Key takeaways
- A torsion spring works in bending, so the body, the leg roots and the anchors all see the same moment.
- Wind direction must be stated with the viewing end and the torque arrow, or the part can be made as a mirror image.
- Leg bends concentrate stress; a generous radius at the root does more for life than a change of wire grade.
- Specify torque at two angles plus the free leg position, because one torque value cannot define a rate.
What a torsion spring actually does
A torsion spring stores energy by bending a coiled length of wire. The coil looks like a compression spring, but the load path is different. Torque is applied at the legs, the legs feed that torque into the body, and the body resists it by bending across the coil diameter. The wire is not twisted along its axis in the way the name suggests.
That distinction matters. A compression spring is sized by shear stress and solid height. A torsion spring is sized by bending stress, leg geometry and angular travel. Its four zones are the body, the leg roots, the legs and the anchors, and each can decide whether the part survives.
Leg geometry: the shape that carries the moment
The leg is not a handle. It is a beam that must transmit the working moment without yielding, rubbing or losing its angle. Most leg problems come from a shape that suits the assembly drawing but loads the wire in a way the body was never sized for.
- A straight radial leg is the simplest and stiffest. A long leg deflects more and changes the effective rate.
- An offset leg moves the contact point away from the coil and adds a moment at the root even before the spring is loaded.
- A hooked or looped end spreads the contact load and reduces wear, but adds a bend that needs its own radius and a crack check.
- A double leg halves the load per leg and balances the reaction. It needs a wider envelope and a clear datum leg.
- A leg root radius below about half the wire diameter is a stress raiser, especially after plating.
Define the leg angle at free length, not at assembly
The free leg angle is a manufacturing dimension. The installed angle is an assembly condition. If the drawing gives only the installed angle, the supplier has to guess the preload and the torque will drift from batch to batch.
Wind direction and handedness
Wind direction describes the way the wire is coiled. Looking at one end, the wire can travel clockwise or anticlockwise as it moves away from the viewer. The same spring viewed from the other end appears to run the other way, so a drawing that says only clockwise is ambiguous unless it names the end and the viewing direction.
Handedness sets the relationship between torque direction and coil diameter. Apply torque in the winding direction and the body shrinks, the coils close and the rate rises slightly. Apply it in the unwinding direction and the diameter grows, the legs move outward and the body can bind in its housing. Draw the spring installed, mark the viewing end, add the torque arrow and state the hand in words.
Stress in the body and at the leg root
Bending stress in round wire is 32 times the moment divided by pi times the diameter cubed, with a curvature correction applied. The correction grows as the spring index falls, so tight small coils run hotter than the nominal figure suggests. As an illustrative example, 1.2 mm wire on a 9.0 mm mean diameter at 60 N mm gives about 354 MPa, or roughly 389 MPa with a factor of 1.1. Those values are illustrative.
The leg root is a different calculation. The section is still the wire, but the moment arm includes leg length and any offset. As an illustration, a 20 mm leg with a 3 N tip load applies 60 N mm at the root. A radius, a shorter leg or a thicker wire are the usual fixes.
The calls that decide whether torque repeats
Two suppliers can make a spring to the same wire diameter and coil count and still deliver different torque. The difference is usually in the calls that were left loose.
| Feature | What to state | Effect if left loose |
|---|---|---|
| Wind direction | Hand, viewing end and torque arrow | Mirror image part that will not assemble |
| Free leg angle | Angle between legs at zero load | Preload and installed torque shift |
| Active coils | Count and tolerance | Rate and body length change |
| Inside diameter | At free position and working angle | Shaft fit and coil bind |
| Leg root radius | Minimum radius, no sharp corners | Crack initiation under fatigue |
| Material and temper | Grade, temper and wire standard | Stress allowance and springback change |
| Finish | Type, thickness and masked areas | Friction, corrosion and hydrogen risk |
| Load direction | Arrow on the installed view | Coil grows when it should close |
Torque is a function of all of these. A tolerance on torque alone does not control the part.
Rate, travel and the solid limit
The angular rate is set by the wire, the mean diameter and the active coil count. A common approximation is E times d to the fourth, divided by 3667 times the mean diameter times the active coils, with E in MPa and dimensions in millimetres, giving N mm per degree. It ignores the legs, coil friction and the change in diameter as the spring winds.
Treat that figure as a starting point and confirm it on a load test. Travel ends when the coils close. Keep a gap of a fraction of the wire diameter at full load, or the rate rises sharply and the spring binds.
Inspection: what proves the spring will work
A torsion spring is verified by function, not by a stack of linear dimensions. The first article should record the free leg angle, the torque at two specified angles, the inside diameter at the working position and the hand of the coil.
The fixture matters as much as the gauge. Support the legs at the same points and distance from the coil that the assembly uses. Apply the load in the service direction, because the diameter and rate change with direction. For fatigue parts, cycle at the working angle and inspect the leg roots afterwards.
What to send for a torsion spring quote
A useful enquiry gives the function as well as the geometry. State the torque at the free position and at the working angle, the maximum travel, the direction of wind, the anchor arrangement and the environment. Add the annual volume and the expected life if the spring is cycled.
- Drawing at the current revision, with the installed orientation and torque arrow
- Wire diameter, material grade, temper and wire standard
- Free leg angle and the angle at working load
- Torque at two angles, or a rate and one torque point
- Inside diameter and the shaft or housing fit
- Finish, plating thickness and any bake requirement
Frequently asked questions
How do I specify wind direction on a torsion spring drawing?
Draw the spring in its installed orientation, mark which end faces the viewer, add the torque arrow and state the hand in words. A note that says clockwise without a viewing direction can be read two ways, and the result is a mirror image part.
Why does my torsion spring lose torque after installation?
The usual causes are yield at the leg root, the legs bedding into the anchor, or the spring being installed in the unwinding direction so the coil grows. Check the installed angle against the free angle and inspect the leg roots for deformation before changing the wire grade.
Does a larger wire diameter always increase torsion spring torque?
It increases moment capacity sharply, because bending stress depends on the diameter cubed. It also raises the rate and reduces travel before coil closure. If the envelope is fixed, a larger wire may not fit, and a larger mean diameter or better leg geometry is often the better change.
Should the spring wind tighter or looser under load?
Design for the winding direction so the coil closes under load. That keeps the diameter predictable and stops the body growing into the housing. Winding open is sometimes unavoidable, but then the housing clearance must allow for the increase in diameter.
Sources and further reading
- ISO 26909:2009, Springs, Vocabulary
- ISO 2162-1:1993, Technical product documentation, Springs, Simplified representation
- Qumek torsion spring drawing review and DFM notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Have a torsion spring that will not hold its torque?
Send the drawing, the installed angle and the torque readings. The engineering desk will review leg geometry, wind direction and stress.
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