Plastic spring and method and apparatus for making the same
This invention provides a helical spring formed of plastic material, comprising a pair of end coils situated at respective ends of said helical spring; at least one full pitch active coil between said pair of end coils; a pair of transition coils, each transition coil interconnecting an end of a respective end coil to an end of a respective proximate full pitch active coil, and having a varying pitch.
1. A helical spring formed of plastic material, comprising:
a pair of end coils situated at respective ends of said helical spring;
at least one full constant pitch active coil between said pair of end coils;
a pair of transition coils, each transition coil interconnecting an end of a respective end coil to an end of a respective proximate one of said full constant pitch active coils, and
wherein the pitch of each of said transition coils varies gradually and continuously from substantially said full pitch at a first point of connection of said transition coil to said respective proximate full constant pitch coil to a pitch angle of substantially zero at a second point of connection of said transition coil to said respective end coil.
2. A helical plastic spring as recited in claim 1 wherein said plastic material comprises a high strength thermoplastic material.
3. A helical plastic spring as recited in claim 2 wherein said thermoplastic material comprises polyetherimide.
4. A helical plastic spring as recited in claim 1 wherein each of said end coils is flat and tapered on its inner side towards a free end of said end coil.
5. A helical plastic spring as recited in claim 1 wherein said spring coils have a substantially rectangular cross section.
6. A helical plastic spring as recited in claim 1 wherein said spring coils have a substantially trapezoidal cross-section tapering in an outward direction.