THE SPRINGING OF COMMERCIAL MOTORS.
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An Article Based upon the Contention Advanced by J. L Thornycroft and Co., Ltd., that the Spring which is Flat under Working Load is the Most Satisfactory.
IT is impossible to exaggerate the importance which the quality of the springing of a vehicle bears to its performance in service. It is thought that a description of the methods of design and manufacture pursued by the makers of such a successfully sprung vehicle as the Thornycraft would be of great interest to readers of this paper. We give below, therefore, some notes of the views and principles which are worked to at the Basing-stoke works of the company regarding this subject.
There are not wanting lorry users whose one idea seems to be that springs should be so still that, whatever load they choose to put on the vehicle, there shall be no sign of overloading so far as the springs are concerned ; nor, in. the opinion of such people, should the springs break down with any load, which circumstances, however unreasonable, may cause to he nut on them.
This, however, is far from being the right view, the consensus of opinion amongst wise users being that a well-sprung vehicle isone in which the driver cuuld make a run of 100 or more miles a day without getting tired or " fed up." The man who is tired is demoralized to a certain extent, and the same thing applies to the inanimate parts of a motor lorry, as both the man and the parts which are continuously suffering from fatigue induced by continued vibration are liable to sudden and disastrous failures.
The proper function of the springs is to protect the driver, the vehicle and the load from the vibration and shock effects of high-speed travel on uneven road surfaces.
Superficially, there .does hot appear to have been any marked improvement in springing for many years, and it is quite true that the visible changes in design are remarkably few. This is partly due to
the fact that the art of spring mounting and the use of spring steel, as compared with the other materials of engineering, were brought to a very high state of perfection before the motor era, and comparatively small advances have since been made in the quality of spring steel, and. even now it is held that the oldfashioned plain carbon steel spring is capable of holding its own, and is, in the opinion of many engineers, the equal in ninny ways of any alloy spring.
The Qualities of Good Springing.
The qualities of a well-sprung vehicle cannot be ascertained by mere inspection, and it is true that vehicles on which the. springing appears to be very similar may differ in practice very greatly as to their riding qualities. These differences are due to a large number of more or less obscure detailed differences of design, the majority of which have been evolved by trial and error in the course of time, in many instances long before any theory to account for the change was thought out. However, a groat deal more of what is requisite to construct a well-sprung vehicle is known by technical men at the present time than was the ease some years ago, although there doubtless remains a large quantity of information which is as yet undiscovered.
There are, moreover, several very simple and well-recognized principles of springing which are applicable to all vehicles, a few of which will be dwelt upon in the discussion which follows on the springing system employed on the Thornyeroft vehicles.
One of the well-established and fundamental needs of a well-sprung vehicle is flexibility of the springs. This should be as great as the practical considerations of design will allow. There are sevetal reasons which limit the amount of flexibility which, in practice, can be given to snrings, such as the limitation to the thickness of plates, length and width of the spring, and weight of materials, which can be allowed without increasing unduly the chassis weight.
An important point has reference to the periodicity —or, in other words, the time taken for complete vibration up and down of the spring and axle. Not only is as low a period of vibration as possible desirable in springs, but it is necessary so to choose the periodicities of the leading and driving springs that they do not vibrate at the same rate, as otherwise they would be liable to produce pitching.
One of the features which tend to increase the flexibility of a spring is, of course, the reduction of the thickness of the leaves, which should be as small as can be conveniently used. Naturally, there are limits to this, as to the other desirable features, such as increase of weight, which would be the consequence of excessive progress in this direction. The length of the spring is also a very important point, but the same considerations—i.e., the question of weight— will place a limit on this feature, which, if otherwise, would doubtless be increased to gain the improved qualities which are obtainable thereby.
Width and Number of Plates in a Spring. '
The influence of the width and number of the plates is largely one of friction, which, if not carried to excess, has a beneficial damping effect on the vibration. It is clear, if a spring is made of a large number of comparatively narrow plates, that the frictional effect which is caused by the sliding of these plates on one another is greater than if the spring is constructed of comparatively few plates of greater width.
People with practical experience of behaviour of springs are well acquainted with this sliding motion of the plates which takes place with the up-and-down movement. This feature can be readily observed by looking at a vehicle which is comparatively new, or has been recently painted, -when the evidence of the sliding of the leaves will be clearly shown on the paint near the ends of any leaf, but more particularly on the longest leaves. This movement, as is well known, explains the difference in riding qualities between a new spring and one which has become rusty between the leaves. When originally put together, the springs are liberally painted with a white lead paint, which acts as a lubricant ; but, in course of time, this film of lubricant is destroyed, and its place is taken by rust.
The Advantages Offered by Greater Width in Spring Plates.
This particular feature is a reason why, generally speaking, wide spring plates, as fitted to Thornycroft chassis, are preferable to narrow ones, as it is clear that a wide plate will retain the lubricating paint for a longer period, and will not so easily become rusty or tight, and also, when it does become rusted, there are not so many frictional surfaces to give the stiffening effect which makes a spring harsh.
The amount of movement of these plates, as also the amount of movement of the shackles of a spring in the fore and aft direction, is, to some extent, a guide as to the correctness of the design of the springing, since spring systems which show a very large amount of shackle movement, or plate sliding, greater than usual, are invariably bad as regards the fore and aft movement of the axle, and also with respect to their general riding qualities. B8 This point, which is attracting a good deal of attention at the moment, is the movement which is imparted in a fore and aft direction to the axles of the vehicle, in consequence of the vertical spring motion. In order to make it quite clear what are the features which influence this, we propose to go somewhat fully into the matter, and describe in detail certain experiments which have been made to show exactly how much this amounts to on one particular vehicle spring. The spring chosen for this experiment is the rear spring of the recently designed Thornyeroft 2-tonner, type BT. The spring in question is partly shown at rest on the ways of a springtesting machine in Fig. 1. it will be noticed that it has 10 leaves of equal thickness, with the exception of the top leaf, which is Ain. thicker than the others. The camber, at rest, as shown in this figure, though it cannot be clearly read from the photograph, is 22? ins. The 5-ft. rule shown in the photograph has the far end resting on a mandrel similar to the one which is seen. The end of the scale is touching a similar square to the one seen in the photograph, which is in an exactly similar position beyond the mandrel, so that the reading,which is shewn by the edge of the square—i.e., 3 ft. 11* ins.—is the exact centre distance of the spring when at rest.
A Well-tried Method of Testing Springs.
It is clear that, if a load is applied by the machine to the spring until the camber is reduced to any required amount, and if the squares are maintained in position against the mandrels as was the case for these three photographs, the rule will show the variation which occurs in the length between the spring eyes.
Fig. 2 shows this dimension when the spring has been loaded sufficiently to make the top plate of the spring, which is, of course, the lowest one in the photograph, perfectly flat throughout its length. This position is the normal loaded position of the spring when in place on the lorry carrying the catalogue load, and it will be noted from the photograph that in this position the spring centres are 4 ft in. apart.
Fig. 3 shows a further 4-in, increase of camber, causing a reversal of curvature with a corresponding increased load on the spring, and it will be seen that the spring centres are 3 ft. 1114 ins. These positions, together with the intermediate points which were also noted during the experiment, and which are indicated by dots on the curve, are shown on Fig. 4 (which is a chart showing the complete movement of the eye of the spring remote from the fixed forward end). From this diagram the length of the spring can be read to correspond with any deflection from ; the position when at rest to that when a total of 6-1 ins, of vertical movement due to load, has been imparted.
The next important point to be noticed is that this motion, small though it is, is twice the amount of the fore and aft motion of the centre of the spring to which the axle is fixed, and, since the spring palm and part of the spring to which it is attached remain throughout their travel parallel to the original centres of the spring, the movement of the axle is exactly similar to the movement of the centre of the spring.
Springs Flat in Loaded Position give Less Shackle pin Wear.
As shown on the diagram (Fig. 4), the total movement of the end of the spring eye is only c in., and, therefore, the fore and aft movement of the rear axle of the Thornyeroft BT chassis is only 9-32 in., reckoned from the no-load position of the spring to an overload or bump represented by a compression of 4 ins, beyond the loadedposition. As, in practice, the up-and-down motion of the spring is considerably less than this at all normal loads and speeds, it will be realized how extremely small the fore and aft travel of the axle may be with the well-chosen springing system such as this fitted to the Thornycroft BT vehicle.
For comparative purposes a spring diagram is shown in Fig. 5 for a spring with very much greater camber; that. is to say, a camber in the loaded position of 5 ins, and making a total movement due to load and bump of 6 ins.—Le., 3 ins, each side of loaded position.
The fore and aft movement is 31 at the spring eye, and it will be noted that the motion is, in this case, very much greater than with a fiat spring, being over sieand a half times as much. It is unnecessary to point out the great advantage which the latter design has in the matter being discussed.
This fore and aft movement is of even greater importance on the leading axle, and the exact motion imparted to the axle by the spring has to be carefully investigated by the designer of the steering gear, so that he can select suitable locations for the joint pins of the fore and aft tube for the steering control. Thornycroft chassis are exceptionally good in this respect, but, if selected locations are tumult
able, a very uncomfortable and unsteady swaying motion is imparted to the leading wheels when on rough roads, as, at such times, there will be considerable up-and-down movement on the front axle of the vehicle. This, of course, interferes very badly with the steering, and is not only dangerous, but is also one of,the causes of the tiredness which drivers experience after driving vehicles in which these features have not been properly arranged in the design.
The Stress on the Material of the Springs.
Another point of spring design which requires careful attention, and which it may be interesting to explain as regards the Thornycroft practice, is the matter of stress allowed in the material of the springs. It is essential that the stress allowed in springs should be fairly high, as it is only by making this stress fairly large that the full elastic properties of the material are utilized. The judgment of the manufacturer, or his designer) is exercised in so selecting the stress that the springs themselves will give a service of sufficient length of life and yet retain fully the qualities of springing which go with a, high stress, and on this particular point we would mention that no really well-designed spring can be expected to last as long as the majority of the other parts on the chassis which it is designed to protect, and that with details which are stressed as highly as these parts are a certain percentage of failure is
bound, in. time, to occur. This, is only in accordance with what might be expected, having regard to the nature of the duty which they are called upon to perform. At the same time, Thornycroft springs can be relied on to last for many thousands of miles, if not overloaded or overdriven, before showing signs of fatigue.
Securing Flexibility and Long Life in Springs.
This is one reason why it is important that spring material should always be of the highest possible quality, as by this means the stress could be kept at a high figure and the greatest measure of flexibility obtained from the same weight of material, while retaining a life correspondingly-Jong. Such considerations as those we have just . enumerated explain why a spring cannot be persistently overloaded without trouble occurring, and also explains why springs which are so designed that they are underloaded when they have the makers' proper working toad on them are not effective or desirable from the user's Point of view. At lighter loads than the correct one the springs will be too stiff, and the vibration will be too rapid for the maximum of comfort, and, at heavier loads than designed for, the stress in the material of the springs will be too great for a reasonable length of life.
It is obvious, therefore, that those springs which stand any amount of overloading and overdriving and do not give any trouble owing to breakage 'cannot possibly be giving their best service as absorbers of vibration.
In connection with this question of spring breakage it is alwaysadvisable to fit a new complete spring rather than to patch it up at home or at the local garage with a new plate to replace the broken one. The reason for this is that breakage, when it does occur, is usually, and should of course only occur, after the spring has done a very considerable mileage ; and there is every likelihood that the other plates in the spring are in a fatigued condition, or may even have incipient cracks which can easily escape detection except by a spring expert.
If a new spring is fitted the old one can be returned to the makers for repair and reconditioning, by whom it will be examined, have defective plates removed, and the others heat-treated to restore the original spring qualities which have become deteriorated in use. After being re-built the spring will be retested, and it can be relied on to stand up to the original test loads as applied when new, and when returned to the owner will be to all intents and purposes a new spring.
Experience has shown that the first breakage of a plate in a spring is often followed, if repaired by fitting a new plate, by a further series cif failures at short intervals ; this experience is avoided by the recommended procedure given above.
It would be quite easy, from what Thornycrofts tell us, for them to fit their chassis with springs which would give no trouble in themselves, even when overloaded to any, practicable degree; but they assure us that they would be extremely sorry to do this, and we are sure that in the end their customers would be sorry also.
Attachment of Spring to Spring Palm.
The last feature af Thornycroft spring design which we shall discuss here is the question of the attachment of the spring to the spring palm. This matter has received a tremendous amount of attention for some time. It is well known to users, and to those who, from time to time, have to assemble or repair heavy lorries, that in order to tighten up the spring bolts of the usual pattern, the spring has to be flattened down to the palm, often from quite a considerable amount of free curvature, by the bolts which hold it to the palm. This entails nutting up with a spanner of considerable power, .and, of course, the al 0
tension in the bolts induced thereby is very considerable.
It is very important, therefore, that the material of spring-bolts should be of the highest quality, and for some years it has been the practice of Thornycroft to make these bolts of a high quality nickel-chrome steel, carefully heat-treated to obtain the maximum of strength and toughness. Any ordinary kind of steel is not of the least use for this purpose.
Further, the design of spring palms has been looked into, and the firm have recently introduced a modification of design, for which patent application has been made, which considerably reduces the stresses and strains which these bolts are called upon to sustain. This design is illustrated in Fig. 6, in which it will be noted that the tightening up of the spring bolts to the extent that is necessary to obtain a thoroughly strong grip of the springs does not put any constraint whatever on the spring as regards its curvature. Now this feature is very important, and it has been found in practice, with this device, that spring bolts which, before the scheme was introduced, were continually becoming slack, do not,. after the device is fitted, give any further trouble in this respect. The explanation of why this is so is interesting. It is clear that the immense stress which is to be put into the bolts to flatten down the spring on the palm when nutting up -originally is partly relieved when a load is placed on the vehicle, and with any ordinary design of spring palm it is quite possible, after nutting the bolts up tightly on the unloaded vehicle, to find that the mere 'application of load to the platform has decreased the eurvature of the spring sufficiently to slacken off the bolts so that a couple of extra turns are required to produce tightness.
This is such a well-known feature that it is common practice to nut up the spring bolts in position with the load on the platform in order that the slackness may not occur.
What has been said above regarding this matter leads, naturally, to the conclusion that the conditions under which these boltshave to work are such that they are extremely likely to becomeslack, seeing that the variations of spring position on the road and with the varying loads on the platformof the vehicles are continually stretching and relieving the bolts. Such conditions are so severe on th'e threads and nuts that it is uncommon to find a chassis of conventional design which has been in use for any time which has not at least one of the spring bolts lack when -examined. More often than not every belt on the chassis is slack, unless the vehicle is in charge of someone who has had his attention drawn very strongly to the necessity of constant attention to this matter It will be realized, therefore, what a step forward it has been so to arrange these bolts that there is not of necessity any great variation of stress in them under any ordinary working conditions.