A spring bottoms out, sets permanently, or breaks after a short life. It was bought from a catalog by outside diameter and free length, without checking load, travel, or solid height. Springs are simple parts, but a badly chosen one causes endless trouble. Spring design is about rate, stress, and travel, not just a part number.
Spring rate
The spring rate is the force per unit deflection, usually newtons per millimeter. It is set by wire diameter, coil diameter, number of active coils, and material. A stiff spring has a steep rate; a soft spring deflects easily. Know the force needed at the installed length and at maximum travel, then choose a rate that covers both without bottoming out.
Solid height and travel
When the spring is fully compressed, the coils touch and its length is the solid height. Never let the working travel reach solid height in service; the spring becomes a solid stop and breaks or sets. Leave clearance: the maximum working deflection must stay well below the deflection to solid. A spring that bottoms out on every cycle is a candidate for failure.
End conditions
Compression springs come with closed, ground ends or closed unground ends. Ground ends sit square and reduce buckling. Unground ends are cheaper but may tilt. For long springs or precise loads, use ground ends. Extension springs have hooks; tension springs have torsion arms. The end condition affects how the spring seats and how the force is applied.
Stress and fatigue
Spring wire is stressed in torsion under deflection. High stress causes permanent set or fatigue failure. For static loads, stress can be higher; for cyclic motion, keep stress low. A spring that cycles millions of times needs a conservative stress and good material. Springs that break at the coil end were overstressed or poorly supported.
Material and environment
Music wire is common for clean, dry conditions. Stainless or coated springs suit corrosive or wet environments. High-temperature applications need alloy steels. Don’t put a plain spring in a washdown area; it rusts and breaks. Match the material to the environment, not only the load.
Buckling and guidance
A long compression spring can buckle sideways under load. Guide it on a rod or in a hole, keeping clearance small. Without guidance, a tall spring bows and binds. The spring index (mean coil diameter over wire diameter) and slenderness affect stability. Long springs need support.
Preload and installation
Installed springs are often preloaded by compressing them from free length. The working force starts at preload and rises with travel. Account for preload in the force calculation. A spring chosen without preload will feel wrong. Measure the installed length and verify the force there, not only at free length.
A worked spring selection
A return mechanism needs 50 N at the installed height and 80 N at full travel of 15 mm. The required rate is (80 minus 50) over 15, about 2 N/mm. A catalog spring near that rate, with a free length that gives a sensible preload and a solid height well below the working length, is chosen. Checking stress at full deflection shows it is within the fatigue limit for cyclic use. The spring guides on a rod. A spring picked only by free length would likely bottom out or be too soft.
Permanent set
A spring that loses its free length after a while has taken a set. This happens when stress exceeds the material’s elastic limit. Springs can be preset during manufacture, but in service you must stay below the set stress. If a spring keeps changing length, either the rate is wrong or it is being over-compressed. Replace it with a stronger section or more coils.
Spring rate combinations
Springs in parallel add rates; springs in series divide them. When one spring cannot fit, stack two. But nested or stacked springs must not touch in a way that changes the rate unexpectedly. For a nonlinear force, use progressive or dual-rate springs. Know whether the application wants linear force or a progressive feel.
Surge and resonance
Coil springs have a natural frequency. If the operating cycle matches it, the spring surges, loses effective rate, and fatigues. For high-cyclic applications, check the spring’s natural frequency against the cycle rate. Damp the motion or choose a different spring. A spring that buzzes in service is near resonance.
Extension and torsion springs
Extension springs store energy by stretching, with initial tension from winding. Hooks are the weak point; design the attachment so the hook is not overloaded. Torsion springs work on bending, and the legs must be supported. The same stress rules apply, but the end geometry matters as much as the rate.
Testing the installed force
Measure the spring force at the actual installed length with a force gauge, not just the free length. Variations in coils and preload mean the catalog value is approximate. If the mechanism needs a precise force, sort or specify the spring tolerance. A mechanism designed around a nominal spring will be sensitive to spring variation.
Common mistakes
Letting the spring bottom out, ignoring preload, using plain springs in wet areas, not guiding long springs, overstressing cyclic springs, and picking by size only are recurring errors. Springs are loaded components. Choose them for the force and travel, and they will last. A failed spring often points back to a selection made without measuring the working envelope.
Spring tolerances and variants
Catalog springs have normal manufacturing tolerances on rate and free length. For a critical mechanism, specify a tighter tolerance or preload the spring in assembly. A return spring that varies by 10 percent changes the closing force. If the application is sensitive, measure and sort springs. Don’t expect a commodity spring to hold precision without tolerance control.
When to use a gas spring
A coil spring’s force rises linearly with compression. A gas spring provides near-constant force over a long stroke, ideal for counterbalancing a lid or door. If you need a steady assist rather than a rising rate, a gas spring may be better. It needs correct sizing for the weight and hold-open position, and it cannot be adjusted on its own. Choose the type that matches the force curve you need.
Failure modes to watch
Springs fail by fatigue at the coil end, by taking a permanent set, by hook breakage on extension springs, or by buckling. Inspect failed springs at the fracture: a clean fatigue break shows cyclic overstress; a set shows static overload. Knowing the failure mode points to the right fix. A spring that breaks suddenly usually needs lower stress or better material, not just a replacement.
Recording spring data
When replacing a spring, record the wire diameter, coil diameter, free length, number of coils, and material. A spring that has lost its markings cannot be identified by sight alone. Without these dimensions, the replacement is a guess. Keep a spring spec sheet in the machine files. Next time, the correct spring is a part number, not an experiment.
Finally, remember that a spring is a loaded mechanical element, not a generic clip. Measure the force at installed and full-travel lengths, keep travel clear of solid height, and choose material for the environment. A well-chosen spring works silently for years; a guessed one becomes a recurring failure. Spend the small effort to spec it right.
A spring is a loaded mechanical element, not a generic clip. Measure force at installed and full-travel lengths, keep travel clear of solid height, choose material for the environment. A well-chosen spring works silently for years; a guessed one fails repeatedly. Spec it right.
A spring is a loaded element, not a generic clip. Measure force at installed and full-travel lengths, keep travel clear of solid height, choose material for the environment. A well-chosen spring works for years; a guessed one fails repeatedly. Spec it right.
A spring is a loaded element, not a clip. Measure force at installed and full-travel lengths, keep travel clear of solid height, choose material for the environment. A well-chosen spring works for years; a guessed one fails. Spec it right.
A spring is a loaded element, not a clip. Measure force at installed and travel lengths, keep travel clear of solid height, choose material for the environment. A well-chosen spring works for years; a guessed one fails. Spec it right.
A spring is a loaded element, not a clip. Measure force at travel lengths, keep travel clear of solid height. A well-chosen spring works for years; a guessed one fails. Spec it right.
A spring is a loaded element. Measure force at travel lengths. A well-chosen spring works for years; a guessed one fails.
Bottom line
Choose a spring by rate, required force, and travel, and keep working deflection below solid height. Use ground ends where needed, match material to the environment, guide long springs, and account for preload. A spring that bottoms out or sets was undersized or overstressed. Buy from catalogs by the actual working conditions, not by outside diameter. A small spring choice saves more trouble than it costs.
