Solid height and free length: the two numbers that decide fit

Two numbers, two different jobs

Solid height is the length of a compression spring when every coil is touching. It is a hard limit: push the spring shorter and the coils cannot move further, so the load path changes from the wire bending to metal bearing on metal. Free length is the length of the spring with no load applied. It is the number that decides whether the part fits between the seats before anything moves.

The numbers are related but not interchangeable. A generous free length can still clash if the working travel leaves no room above solid. A short solid height can still fail to fit if the free length exceeds the space between the seats.

Calculating solid height

For a spring with closed and ground ends, solid height is normally the total number of coils multiplied by the wire diameter. Total coils include the active coils plus the two end coils that do not contribute to rate. A spring with 10.5 total coils and 1.2 mm wire therefore has a nominal solid height of 12.6 mm.

That figure is nominal. Grinding removes material from the ends, so a ground spring can sit slightly shorter than the calculation. Wire diameter tolerance moves it again: a 1.2 mm wire with a plus tolerance gives a taller stack across ten coils. The drawing should give solid height as a maximum, not a nominal, because a shorter stack is rarely a problem and a taller one causes clash.

Open ends, closed ends without grinding and tapered ends all change the count. Use the number of coils that actually stack metal on metal, and confirm the end condition before the drawing is released.

Free length and the length stack

Free length is built from the bottom up. Start with solid height, add the clash allowance, add the working travel from the installed height to the loaded height. Then add the preload travel the assembly needs.

A common mistake is to set free length from the available space rather than from the load requirement. The space sets the maximum; the rate and the load set what the spring needs. If the two disagree, change the wire, coil count or installed height rather than asking the supplier to make it fit.

The clash allowance is the gap left between coils at full load, usually a fraction of the wire diameter. It stops the spring reaching solid in normal service, including tolerance and overload. Without it, the rate rises sharply at the end of the stroke and the seat takes an impact.

Why both numbers move after coiling

A coiled spring is not a stable elastic body when it leaves the coiler. The wire has been bent beyond its elastic limit, and the coils retain residual stress. Stress relief and setting change the free length, sometimes by several per cent. A spring measured straight off the machine and a spring measured after heat treatment are not the same part.

Setting, sometimes called presetting or scragging, compresses the spring to solid or beyond so the coils take a permanent set. That stabilises the free length and reduces the loss of load in service. It also means the free length after setting is the number that matters, and it should be the number on the drawing and on the first article.

Grinding is the other process that moves solid height. A spring that is not ground sits on the wire round and can lean; a ground spring stands square and stacks predictably. For a flat seat or work close to solid, grinding is usually worth specifying.

The fit checks that catch collisions

Check the spring against the space at both ends of the tolerance stack. At maximum free length and minimum cavity depth, the spring may not fit between the seats. At maximum solid height and maximum travel, the spring may clash before the mechanism stops. Both are assembly or function failures, and both are visible on paper.

Check the diameter at the same time. A compression spring grows in diameter as it is compressed, so the bore clearance must allow for the growth at the loaded height, not at free length. A spring that is free to rotate in the bore at rest can rub when loaded.

If the spring sits on a rod, check the rod diameter against the inside diameter at the loaded height. If it sits in a bore, check the outside diameter at the loaded height against the bore. Add plating thickness to both, because it reduces the gaps.

Measuring free length and solid height

Free length is measured with the spring standing on a flat surface, unloaded, with the axis vertical. A spring measured lying on its side reads longer because the coils sag. Use a flat plate and a height gauge, or a fixture with a known light preload.

Solid height is measured under a load high enough to close the coils, or calculated from the coil count and checked against a sample compressed between plates. Do not infer it from a load test at an arbitrary height, because the coils may not be fully closed.

Both measurements belong on the first article with the gauge and the load stated. If the free length is critical, ask for it to be recorded after setting and stress relief, because that is the condition the customer receives.

What to put on the spring drawing

Give solid height as a maximum, free length with a tolerance, and the load at one or two installed heights. State the end condition, ground or not, and the direction of the load. Add the bore and rod clearance if the spring runs in a housing.

• Wire diameter and material grade
• Outside or inside diameter with tolerance
• Total coils and active coils
• Solid height, maximum
• Free length with tolerance, after setting
• Load at installed height and at loaded height
• End condition: closed and ground, closed, or open