Solid Height of Compression Springs: How to Calculate It and Why It Kills Designs
Solid height is the compressed length at which every coil of a compression spring touches its neighbor. Designs fail when the assembly needs more travel than the spring has before going solid.
How to calculate
- Closed & ground ends: solid height ≈ total coils × wire diameter
- Closed, not ground: ≈ (total coils + 1) × wire diameter
- Plated or coated springs: add twice the coating thickness per coil
Example: 10 total coils of 1.2 mm wire, closed and ground → about 12 mm solid.
Why hitting solid is a problem
- Forces spike unpredictably — the spring becomes a rigid column and transmits shock into surrounding parts.
- Stresses exceed design values, causing set: the spring is permanently shorter afterward.
- Repeated solid contact chews plating and generates debris in clean mechanisms.
Design rules
- Ensure available travel = free length − solid height is at least what the mechanism needs, with margin.
- Keep the maximum working deflection at or below ~80–85% of deflection-to-solid.
- If the spring can be compressed solid during assembly or abuse (service technicians, end stops), tell the manufacturer — we can preset the spring so a trip to solid does not change its free length.
When space is too tight
If required travel does not fit, options include: smaller wire with better material, reduced coil count with higher stress allowance, nested springs, or a change to a wave spring which delivers similar force in about half the solid height. Send the space constraints with your RFQ and we will propose options.
Frequently Asked Questions
Is solid height a guaranteed dimension?
Treat it as a maximum, not a nominal. Manufacturers guarantee "solid height not to exceed X" because wire tolerance and end grinding vary; specify it that way on drawings.
Can a spring be used at solid intentionally?
Some designs use solid as a positive stop, which is acceptable if the spring is preset and stresses are checked. Say so explicitly — it changes material and process choices.
Does grinding ends reduce solid height?
Yes. Grinding removes roughly one wire diameter from the total stack and gives flatter seating; that is why closed-and-ground is standard for precision compression springs.