Machinery's Handbook, 31st Edition
336
Spring Design
Deflection F°
Free Position
90°
16 lbs. = P
d
= R ” 5 8
Loaded Position
” 1 2
” 21 32
To Fit Over
Left Hand
rod ” 7 16
Fig. 24. Torsion Spring Design Example. The Spring is to be Assembled on a 7 ∕ Method 1, using table: From Table 14, page 339 , locate the 1 ∕ 2 inch inside diameter for the spring in the left-hand column. Move to the right and then vertically to locate a torque value nearest to the required 10 pound-inches, which is 10.07 pound-inches. At the top of the same column, the music wire diameter is found, which is Number 31 gauge (0.085 inch). At the bottom of the same column the deflection for one coil is found, which is 15.81 degrees. As a 90-degree deflection is required, the number of coils needed is 90 ∕ 15.81 = 5.69 (say 5 3 ∕ 4 coils). The spring index . . . . d D 0085 0500 0085 688 = + = and thus the curvature correction 16 -Inch Support Rod. factor K from Fig. 20 = 1.13. Therefore the corrected stress equals 167,000 3 1.13 = 188,700 pounds per square inch which is below the Light Service curve (Fig. 7) and there- fore should provide a fatigue life of over 1,000 cycles. The reduced inside diameter due to deflection is found from the formula in Table 13:
free
N ID ^
. 575 0500 0479 . #
h
in
.
.
ID
=
=
=
. 360 575 360 90 +
1
N F +
This reduced diameter easily clears a suggested 7 ∕ 16 inch diameter supporting rod: 0.479 − 0.4375 = 0.041 inch clearance, and it also allows for the standard tolerance. The overall length of the spring equals the total number of coils plus one, times the wire diam- eter. Thus, 6 3 ∕ 4 3 0.085 = 0.574 inch. If a small space of about 1 ∕ 64 in. is allowed between the coils to eliminate coil friction, an overall length of 21 ∕ 32 inch results. Although this completes the design calculations, other tolerances should be applied in accordance with the Torsion Spring Tolerance Table 16 through Table 17 shown at the end of this section. Longer fatigue life: If a longer fatigue life is desired, use a slightly larger wire diameter. Usually the next larger gage size is satisfactory. The larger wire will reduce the stress and still exert the same torque, but will require more coils and a longer overall length. Percentage method for calculating longer life: The spring design can be easily adjusted for longer life as follows:
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online