Torsion Spring Torque Tolerance: Why Wire Diameter Sets the Limit

Most custom torsion spring disputes are not about material grade. They are about torque measured on the customer's fixture falling outside the drawing tolerance. The reason is almost always the wire, not the coiling machine: torque depends on wire diameter to the fourth power, so every micron of diameter variation is amplified before the spring is even formed.

This article explains the tolerance chain behind torsion spring torque, and how to specify wire so that the finished spring passes the gauge the first time.

The Fourth Power Rule

For a round wire torsion spring, torque is proportional to the elastic modulus, the wire diameter to the fourth power, the deflection angle, and inversely proportional to mean coil diameter and active coils. The practical consequence: a 1% change in wire diameter changes torque by about 4%, while a 1% change in coil diameter changes it by 1%.

On 1.0 mm wire, a standard h9 tolerance of 0.025 mm is already 2.5% of diameter. That converts to roughly 10% torque variation, before any contribution from modulus scatter or leg geometry. This is why a torque tolerance tighter than plus or minus 10% cannot be delivered on standard commercial wire.

The Full Tolerance Chain

Wire diameter

h9 is the normal starting point for spring wire. Where torque tolerance below 8% is written on the drawing, we recommend h8 or a fixed diameter band agreed with the wire mill, for example 1.00 mm plus or minus 0.008 mm. The tighter band must be ordered as a mill lot, because mixed lots reintroduce the scatter.

Elastic modulus

Cold drawn 302 and 304 sit around 193 GPa with typical lot scatter of plus or minus 2%. That scatter moves torque by the same 2%, so it must be included in the stack-up rather than measured once and assumed constant.

Leg geometry

Leg length, leg angle and bend radius at the leg root all affect where the spring contacts the mating part. We hold leg angle to plus or minus 2 degrees and leg length to plus or minus 0.3 mm on springs above 2 mm wire, tightening both figures on request. The bend radius at the leg root should not be smaller than half the wire diameter, otherwise the leg becomes the first fatigue site.

Stress relieving after coiling

Coiling leaves residual stress that relaxes over the first few hundred cycles, so torque measured cold is not the torque the customer sees in service. A low temperature relief at 200-300 degrees C for 30-60 minutes stabilises the geometry and reduces initial torque loss, particularly on springs with a high index.

Typical Specification Window

Applications

• Clips, latches and damper mechanisms
• Door and lid hinges in appliances
• Retaining clips for automotive interiors
• Medical and instrument closures
• Electrical switch and contact springs

How to Write the Drawing

Give wire diameter and tolerance, outside diameter, number of coils, leg lengths and leg angle, working deflection angle at which torque is checked, and the fixture gap used for measurement. State the torque tolerance against a defined deflection angle rather than a free position, because free angle moves after stress relieving while torque at a fixed angle does not.

TOPONE has produced stainless steel wire since 1993 and supplies both the wire and coiled torsion springs, which means wire tolerance and spring tolerance are agreed in one place instead of being negotiated between two suppliers.