How to Specify a Custom Spring: The 8 Parameters You Must Get Right

When a buyer approaches a spring manufacturer for a custom spring, the quality of the specification determines the quality of the outcome.

A spring that looks correct — same wire diameter, same outer diameter, same length — may still fail in service if the load and deflection requirements were not specified correctly.

This guide covers the 8 parameters your manufacturer needs. Providing all of them upfront avoids back-and-forth, reduces sampling time, and ensures the spring performs correctly in your application.

Note: The parameters below apply to compression and extension springs. Torsion spring parameters are covered separately at the end of this article.

Parameter 1  —  Wire Diameter (d)

Wire diameter is the thickness of the wire used to coil the spring. It is measured in millimetres.

This is one of the most critical dimensions. Wire diameter directly determines:

•  The stiffness (spring rate) of the spring

•  The load capacity — how much force the spring can carry

•  The fatigue life — how many cycles the spring will last

Changing wire diameter by even 0.1 mm can noticeably change spring performance.

If you are replicating an existing spring, measure the wire diameter with a micrometer — not a ruler. Visual estimates are not accurate enough.

At Super Springs, we manufacture in wire diameters from 0.3 mm to 12 mm depending on the spring type.

Parameter 2  —  Outer Diameter (OD)

Outer diameter is the outside measurement across the spring coil.

This dimension is important when the spring must fit inside a hole (bore) or slide over a shaft.

•  If the spring fits inside a bore — specify the bore diameter. The manufacturer will confirm the OD fits with the required clearance.

•  If the spring fits over a rod or shaft — specify the rod diameter. The inner diameter (ID) of the spring must be larger than the rod.

Inner diameter (ID) = Outer diameter (OD) minus 2 times the wire diameter.

Always specify OD. Do not specify ID — OD is the standard reference dimension used in manufacturing.

Parameter 3  —  Free Length (L)

Free length is the length of the spring when no load is applied — its natural, unloaded state.

This is different from the installed length or the compressed length.

•  For compression springs: free length is measured end to end when the spring is sitting on a flat surface with no force applied.

•  For extension springs: free length is the total length including hooks, measured end to end with no load.

Always specify free length. If you provide installed length instead, state clearly that it is the installed (not free) length so the manufacturer can calculate correctly.

Parameter 4  —  Load and Deflection

This is the most important functional parameter — and the one most commonly missing from specifications.

Dimensions alone tell the manufacturer what the spring looks like. Load and deflection tell the manufacturer how the spring must perform.

For compression and extension springs:

State the required load at a specific deflection. For example:

•  Load of 15 N at 20 mm compression from free length

•  Or: 10 mm deflection under a load of 8 N

Both statements define the spring rate (stiffness). Spring rate = Load ÷ Deflection.

If you specify a spring rate directly (e.g. 0.75 N/mm), that is also acceptable.

For torsion springs:

State the required torque at a specific angular deflection. For example:

•  Torque of 50 N·mm at 45 degrees of rotation

•  Or: angular spring rate of 1.2 N·mm per degree

If you do not know the exact load values, describe the application — the weight being supported, the force being applied, or the mechanism the spring is part of. A manufacturer with engineering experience can help calculate the required spring rate from this information.

Do not leave load and deflection unspecified if your application has any functional requirement. A spring that meets all dimensions but has the wrong spring rate will not work correctly in service.

Parameter 5  —  Number of Turns (N)

The number of turns (also called active coils) is the count of coil windings that contribute to the spring's deflection.

Together with wire diameter and outer diameter, the number of turns determines the spring rate.

•  More turns = softer spring (lower spring rate)

•  Fewer turns = stiffer spring (higher spring rate)

For compression springs, total coils include the active coils plus the end coils (which are closed and do not contribute to deflection). When specifying, clarify whether you mean total coils or active coils.

If you have specified the load and deflection (Parameter 4), the manufacturer can calculate the correct number of turns. You do not need to specify both independently unless you have a physical constraint on the number of coils.

Parameter 6  —  End Type

The end type determines how the spring terminates at each end. This affects how it seats in the assembly and, for compression springs, how accurately it applies force.

For compression springs, the main end types are:

•  Closed and ground — the end coils are closed (tightly wound) and the ends are ground flat. This gives a flat seating surface and is preferred for precision applications where the spring must sit squarely.

•  Closed, not ground — end coils are closed but not ground. Suitable for less critical applications. Lower cost.

•  Open ends — the coil pitch continues to the end with no closing. Used in specific industrial applications.

For extension springs, the end type is the hook configuration:

We manufacture extension springs with 8 hook types:

•  Closed German Loop

•  German Side Loop

•  Raised Hook

•  Half German Hook (open loop)

•  Double German Loop

•  Extended German Side Loop

•  English Loop (full loop with offset)

•  Extended Side Hook (raised side hook)

Also specify the angle between hooks if the two hooks must be oriented at a specific angle relative to each other.

Parameter 7  —  Material Grade

The material grade determines the mechanical properties of the spring — its strength, fatigue life and corrosion resistance.

The most common options are:

•  SM/DM (medium carbon spring steel) — suitable for general indoor industrial applications. Standard grade, cost-effective.

•  SH/DH (high carbon spring steel) — higher tensile strength and better fatigue life. Recommended for agricultural equipment, outdoor use, high-cycle applications and heavy-duty assemblies.

•  SS304 stainless steel — corrosion resistant. Use where the spring is exposed to moisture, chemicals, food contact or outdoor conditions without protective coating.

•  SS316 stainless steel — for more aggressive corrosive environments or medical applications.

•  EN 10270-1 (music wire) — high precision, for small springs in electronics and instruments.

•  EN 10270-2 (oil tempered wire) — for high fatigue, high cycle applications.

If you are unsure which grade is appropriate, describe the operating environment and we will recommend the correct material.

Parameter 8  —  Surface Finish

Surface finish provides corrosion protection and, in some cases, improves fatigue performance.

Common options:

•  Zinc white plating — standard finish for indoor and mild outdoor applications. Provides basic corrosion protection.

•  Zinc black — same protection as zinc white with a black appearance.

•  Powder coating — for colour matching and enhanced outdoor protection.

•  Passivation — used on stainless steel springs to enhance corrosion resistance.

•  Shot peening — improves fatigue life for high-cycle applications. Used on automotive and critical industrial springs.

•  Black oxide — for mild corrosion resistance with a dark appearance.

•  No coating — for applications where the spring is already in a lubricated or protected environment.

For export to humid climates — including the Middle East, Southeast Asia, West Africa and Latin America — zinc white plating is recommended as a minimum.

Specifying Torsion Springs

Torsion springs have different geometry from compression and extension springs. The key parameters are:

•  Wire diameter (d)

•  Outer diameter (OD) of the coil body

•  Number of turns (N)

•  Leg length 1 (L1) — first leg extending from the coil

•  Leg length 2 (L2) — second leg extending from the coil

•  Leg angle — the angle between L1 and L2 in the unloaded state

•  Torque at specified angular deflection (the load parameter for torsion springs)

•  Material grade and surface finish

Double torsion springs — used in agricultural cultivators and heavy mechanical assemblies — have two coil bodies. Additionally specify the width between the two coil bodies and the direction of wind (left-hand or right-hand).

What If You Do Not Have All the Parameters?

You do not need to provide all 8 parameters to get started.

There are three ways to place a custom spring order with us:

•  Send a physical sample — we measure all dimensions, test the load and deflection, and replicate the spring to the same specification.

•  Send a drawing — any format is accepted, including PDF, DXF, STEP, or a clear hand sketch with dimensions marked.

•  Describe the application — tell us what the spring does, the space available, and the load or force required. Our team will propose a specification for your approval before manufacturing begins.

Prototype samples are available with low minimum order quantities before committing to bulk production.

About Super Springs

Super Springs has been manufacturing custom springs and wire components from our factory in Gurgaon, India since 1984.

We are an ISO 9001:2015 certified manufacturer supplying OEMs, brands and distributors across 14 countries.

We manufacture compression springs, extension springs, torsion springs, drawbar springs, trampoline springs and long industrial springs — all to customer drawings, samples or specifications.

View our spring range →  superspringsglobal.com/category/springs

Send us your drawing or sample →  superspringsglobal.com/contact-us