Wave springs vs coil springs: height, load and cost

The problem is usually axial height

Most wave spring enquiries start with a height problem. The housing has a fixed depth, the travel is small, and a coil spring with enough free length will not fit once it is compressed to solid. A wave spring is a flat, corrugated ring that deflects in the axial direction, so it can provide a short working stroke in a very thin package.

The comparison is therefore not fair unless it is made at the same installed height and the same travel. A wave spring that fits in 3 mm cannot be compared with a coil spring that needs 12 mm of free length. Compare the two at the envelope the product actually has, and compare the load at the working height, not the catalogue free height.

How a wave spring works

A wave spring is made from flat strip or wire formed into a ring with a series of waves around the circumference. The number of waves, the wave height, the strip thickness and the ring diameter set the rate. Under axial load, the waves flatten and the ring behaves as a set of small beams in parallel.

Because the spring is flat, the solid height is close to the strip thickness multiplied by the number of layers. A single turn wave spring may have a solid height of one strip thickness plus the wave amplitude. A multi turn wave spring stacks several rings to increase travel while keeping the height low. The trade-off is friction between the turns and a more complex part.

Solid height and installed height

The clearest difference is the stack. For illustration, a coil spring with 1 mm wire, a 20 mm outside diameter and five active coils might have a free length of 12 mm and a solid height of about 6 mm. A wave spring with a 0.5 mm strip, a 20 mm outside diameter and three waves might have a free height of 4 mm and a solid height of about 1.5 mm for the same 2 mm of working travel.

Those numbers are illustrative, but they show the scale of the saving. The wave spring gives back several millimetres of housing depth. That can be the difference between a product that fits a standard enclosure and one that needs a new moulding. The saving is real, and it is the main reason the part exists.

Load and rate behaviour

A wave spring is stiff for its height. The working travel is small, often a fraction of a millimetre to a few millimetres, and the load rises quickly once the waves flatten. That suits a preload or a take-up function, where the spring has to hold a defined force over a small movement. It suits less well an application that needs a long, soft stroke.

A coil spring has more travel and a more predictable rate. It can be designed for a nearly linear curve over a long stroke, and it tolerates a wider range of installed heights. If the application needs to absorb a large tolerance stack or a long actuation, the coil spring is usually the simpler answer.

Stability and bore contact

A wave spring sits flat in a bore and does not buckle in the way a tall coil spring can. That makes it useful in a shallow cavity where a coil spring would need a guide or a sleeve. The flat ring also distributes load around the circumference, which helps when the mating part is a soft cover or a bearing preload ring.

The flat shape has its own risk. If the bore is not square, the ring can tilt and load one side. If the mating face is rough, the wave crests can wear. A wave spring usually needs a flat, parallel seat, and the seat finish matters more than it does for a coil spring with a rounded end.

Cost: tooling, material and volume

At low volume, a coil spring is usually cheaper. The coiler setup is quick, standard wire is available, and the part can be made in small batches. A wave spring often needs a dedicated form tool, a setup that is specific to the diameter and wave count, and a longer prove-out. That tooling has to be spread over the order.

At higher volume, the picture can change. A wave spring uses less material for the same installed height, and it can replace a taller coil spring plus a guide sleeve. If the design also avoids a machining operation on the housing, the total cost can favour the wave spring even when the piece price is higher. Compare the installed cost, not the spring price.

Fatigue, creep and temperature

Both types are springs, so both are subject to fatigue and creep. A wave spring works at a high stress for its size and has a small travel, so the stress range can be high if the application cycles through the full wave height. A coil spring has more material to share the load but can still fail at the end coils or at a corrosion pit.

Temperature changes the material properties and the installed height. A wave spring in a hot housing can lose load as the strip relaxes, and a coil spring can do the same. If the application is hot or heavily cycled, test the actual spring at the actual travel and temperature. A catalogue rate at room temperature is not a validation.

How to choose

Start with the envelope. If the axial space is the binding constraint and the travel is small, a wave spring is worth pricing. If the travel is long, the load has to be soft, or the volume is low, a coil spring is usually the better starting point. Then check the seat, the bore and the mating face, because a wave spring is less forgiving of a poor seat.

Write the requirement as a load at an installed height with a travel range, not as a spring type. That leaves the supplier free to propose the shape that meets the function. The comparison then becomes a real engineering decision rather than a preference.

• State the installed height, the travel and the load at height
• State the bore, the seat flatness and the mating face
• State the cycle count and the operating temperature
• Ask for the solid height and the load at solid
• Compare installed cost, including any guide or sleeve removed