Katy Spring is a premier source of extension springs information. We assist customers in developing the proper initial tension needed for a given extension spring.
Extension Springs Information — Helpful Details
We specialize in custom extension springs. Initial tension is the force which presses the coils of a closed-wound extension spring, as well as the force needed to open the coils. Initial tension can be wound into cold-coiled extension springs made from hard-drawn or oil-tempered wires only. Examples include music wire, 300 series stainless steel, phosphorous-bronze, Monel, Inconel, and their pre-hardened materials.
The amount of initial tension that can be wound into an extension spring depends principally upon the spring index — the ration of wire size to extension spring outside diameter. A tighter index spring needs less initial tension. It cannot be wound into extension springs made from annealed materials or those materials that require hardening after coiling, including hot coiled springs.
Hooks and Crossover Hooks
A hook is open to fit over a projection while a loop is a closed hook. A regular machine hook or loop is made on an automatic looper and is the least expensive. Often, it is the most satisfactory type. Hook stresses can be reduced by reducing hook diameter, using swivel hooks (which can be expensive) and minimizing sharp radius bends.
Generally, crossover hooks are easier and less expensive to manufacture than full loops over center, and if made without sharp radius bends they often last longer than the full loops over center. Calculations to determine exact stresses in hooks are quite complicated and assistance should be obtained from Katy Spring.
Also, bend radii forming the hooks are difficult to determine and frequently vary between specifications on a drawing and actual samples. However, regular hooks are higher stressed than the coils in the body of the spring and are often the cause of failure. Crossover center hooks, when manufactured without sharp bends, last longer than regular hooks. The bending stresses are the same and the life should be the same.
However, if a large bend radius is made on a regular hook, the bending stresses often coincide with some torsional stresses, thereby explaining the reason for earlier breakage. If sharper bends were made on the regular hooks, the life should be the same.
Contact Katy Spring to ask questions, request additional extension springs information, or place your order.