These are generally made of rectangular section material, flat strip with round edges or normal slit edges with equal or increasing space between coils. These are wound flat to achieve torque. For first 360* , torque delivered per revolution is linear. Therefore, these are generally used for less than 360 rotational applications. If these springs are rotated more than one turn the coils will touch each other causing friction leading to erratic loads. They generally have a lug across the I.D. of the spring. Which engages a slot in the shaft, over which it works. The I.D. can be made with holes to engage an arbor protrusion or can be made in a special shape to fit over a similar shaped shaft. The outside end can be formed as per customer’s requirement.