invites curling. If torque is required, the motor form is recommended. A torque or friction load will also cause a "hysteresis" effect. In an extreme case this condition can render the spring non-functioning.

STABILITY 
Unless properly designed, a long, free extended portion of a spring may have a tendency to buckle or curl-particularly at the start of a return stroke. While there is no theoretical limit to such a spring's extension, it is advisable to confine any long extended portion in a way that will prevent curling. This can be done by providing some external support or guide. Stability increases as the width is increased in its relation to thickness. Width should be at least 50 times the thickness. A width to-thickness ratio of 250 is a recommended maximum. Good drum design, good bearings and careful attention to avoid high mass
inertia, will contribute to stability.

INITIAL LOAD 
From its relaxed condition, the coiled-band extension spring must be withdrawn at least 1.25 times its drum diameter before developing full rated load. After this extension is made, the force will be maintained
at any further extension. (Figure 13)
             

Figure 13 To Develop Rated Load, the Spring Must be Withdrawn 1.25 Times Its Drum Diameter.

5.0 DESIGN PROCEDURE 
It is convenient to proceed in a stepwise fashion as follows:
Step 1-Selection of material: 
There are two basic materials to choose from; High-Carbon Spring Steel (SAE 1095) and Stainless Steel Type 301/302. Since there are sometimes major differences in cost, compact-ness and availability between the two, it is advisable to complete the selection procedure for both materials, allowing for a final comparison. High-Carbon Spring Steel (SAE 1095): Dark metallic blue or black, after temperature processing; generally more economical in the thicker stock.
Stainless Steel: Better resistance to corrosion; better appearance; more economical in thinner stock. Types 301/302: Light metallic amber to uneven dark brown, after heat treating. Type 301 "High Yield": Used for highest possible stress ratings; color similar to regular Types 301/302.