Belleville spring H h t f Garter spring g Constant force spring h Constant

# Belleville spring h h t f garter spring g constant

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) Belleville spring H = h + t ( f ) Garter spring ( g ) Constant force spring ( h ) Constant force spring motor FIGURE 18–2 Several types of springs
CHAPTER EIGHTEEN Springs 597 without grinding. In unusual cases the ends may be ground without squaring, or they may be left with plain ends, simply cut to length after coiling. You are probably familiar with many uses of helical com- pression springs. The retractable ballpoint pen depends on the helical compression spring, usually installed around the ink supply barrel. Suspension systems for cars, trucks, and motor- cycles frequently incorporate these springs. Other automotive applications include the valve springs in engines, hood link- age counterbalancing, and the clutch pressure-plate springs. In manufacturing, springs are used in dies to actuate strip- per plates; in hydraulic control valves; as pneumatic cylinder return springs; and in the mounting of heavy equipment for shock isolation. Many small devices such as electrical switches and ball check valves incorporate helical compression springs. Desk chairs have stout springs to return the chair seat to its upright position. And don’t forget the venerable pogo stick! The following paragraphs define the many variables used to describe and analyze the performance of helical compression springs. Diameters Figure 18–5 shows the notation used in referring to the characteristic diameters of helical compression springs. The outside diameter ( OD ), the inside diameter ( ID ), and the wire diameter ( D w ) are obvious and can be measured with standard measuring instruments. In calculating the stress and deflection of a spring, we use the mean diameter, D m . Notice that Spring Diameters OD = D m + D w ID = D m - D w of the coil and on the elastic modulus of the spring material. Basically, the force is related to the deformation of the strip from its originally curved shape to the straight form. Power springs, sometimes called motor or clock springs, are made from flat spring steel stock, wound into a spiral shape. A torque is exerted by the spring as it tends to un- wrap the spiral. Figure 18–2 shows a spring motor made from a constant-force spring. A torsion bar, as its name implies, is a bar loaded in tor- sion. When a round bar is used, the analyses for torsional stress and deflection are similar to the analysis presented for circular shafts in Chapters 3 and 12. Other cross-sectional shapes can be used, and special care must be exercised at the points of attachment. Information about commercially available springs can be found in Internet sites 3–5 and 13. The methods of ana- lyzing and designing springs used in this book have been de- veloped by applying principles from References 1–8. 18–3 HELICAL COMPRESSION SPRINGS In the most common form of he- lical compression spring, round wire is wrapped into a cylindri- cal form with a constant pitch between adjacent coils. This basic form is completed by a variety of end treatments, as shown in Figure 18–3. For medium- to large-size springs used in machinery,