US2649298A

US2649298A - Volute spring

Info

Publication number: US2649298A
Application number: US161706A
Authority: US
Inventors: Cal W Wulff; Frank G Newman
Original assignee: Holland Co
Current assignee: Holland Co
Priority date: 1950-05-12
Filing date: 1950-05-12
Publication date: 1953-08-18
Anticipated expiration: 1970-08-18
Also published as: BE524034A

Description

3 Sheets-Sheet -1 I I I I I I I l I I I I I I I jhm@ @i fvwm VOLUTE SPRING C. W. WULFF ET AL Aug. 18, 1953 Filed May i2, 195o c. w.wu1.r-'F ETAL $2,649,298

VOLUTE SPRING Aug. 18,v 1953 "3 Sheets-Sheet 2 Filed May 12, 1.950

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Aug.' 18,- 1953 c. w. wuuFFET AL 2,649,298"

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1N VEN TORS Patented Aug. 18, 1953 UNITED STATE-S PATENT GFFIC E vomrrn SPRING Illinoisy ApplicationlMay- 12 1950;,v Serial No. 16l70&

This invention relates to. volute. springs, either of the type designed Withbuilt-in absorption to carry live loads, or of the. type designed.' lto act as snubbers with other `parallelY mounted springs carrying major portion of the load.

The primary object of the invention is t'o, pro.- vide a spring of these types'. which has superior damping properties and. characteristics when compared with volute springs now in use, and which because of their. built-inabsorption characteristics are capableY of absorbing, vastlyY greater work energy over. their effective lifi.` cycle than prior art volute springs of comparable dimension andA material.

These and further objects. and advantages will become apparent as the disclosure proceeds andi the description is read. in conjunction with. the accompanying drawings. in which Figure l is a side elevational view partly section showing a volute snubber spring made in accordance with this invention, the spring being shownat its free height. after being. given av cold. set;

Figure 2 isa plan View ofthe. samek with the dead coilsl being indicatedby distinctive crosshatching;

Figure 3' is a side.. elevational View ofj the springl shownvin Figures 1. and' 2V but. shows, the. form of the coil as wound,A and before being given a coIdj set;

Figure 4 (subdividedl into Figures. 40t-v to 4e inclusive) shows the profile as coiledY slope`of the developed bar for various types of, volute, springs, as indicated by suitable legends; and

Figure 5 is a graph showing, in. comparison with a conventional tight-woundl volute spring,l the load-deflection characteristics (bothon compression and release) of.' a volute. spring made in accordance withk this invention.

It should be understood that the selection of` a. particular volute spring for illustration in the drawings and specific description, in the specification is for the purpose of' complying with section 4888 of the revised statutes andshould not be construed as imposing limitations on the ap` pended claims, except as may be requiredby'the prior art.

The design theory of volute springs is not tooV well" known andthe fact is that in considering volute springv formulas there are, in addition to.- the primary torsional stress 'produced by a cen.- tral load, manysecondary. stresses which. arise out of the variation. ofthe twisting torquefrom.vr section to section, by iiexibility of. they dead. coils and by misalignment. of. the. springL seats., By

2? way of example, cone, and. arch stresses are typically secondary stresses.

The friction developed in. volute springs hasy always been consideredK more or less undesirable because. of its, effect on fatigue endurance; As a matter ofA fact', volute spring designs for use as load-carrying springsl have normally avoided' contact between adjacent .coils because experience demonstrated' that the contacting, surfaces were non--unifornny with the result that gougini occurred, producing excessive wear and ac celeratedv failure.`

We. have found thatv instead of avoiding fric@ tional contact between the' adjacent active'y coils of a volute spring, we can provide a volute spring of substantially greater workeabsorbing. capacity by buildingl certain fricti'onal4 adsorption forces intothe spring and controllingl its characteristics: so that the frictional forces are` substantial', are evenly spread'over wide areasLwillnot he effec*- tively l'ost as the spring wears, and which pro*-y vi'de vol'utezspring characteristics never before at'- tained'. Essentially, our spring is a. non-har"- monic snubber spring in whichY energy absorption through friction is of greatest. importance.

This is accomplished by employing whatwe term negative ,helix angle when winding the volute spring while. hot; By negative lielixY angle is meant a7 deyei'oped bar whose proie before setting curves upwardly'from outer activev 'coil to innerA active coil; In other words, the

helix angle increases from outer active` coil to4 inner active coil in the as-coi'l'ed condition and before cold set; The advantages of` this negative orY increasingl helixangle may bestV be understood by' referring tot Figure 4- and the graphstherein shown.

As is' well\ understood; in every volute` spring a certain portion of the outer coil' anda certain portion of the inner coil7 arev inactive in so far as their load-carrying capacity is` concerned, and these coils or portions of coils are commonly calleddead-coils. The intermediate turns or coils of the spring," are active coils and usuallyv they include an outer' active coil, a middle active coil and' an inner active coil- In orderk tob distinguish ourI improved' volutespringl from those` heretofore known, one may refer toFigures 4a to 4e4 inclusive which compare theA as coiled slope of developed bars having various helix angles. and pitch characteristics.

First consider Figure 4a. The graphline lo, is a straight line and for that, reason the. spring may besaid. to, have. a constant. helix angle.

Howeven. itwill, be, seen that the axial pitch indicated by the lines I la, IIb and llc diminish from the outer active coil to the inner active coil. The result is that when the spring is deflected to solid, the active coils are non-uniformly stressed, with the result that the maximum eciency of the spring material is not fully attained.

For this reason some manufacturers make what is known as a spring having a positive helix angle, as shown by the prole graph line i2 in Figure 4b. Here it Will be seen that the prole,

of the developed bar as coiled curves downwardly from outer active coil to inner active coil. In this case the axial pitch decreases, as in the. case of the constant helix angle coil illustrated;y

in Figure lla, but the pitch indicated by the lines I3a, 13b and l3c diminishes at such a rate in progressing from the outer active coil to the inner active coil that all sections of the active spring are stressed uniformly at solid.

It is possible to build other desired characteristics into volute springs 'using a positive helix angle such as shown by the graph line I4 in Figure 4c, and in this case the axial pitch indicated by the lines 15a, [5b and 15o decrease at some rate which does not produce uniform stress indicated by the graph line I6 the profile of a` developed bar as coiled, and in this case the pitch, indicated by the lines lla, Hb and Hc, is constant. This means, of course, that at solid the inner active coil will be more severely stressed than the middle active coil, and this latter coil will in turn be more highly stressed than the outer active coil.

This condition may be accentuated still further by providing not only a negative helix angle but also a negative or reverseV pitch, as indicated by the graph line i3 in Figure 4e. In this case the axial pitch, indicated by the lines 19a, i911 and l9c actually increases from outer active coil to inner active coil.

Thus it will be seen that the prior art has always employed either a constant helix angle or a positive helix angle which resulted in axial pitch diminishing from outer active coil to inner active coil, whereas in our improved design a negative helix angle is used which ordinarily results in either a constant axial pitch or a reverse pitch,

the latter pitch being defined as one that in wound to provide a reverse pitch, negative helix angle in the as coiled condition. Such a spring is shown in Figure 3 and the vouter Vdead coil is indicated at 20, the inner dead coil at 2l, the outer active coil atw22', the middle active coil at 23 and the inner. active ,coil at 24. Although it is not possible to accurately see the negative helix angle in thislgurait is easy. to vsee that the coil has reverse axial pitch, i. e., the inner active coil is more greatly pitched than the middle active coil 23, and this latter coil is more greatly pitched than the outer active coil 22.

By way of example, volute springs constructedV Let us consider now a volute spring that is in accordance with this invention and made of C1085 spring steel may be coiled at about 1800 F. (which is well above the critical temperature for this particular material) and then quenched, after which the springs are tempered by reheating to a temperature of about 750 F., the Precise temperature being determined by the ultimate physical properties desired. When, after tempering, the spring is given a cold set, i. e., compressed to solid and then released and vibrated to nd its true free height, it will be found that the elastic transformation of the steel that has taken place on cold setting gives it a pitch somewhat as shown in Figure 1. In other words, on cold setting trapped stresses are set up within the material itself which have an important bearing on fatigue endurance. The particular pitch for the coils of the nished spring will depend upon the materials used, smoothness of the material, etc., but it will be found that when the spring in its coiled condition is given a particular coil pitch relationship, the free height of the finished spring will be the same for any given spring material (assuming uniform manufacturing procedures).

For example, let us assume that a steel bar 5% wide, .343" thick is tightwound when hot on a substantially cylindrical mandrel to provide a 51/2 outside diameter for the spring, and that while Winding the active coils are pitched progressively (from outside active coil to middle active coil to inside active coil) from .937 to 1.56" to 1.75", respectively. If a C1085 spring steel is used, this spring upon cold set will assume an axial pitch which from outside active coil to inside active coil progresses approximately from .62" to .68 to .70", respectively. If steel used is A9262 spring steel, the axial pitch of the iinished spring will progress approximately from .68 to .75" to .81, respectively, from outside active coil to inside active coil.

As stated before, the springs, after coiling, and before being given their cold set are heat treated according to particular requirements, and one of the advantages of the reverse axial pitch is that it exposes more of the friction surface to the desired heat treatment before the spring is given its cold set, thereby improving the physical properties ofthe spring itself. Furthermore, the ex posure of greater areas of the inner and middle coils for heat treatment is fortuitous because surface and depth hardness are of extreme importance.

The particular axial pitch given to the several active coils is determined by the end characteristics desired but, generally speaking, the inner activev coil is designed Vso that when the spring is given its cold set it Will be stressed almost to maximum capacity of the material and the middle active coil will be stressed somewhat less with the outer active coilbeing stressed very lightly during the cold set. The result is that the outer active coil together with the outer dead coil acts as a resilient casing for the middle and inner active coils during their deflection, and the constant or increasing pitch given tothe active coils, as coiled, tends, upon cold setting, to packthe inner active coil tightly intok the middle active coil, and the middle active coil tightly (although slightly less so) intoV the outer active coil, with the result that there is an inherent radial pressure exerted between the active coils which maintains them in tight frictional contact throughout the life of the spring, that is, each active coil is contained under positive radial compression within its adjacent outer coil. Y'

garages i It has: been foundA empirically that; even when springs made in accordance with our invention Wear, they lose little of" their frictional absorption characteristics due to this inherent radial pressure created by the reverse axial' pitch built.

into the spring when. tightvvoundv andcoiledV hot and subsequently cold pressed.

There is another factor which isbeleved to bev secondary stress in the spring and which. tends tol produce gouging due tov the uneven contact between adjacent turns of the spring brought about by non-axial eccentric loading. When, however, the spring is Wound with a negative or increasing helix angle, this coning effect diminishes, with the result that there is a more uniform contact between adjacent turns of the volute and diminished eccentricity on loading, which thereby distributes the Afriction forces more evenly, and prevents the gouging that causes excessive Wear and rapid loss of absorption.

lThis diminished coning ei'iect taken in conjunction with the built-in radial pressure from inner active coil to middle active coil, and from middle active coil to outer active coil gives the spring an unusually high capacity to absorb energy, particularly at small spring deflections where it has been unusuallyy diiiicult in the past to provide for such energy absorption.

This is indicated in Figure 5 which shows a load deiiection curve, both on compression and release, for a spring made in accordance with our invention-this in comparison with a conventional tightwound volute spring of equi-valent dimension and material. For clarity, the characteristic curve for the improvedY spring made in accordance with this invention is shown in full lines, whereas the characteristics of a conventional springr are indicated in dotted lines.

As is well known, the area under a compression curve is a measure of the work vdone or energy expanded in deiecting the spring. The area under the release curve represents the work expended by the spring in returning to its original free height against a resisting load. In all springs, and particularly snubber springs, the work expended by the spring in returning to its free height will be less than the work expended in compressing the spring. rihe difference in the compression and release work is the energy' or work absorbed by the spring. The compression and release curves for a given spring are commonly called the hysteresis loop for that spring, and it is essential in a snubber springY to have the area between these two curves ofV the desired magnitude, taking into consideration the particular application of the snubber spring. This represents the work absorbed by the spring.V

In Figure 5 the compression curves of a spring made in accordance with this inventori are shown in comparison with a compression curve of the conventional tightwound volute spring of equivalent dimension and material. These two curves are designated E6 and 2l, respectively. The release curves for these springs are indi;

cated at 23 and 29, respectively. vIn comparing the hysteresis` loops over the range of' deiectio'n from free height to solid it is apparent that' a rrr 6? spring made in accordance with this invention; has much greater energyr absorption than aconventional spring of equivalent. dimension and material.

This point is all' the more clear. whenl onecon.- Siders the portions of the hysteresis loops whichv lie between spring heights of 81/4," and` '74" which. represent the normal overall operating` range. For clarity, the portion of' the hysteresis loop within this range for the improved volute spring is indicated by crosshatching bounded by points A, B, C and D, and similarly the portion of the hysteresis loopior thev comparable conventional tightwoundvolute spring is represented by yother crosshatching boundedby pointsl E. F. G and H. It is apparent that within the range indicated. our improved volute spring is capable of absorb ing over twice the energy which the conventional tightwound volute spring can absorb within that range. This improved energy absorptionis directly attributable to each inner active coil being contained under elastic radial compression Within the adjacent larger coil.

Another important feature oi' our inventionis that compared with conventional tight-wound volute springs, the energy of absorption for our spring under small deiiections will be muchgreater percentage-ivise than at larger deections; By comparing the portion of the hysteresis loops, for our improved spring and for thev conventional spring in Figure 5 within the rangey of spring height of 8% to 8^', it will be noted that the absorption of energy bythe improved: spring is approximately four timesy as great as f' that of the conventional` spring. Even morepronounced results vwill be observed for smaller deiiections. inasmuch as high energy absorption characteristics at small deiiectionsis an' especially desirable feature for snubber springs, it will be seen that our improved spring has exceptionally desirable characteristic-s for such use, and actual tests have verified the greatly' improved energy absorption characteristics ofthe improved helix angle, the spring were hotA wound to pro-v vide a negativo helix angle with constant axial pitch, the spring would not be as effective as al snubber but more suitable as a non-harmonicV `load-carrying spring. It would, for example, be"

superior to conventional volute load springs because of itsinherent frictional absorption characteristics at low spring deiiections which persist throughoutv the effective life cycle of' the spring, but would not possess the high energy absorption characteristics desirable in a spring which is mounted in parallel,y with other load-carrying springs for action as a snubber.

It will'be understood that in manufacturing a spring in accordance with our invention, the bar from which the spring is to be made is iirst scarfed to provide a blank such as shown on page 361 of Mechanical Springs by A. M". Wahl, 19a-e' ed., and' the spring is then tightly wound about a substantially cylindrical mandrel provided with the usualv stripping taper. The bar is heated well above critical temperature, say, to about D F. and tightly wound under exwith the'scarfedportion of the inner inactive coil normal to the axis of the mandrel. If no restraint or guidance is given to the bar as it is wound about the mandrel, it will have a constant free helix angle, but this angle may be changed by guiding the bar as it is wrapped around the mandrel, this being accomplished with a lead screw having the desired pitch or rate of pitch. In order to provide the ordinary positive helix angle, a lead screw is used which has increasing pitch so that the bar as it is Wrapped around the mandrel is extended along the length of the mandrel. In our invention, the lead screw is, in a sense, reversed because, in order to obtain a negative helix angle, it is necessary to restrain rather than augment the distance which the spring extends itself along the axis of the mandrel during coiling. All of this will be clearly understood by those skilled in the art.

It should be understood that reference in the specification and claims to outer active, middle active and inner` active coils should not be interpreted to mean that the outer active coil and the inner active coil must necessarily be a full 360 coil, because in some cases either of these two coils may be something less than 350 in extent.

We claim: l. In a volute spring, the combination of inactive outer and inner coils and at least three active intermediate coils, all tightly wound, the outer active coil being low-stressed, the middle active coil highly stressed vand the inner active coil still more highly stressed, whereby the outer active coil acts as a resilient casing to yieldably oppose unwrapping of the middle and inner active coils, said middle and inner active coils maintaining firm frictional contact with each other even after substantial wear due to the high stressing of said middle and inner active coils, and said spring being hot-wound with an increasing helix angle from outer active coil to inner active coil, and then cold set, whereby secondary stresses due to coning are attenuated.

2. In a volute spring, the combination of inactive outer and inner coils and at least three active intermediate coils, all tightly wound, the outer active coil being low-stressed, the middle activercoil highly stressed and the inner active coil still more highly stressed, whereby the outer active coil acts as a resilient casing to yieldably oppose unwrapping of the middle and inner active coils, said middle and inner active coils maintaining rm frictional contact with each other even after substantial wear due to the high stressing of said middle and inner active coils, and said spring being hot-wound with an increasing axial pitch from outer active coil to inner active coil and then cold set, whereby secondary stresses due to coning are attenuated.

3. A tightly wound volute spring comprising inactive outer and inner coils and at least three intermediate active coils, said spring after being tightly wound while hot and before cold setting being characterized by having its active coils provided with constant or increasing axial pitch from outer active coil to inner active coil, whereby upon compression of the spring the inner and middle active coils expand radially outwardly more rapidly than their adjacent enveloping coils to provide a continuous radialV pressure between said adjacent coils.

4. A tightly wound volute spring comprising inactive outer and inner coils and at least three intermediate active coils, said spring after being tightly Wound while hot and before cold setting beingl characterized by having its active coils provided with an increasing helix angle from outer active coil to inner active coil, whereby upon compression of the spring the inner and middle active coils expand radially outwardly more rapidly than their adjacent enveloping coils to provide a continuous radial pressure between said adjacent coils.

5. A volute snubber spring comprising inactive outer and inner coils and at least three active intermediate coils, including an active outer coil, an active middle coil, and an active inner coil, said active coils having overlapping, substantially cylindrical, working surfaces in rm, substantially uniform contact with each other and under intense inherent radial compression when the spring is at free height, whereby the spring has a relatively high energy absorption under light loads.

6. A volute snubber spring as set forth in claim 5 in which the active coils are of generally rectangular cross-sectional shape and of substantially uniform thickness.

'7. A volute snubber spring as set forth in claim 5 in which the material distribution within the active coils is such that the active coils maintain firm uniform contact with each other during deflection of the spring due to said. inherent initial radial compression forces.

8. A volute snubber spring as set forth in claim 5 in which the spring in its as-coiled condition and before cold-set is characterized by an increasing helix angle whereby after cold setting the material distribution within the active coils is such that upon compression of the spring, the inner and middle active coils expand radially outwardly more rapidly than their adjacent enveloping coils to provide a continuous radial pressure between said adjacent coils as the spring is compressed.

9. A volute snubber spring 'as set forth in claim 5 in which said spring is hot-wound and then cold set, and in which the spring structure in its as-coiled condition before cold setting is characterized by a constant or increasing axial pitch from outer active coil to inner active coil.

10. A volute snubber spring as set forth in claim 5 in which said spring is hot-wound and then cold set, and in which the spring structure in its as-coiled condition before cold setting is characterized by an increasing helix angle from outer active coil to inner active coil.

11. In a volute spring, the combination of inactive outer and inner coils and at least three active intermediate coils, said spring being tight wound while hot with an increasing helix angle and then cold-set, whereby the outer active coil is low-stressed, the medium active coil highly stressed, and the inner lactive coil still more highly stressed, with the outer active coil acting as a resilient casing to yieldably oppose unwrapping of the middle and inner active coils, said spring being further characterized by relatively high inherent radial compression forces between the active coils when the spring is at free height.

12. In a volute spring, the combination of inactive outer and inner coils and at least three active intermediate coils, all tightly wound, the outer active coil being low-stressed, the middle active coil highly stressed and the inner active coil still more highly stressed, whereby the outer active coil acts as a resilient casing to yieldably oppose unwrapping of the middle and inner active Y coils, said middle and inner active coils maintaining rm frictional Contact with each other even after substantial wear due to the high stressing of said middle and inner active coils, and said spring being hot-wound with an increasing helix angle from outer active Coil to inner active Coil, and then cold set, said active coils after cold setting being under relatively high, continuous, radial compression.

13. In a volute spring, the combination of inactive outer and inner coils and at least three active intermediate coils, all tightly Wound, the outer active coil being low-stressed, the middle active coil highly stressed and the inner active coil still more highly stressed, whereby the outer active coil acts as a resilient casing to yieldably oppose unvvrapping of the middle and inner aetive coils, said middle and inner active Coils maintaining firm frictional Contact With each other even after substantial Wear due to the high stressing of said middle and inner active coils, and said spring being hot-Wound with an 19 increasing helix angle from outer active Coil to inner active coil, 4and then cold set, said spring being under relatively high radial compression zero load due to the radial expansion during cold setting of at least one of said active coils at a greater rate than its enveloping adiacent eoil.

CAL W. WULFF. FRANK G. NEWMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 432,342 'rimmis July 15, 1890 2,075,815 Knox Apr. 6, 1937 2,176,719 Peyton Oct. 17, 1939 2,220,857 Weber Nov. 5, 1940 2,390,937 Holland Dee. 11, 1945 OTHER REFERENCES SAE Journal (Transaction), vol. 51, No. 9, September 1943, pages 317 to 328.