US3339908A

US3339908A - Tapered leaf springs

Info

Publication number: US3339908A
Application number: US449486A
Authority: US
Inventors: Rostislaw S Komarnitsky
Original assignee: Rockwell-Standard Corp
Current assignee: Rockwell-Standard Corp
Priority date: 1965-04-20
Filing date: 1965-04-20
Publication date: 1967-09-05
Anticipated expiration: 1984-09-05
Also published as: FR1463597A

Description

R. s. KOMARNITSKY 3,339,908

TAPERED LEAF SPRINGS Sept. -5, 1967 Fil ed April 20, 1965 3 Sheet s-Sheet l Heat Treatment Furnace Induction Heating Furnace INVENTOR HOST/SLAW 5. KOMAR/V/7ISK) BY ,Jmdjiam, M

' aux/. 2

ATTORN Y5 Sept. 5, 1967 R. s. KQMARNIITSKY 3,339,908 TAPERED LEAF SPRINGS Filed April 20,. 1965 5 sheets-Sheet 2 45 I GRIND TENSION SURFACE usf 4l GRIND 1 TOHERE INVENTOR HOST/SLAM S. KOMA/PN/TSKY BY WWW/7M Wake/MID ATTORNEY 5 Sept. 5, .1967

R. s. KOMARNITSKY 3,339,908

TAPEREQ LEAF SPRINGS Filed April 20, 1965 5 Sheets-Sheet 5 IN VENTOR HOST/SLAM SI KOMAR/V/TSKY MWMM 7/42 MW ATTORNEYS United States Patent 3,339,908 TAPERED LEAF SPRINGS Rostislaw S. Komarnitsky, Bloomfield Hills, Micl1., as-

signor to Rockwell-Standard Corporation, Pittsburgh, Pa., a corporation of Delaware Filed Apr. 20, 1965, Ser. No. 449,486 11 Claims. (Cl. 2.67-47) ABSTRACT OF THE DISCLOSURE A finished steel spring leaf preferably formed by taper rolling and having the side which is to be the tension surface in operation, as Well as the opposite edge surfaces extending adjacent and along said tension surface, ground and subsequently stress peened.

The present invention relates to improved tapered leaf springs and particularly relates to special leaf spring structure for single or multiple taper leaf spring assemblies for vehicles.

Over the years, it has been standard practice to use conventional multileaf spring bundles in vehicles for absorbing road shocks. While such spring bundles are generally satisfactory for absorbing a large percentage of normal road shocks, they are undesirably heavy and they produce a considerable amount of interleaf friction which objectionably affects the spring deflection by increasing the spring rate. Another serious objection of multileaf springs is that it is generally impossible to obtain a uniform spring rate under various road conditions wherein the same incremental variation in load produces the same incremental spring deflection.

In an effort to eliminate these problems existent with multileaf springs, recent eiforts have been directed to the development of parabolic tapered leaf steel springs, and the present invention is concerned wth a further improvement of such springs to materially increase the fatigue life thereof. Among the chief factors to which reduction of spring fatigue resistance and consequent failures are generally attributed are decarburization of metal surface layers, surface irregularities including rolled-in scale and oxide penetrations, and the effect of certain mechanical treatments.

To improve fatigue resistance, prior to this invention spring leaves were stress peened following heat treatment. However, the expected percentage increase in the useful life of the spring leaf afforded by stress peening was not fully obtained in prior fabricating methods.

The present invention contemplates novel improved tapered leaf springs wherein the spring leaf has been subjected to grinding operations at least on certain surfaces to remove decarburization films, surface imperfections, scale and oxide prior to the taper rolling and shot or strain peening operation after heat treatment. It has been found that by so grinding the blank before taper rolling and shot or strain peening, a tapered leaf spring of unexpectedly and immensely increased fatigue life is obtained particularly in comparison to the life of tapered spring leaves which are shot or strain peened without the novel grinding operation. With the present invention, therefore, preparatory grinding is utilized to great advantage in prolonging the useful life of the spring leaf. While it is preferred to carry out the grinding operation prior to the sequential operations of taper rolling, heat treating and shot or strain peening, the grinding operation may alternatively follow the taper rolling operation.

The invention herein is an improvement over that disclosed in U.S. Letters Patent No. 3,238,072 issued Mar. 1, 1966, to Greene et al. and assigned to Rockwell- Standard Corporation, assignees of the present application. Said Patent No. 3,238,072 and the present application were copending in the Patent Oflice.

Related to the present application is the inventors Ser. No. 449,585 filed Apr. 20, 1965, which claims the method phase of the invention.

It is the primary object of the present invention to provide a novel tapered leaf spring made from a spring steel blank that has been sequentially roll tapered and shot or strain peened at least on the side thereof which is the tension surface in the finished spring leaf, and wherein said tension surface is subjected to a grinding operation prior to shot or strain peening so that the shot or strain peened surface is substantially free of decarburized metal and surface imperfections.

A further object of the invention is to provide a novel spring leaf wherein the tension surface and the adjacent side edges are ground prior to shot or strain peening the tension surface.

Another object of the invention resides in a vehicle suspension assembly in the provision of spring liners attached to the spring leaf of the present invention at the intended intermediate clamp area to prevent the formation of high stress concentrations in that area.

Further objects of the invention will appear as the description proceeds in connection with the appended claims and the annexed drawings wherein:

FIGURE 1 is a diagrammatic view illustrating the preliminary surface grinding operation of a flat spring steel blank in accordance with the present invention;

FIGURE 2 is a diagrammatic view of an induction heating furnace for heating the ground spring leaf blank to hot Working temperature;

FIGURE 3 is a diagrammatic view of a taper rolling apparatus;

FIGURE 4 is a diagrammatic view of an optional spring leaf piercing and drilling apparatus which is used when a spring leaf is provided with a locating hole if required;

FIGURE 5 is an elevation of a tapered spring leaf following the taper rolling operation and piercing;

FIGURE 6 is a diagrammatic view of an optional heating furnace for preparing the spring leaf for an attachment eye forming operation if so required;

FIGURE 7 is a diagrammatic view of an optional apparatus for forming attaching eyes if so required;

FIGURE 8 is a diagrammatic view of a furnace for heating a spring leaf preparatory to a cambering operation;

FIGURE 9 is a diagrammatic view of a spring leaf cambering apparatus;

FIGURE 10 is a diagrammatic view of tempering furnace;

FIGURE 11 is a diagrammatic view of a spring leaf strain peening apparatus;

FIGURE 12 is a diagrammatic view of a pre-setting apparatus;

FIGURE 13 is a diagrammatic view of a tion station;

FIGURE 14 is a diagrammatic view of a spring leaf a spring leaf leaf inspeccoating station for applying a corrosion resistant film to inserted between the leaf and the clamp.

Referring to FIGURE 1, a flat sided spring steel blank or billet 40 cut to proper length is placed in a grinding apparatus 42 for grinding the surface 44 of blank 40 which will be the tension surface of the spring leaf as finally formed. Apparatus 42 preferably comprises grinding belts 46 which are adapted to engage surface 44. The grinding apparatus 42 also includes grinding belts or the like (see FIGURE for grinding both

edge surfaces

41 and 43 adjacent the tension surface of the blank. While for some springs it may be acceptable to grind only the tension surface for the purpose it has been discovered as a very important part of the invention that grinding of both the tension surface 44 and portions of

edge surfaces

41 and 43 adjacent the edge 45 is extremely beneficial to further increase the fatigue life of the spring. Other forms of grinding machines (not shown) may also be used, such as, for example, grinding Wheels. Grinding of surface 44 and

edge surfaces

41 and 43 in accordance with the present invention removes any decarburization films, surface imperfections, scale and oxide to provide smooth flat surfaces. For example a typical spring steel leaf was found to have a decarburized surface layer about 0.015" deep which ranged in hardness from an outer surface hardness of about 30 Rockwell C to the inner uniform leaf hardness of about 46 Rockwell C. This harmful layer was removed from the tension surface and adjacent edges by the grinding operation to produce a leaf spring of high quality.

With particular reference to FIGURES 15 and 16 the blank 40 shown in cross section comprises a lower surface 47 which will become the compression surface and an upper surface 44 which will become the tension surface. Both surfaces are laterally connected by

opposite edge surfaces

41 and 43 respectively.

To grind the tension surface 44 the blank 40 is placed on the grinding machine table, conveyor, or the like 116 with its tension surface up and a number of grinding discs, belts, etc. 46 are brought in contact with the tension surface 44. The outer most 49 of the grinding devices extend around the edges 45 of the

edge surfaces

41 and 43 to simultaneously grind the upper portions of the sides immediately adajacent the tension surface 44.

Referring to FIGURE 16 the blank 40 when it comes from the rolling mill shows a surface layer of decarburized material .118 which may extend continuously around all surfaces or only part of the surfaces. This surface decarburization is harmful and prevents effective surface treatment of the leaf spring blank such as taper rolling, shot peening, etc. especially at the side which is to become the tension surface. As for instance during taper rolling, flakes of hard mill scale may be pressed into the surface and thus cause the occurrence of stress raisers in the tension surface layer, which may become the nuclei of fatigue cracks and lead to early fracture of the leaf spring.

Although the decarburization may be removed by grinding all surfaces of the blank it is necessary only to remove it from the side which is to become the tension surface and portions of the edge surfaces immediately adjacent the tension surface as indicated in FIGURE 16 since these portions of the edges extending from the tension surface are subject to tension stresses and if only the tension surface would be ground leaving decarburiza tion on the

edge surfaces

41 and 43 early fatigue failures may occur due to the surface imperfections at the highly stressed portions of the edge surfaces adjacent the tension surface.

Preferably the extent of side edge grinding indicated in FIGURE 16 is about 35% of the distance downwardly from edge corners 45, which latter are preferably rounded to eliminate sharp edges where stresses may concentrate.

The side 47 of blank 40 opposite from tension surface 44 which is to become the compression surface also may be ground in the same manner as surface 44 although not necessary or it may be shot blasted before final thermal treatment to clean the blank and remove any scale left from the steel mill rolling operation. However, grinding or shot blasting of the side 47 of blank 40 opposite from tension surface 44 will normally not be necessary, since, presence of decarburized layer and stress raisers on the surfaces, which are subjected only to compression stresses, are not detrimental to the longevity of the spring.

Following the grinding operation, blank 40 is then heated in a suitable furnace 54 (FIGURE 2) to a temperature of from 1200 F. to 2250 F. in preparation for a hot taper rolling operation. Heating of blank 40 is advantageously, though not necessarily, accomplished by induction heating to produce a gradual increase of temperature extending towards the ends of blank 40 depending upon the amount of hot rolling to be done in the tapering operation. This treatment minimizes grain growth in spring blank regions subjected to comparatively little hot working in the rolling operation and is readily accomplished by special design of induction heating units.

After blank 40 is heated to its hot working temperature it then is conveyed to a taper rolling machine 56 (FIG- URE 3) where it is hot taper rolled. Alternatively, surface 44 and adjacent edge surfaces 41 and 43 may be ground following the heat treatment in furnace 54 and preceding the taper rolling operation in machine 56.

In tapered leaf springs, the spring must be designed so that the operating stresses are uniformly distributed throughout the length of the leaf. In other words, the leaf must be of variable thickness or variable width, or both. To facilitate manufacture and reduce cost and for other reasons, it is preferred to produce a tapered spring leaf which varies in thickness only. The taper extends preferably from a maximum thickness at or near the axle mounting or spring seat portion towards a minimum thickness at or near the end or ends. The leaf ends may be of constant thickness to form attachment eyes.

To obtain the foregoing tapered configuration, blank 40 may be preferably roll tapered by the method and apparatus disclosed in United States Letters Patent No. 3,145,591 issued Aug. 25, 1964, to F. R. Krause, to which reference should be made for complete details, such method and apparatus being only schematically illustrated in FIGURE 3 to an extent deemed necessary to understand the present application. The foregoing apparatus is suit-able to produce heavy duty spring leaves, however, for light duty spring leaves a taper roll machine as disclosed in copending application Ser. No. 205,333 filed June 26, 1962, now matured in U.S. Letters Patent No. 3,233,444 dated Feb. 8, 1966, assigned to applicants assignee, may be adapted.

To produce a single leaf spring having satisfactory suspension qualities over an appropriate range of deflection, the thickness tolerance of the spring throughout its taper must be held to plus or minus 0.005 inch from a desired theoretical parabolic contour. For this reason, among others, the surface of blank 40 to be tapered which may become either the tension or compression surf-ace must be essentially free from stress raising surface defects and must be maintained in this condition as the forming roll advances over it. In actual application the taper formation may appear on either the tension surface or the compression surface of the leaf spring.

As shown in FIGURE 3, the hot blank is placed in a. die 58 seated on a support base of machine 56. A forming roll 62, mounted for rotation about an axis transverse to the longitudinal axis of blank 40, is then rolled longitudinally along the blank starting from the central portion of the blank and traveling first towards one end and then towards the other end. As previously mentioned, the spring leaf may terminate at both ends in a uniform thick portion in order to form attachment eyes. If a single forming roll 62 is used, the taper on one half of blank 40 is formed. The position of blank 40 is then reversed end for end and the other half of blank 40 is tapered. In a double end machine reversal of the blank is not necessary. Roll 62 is pressed against blank 40 by means of a contoured cam 64 which is suspended above die 58 and which is mounted for movement longitudinally of the blank. As cam 64 moves to the right as viewed in FIGURE 3, it forces the roll 62 against blank 40 which is held against movement. Roll 62 is thus advanced over surface 44 of blank 40 by frictional engagement between roll 62 and blank 40 and between cam 64 and cam follower rollers (not shown) integral with and at each end of roll 62.

To maintain the accuracy of the taper, it is very important that no scale or other dirt come between the contacting surfaces of forming roll 62 and blank 40 or cam 64 and the cam follower rollers. The contour of cam 64 is such that, as it engages and advances roll 62, roll 62 shapes blank 40 to the desired tapered form described above, thus providing a tapered spring leaf indicated at 65 in FIGUR'ES 4 and 3 and having tapered tension surfaces 66. When it is desired to have the leaf of constant width and tapered only in thickness, the die 58 will be provided with a longitudinal cavity such that it effectively prevents the metal from spreading out laterally on the sides, as more fully described in the above referred to Krause patent.

Following the taper rolling operation, the semi-finished tapered leaf 65 is conveyed to a heating furnace 68 (FIG- URE 8) where it is heated to austenitizing temperature as a preliminary step to a subsequent camber forming and quenching operation. As shown in FIGURE 9, the camber is formed by mounting and clamping the thus heated spring leaf between curved complementary upper and lower die

fixtures

72 and 74.

Fixtures

72 and 74 are so formed relative to the desired curvature of the final spring leaf as to allow for changes in height of the camber particularly during a presetting operation to be described later on. The assembly comprising die

fixtures

72 and 74 with the spring leaf 65 clamped in place is immersed in quenching oil for a period of time sufiicient to obtain a martensitic transformation on the order of 95 or more. While this transformation is taking place, th semifinished spring leaf 65 is retained under high clamping pressure between

fixtures

72 and 74 to prevent distortion. After removal from the quenching oil, spring leaf 65 is released from the

fixtures

72 and 74 and is conventionally tempered in .aconveyor type tempering furnace 76 (FIG- URE. 10) to relieve stresses imposed upon the spring during the forming and quenching operations.

Following the taper rolling, cambering and tempering operations, spring leaf 65 is shot or strain peened at least on its ground tension surface 44 and also on and along both longitudinal ground adjacent portions of edge surfaces 41 and 43. The peening apparatus may be of any conventional construction and may contain one or more shot throwing wheels, two of which are indicated at 77 and 78 in FIGURE 11. The tension surface 44 is pelted with shot from wheels 77 and 78 while the spring leaf is held by suitable clamps in a stressed condition wherein it is subjected to longitudinal bending. These wheels may be posiitoned at an angle to simultaneously peen the tension surface 44 and adjacent edge surfaces 41, 43, or added wheels may be provided for thise purpose. When the strain under which the leaves are to be peened is specified, it is, preferably at a stress which is as close to the yield point of the steel as conditions will permit.

The spring leaf 65 which is cambered prior to the shot or strain peening operation, is preferably clamped in a substantially flat position by any suitable means. It will be appreciated that other conventional forms of shot peening apparatus may be employed, such as, for example, air nozzles. Shot peening relieves harmful residual tension stresses which could be present in the surface layer and introduces beneficialcompressive stress.

Following the shot or strain peening operation the spring leaf is preferably transferred to a presetting'or bulldozing assembly '86 illustrated in'FIGURE 12..Assembly 86 comprises a rigid fixture 88 having a curved top surface over which the cambered spring leaf is reversely deflected in the direction of service loading by push rods 90 or other suitable means. The spring leaf is deflected by an amount exceeding the designed maximum service deflection and the yield point of the material. In this presetting or bulldozing operation, the cambered spring leaf is deflected from its curved unloaded configuration, through a flat configuration, to the reversely curved configuration shown in FIGURE 12. The presetting operation readjusts the normal camber height, reduces the possibility of permanent set from occuring in the spring leaf during service, and introduces residual compression stresses in the tension surface layer in addition to that introduced by the shot or strain peening operation. The spring leaf when released from the presetting fixture returns to a cambered shape which is slightly relaxed from the cambered shape shown in FIGURE 9.

After the initial grinding operation and before taper rolling, at least the ground tension surface 44 and portions of the adjacent edge surfaces 41 and 43 of blank 40 may be treated to resist the formation of scale and oxide particularly while the blank is being taper rolled. This may be effectively accomplished for instance by contacting the ground blank with lithium vapor while the blank is being heated in furnace 54. The lithium vapor sticks to and coats the blank with a film which, in addition to preventing formation of scale and oxide, also acts as a lubricant in the rolling process.

Following the scale prevention treatment and before or after hot rolling, blank 40 may in certain instances be provided with a locating or anchor bolt hole or other known locating or centering means as shown in FIG- URE 4, For reasons stated below in the fabrication of single spring leaves, the prior conventional locating of anchor bolt hole is preferably omitted and instead the leaf is provided with one or more impressions, dimples or studs preferably on the neutral axis of the side edge surfaces of the leaf or on the compression surface for locating and centering purposes to reduce the high stress raising effect caused by the conventional center bolt hole in the anchor area, as more fully illustrated and explained in copending application Ser. No. 411,285 filed Nov. 16, 1964, since issued as US. Letters Patent No. 3,305,231 dated Feb. 21, 1967.

After the taper rolling ope-ration the partly finished spring leaf 65 may be optionally subjected to a further grinding operation to remove any surface imperfections caused by the taper rolls particularly on the tension surface indicated at 44 and the adjacent edg surfaces 41 and 43. The spring leaf 65 then may be conveniently conveyed to a taper inspecting station (FIGURE 13) where the contour of the spring leaf taper is checked as by means of accurate electronic thickness measuring devices 98 or by any other conventional measuring apparatus. As explained above, maintenance of accurate thickness dimensions throughout the taper is very irnportant.

Prior to cambering, spring attachment eyes may be formed, if desired, as shown in FIGURES 6 and 7 by first heating spring leaf 65 in conveyor type right-hand and left-hand dual furnace 100 shown in FIGURE 6 and comprising two

opposed heating units

102 and 104 respectively located on opposite sides of a conveyor 105. Leaf spring 65 is moved laterally between

furnace units

102 and 104 to heat the spring end regions indicated at 106. To facilitate formation of eye attachments, end regions 106 are preferably of uniform thickness.

Furnace units

102 and 104 may be of any suitable and conventional type and are preferably gas-fired to provide high intensity heat. Immediately after heating, the spring leaf end regions are formed simultaneously into attachment eyes 108, as shown in FIGURE 14. The eye forming operation is performed by any conventional auto-' 7 matic double-end three-pass forming machine having forming rolls indicated at 110 in FIGURE 7.

After the presetting operation, the finished spring leaf indicated at 112 in FIGURE 14 is advantageously provided with a corrosion and scar resistant coating by immersing the spring leaf in a suitable coating solution 113 contained in a tank 114.

While it is preferred to grind tension surface 44 and the adjacent edge surfaces 41 and 43 before the taper rolling operation, these surfaces may alternatively be ground after taper rolling to the form of FIGURE and preceding the shot or strain peening operation.

The feature of the invention residing in grinding and stress peening the side edge surfaces adjacent and along the tension surface over and above the grinding and stress peening of the tension surface disclosed and claimed in said Patent No. 3,238,072 results in elimination of any tendency for the finished spring to crack at those edges during operation, and so it contributes materially to even greater useful life of the spring.

With further reference to FIGURE 17 the finished spring leaf 112 is shown installed at the rear axle of a vehicle indicated by the vehicle frame 120.

The rear spring end 122 of the spring leaf 112 is attached to a shackle 124 pivotally suspended from the frame 120 and the front end is attached to a hanger bracket 126 depending from the frame 120.

Intermediate its ends the spring leaf of the present invention is supported on the vehicle axle 128 on a spring seat 130 attached to the axle to which the leaf is clamped by means of a lower pressure plate 132 and secured there-to by U-bolts 134.

Due to the high stress concentration at the intermediate mounting block area it is further proposed within the purview of the present invention to provide spring liners 136 of plastic material (such as polyethylene or the like) between the spring seat 130 and the upper surface of the spring leaf and between the pressure plate 132 and the lower surface of the spring leaf. As will be noted from the figure the liners 136 preferably extend a distance beyond the mounting block area to assure complete coverage over the entire high stress area. The mounting block liners are preferably bonded to the spring leaf and thus form an inherent part of the leaf itself.

The liners in the present invention do not merely serve as a means to prevent corrosion and fretting at the mounting block area but are provided for the main purpose of redistributing the stresses in this area more evenly to avoid high localized stresses. The liners 136 are surface bonded along the upper and lower surfaces of the spring leaf at the mounting block region and they may flex with spring movement. It has been discovered that use of these liners, besides the foregoing protective action considerably contributes to unexpectedly improve the useful spring lite and further prevent premature failure at this region.

This premature failure at the mounting block area was due to the high residual compressive stresses introduced by the clamping members to which is added the working stress (either compression or tension) during spring flexure causing peaks of extremely high stress concentrations especially immediately adjacent the U-bolts. The liners 136 provided in this area effectively redistribute the stress concentrations more evenly throughout the mounting block area.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

1. A finished spring leaf comprising a mechanically worked heat treated length of steel having one surface taper rolled to desired contour, and its tension surface and adjacent longitudinal side edge surfaces ground and mechanically worked to introduce beneficial residual stresses.

2. A finished spring leaf comprising a mechanically worked heat treated length of steel having its tension surface and its adjacent longitudinal edge surfaces ground and shot peened.

3. A finished spring leaf comprising a mechanically worked heat treated length of steel having a flat taper rolled surface and adjacent opposite longitudinal edge surfaces, all of said surfaces being ground and strain peened after grinding.

4. In the finished spring leaf defined in claim 3, said taper rolled surface being the tension surface of the spring.

5. A finished spring leaf comprising a length of steel having an accurately rolled taper along one side, and a tension surface, said surface being ground to remove decarburized layers and surface imperfections and provide a smooth surface and said smooth ground surface being strain peened, and the opposite edge surfaces of said spring leaf adjacent said tension surface being ground to provide edge surfaces that are free of decarburized layers and surface imperfections and stress peened after grinding.

6. A steel spring leaf that has been successively ground on the tension surface and its adjacent edge surfaces to remove decarburization, scale and surface imperfections and irregularities, hot taper rolled on said tension surface, and then strain peened on said ground and taper rolled surfaces.

7. In combination: a vehicle axle, a mounting block on said axle, a single spring leaf having a compression surface and ground tension and adjacent edge surfaces which are taper rolled and strain peened, the combination comprising liners of plastic material between said single spring leaf and said spring mounting block.

8. A spring leaf as defined in claim 3 wherein both opposite surfaces at the area of maximum thickness of said spring leaf are provided with liners of plastic material permanently bonded to said surfaces.

9. A finished steel spring leaf comprising a single length of steel of uniform width having a taper rolled surface on the side which is to be the tension surface in operation, said surface and the opposite longitudinal side edge surface areas immediately adjacent and along the length of said surface being ground smooth to remove decarburized layers and surface imperfections, and said ground surfaces being strain peened.

10. The spring leaf defined in claim 9 wherein said ground longitudinal side edge areas extend away from the corners where they intersect said tension surface a distance of about 35 percent of the thickness of the spring leaf.

11. The spring leaf defined in claim 9, wherein the thickest section of said leaf comprises the spring seat mounting region, the side opposite said tension surface is the compression surface in operation and wherein liners of solid flexible plastic material are surface bonded to said tension surface and the opposite compression surface of said spring at said region.

References Cited UNITED STATES PATENTS 3,238,072 3/1966 Greene et al 29173 FOREIGN PATENTS 959,801 6/1964 Great Britain.

ARTHUR L. LA POINT, Primary Examiner.

R. M. WOHLFARTH, Assistant Examiner.

Claims

1. A FINISHED SPRING LEAF COMPRISING A MECHANICALLY WORKED HEAT TREATED LENGTH OF STEEL HAVING ONE SURFACE TAPER ROLLED TO DESIRED CONTOUR, AND ITS TENSION SURFACE AND ADJACENT LONGITUDINAL SIDE EDGE SURFACES GROUNN AND MECHANICALLY WORKED TO INTRODUCE BENEFICAL RESIDUAL STRESSES.