Machinery's Handbook, 31st Edition
Involute Splines
2349
x
D
F
r 1
r 2
A
Section x-x
x
The accompanying diagram of a crowned spline shows the radius of the crown r 1 ; the radius of curvature of the crowned tooth, r 2 ; the pitch diameter of the spline, D ; the face width, F ; and the relief or crown height A at the ends of the teeth. The crown height A should always be made somewhat greater than one-half the face width multiplied by the tangent of the misalignment angle. For a crown height A , the approximate radius of curva ture r 2 is F 2 ÷ 8 A , and r 1 = r 2 tan φ , where φ is the pressure angle of the spline. For a torque T , the compressive stress on the teeth is: S T DNhr 2290 2 c 2 ' = and should be less than the value in Table 11. Fretting Damage to Splines and Other Machine Elements.— Fretting is wear that occurs when cyclic loading, such as vibration, causes two surfaces in intimate contact to undergo small oscillatory motions with respect to each other. During fretting, high points or asper- ities of the mating surfaces adhere to each other and small particles are pulled out, leaving minute, shallow pits and a powdery debris. In steel parts exposed to air, the metallic debris oxidizes rapidly and forms a red, rustlike powder or sludge; hence, the coined designation “fretting corrosion.” Fretting is mechanical in origin and has been observed in most materials, including those that do not oxidize, such as gold, platinum, and nonmetallics; hence, the corrosion accompanying fretting of steel parts is a secondary factor. Fretting can occur in the operation of machinery subject to motion or vibration or both. It can destroy close fits; the debris may clog moving parts; and fatigue failure may be accelerated because stress levels to initiate fatigue in fretted parts are much lower than for undamaged material. Sites for fretting damage include interference fits; splined, bolted, keyed, pinned, and riveted joints; between wires in wire rope; flexible shafts and tubes; between leaves in leaf springs; friction clamps; small amplitude-of-oscillation bearings; and electrical contacts. Vibration or cyclic loadings are the main causes of fretting. If these factors cannot be eliminated, greater clamping force may reduce movement but, if not effective, may actu ally worsen the damage. Lubrication may delay the onset of damage; hard plating or sur face hardening methods may be effective, not by reducing fretting, but by increasing the fatigue strength of the material. Plating soft materials having inherent lubricity onto con tacting surfaces is effective until the plating wears through. Involute Spline Inspection Methods.— Spline gages are used for routine inspection of pro- duction parts. Analytical inspection, which is the measurement of individual dimensions and varia tions, may be required: a) To supplement inspection by gages, for example, where NOT GO composite gages are used in place of NOT GO sector gages and variations must be controlled. b) To evaluate parts rejected by gages. c) For prototype parts or short runs where spline gages are not used.
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