Machinery's Handbook, 31st Edition
Spring Design 325 Extension Springs.— About 10 percent of all springs made by many companies are of this type, and they frequently cause trouble because insufficient consideration is given to stress due to initial tension, stress and deflection of hooks, special manufacturing meth- ods, secondary operations and overstretching at assembly. Fig. 15 shows types of ends used on these springs.
Machine loop and machine hook shown in line
Machine loop and machine hook shown at right angles
Small eye at side
Hand loop and hook at right angles
Full loop on side and small eye from center
Small eye over center
Double twisted full loop over center
Single full loop centered
Reduced loop to center
Full loop at side
Small off-set hook at side
Machine half-hook over center
Hand half-loop over center
Plain square- cut ends
All the Above Ends are Standard Types for Which No Special Tools are Required
Long round-end hook over center
Long square-end hook over center
V-hook over center
Coned end with short swivel eye
Coned end with swivel bolt
Extended eye from either center or side
Straight end annealed to allow forming
Coned end to hold long swivel eye
Coned end with swivel hook
This Group of Special Ends Requires Special Tools
Fig. 15. Types of Helical Extension Spring Ends Initial tension: In the spring industry, the term “Initial tension” is used to define a force or load, measurable in pounds or ounces, which presses the coils of a close wound exten- sion spring against one another. This force must be overcome before the coils of a spring begin to open up. Initial tension is wound into extension springs by bending each coil as it is wound away from its normal plane, thereby producing a slight twist in the wire which causes the coil to spring back tightly against the adjacent coil. Initial tension can be wound into cold-coiled
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