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Extension Springs
All extension springs are closed coiled helical springs. They offer resistance to a pulling force. These springs work opposite from a compression spring, which works with a pushing force. This type of coil spring could be described as closed wound. This means the coils are wound tightly together so they are in contact with each other. In some cases the coils could be wound so tightly that it requires an effort just to pull them apart. This is what is known as initial tension. I believe this is why I have heard these springs sometimes called tension springs. The spring manufacturer can control this initial tension in his manufacturing process. 
The loops or hooks on each end come in a variety of shapes. The most used types are what is called German(or twist) and English(or cross). The various other types are extended English, enlarged or reduced German, double twist, full side loop, a swivel hook added into the spring, or even a threaded plug with a loop on it fitted into the spring.
Rate and Initial Tension Rate calculation formulas used for extension springs are the same ones used for compression springs. Most extension springs have no pitch between the coils. They are formed with the coils wound tightly against each other. This is sometimes called "closed wound". The load needed to separate these closed wound coils is called initial tension. Initial tension affects the load/deflection plot as a preload shown in the graphic to the right. Once the initial tension load is exceeded the load/deflection line is straight. Initial tension is measured by checking two load/deflection points at loads where the coils are open and extending the rate line to the intercept at zero deflection. This gives the initial tension load as shown in the graphic. Initial tension is necessary in extension springs to help maintain control of free length. The best range to keep initial tension as is a stress value between 40% and 80% of the tensile strength of the material divided by the spring index. The designer needs to select a load value corresponding to stress in this range. Free Length Free length of an extension spring is specified between the inside of the spring loops. When you want the spring to meet specific loads, then make the free length a reference dimension. The loads should be specified at a definite extended length and not as a deflection from free length. Key to the graphics:
Stress In the body of a helical extension spring the wire is stressed in torsion. You must include the stress from initial tension to calculate the total stress. When estimating the maximum allowable stress, the stresses in the hooks or loops must also be considered(see graphics below). Take note that in some cases the spring is extended beyond the maximum service height in installation. This will result in the spring being stressed beyond the specified maximum service stress. Go to this page for a further discussion on stress levels - Stress Level Considerations in Spring Design Square or Rectangular Wire The same considerations apply here as they do for compression springs. You can go to this page on
Compression Spring Design
for a full explanation on use of square or rectangular shaped wire.
Hook/Loop and End Design Today, most hooks, loops, and bends on extension and torsion springs can be made complete on the spring machine. Though, many spring manufacturers still use seconday equipment for short orders. When the stresses in hooks and loops exceed the body stresses they must be considered in the spring design. Critical stresses can occur at Sections A and B shown in the graphics. The practice that is recommended is to make r4 greater than 2d. Another practice is to wind the last few body coils on each end with a decreasing diameter. This will reduce the mean diameter D so that the stresses in the hooks can be minimized. This will often solve a hook breakage problem. 
Clearance Extension springs, when defelcted, do not have a buckling problem like compression springs. What needs to be considered is clearance between adjacent parts. When a spring is dynamically loaded and then suddenly unloaded, it may vibrate laterally. If clearance is not allowed this vibration may be quite noisy and could result in premature spring failure from the spring rubbing on adjacent parts. Maximum Deflection Without Set There is no definite deflection stop for extension springs like a compression spring has. When they are working they are not compressed to a solid height. They are extended. Therefore the maximum extended length without set should be calculated and specified. If a solid stop is needed a compression spring can be converted to act like an extension spring using a device made with a couple of hooks installed in the spring. This device is called a drawbar spring(see sketch above). SPECIAL NOTE: The design formulas given here do not apply if the helix angle exceeds 12 degrees at the test deflection. Above 12 degrees large errors are introduced due to the bending stresses. Some additonal hook and loop formations that are used most often:
The links below will take you to additional reference pages that you should review to get a complete picture of all aspects of these springs and their design. -for material references: Spring Steel Types Used In Springmaking
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