Extension springs are closed-coiled helical springs that extend under a pulling force.
Many design considerations must be made with extension springs including; initial tension, stress and deflection of hooks, hook end types.
There are a variety of extension spring end types.Machine loop and hook over center, side loop, full loop over center, no hook, extended hook, hook end with swivel,
v-hook over center, double twisted full loop over center, and many more.
A hook is open to fit over a projection; a loop is a closed hook. A regular machine hook or loop is made on an automatic looper and is the least expensive
and often 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.
Initial tension is the force which presses the coils of a closed-wound extension coils together. Initial tension is the force needed to open these coils.
Initial tension can be wound into cold-coiled extension springs made from hard-drawn or oil-tempered wires only, such as music wire, 300 series stainless steel,
phosphorous-bronze, Monel, Inconel, and their pre-hardened materials.
Several methods for designing extension springs can be used. The easiest is to contact Katy Spring with parameters including; force needed,
space limitations, free length, outside diameter, initial tension, and working environment. Force calculations can easily be made using spring design software.
Ordering Extension Springs
Extension springs take longer to design because consideration must be given to stress due to initial tension, stress and deflection of hooks,
special coiling methods, secondary operations, and allowance for overstretching in assembly.
There are many hook configurations to consider in designing extension springs. Katy Spring can assist in making the right choice for a hook.
Generally, crossover hooks are easier and less expensive than full loops over center, and if made without sharp radius bends they often last
longer than the full loops over center..
Initial tension force can be wound into an extension spring by bending each coil as it is wound, away from its normal coiling direction,
thereby causing a slight twist in the wire which makes the coils spring back tightly against the adjacent coil. This force should be wound into an
extension spring. It cannot be wound into extension springs made from annealed materials or those materials that require hardening after coiling,
including hot coiled springs. Some initial tension should be coiled into extension springs to hold the spring together. If not, a long
spring with many coils will have a different free length when positioned vertically rather than horizontally. 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. Katy Spring can assist customers in developing the proper initial tension needed for a
given extension spring.
Calculations to determine exact stresses in hooks are quite complicated and assistance should be obtained from Katy Spring. Also, bend radiuses
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.
To follow is a list of notes to keep in mind when ordering extension springs from Katy Spring:
- Extension springs should be stressed about 10-15 percent lower than compression springs to allow for
overstretching at assembly and to reduce hook stresses.
- At least 10 percent of the maximum force should be in an extension spring's initial tension.
- Full hooks deflect under a load equivalent to about half a coil, therefore deduct one coil from the calculated number of coils determined by design to allow for deflection of two hooks. For example, if not allowed for, a ten coil spring would be ten percent low of load. Each half hook deflects approximately equal to one tenth of a coil.
- All coils are active in an extension spring, allowance should be made for hook deflection.
- For regular hooks, the distance from the inside the hook to the body of the extension spring is about 75 percent of the inside diameter.
- Specifying the relative position of the hooks adds cost to extension springs. Do not specify position unless it is important. Also, avoid using large, extended or special hooks where possible, as they add cost to extension springs.
- Keep the outside diameter of a hook the same as the extension spring OD so the hook can be made by bending up a regular coil. Keep the tolerances high in regards to hook opening. Swivel hooks and coned ends reduce breakage, but are often quite expensive. An extension spring with a reduced OD may often suffice to reduce breakage.
- Electroplating does not deposit a thorough coating between the coils of extension springs, such springs should be extended during plating.
- Specify forces at extended lengths between hooks, not at amounts of deflection.
- If high stresses cannot be achieved consider using compression springs fitted with drawbars.
- Specifying two forces is often expensive, consider using a rate when appropriate.
Extension Springs | Custom Extension Spring
Stainless Steel Extension Springs
Large Heavy Duty Extension Springs
Extension Springs With Special Ends