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).
Technical discussion on the installation, service, and riding quality of air-springs.
| Identifier | ExFiles\Box 43\3\ Scan022 | |
| Date | 28th September 1923 guessed | |
| Page 18 AIR-SPRINGS AND RIDING-QUALITY workmanship must be maintained throughout the course of manufacture. Each element that is to carry air pressure must be tested thoroughly to assure its being air-tight before it passes on to the assembling. Where a leather cup-washer is used, it is even necessary to treat each piece with a preparation that fills the pores of the hide. Next in importance is the finish given to the cylinders. These are ground to close limits both internally and externally. Any scratches, nicks or flaws in the material that might allow the escape of air or cause damage to the cup-washer cannot be allowed. In the machining operations, particular attention must be given to the alignment of the surfaces, to facilitate assembling and secure flexibility of the spring. The assembling department is responsible for building the spring up from the finished parts, testing the semi-finished and the finished product, and putting on the finishing touches such as painting. Fig. 26 shows a direct-acting spring being assembled. The piston-rod is held in the stand and the cup-washers, expanders and followers are assembled in order. The cylinder with the dome assembled is about to be slipped on over these parts. From this step the spring goes to the "dancers" to be worked-in. At the left of Fig. 27 are three springs of another type, but at the same stage of assembly, being run-in on the dancer. After being run-in, the springs are put into the frames, shown in the center of this view. Here they are put through an endurance test; that is, they are allowed to stand under pressure for hours and are inspected then for loss of pressure. The springs then encounter the final operation, where the guide or bottom is assembled. Fig. 28 shows the assembled spring again on the dancers. The purpose of this second run-in is to find any spring that would be a "sticker" under working conditions. Before the spring goes to the paint-room, it again goes through an over-night endurance test. INSTALLATION AND SERVICE The air-spring is, obviously, not a product to be sold by hit-or-miss methods. The one way to sell the air-spring properly is installed on the car. Only factory branches and qualified licensed representatives should make the installations. The labor involved is confined chiefly to fitting the brackets to the frame of the car. This must be done with care, so that the part of the bracket known as the pad, to which the air-spring is bolted, shall line-up perfectly with the frame and with the steel springs. Once the brackets are installed, the work of attaching the air-springs takes 1 hr. Occasionally, an objection is made against cutting the frame of the car. It is not always necessary to do this when installing air-springs. Some frame makers, however, run the frames out so close to the spring-eyes that to attach the air-spring no alternative other than making a new cut-off is practicable. By so doing it is possible to build the bracket and the air-spring into the car so that the equipment presents the appearance of being really a part of the car and not an after-thought, as it would be if the original spring-hangers had been retained. Should a customer, in disposing of his car, wish to keep his air-springs for future use, we can always weld the pieces of frame back on the car, restor'ng its original condition. Owners have been known to resort to this step, even several times, before they could be prevailed upon to buy a new set of air-springs. A built-in-bracket job that necessitates cutting the frame is always designed to keep the body in its original position; and, in some cases where the mudguards and body clearance will allow it, actually to lower the center of gravity. A good rule to follow is that the maximum increase in height over the axle must not be more than ¾ in.; and, if it is possible to design a "no-cut" bracket that will stay within this limit, by all means do it. But if the increase is more than ¾ in., by all means cut the frame and keep the body low. The importance of proper installation cannot be over-stressed, for it is so very closely allied with sales that, of the two, one can hardly exist without the other. A poor installation should never be made, for it not only increases sales resistance on the particular make of spring but also brings discredit upon any air-spring maker, whether he is responsible or not. Where the installation ends, the responsibility for service begins. Despite the fact that the layman is qualified to give his air-springs whatever attention they may need, it is important that the manufacturer be in a position to render this service. Air-springs require a certain amount of attention to maintain the correct adjustment of riding position. About once a month it may be necessary to add a certain amount of air. Oil-cup lubrication demands attention also to avoid the spring becoming inoperative because of lack of oil. The inside of the spring, once it is adjusted to the proper amount, requires no attention other than to change it once or twice annually, depending on mileage and temperature variation. Shackle-bolts, bushings and the like must be renewed the same as other standard parts of the car. The life of the air-spring itself is dependent upon the usage to which it is put and the care it receives. I again call attention to the riding characteristic curve of the air-spring combination and hope that its general characteristics will be studied and its natural advantages utilized to produce better riding-quality. FIG. 28—FINAL TEST AND ASSEMBLY DEPARTMENT. This View Shows the Dancing Machines That Are Used To Work-Out the Air-Spring by Giving It the Same Up-and-Down Motion Under Pressure as it Receives in Actual Service. This Is a Severe Test To Find Any Spring That Would Stick under Working Conditions Page 19 AIR-SPRINGS AND RIDING-QUALITY THE DISCUSSION QUESTION:—What is the amount of air pressure in the air chamber? J.{Mr Johnson W.M.} J.{Mr Johnson W.M.} McELROY:—The air pressure varies with the weight of the car. Under normal load we generally try to carry between 60 and 75 lb. per sq. in. The pressure range, then, under actual operating conditions, is from 50 under expansion to 200 lb. per sq. in. under compression. We build 2½, 3 and 3½-in. diameter air-springs to take care of the cars of different weights. QUESTION:—How long can the air-spring be run with the same air? MR. McELROY:—That depends on the mileage and the use. I have known cars to run 1 year or more. Generally speaking, any air-spring in fair condition ought to hold up 2 months without putting in additional air. QUESTION:—Is any effort made to put the car oilers into good condition while testing? Do you use oil? MR. McELROY:—If the steel springs come into our service-station in good condition, we do not touch them. If they come in rusted-up, we lubricate them with oil. In most of the tests we test the cars "as is," without attempting to lubricate the steel springs. However, by changing the tire pressure you can get better riding-quality. QUESTION:—Do you install air-springs on the rear of cars that have cantilever rear-springs? MR. McELROY:—No. It costs too much to rebuild steel springs and that has to be done to put air-springs on the rear of such cars. QUESTION:—What allowance is made for riding clearance, in consideration of the increased flexibility? MR. McELROY:—The average car has ample clearance on the front end but, in some cases where the front hanger has little drop, we increase the clearance about ¾ in. over the axle to take care of the initial shock movement. After the first initial shock, the tendency with the air-spring is to rebound and work the axle farther away from the chassis. The single-bump curves shown illustrate how one single-bump back-reaction has a tendency following it to float the chassis slightly higher. Also, the increase from the initial shock on the up-stroke is considerably less than the increase on the down-stroke; so, the total increase of 25 per cent only amounts to an average of 10 per cent on the up-stroke. Sometimes, especially on custom-built jobs, we have trouble with the wheels hitting the fenders in the rear, the body being built too close for increased amplitude. Generally speaking, we try to keep the car at a normal level; but, if it is possible and we have plenty of mudguard clearance, we lower it a little, say ¾ in., at the rear, in which case we add extra leaves to the steel spring. QUESTION:—Is the tendency of air-spring equipment sufficiently definite to take advantage of designing the car 30 per cent lower to begin with and thereby prevent the abnormal raising while running? MR. McELROY:—Yes, if the car is so designed from the beginning; but I doubt if any material advantage would be gained, as the increased axle clearance obtained when the air-spring is working on a rough road is only about one-third of the additional axle movement, say 10 per cent in this particular case. QUESTION:—How about an allowance for initial shock in such a case? MR. McELROY:—I did not take into consideration the additional percentage of initial shock, which would be 3/8 in., as I had in mind the knowledge that we do lower cars occasionally, sometimes as much as 5/16 in. all around, due to the fact that they originally had more clearance than was necessary. In the case of a new car layout, I do not see how any saving could be obtained in axle clearance. Theoretically, it would have to be increased to take care of the initial shock-movement. QUESTION:—Why, then, are extra leaves put into the steel springs when you make installations? MR. McELROY:—On the average car the front steel spring had a 450-lb. spring-scale. The air-spring combination with it will reduce it to about a 200-lb. spring-scale with the air-spring scale from there on. The rear spring has a spring scale of about 165 lb., and often it is necessary to put in extra leaves to get the proper combination; otherwise there would be too much flexibility and the movement on the rear end would be too great. We put less work on the steel spring, stop the flexing and throw the shock into the air-spring to get the proper combination. Usually, we figure that the best working condition is at least a 200-lb. scale rear-spring. QUESTION:—Would the air-spring alone be a better job if it were free from friction? MR. McELROY:—I think not. Some friction or damping is apparently necessary, as an air-spring entirely free from friction would have a high period and I fear that it would transmit its vibrations to the steel spring unless some provision were made to cut down the duration of the vibration. On the other hand, too much friction is a serious matter, especially in view of the fact that the static friction of an air-spring after it remains for some time in a cold garage is at least 10 times greater than the sliding friction. A MEMBER:—Complete tests and descriptions of an interesting pneumatic suspension for bicycles and automobiles are reported in a paper by Archibald Sharp, an English engineer, its title being Pneumatic Springs for Road Vehicles.* The piston cylinders have no contact with one another and are attached with what Mr. Sharp terms a "rolling packing mitten." One side of it is attached to one member, the other side to the other member, and the mitten rolls over on itself, as a golf stocking is rolled. Two advantages are said to result, approximate elimination of friction and permanent trapping of the air inside the mitten. QUESTION:—The central oil-plunger is designed to care for 17 lb. per sq. in. pressure. Does this not destroy the normal action of the steel springs to that extent? MR. McELROY:—If the little pump inside the air-spring were capable of pumping say to a pressure of 100 lb. of air per sq. in., it would pump the air-spring out and extend it, consequently, its volumetric efficiency is reduced so that it cannot pump more than to a pressure of 17 lb. per sq. in. That has nothing to do with the operation of the air-spring. The pump diameter is only 5/16 in. The amount of additional restriction due to the action of the pump, that is, to the oil that passes through from the pressure chamber to the lower chamber, is practically negligible. QUESTION:—Why it is not possible to get the same effect with a flexible steel-spring as with an air-spring? MR. ELROY:—A flexible steel-spring has a constant scale unless it is built up so that it is progressive in its action. In combination with the steel spring, the air-spring has an increasing scale. If the front suspension of a car were designed to have a spring of a scale equal to that of the air-spring combination at rest position, it would be too flexible and no added resistance would *See Proceedings of the Institution of Automobile Engineers, vol. 4, p. 97. | ||