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).
Analysis of air-spring suspension systems, comparing their load-deflection characteristics and vibration damping effects against traditional steel springs.
| Identifier | ExFiles\Box 43\3\ Scan015 | |
| Date | 1st September 1924 guessed | |
| 4 AIR-SPRINGS AND RIDING-QUALITY FIG. 2—CHARACTERISTIC LOAD-CURVE OF THE AIR-SPRING. The Lower Curve Is the Typical Compression Curve of an Air-Spring Alone; the Upper Curve Shows the Same Air-Spring Effect at the Axle if a Rigid Beam Was Used Instead of the Steel Spring. grammatic drawing showing the combined action of the air-spring and the steel spring. The load on the air-spring, considering the axle in the middle of the steel spring, is one-half the load at the axle. In Fig. 1 we have illustrated a 400-lb. scale steel-spring which is compressed 1 in. under load. An additional load will compress both the air and the steel spring. If the air-spring is compressed 1 in., the axle moves up ½ in. and, if it takes 240 lb. at the axle to compress the air-spring 1 in., it will compress the steel spring to the extent of 240 lb. or 0.6 in. This results in a total upward axle movement of 1.10 in. for an additional applied load of 240 lb. A 400-lb. steel-spring alone would need to have a 440-lb. additional load to compress it 1.10 in. The central and right views in Fig. 1 show the combined spring action when passing over a bump and going into a depression. The lower curve in Fig. 2 is the typical compression-curve of an air-spring; the upper curve is the air-spring compression-curve as at the axle, if we consider the steel spring a rigid member. Fig. 3 is the ordinary FIG. 3—CHARACTERISTIC LOAD-CURVE OF THE COMBINED AIR-STEEL-SPRING-SUSPENSION CONTRASTED WITH THE STRAIGHT-LINE LOAD-CURVE OF THE SAME STEEL SPRING THAT IS USED IN THE COMBINED SUSPENSION It Should be Noted in This Connection That the Load-Curve of the Combination Has an Ever-Changing Scale in Both Deflection and Load-Curve Excepting at Its Minimum Scale Is That of the Steel Spring and Its Maximum When the Air-Spring Has Reached the Full Extent of Its Stroke. straight-line load-curve of the spring, contrasted with the load curve of the air-spring combination. The load curve of the combination has an ever-changing scale that increases in stiffness as the load or shock increases, and its maximum scale is that of the steel spring, this occurring only when the air-spring has reached the full extent of its stroke. If the air-spring is designed so that it is impossible to complete its full working-stroke, the spring scale of the combination is always under that of the steel spring alone. The combination suspension has a long range and breaks off abruptly at the bottom of the curve into steel spring action alone; the scale then instantly changes to that of the steel spring. This occurs when the air-spring has reached the end of its downward stroke. The air-spring and steel-spring combination has a characteristic load-curve that allows maximum flexibility in the general working-range of the axle, yet has an increasing resistance to dissipate large shock-loads. By varying the compression volume in the air-spring, the load-curve of the combination can be made more flexible or stiffer, FIG. 4—CHARACTERISTIC LOAD-CURVES SHOWING THE FUNCTION OF THE AIR AND THE STEEL SPRING ALONE AND IN COMBINATION These Curves Were Obtained as the Result of a Laboratory Test on a 2½-in. Direct-Acting Air-Spring in Combination with a Standard Dodge Front-Spring. This Friction Is Static, Test Readings Being Made at 1-hr. Intervals. The Actual Sliding Friction of the Spring Alone Is Approximately One-Tenth of the Static Friction. as occasion demands. The proper combination will always give remarkable results. Fig. 4 shows the results of a laboratory test on a 2½-in. direct-acting spring working with a standard Dodge front steel-spring. The Dodge spring has a 36-in. over-all length, and the axle is located 15 in. back from the front eye, leaving the rear half 21 in. long. The vibration period and the duration of the vibrations are recorded musically. This damping effect is shown graphically in Fig. 5, which is a comparison of the vibration period of a leaf spring with that of the air-spring combination. The heavy-line curve in the upper portion shows the vibration of the steel spring alone as recorded on the moving paper. The air-spring and steel-spring combination under the same test is shown in the lower portion of Fig. 5. In this latter case the air-spring movement and the combined movement of both the air-spring and steel spring at the center or axle position were recorded simultaneously on moving paper; and, by making proper corrections, the effective movement of the air-spring at the axle was plotted. The shaded area represents the work of the air-spring at the axle. The difference between the combined movement and the AIR-SPRINGS AND RIDING-QUALITY 5 FIG. 5—ELEMENTARY TESTS SHOWING THE CHARACTERISTIC EFFECTS OF COUNTER SPRING-SUSPENSION. The Upper Set of Curves Illustrates the Difference in Period, Duration and Amplitude of the Same Steel Spring with (Dot and Dash Line) and without (Solid Line) Air-Springs when Passing over an Obstruction of the Same Height. The Combined Action of the Steel Spring and the Air-Spring at the Axle Is Shown in the Lower Portion. The Shaded Area Indicates the Work Done by the Air-Spring. spring effect, indicated by the shaded area, is the flexing or work done by the steel spring. Referring again to the upper portion of Fig. 5, the dot-and-dash line is the steel-spring action; it is the same as the inside boundary line of the shaded area and is moved up into the upper portion to show more clearly the decreased action, or bending, period and duration in the same steel spring when used in connection with the air-spring. In Fig. 5, it will be noted that the steel-spring vibration alone had a duration of 5½ sec. with a period of 87.2 vibrations per min., while the combination showed a duration of 3 sec. with a period of 60 vibrations per min. The steel spring, separated from the combination, showed a similar period; although it lags behind the combination curve slightly. Fig. 6 shows the apparatus used. A standard Dodge front steel-spring was hooked up with a 2½-in. direct-acting air-spring so that either the steel spring alone or the combination of the steel spring and the air-spring could be compressed at the center or axle position through a lever arrangement, as shown. The spring-suspension was given a constant load by applying a permanent weight at the end of the lever, and was then compressed by manpower so that 1½-in. additional compression movement was obtained at the center or axle. This additional force was applied quickly, and then the spring was allowed to vibrate freely at its own period. The recording instrument is a spring motor-driven paper-roll used in connection with an electrical timing-device, marking ½-sec. intervals on the moving paper. The average speed was approximately 15 ft. per min. This test is elementary and is not intended as a true example of actual working conditions; but it is exhibited to bring out the fundamental principle of air-spring suspension and to draw attention to the general characteristic involved in a typical air counter-spring arrangement. This same characteristic reduction in the period and the duration of the vibration is apparent in all the series of tests that have been made. FRICTION CHARACTERISTICS Regarding the friction that is present in the air-spring, I would say that it has a tendency to bind and stick. This is usually due to incorrect alignment, but there is also considerable trouble due to the design and the materials used in the air-spring division. The friction present in the air-spring can be divided into friction due to cup-leather material and design, bearing or mechanical FIG. 6—APPARATUS USED IN MAKING THE LABORATORY TESTS OF EITHER THE STEEL SPRING OR THE COMBINED STEEL AND AIR-SPRING. The Moving Paper Roll Was Originally Designed for Use With the Seismograph That Can Be Seen in the Lower Left Corner. The Movements of Both the Steel and the Air-Spring Are Recorded Simultaneously on the Moving Paper, the Former by the Tubular Member Extending Diagonally Downward from the Air-Spring at the Upper Left to the Paper Roll. The Combined Effect of the Air-Spring and the Steel Spring Is Recorded by the Vertical Wire That Extends Downward from that Point to the Record Roll. | ||