons per millimetre (N/mm) or pounds per inch (lb/in).
Types of Compression Springs
Most stock compression springs are cylindrical helical springs — constant diameter from end to end. Three other profiles exist for specific applications where standard cylindrical springs have limitations.
Cylindrical (helical) compression springs
The standard type. Consistent coil diameter from end to end, predictable linear spring rate, easy to manufacture and stock. This is what most industrial suppliers, including AIMS, carry as standard assortment sizes. Suitable for the vast majority of maintenance and repair applications.
Conical compression springs
Cone-shaped: one end has a larger coil diameter that tapers to a smaller diameter at the other end. When fully compressed, each coil nests inside the next, achieving a solid height as low as a single wire diameter — far lower than a cylindrical spring of equivalent travel. Used where installed height is severely restricted, such as in valve seats, battery contacts, and circuit breakers. Conical springs also have inherently higher lateral stability and resist sideways buckling better than cylindrical springs of the same rate.
Barrel (convex) compression springs
Coil diameter is smallest at both ends and largest in the middle, like a barrel. The geometry reduces the tendency to buckle under load and provides a progressive spring rate — the rate increases as the spring is compressed because the outer coils close off first. Used in vehicle seats, mattresses, and applications needing anti-buckling without a guide rod. Also called convex or cushion springs.
Hourglass (concave) compression springs
The inverse of the barrel — largest diameter at both ends, smallest in the middle. Like conical springs, hourglass springs have improved lateral stability and resist buckling. The nested coil geometry also allows a very low solid height. Less common in standard stock; usually specified or custom-made for particular applications.
Compression Spring End Types
How the ends of a compression spring are formed has a direct effect on how it seats, how square it sits under load, and whether it needs a guide rod or housing. There are four configurations.
Open ends (plain)
The coil pitch continues right to the end of the wire — no change in spacing, no closing of the final coil. The end of the wire is simply cut. Open-end springs are the cheapest to manufacture but do not sit flat. They are designed to operate over a rod or inside a housing that controls alignment. Not suitable for free-standing applications where squareness under load matters.
Closed ends (squared)
The final coil at each end is wound tight against the adjacent coil, closing off the pitch. This creates a flatter bearing surface and makes the spring more self-supporting. Closed ends are the most common configuration in stock springs and general-purpose applications. Also called squared ends.
Closed and ground ends
After the end coils are closed, the ends are precision-ground flat and perpendicular to the spring axis. This is the most precise configuration — it maximises squareness under load, minimises buckling tendency, and ensures consistent contact with the seat. Specified where accurate load positioning and long fatigue life are required, such as in precision machinery and valve springs. Adds cost over plain-closed ends but is often worth it in production or high-cycle applications.
Open and ground ends
Open-pitch ends that have been ground flat. Less common than the three configurations above. Used in specific applications requiring a low solid height with a flat bearing surface.
Practical rule: For most workshop maintenance and general industrial repair work, closed ends (squared) are correct. If you are replacing a precision spring in machinery — especially anything with a defined seat — check whether the original is ground. Using an unground spring in a ground-spring application can introduce lateral error and accelerate wear.
Key Dimensions Explained
Six measurements define a compression spring. You need all six to specify a replacement correctly.
| Dimension | What it is | Why it matters |
|---|---|---|
| Free length (FL) | Length of the spring with no load applied | Must fit the available installed height in its uncompressed state |
| Outside diameter (OD) | Outer diameter of the coil | Must fit inside a housing or bore without binding |
| Inside diameter (ID) | Inner diameter of the coil | Must clear a rod or shaft that the spring seats over |
| Wire diameter (d) | Diameter of the wire used to wind the spring | Directly determines stiffness — small changes have a large effect on spring rate (rate varies with d⁴) |
| Active coils (Na) | Number of coils that actually deflect under load (total coils minus dead end coils) | More active coils = lower spring rate; fewer = stiffer |
| Solid height (Ls) | Length when all coils are touching (fully compressed) | The spring must never be compressed to solid height in service — this causes permanent set or failure |
Note on OD vs ID: Standard spring catalogues list OD. When measuring a spring to go over a rod, work from ID outward. Add at least 0.5–1.0 mm clearance between the rod and the spring ID to prevent binding as the spring deflects and its coils expand slightly in diameter.
Working travel: The usable deflection range is the difference between free length and solid height, minus a minimum clearance of around 15–20% of that travel. Operating a spring repeatedly to its solid height causes coil clash, work-hardening, and permanent set. Size for the application load well within the working travel range.
Spring Rate: What It Is and How to Calculate It
Spring rate (k) is the force required to compress or extend a spring by one unit of length. In metric terms:
k = F / x
Where k = spring rate (N/mm), F = applied force (N), x = deflection from free length (mm)
For a helical compression spring, the spring rate is determined by four geometric and material factors:
k = (G × d⁴) / (8 × D³ × Na)
Where G = shear modulus of the material (N/mm²), d = wire diameter (mm), D = mean coil diameter (mm), Na = number of active coils
You do not need to calculate this from first principles for a replacement spring — but the formula tells you what the variables are and how sensitive rate is to each:
- Wire diameter has a fourth-power effect — increase wire diameter by 10% and spring rate rises by about 46%. A very small change in wire size produces a large stiffness change.
- Mean coil diameter has a cubic inverse effect — wider coils produce a softer spring.
- Adding coils softens the rate proportionally; removing coils stiffens it.
When selecting a replacement, match the spring rate as closely as the available stock allows. A spring with a significantly higher rate than the original will apply too much force at the working deflection; one with a lower rate may not generate enough closing or return force for the mechanism to function correctly.
Materials
Most stock compression springs are made from one of three materials. The right choice depends on the operating environment.
| Material | Also called | Best for | Avoid when |
|---|---|---|---|
| High-carbon steel (music wire) | Music wire, hard-drawn wire, carbon steel spring wire | Indoor, dry environments. Highest tensile strength of any spring wire. Excellent fatigue life. Best value for standard workshop and machinery applications. | Exposed to moisture, chemicals, or corrosive environments — will rust without surface treatment. |
| Stainless steel 316 (A4) | SS316, marine grade stainless | Wet, marine, food processing, or chemically exposed environments. Good corrosion resistance. Slightly lower tensile strength than music wire for the same diameter. | High-temperature applications above ~300°C (316 loses temper). Also costs more than carbon steel. |
| Stainless steel 302/304 (A2) | SS302, SS304 | General corrosion resistance where 316 is not required. Common in food and light industrial environments. | Marine or chloride-heavy environments — 302/304 is less resistant to chloride pitting than 316. |
| Phosphor bronze | PB, CuSn alloy | Electrical conductivity requirements, seawater immersion, non-magnetic applications. Good corrosion resistance in marine environments. | High-load applications — lower tensile strength than steel. Higher cost than stainless. |
For the majority of Australian industrial and workshop applications — plant maintenance, jigs and fixtures, tooling, general machinery — high-carbon steel springs are the standard choice. Upgrade to 316 stainless for any outdoor, wash-down, coastal, or food-production environment.
How to Select the Right Compression Spring
Follow these steps in order to identify or specify a replacement spring.
Step 1 — Measure free length
With no load on the spring, measure end to end. This is your starting point. If you are measuring a failed spring, check whether it has taken a permanent set — a spring that has shortened under overload will give a false free length reading.
Step 2 — Measure OD and ID
Use calipers for accuracy. Note both OD and ID, then confirm which dimension is constrained by the application (inside a bore = OD critical; over a rod = ID critical). Allow 0.5–1.0 mm clearance for deflection.
Step 3 — Measure wire diameter
Calipers across a single coil wire. This is the most critical measurement for getting spring rate close to the original. Even a 0.1 mm difference in wire diameter can shift the rate meaningfully on small springs.
Step 4 — Count active coils
Count total coils, then subtract 1.5–2 coils for ground and closed end types (these are the inactive/dead coils at each end). Active coil count, combined with wire diameter and coil diameter, determines spring rate.
Step 5 — Confirm solid height
Compress the spring fully by hand or in a vice until all coils touch. The length at this point must be less than the compressed working height in the application. If solid height is too long for the housing, the spring will bottom out in service.
Step 6 — Match material to environment
Default to carbon steel for dry, indoor use. Specify stainless 316 for any wet, coastal, or chemically exposed location.
Step 7 — Check load or rate requirement
If you know the force the spring must exert at its working length, calculate the required rate: k = F / (free length − working length). Compare this to the rate of the stock spring you are considering. A ±20% tolerance on spring rate is generally acceptable for non-precision replacement work.
Assortment Kits vs. Individual Springs
For workshop maintenance and general repair work, an assortment kit is almost always more practical than specifying individual springs. The reason is straightforward: you rarely know exactly which spring has failed until you are standing in front of the equipment, and ordering individual springs involves lead time that a stocked kit avoids.
AIMS stocks Champion compression spring assortment kits in both carbon steel and stainless steel 316, covering a range of diameters and lengths suited to common industrial and workshop applications. These are the two options:
- Champion CA102 — 72-piece carbon steel compression spring assortment. Covers the most common OD, wire diameter, and length combinations for standard machinery and tooling maintenance.
- Champion CA1802 — 72-piece stainless steel 316 (A4) compression spring assortment. The stainless equivalent for wet, coastal, or food-grade environments.
- GJ Works GKA92 — 90-piece imperial compression and extension spring set, suitable for older machinery and equipment with imperial spring specifications.
Individual Champion carbon steel and stainless 316 springs are also available for applications where a specific size is needed in quantity.
A kit on the shelf beats a lead time every time. For any workshop that regularly services machinery, it is a practical investment.
Custom Compression Springs
Standard stock springs cover the majority of industrial replacement needs. However, there are applications — specific force requirements, unusual dimensions, non-standard materials, or production quantities — where a standard spring cannot be made to work. In these cases, custom-manufactured springs are the right answer.
AIMS may be able to assist with sourcing custom compression springs depending on your specification. Contact the AIMS team with your full spring spec — free length, OD, wire diameter, active coils, material, end type, and required rate or load at deflection — and we can advise on options and lead times.
Common Industrial Applications
Compression springs appear across a wide range of industrial and workshop applications:
- Machine tooling and jigs — return springs in clamps, die springs in punch and press tooling, ejector springs in injection moulds
- Valves and flow control — valve seat springs in pneumatic and hydraulic systems, check valve springs, pressure relief valve springs
- Assembly and fastening — spring-loaded plungers, detent mechanisms, push-button assemblies
- Conveyor and materials handling — tension-take-up systems, over-centre mechanisms, spring-loaded guides
- Electrical and electronics — battery contacts, circuit breaker components, relay springs
- Automotive and mobile equipment — suspension bump stops, throttle return springs, door and hatch mechanisms
- General maintenance — replacing worn or failed springs in any plant or facility maintenance context
Frequently Asked Questions
What is a compression spring?
A compression spring is a helical coil spring designed to resist compressive axial force. Its coils are open (spaced apart) in the free state. When compressed, the coils move together and the spring stores energy elastically. When the load is removed, the spring pushes back to its original free length. Compression springs are the most common spring type in industrial and mechanical applications.
How does a compression spring work?
When a compressive force is applied along the axis of the spring, the coils deflect toward each other in proportion to the force applied. This relationship is linear — described by the spring rate (k = F/x) — meaning twice the force produces twice the deflection within the working range. The spring stores the energy elastically in the wire material and releases it as a push force when the load is removed.
What are the different types of compression springs?
The four main types are: cylindrical (constant diameter, most common), conical (tapers from large to small diameter, very low solid height), barrel or convex (widest in the middle, anti-buckling), and hourglass or concave (widest at both ends, used for specific stability requirements). Standard stock springs are cylindrical. The other three are selected for applications where the cylindrical form has a specific limitation.
What is spring rate and how is it calculated?
Spring rate (k) is the force required to deflect a spring by one unit of length, expressed as N/mm (metric) or lb/in (imperial). It is calculated as k = F / x (force divided by deflection). For a helical compression spring, rate is determined by material shear modulus, wire diameter (to the fourth power), mean coil diameter (cubed, inverse), and number of active coils. Wire diameter has the largest effect: a 10% increase in wire diameter raises spring rate by approximately 46%.
What are the different end types for compression springs?
Four configurations exist: open (plain) ends where the pitch continues to the wire tip — these require a rod or housing for support; closed (squared) ends where the final coil winds tight against the adjacent coil for a flatter bearing surface; closed and ground ends where the squared ends are precision-ground flat and perpendicular — the most precise configuration for load-critical applications; and open and ground ends. For general industrial and workshop replacement work, closed (squared) ends are the standard choice.
What is solid height and why does it matter?
Solid height is the length of the spring when fully compressed — all coils touching. It equals wire diameter multiplied by total coil count. In service, the spring must never be compressed to solid height. Repeatedly bottoming out a spring causes coil clash, work-hardening, and permanent set (the spring stays shorter and loses rate). Always confirm the solid height is smaller than the minimum compressed length in the application by at least 15–20% of the available travel.
What materials are compression springs made from?
Most stock springs are high-carbon steel (music wire) for indoor and dry applications — highest tensile strength and best value. Stainless steel 316 (A4) is specified for wet, coastal, marine, or food processing environments due to its corrosion resistance. Stainless 302/304 (A2) is used for lighter corrosion resistance requirements. Phosphor bronze is used where electrical conductivity, non-magnetic properties, or seawater immersion is required. Chrome silicon and Inconel alloys are used for high-temperature and high-cycle fatigue applications, typically in custom-specified springs.
How do I measure a compression spring for replacement?
Measure six dimensions: (1) free length — overall length with no load; (2) outside diameter (OD); (3) inside diameter (ID); (4) wire diameter — use calipers across a single coil wire; (5) total coil count; (6) solid height — compress fully until coils touch. From these you can calculate spring rate and match to a stock spring. Note whether the ends are open or closed, and whether they are ground. If the spring has failed through permanent set (shortened), estimate the original free length from the application's housing depth.
What is the difference between a compression spring and an extension spring?
Compression springs have open, spaced coils and resist compressive (push) forces. Extension springs have tightly wound coils with formed hooks or loops at each end, and resist tensile (pull) forces — they stretch under load rather than compress. The operating direction is opposite: compression springs push back when squeezed; extension springs pull back when stretched. Extension springs also have an initial tension that must be overcome before the coils begin to open.
What is the difference between a compression spring and a torsion spring?
Compression springs resist axial (push/pull) force along the spring axis. Torsion springs resist rotational (twisting) force — they are designed to wind tighter or unwind when torque is applied to their legs. Torsion springs are found in door hinges, clothespins, window latches, and garage door mechanisms. The wire in a torsion spring is loaded in bending rather than torsion (despite the name), which affects material selection and fatigue behaviour differently from compression spring design.
What happens if a compression spring is compressed too much?
Over-compression causes coil clash — the coils impact each other at solid height — which induces shock loading, surface damage, and work-hardening in the wire material. Repeated over-compression leads to permanent set: the spring takes a shorter free length and reduced rate, meaning it can no longer exert the correct force at the working deflection. In extreme overload, the spring yields plastically or fractures. Always design and select so the working deflection leaves at least 15–20% of available travel as a buffer above solid height.
Can compression springs be custom made?
Yes. When standard stock springs cannot meet the required free length, OD, wire diameter, spring rate, or material specification, custom-manufactured springs are available. AIMS may be able to assist with sourcing custom compression springs for specific applications. Contact the AIMS team with your full specification — free length, outside diameter, wire diameter, number of coils, material, end type, and required rate or load — and we can advise on options and lead times.
Shop Compression Springs at AIMS Industrial
AIMS stocks compression springs in carbon steel and stainless steel 316, available as individual springs and as assortment kits for workshop and maintenance applications.
Browse the full compression springs range at AIMS Industrial — including Champion assortment kits in carbon steel and stainless 316, individual compression springs by size, and imperial spring sets for older equipment.
Need a spring that isn't in our standard range? Contact the AIMS team with your specification and we will advise on custom options and lead times.