Machinery's Handbook, 31st Edition
316 STRESSES IN SPRINGS increased by 75 percent for torsion and flat springs. In using the data in Table 2 it should be noted that the values given are for materials in the heat-treated or spring temper condition. Table 2. Recommended Maximum Working Temperatures and Corresponding Maximum Working Stresses for Springs
Max. Working Temp., ° F
Max. Working Stress, psi
Max. Working Temp, ° F
Max. Working Stress, psi
Spring Material
Spring Material
Brass Spring Wire Phosphor Bronze
150 225 250 300 325 375 400 425 450
30,000 Permanickel a 35,000 Stainless Steel 18-8 75,000 Stainless Chromium 431
500 550 600 700 775 850 900
50,000 55,000 50,000 50,000 70,000 55,000 55,000 75,000
Music Wire
Beryllium-Copper Hard Drawn Steel Wire Carbon Spring Steels Alloy Spring Steels
40,000 Inconel
50,000 High-Speed Steel 55,000 Inconel X
65,000 Chromium-Molybdenum- Vanadium 40,000 Cobenium, Elgiloy
1000
Monel
K-Monel
45,000
a Formerly called Z-Nickel, Type B. Loss of load at temperatures shown is less than 5 percent in 48 hours. Spring Design Data
Spring Characteristics.— This section provides tables of spring characteristics, tables of principal formulas, and other information of a practical nature for designing the more commonly used types of springs. Standard wire gages for springs: Information on wire gages is given in the section begin- ning on page 2702 , and gages in decimals of an inch are given in the table on page 2703 . It should be noted that the range in this table extends from Number 7 ∕ 0 through Number 80. However, in spring design, the range most commonly used extends only from Gage Num- ber 4 ∕ 0 through Number 40. When selecting wire use Steel Wire Gage or Washburn and Moen gage for all carbon steels and alloy steels except music wire; use Brown & Sharpe gage for brass and phosphor bronze wire; use Birmingham gage for flat spring steels, and cold rolled strip; and use piano or music wire gage for music wire. Spring index: The spring index is the ratio of the mean coil diameter of a spring to the wire diameter ( D / d ). This ratio is one of the most important considerations in spring design because the deflection, stress, number of coils, and selection of either annealed or tempered material depend to a considerable extent on this ratio. The best proportioned springs have an index of 7 through 9. Indexes of 4 through 7, and 9 through 16 are often used. Springs with values larger than 16 require tolerances wider than standard for manufacturing; those with values less than 5 are difficult to coil on automatic coiling machines. Direction of helix: Unless functional requirements call for a definite hand, the helix of compression and extension springs should be specified as optional. When springs are designed to operate, one inside the other, the helices should be opposite hand to prevent in- termeshing. For the same reason, a spring that is to operate freely over a threaded member should have a helix of opposite hand to that of the thread. When a spring is to engage with a screw or bolt, it should, of course, have the same helix as that of the thread. Helical Compression Spring Design.— After selecting a suitable material and a safe stress value for a given spring, designers should next determine the type of end coil forma tion best suited for the particular application. Springs with unground ends are less expen sive but they do not stand perfectly upright; if this requirement has to be met, closed ground ends are used. Helical compression springs with different types of ends are shown in Fig. 12.
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