Coil Definitions
Understanding these terms helps in selecting the right coil type for specific applications, whether you need flexibility, strength, or fine dimensional control.
Closed Pitch Coil
Turns are tightly touching, providing strength and crush resistance. Common in components that need to transfer compressive loads such as endoscope shafts, guidewires, or coil pipes / Bowden tubes.
Why it matters: Transfers compressive loads while providing flexibility and crush resistance.
Open Pitch Coil
A coil with space between turns, offering flexibility for bending or navigating curves.
Why it matters: Increases flexibility, allowing the coil to bend more easily for navigation through curves. Also adds space for polymer to flow during lamination.
Single-Filar Coil
A coil made from one continuous wire. This is the simplest coil construction. Single-filar coils can provide crush resistance in catheter shafts or serve as the heating element in miniature electrical heaters.
Why it matters: Offers precision and simplicity, with predictable properties.
Multi Filar Coil
A coil made from two or more wires wound together. In catheter design, multi-filar construction allows engineers to fine-tune reinforcement and flexibility by varying the number of wires, their arrangement, or even their material. Designers can combine different wire types such as round and flat, or stainless steel and nitinol to create unique performance characteristics tailored to the application.
Why it matters: Multi-filar coils give designers a powerful way to balance strength, flexibility, and torque response, opening up a wide range of customizable properties.
Pitch
The distance between wires (center to center).
Why it matters: Fine-tunes flexibility vs push-ability. A tighter pitch is more flexible, while a more open pitch offers better push-ability.
Pitch, Multi Filar
The distance between coils of the same wire (center to center).
Why it matters: Fine-tunes flexibility vs push-ability. A tighter pitch is more flexible, while a more open pitch offers better push-ability.
Closed Pitch, Multi Filar
A coil made from multiple wires wound tightly together with no spacing between turns. This construction is commonly used in flexible torque shafts, where left- and right-handed coils are paired to transmit torque smoothly in both directions.
Why it matters: Enables excellent torque transmission and push-ability while maintaining flexibility. See closed pitch coil.
ID (Inner Diameter)
Internal distance between the coil's inner space.
OD (Outer Diameter)
Total external width across coil.
Winding Angle (closed pitch)
The angle of the wire as it is winds onto the mandrel. The higher the angle, the stiffer the coil. Positive if left of center and negative if right of center.
Why it matters: Adjusting the angle fine-tunes coil stiffness, letting designers balance flexibility with support.
Winding Angle (open pitch)
The angle of the wire as it winds onto the mandrel. For open pitch coils, this angle is always negative.
Why it matters: In open pitch designs, the winding angle controls the pitch. See pitch definition.
