From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The formulas and considerations for designing leaf springs, including stiffness and plate overlap.
| Identifier | ExFiles\Box 43\2\ Scan266 | |
| Date | 12th December 1930 guessed | |
| - 3 - This is not strictly accurate as it takes no account of the stress in the individual plates when the spring is unloaded. An allowance should be made for this. 2. For an unequal spring, we have a formula: Stiffness = 1/EI * (L1^2 L2^2) / (2(L1 + L2)) where L1 and L2 are half lengths. (Note for equal ends this becomes: Stiffness = L^3 / 32EI where L = total length of spring) For a desired stiffness we can now calculate the value of I I = L^3 / (32E x Stiffness) I is the sum of the moments of inertia in bending of all the plates, and from this the plate thicknesses are chosen. We have found that possibly owing to inaccuracies in our constants or variations in thickness of plates, the stiffness of the final spring comes about 10% higher than the calculated stiffness and according design for 90% of the stiffness we require. The thickness and number of the plates is the only factor that affects the stiffness of the spring. 3. To obtain the free shape of the plates, and ensure that the spring will be flat in the flat condition - we estimate the total bending moments on the spring at the point of support (the end of the clamp or more accurately the center of the clamping bolt), and apportion this among the leaves according to their capacity (i.e. their individual moment of inertia and the stress desired to work to). Having assigned to each plate its bending moment at the center its free curvature is given by 1/R = M/EI This gives us the working radius of each plate. This has to be corrected for permanent set that it takes at first loading, and opening in quenching, each of these has the effect of reducing the so called "camber" by about the thickness of the plate. 4. Overlap of plate is Bending moment on plate when flat. Load at eye when flat. The leaves are made 3/8" longer at each end, allowing for average bedding 1/8" from end of plates. (Continued) | ||