Step 2: Estimation of fatigue life in cycles
A cycle is a complete change of curvature from the coiled to the straightened and back to the coiled
condition. Table 1 shows the load (or pull), P, as a function of spring parameters for high-carbon spring
steel (charts B, C, D, E, F, G) and stainless steel (Type 301/302: charts K, L, M, N, P; type 301 "High yield":
charts H and J). Each chart corresponds to a predetermined minimum fatigue life in cycles.
Step 3: Determination of load (P).
This is a function of the application.
Step 4: Determination of drum diameter (D2), natural coil diameter (2Rn), spring-stock thickness (t) end width (b). These quantities are obtained from the charts in Table 1 as a function of load (P). Acceptable values of
the design load should be at or slightly above the nominal value. Too low a load capability is unacceptable
and too high a capability results in a larger than necessary and more expensive spring. In the design
charts: D2 is given in column 3 2Rn is given in column 2 t is given in column 1 and b is given in the horizontal entries marked B, C, D......Y at the top of the charts on each page of the
table.
Step 5: Determination of total length of spring
This is equal to the total desired working deflection plus the following:
(i) 1.25 diameters (D2), which are required to develop full force.
(ii) About 5 diameters (or 1½ turns), which should remain on the bushing when the spring is in the fully
extended position.
5.1 Example
Consider the case of a 20 lb. vertical door counterbalanced by two NEG'ATOR springs on two 24-inch vertical
tracks, which are 1" wide.
A maximum operating life of 2000 cycles is considered adequate and the door
operates in a humid atmosphere.
Step 1: Material selection
Because of the humid atmosphere we select stainless steel.
Step 2: Estimation of fatigue life in cycles
This is given as 2000 cycles. Chart H of Table 1 applies to stainless steel and a fatigue life of 2500
cycles. The latter is a reasonable estimate for preliminary design purposes (i.e. it is slightly higher than the
nominal design value). We shall therefore use chart H in the design calculations.
Step 3: Determination of load P
Consider a spring material of width b = 3/4" (Column K of table H) and thickness t = 0.020". According to
the chart this will develop a pull of 9.9 lbs., which seems adequate (since there are two tracks and each has
a NEG'ATOR spring). A spring with less capacity will not be safe and one with a greater capacity will be
unnecessarily large as well as more expensive.
Step 4: Determination of spring parameters
The width (b) and thickness (t) have been obtained in Step 3 in this example. For these values of band t
chart H shows a natural spring diameter of 1.46" and a drum diameter (D2) of 1.75". |
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