US 3238072 A
Description (OCR text may contain errors)
8 0m mm om w K 9.
R. R. GREENE ETAL METHOD OF MAKING TAPER LEAF SPRINGS Filed June 12, 1963 March 1, 1966 INVENTORS Robert R. Greene F reden'c/r 7. Rowland 9% flavrw h m-lu H MmmooEsu mE m m:
m wv 0581 c2 35 ATTORNEYS United States Patent METHOD OF MAKING TAPER LEAF SPRINGS Robert R. Greene, New Castle, and Frederick T. Rowland, New Wilmington, Pa., assignors, by mesne assignments, to Rockwell-Standard Corporation, a corporation of Delaware Filed June 12, 1963, Ser. No. 287,328 Claims. (Cl. 148-12) This application is a continuation-in-part of our application Ser. No. 23,471, [filed Apr. 20, 1960, now abandoned, for Spring Leaf and Method for Making.
The present invention relates to an improved method of making taper leaf springs and is particularly applicable in the manufacture of single and dual 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 interle-af 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 wherein the same incremental variation in load produces the same incremental spring deflection.
In an effort to eliminate these problems existant with multileaf springs, recent efforts have been directed to the development of parabolic taper single leaf steel springs. Most of these efforts, however, have been generally unsuccessful in that early structural failures resulting from poor fatigue resistance are frequently encountered to render previously proposed single leaf springs commercially unattractive.
Among the chief factors to which reduction of spring fatigue resistance and consequent failures are generally attributed are decarbur-ization of metal surface layers, surface irregularities including rolled-in scale and oxide penetrations, and the effect of certain mechanical treatment processes such as grinding. Grinding, for example, is generally regarded by such authoritative sources as the 2nd edition of the Metals Handbook (pages 238 and 252), published 1954 by the American Society for Metals to cause unfavorable residual stresses which augment the working stresses to produce early service failures. Consequently, grinding, for any purpose, previously was not desirable since it was regarded to impair the fatigue resistance of the spring To improve fatigue resistance, prior to this invention spring leaves were stress peened following heat treatment. The percentage increase in the useful life of the spring leaf alforded by stress peening according to prior methods, however, is not sufficient to render the leaf entirely acceptable particularly in single taper leaf springs.
The present invention contemplates a novel method of making taper leaf springs in which the spring leaf is subjected to a grinding operation to remove decarbu-rization films, surface imperfections, scale and oxide prior to the aforementioned stress peening operation. Contrary to the previous teachings objecting to grinding operations, it has been found that by so grinding the blank before stress peening, a single taper leaf spring of unexpectedly and immensely increased fatigue life is obtained particularly in comparison to the life of tapered spring leaves which are stress peened without the grinding operation. With the present invention, therefore, preparatory grinding, which previously was regarded to be unfavorable, is utilized to great advantage is 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 stress peening, the grindc we ing operation may alternatively follow the taper rolling operation.
Accordingly, it is the primary object of the present invention to provide a novel method of making a taper leaf spring wherein a steel spring blank is sequentially tapered, heat treated and then stress peened at least on the side thereof which is the tension side in the finished spring leaf, and wherein at least the tension side is subjected to a grinding operation prior to stress peen-ing.
More particularly, it is the object of the present invention to improve the spring fatigue resistance by subjecting an unfinished spring leaf to a novel grinding operation prior to a stress peen-ing operation in a method wherein the stress peeni-ng operation follows the sequential steps of taper rolling and heat treating.
\Fu-rther 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 steel spring 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 a spring leaf piercing and drilling apparatus;
FIGURE 5 is an elevation of a tapered spring leaf following the taper rolling operation and piercing;
FIGURE 6 is a diagrammatic view of a heating furnace for preparing the spring leaf for an attachment eye forming operation;
FIGURE 7 is a diagrammatic view of an attachment apparatus for forming eyes;
FIGURE 8 is a diagrammatic view of a furnace for heating a spring leaf preparatory to a cambening operation;
FIGURE 9 is a diagrammatic view of a spring leaf cambering apparatus;
FIGURE 10 is a diagrammatic view of a spring leaf tempering furnace;
FIGURE 11 is a diagrammatic view of a spring leaf stress peening apparatus;
FIGURE 12 is a diagrammatic view of a pre-setting apparatus;
FIGURE 13 is a diagrammatic view of a leaf inspection station; and
FIGURE 14 is a diagrammatic view of a spring leaf coating station for applying a corrosion resistant film to the finished spring lea-f.
Referring to FIGURE 1, a flat sided steel spring leaf blank or billet 40 sheared to proper length is placed in a grinding apparatus 42 for grinding at least the surface 44 of blank 40 which will be the tension side of the spring leaf as finally formed. Apparatus 42 preferably comprises grinding belts 46 which are adapted to engage surface 44. Other forms of grinding machines (not shown) may also be used, such as, for example, grinding wheels. Grinding of surface 44 in accordance with the present invention removes any decarburization films, surface imperfections, scale and oxide to provide a smooth fiat face.
The compression side of blank 41 opposite from surface 44 also may be ground in the same manner as surface 44 or it may be shot blasted when blank 40 is unstressed particularly to clean the blank and remove any scale left from the steel mill roll-ing operation. Grinding or shot blasting of the side of blank 40 opposite from surface 44 may be omitted for certain spring applications without affecting the fatigue resistance of the finally formed tapered spring leaf.
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 compartively 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, side 44 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 saddle 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 is preferably roll tapered by the method and apparatus disclosed and claimed in copending application Serial No. 851,385, filed November 6, 1959, of Robert R. Greene and Thomas McClain and entitled Method and Apparatus for Roll-tapering Leaf Springs, to which reference should r 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.
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, surface 44 of the tension side of blank 40 to be tapered must be essentially free from stress raising surface deflects and must be maintained in this condition as the forming roll advances over it.
As shown in FIGURE 3, the hot blank is placed in a die 58 seated on a support base 60 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. If two separate forming rolls are used, 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 FIGURES 3 and 4 and having tapered tension side surfaces 66. Under certain conditions, it may be desirable to provide die 58 with a longitudinal cavity such that it effectively prevents the metal from spreading out laterally on the sides.
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 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 fixture 72 and 74 with the spring leaf 65 clamped in place is immersed in quenching oil for a period of time suflicient to obtain a reasonably complete martensitic transformation on the order of %95%. While this transformation is taking place, the semi-finished spring leaf 65 is retained under high clamping pressure between fixture-s 72 and 74 to prevent distortion. This operation produces a formed cambered spring leaf containing a very high percentage of martensitic structure which is very desirable in high quality spring material.
After removal from the quenching oil, spring leaf 65 is released from the fixtures 72 and 74 and is conventionally tempered in a conveyor type tempering furnace 76 (FIGURE 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 stress peened at least on its tension side 66. 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 side 66 is pelted with shot from wheels 77 and 78 while the spring leaf is held by suitable clamps in a stressed condition. Preferably, spring leaf 65 is stressed for peening as close to the yield point of the steel as conditions will permit. Stressing the leaf to at least 75% of the yield point stress generally is satisfactory.
If spring leaf 65 has been cambered prior to the stress peening operation, it is preferably clamped in a substantially flat position by any suitable means. Shot wheels 77 and 78 are preferably set at slight angles to a plane passing normally through the tension side 66 so that the adjacent edges of tension side 66 are also peened. It will be appreciated that other conventional forms of shot peening apparatus may be employed, such as, for example, air nozzles.
Following the stress 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 camber 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 and reduces the possibility of permanent set from occurring in the spring leaf during service. Primarily, this presetting operation introduces additional beneficial residual stresses in the leaf which effectively oppose the working stresses imposed upon the leaf during service. The spring leaf when released from the presetting fixture returns to the cambered shape shown in FIGURE 9.
After the initial grinding operation and before taper rolling, at least the ground surface 44 of blank 40 is advantageously treated to resist the formation of scale and oxide particularly while the blank is being taper rolled. This is effectively accomplished 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 lithium treatment and before or after hot rolling, blank 40 may be advantageously provided with a locating or anchor bolt hole at a metal piercing or drilling station 92 shown in FIGURE 4. In the fabrication some single spring leaves, the locating or anchor bolt hole may be replaced with one or more impressions or dimples preferably on the neutral axis or on the compression side for locating and centering the leaf.
After the taper rolling operation the partly finishing 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 tapered tension side indicated at 66. The spring leaf 60 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 important.
Prior to cambering spring attachment eyes may be vided with a corrosion and sear resistant coating by immersing the spring leaf in a suitable coating solution 113 contained in a tank 114.
While it is preferred to grind at least side 44 before 5 the taper rolling operation, the side to be the tension formed, if desired, as shown in FIGURES 6 and 7 by first heating spring leaf 65 in a 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 side in the finished tapered spring leaf may be alternatively ground after taper rolling and preceding the stress peening operation.
A number of tapered spring leaf specimens identically made in accordance with the foregoing method were tested for fatigue resistance on a conventional fatigue testing machine. These spring specimens each were made of steel having a yield point stress of approximately 185,000 p.s.i. and were stressed to 160,000 p.s.i. while being peened following the successive grinding and hot taper rolling operations. The fatigue loading of each test specimen was cyclic form 3450 pounds to 16,000 p.s.i. pounds. The number of cycles required to cause failure and the location of the failure are indicated in Table I below.
To exemplify the superiority of tapered spring leaves made according to the foregoing method of the present invention and particularly to emphasize the essential importance of the preliminary grinding operation, a second set of tapered spring leaf specimens were made in the identical manner as the Table I specimens except that the spring leaf blanks were not ground prior to the taper rolling or stress peening operations. This second set of spring leaf specimens were submitted to fatigue tests under the same conditions as the spring leaves in Table I and the results are indicated in Table II below.
To particularly emphasize the combined importance of grinding and stress peening, a third set of tapered spring leaf specimens were made in the identical manner as the Table I specimens except that the taper rolled spring leaves were not stressed while being peened.
The third set of spring leaf specimens were submitted to fatigue tests under the same conditions as the Table I and 11 specimens and the results are indicated in Table III below.
TABLE I Number of Specimen No. Surface treatcycles required Location of failure ment for failure g gga gg 3,196,282 4% from center bolt hole. strss 1,630, 178 51%;: from center bolt hole. peened 4,011,445 34 from center bolt hole.
TABLE II g ggg 67,698 4.0" from center bolt hole. strss 110, 748 3.8 from center bolt hole. peened 80,910 4.1 from center bolt hole.
TABLE III Belt ground, 44, 671 6M5 from center bolt hole.
unstressed 64, 579 3%" from center bolt hole. shot peened. 68,302 4%" from center bolt hole.
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 automatic double-end three-pass forming machine having forming rolls indicated at 110 in FIGURE 7.
After the presetting operation, the finished spring leaf From Tables I, III and II, the average fatigue lives of the test specimens were respectively 2,945,968 cycles,
59,184 cycles and 86,452 cycles. Thus, it is established that the average fatigue life of the Table III specimens which were ground but not stress peened was the shortest. By stress peening but not grinding the Table II specimens, the average fatigue life was increased by 26,268 cycles or 44.5 percent. When, however, the spring leaf specimens were ground preceding the taper rolling operation, and stress peened after taper rolling, the average fatigue life was immensely and unexpectedly indicated at 112 in FIGURE 14 is advantageously proincreased by 2,886,784 cycles over the Table III specimens and by 2,859,516 cycles over the Table II specimens. This represents extraordinary improvements over the Table II and Table III specimens of approximately 3310 percent and 4900 percent respectively.
The test results in the foregoing tables conclusively demonstrate that the grinding operation, which heretofore was generally regarded to impair the fatigue resistance of the spring leaf, is actually essential to improve the spring leaf fatigue resistance to the degree indicated in Table I above. Table II particularly evinces that both grinding which precedes and stress peening which follows the taper rolling operation are essential to achieve the exceptional improvements in fatigue resistance as indicated in Table I.
The invention may be embodied in other specific forms without departing from the spirit or essential characteri'stics 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 United States Letters Patent is:
1.' In the method of making a taper leaf spring wherein a steel blank is heat treated, taper rolled and then stress peened on the side which is the tension side in the finished spring, the step of grinding said tension side prior to stress peening.
2. The method defined in claim 1 wherein said grinding step precedes the step of taper rolling said blank.
3. The method defined in claim 1 wherein said grinding step precedes the steps of heating and taper rolling said blank.
4. The method defined in claim 1, said blank being heated in a protective lithium vapor immediately prior to said rolling for inhibiting oxide and scale formation.
5. The method defined in claim 1 wherein the rolled unfinished spring leaf is stressed for peening to a degree that is near but sufficiently short of the yield point to avoid permanent deformation.
6. The method defined in claim 2 wherein the spring blank is heated to a temperature in the order of1200 F. to 2250 F. prior to taper rolling.
7. The method defined in claim 2 wherein prior to stress peening, the taper rolled leaf is heated to an austenitizing temperature, arcuately confined to form a desired spring camber while so heated, and quenched while so confined.
8. The method defined in claim 7 comprising the step of presetting said spring leaf following the stress peening operation by reversely bending it in the direction of service loading beyond the yield point of the steel.
9. A method of making a tapered spring leaf from a single blank of steel which comprises the sequential steps of (a) heat treating said blank in preparation for roll forming, (b) grinding that side of said blank which is to be the tension side in the finished leaf to remove scale, surface irregularities, decarburized regions and the like, (c) roll tapering said blank along one side from a region of maximum thickness serving as the axle mounting portion toward a minimum thickness at the ends and (d) then stress peening said tension side of the leaf.
10. The method defined in claim 9 wherein said blank is heat treated to a temperature within the range of 1200 F. to 2250 F., in preparation for said taper rolling.
11. In the method defined in claim 9, said blank being roll tapered along said tension side.
12. A method of making a tapered spring leaf from a single blank of steel which comprises the sequential steps of heat treating said blank in preparation for roll forming, mechanically treating the side of said blank which is to be the tension side in the finished spring to remove surface imperfections, decarburized regions and the like for providing a smooth steel surface at said side that is substantially homogeneous with the rest of the blank, hot roll tapering said blank along one side, stressing said rolled spring leaf to a degree that is near but sufficiently short of the yield point to avoid permanent deformation, and then mechanically working said tension side of the stressed rolled spring leaf to relieve localized surface stresses.
13. A method of making a tapered leaf spring comprising the steps of providing a steel blank of adequate size, heat treating said blank in preparation for roll forming, grinding the blank at least on the side thereof which is to be the tension side of the finished spring leaf to remove decarburization film, surface imperfections, scale and oxide, taper rolling the heated blank to provide a tapered spring leaf having a substantially parabolic contour on one side thereof, forming the spring leaf to desired camber, and shot peening the ground tension side of the formed spring leaf while holding the leaf in stressed condition.
14. A method of making a single taper leaf spring for vehicles comprising the steps of sequentially grinding a steel spring blank at least on the side thereof to be used as the tension side in the finished tapered spring leaf to remove any decarburization film, surface imperfections, scale and oxide that may be present, heating the ground blank to a temperature in the order of 1200 F. to 2250 F., treating the blank to resist oxidation and scale formation while being so heated, taper rolling the heated blank to form a tapered spring leaf having controlled substantially parabolic contour at the tension side, heating the tapered spring leaf to an austentizing temperature and arcuately confining the spring leaf while so heated to form a desired spring camber, quenching the heated spring leaf while so confined, tempering the quenched spring leaf, stress peening the tempered spring leaf at least on the tension side thereof to relieve localized surface stresses, and presetting the stress peened spring leaf by reversely bending the leaf in the direction of service loading beyond the yieldpoint of the steel.
15. A method of making a taper leaf spring for vehicles comprising the steps of grinding a steel spring blank at least on the side thereof to be used as the tension side in the finished spring leaf to remove any decarburization film, surface imperfections, scale and oxide that may be present and provide a smooth clean surface along said tension side, heating the ground blank to a temperature in the order of 1200 F. to 2250 F. in preparation for rolling, taper rolling the heated blank to form a tapered spring leaf ha ving controlled substantially parabolic contour along one side, heating the tapered spring leaf to an austentizing temperature and arcuately confining the spring leaf while so heated to form a desired spring camber, quenching the heated spring leaf while so confined, tempering the quenched spring leaf, stress peening the tempered spring leaf at least on said ground tension side thereof to relieve localized surface stresses, and then presetting the stress peened spring leaf by reversely bending the leaf in the direction of service loading beyond the yield point of the steel.
References Cited by the Examiner UNITED STATES PATENTS 2,608,752 9/1952 Schilling 267-47 2,775,152 12/1956 Krause -40 DAVID L. RECK, Primary Examiner.