Org Maq Molas-helicoidais 2011

7/27/2019 Org Maq Molas-helicoidais 2011

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ESCOLA NUTICA INFANTE D. HENRIQUEDEPARTAMENTO DE MQUINAS MARTIMAS

Engenharia de Mquinas Martimas

ORGOS DE MQUINAS

Dimensionamento de molas helicoidais

Victor Franco Correia

(Professor Adjunto)

2005


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Molas helicoidais

Para este tipo de molas, em regime elstico, aplica-se a Lei deHooke e vlida a relao,

= kF

A deformao da mola, , da mola pode ser calculada atravsdo teorema de Castigliano, obtendo-se

4

38

dG

nDF

k

F a=

e a constante da mola , k, ser assim dada pela expresso

anD

GdFk

3

4

8

= .

Em que G o mdulo de elasticidade transversal do material da mola, na o nmerode espiras activas ou teis da mola, D e dso, respectivamente, o dimetro mdio doenrolamento e o dimetro do arame. O nmero de espiras activas igual ao nmerototal de espiras nt menos o nmero de espiras terminais n* que efectivamente nocontribuem para a deformao da mola,

*nnn ta =

O valor de n* depende do tipo de acabamento das extremidades da mola helicoidal.Na figura seguinte indicam-se alguns valores de n* :


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Factores Geomtricos

O ndice da mola,d

DC= , pode ser usado para exprimir a deformao,

k

F

dG

nFC a ==38

A gama de valores usuais para a constante C de aproximadamente 6 a 12.O dimetro do arame, d, deve respeitar os dimetros normalizados.

O comprimento activo do arame, aa nDL = , pode tambm ser usado para obter

uma expresso para a deformao da mola,

4

2

8dGLDF a

= .

Tenses de corte na mola

A tenso de corte mxima na mola pode ser calculada pela sobreposio dos efeitosde corte directo e toro, obtendo-se

A

F

J

dT

+=

2max

A tenso de corte mxima ocorre na face interior do enrolamento (ver figura


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seguinte). Substituindo os termos: 2/DFT= , 2/dr= , 32/4dJ = e 4/2dA = ,obtm-se

23max48

d

F

d

FD

+

= .

Substituindo o ndice de mola C, vem

+

=

Cd

FD 5.01

8

3max.

Alguns autores apresentam a equao de tenso de corte mxima, sob a formaalternativa

3max8dFDkW

=

em que Wk o designado factor de correco de Wahl que pretende ter em

considerao o efeito da curvatura da mola na tenso de corte resultante, sendo dadopela expresso

CC

CkW

615.0

44

14+

= .

(a) Tenses de corte devidas a toro pura.(b) Tenses de corte devidas a corte directo(c) Sobreposio dos efeitos de corte directo e toro pura(d) Sobreposio dos efeitos de corte directo e toro pura considerando

o efeito da curvatura da mola.


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Instabilidade de molas helicoidais

Quando a mola submetida a foras de compresso podem ocorrer condies parainstabilizao como a figura ilustra, que se caracteriza pela ocorrncia de deformaes

no axiais:

Uma vez que a instabilizao se inicia, a deformao lateral progride rapidamente eocorre a falha da mola. Assim, fundamental que o projecto de molas de compressotenha em considerao a probabilidade de ocorrncia de situaes de instabilidade.Basicamente, o processo de instabilizao de molas de compresso similar instabilizao de colunas estruturais. Na prtica, quando o comprimento livre da mola(Lfree) superior a cerca de 4~5 vezes o dimetro nominal do enrolamento D, a molapode instabilizar sob a aco de uma carga suficientemente elevada. As condies deinstabilizao da mola dependem do comprimento livre, dimetro nominal doenrolamento e ainda do tipo de extremidades da mola e do tipo de constrangimentosque estes impem sua deformao (pivot ball permite a rotao; ground &squared no permitem a rotao).Um mtodo rpido para verificar a instabilidade da mola consiste em calcular arelao entre a deformao da mola e o seu comprimento livre (/Lfree) e utilizar atabela abaixo para verificar se esta relao excede o valor mximo admissvel:


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Molas de traco

As molas de traco tpicas tem usualmente o seguinte aspecto:

As molas de traco so normalmente fabricadas com uma traco inicial Fi quepressiona as espiras umas contra as outras no estado livre da mola. Este factotem como consequncia que a relao fora-deformao no sejaverdadeiramente linear, quando medida a partir da posio de repouso. Noentanto uma vez que a traco inicial seja ultrapassada, a mola tem um

comportamento linear.

Tenses de corte

Dado que as molas de traco tm uma traco inicial na sua posio derepouso, tm igualmente uma tenso de corte inicial instalada nas espiras noestado de repouso. A tenso de corte mxima (em repouso), i ocorre na faceinterior das espiras, e dada pela equao,

3

8

d

DFk iWi

=

em que D o dimetro nominal do enrolamento, d o dimetro do arame, e

Wk o factor de correco de Wahl .

Uma vez que a traco inicial ultrapassada, a mola de traco pode seranalisada como uma mola de compresso com uma fora negativa. A tenso decorte mxima (max) na mola aumenta com a fora e dada por,

3max8

d

FDkWi

+=

A deformao da mola dada por,

4

)(8

dG

FFn it =

em que G o modulo de elasticidade transversal e nt o nmero total de

espiras.


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Concentrao de tenses nas extremidades

Consideremos o arco tpico que normalmente existe nas extremidades das molasde traco. A geometria do arco frequentemente causa fenmenos deconcentrao de tenses que podem originar a falha da mola. A ilustraoseguinte mostra a geometria tpica das extremidades desta mola e define osparmetros radiais r1 e r4,

A tenso mxima em flexo no ponto A e a tenso de corte mxima no pontoB podem ser expressas pelas equaes, respectivamente,

Coeficiente de segurana nas molas de traco

Quando ocorre a rotura de uma mola de compresso, a falha catastrfica de todoo mecanismo onde a mesma se insere, normalmente evitada porque oscomponentes que suportam as extremidades da mola, na pior das hipteses

comprimem os restos da mola em rotura.

Com uma mola de traco, no existe este tipo de segurana de carctergeomtrico uma vez que a mola est em traco. Por esta e outras razes, astenses de trabalho das molas de traco so normalmente limitadas a cerca de do valor correspondente para uma mola de compresso com geometriasimilar e do mesmo material.


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Fadiga de Molas helicoidais

O fenmeno da fadiga constitui um problema em molas sujeitas a cargas

cclicas, em que as foras variam entre um valor mximo e um valor mnimo.As molas do sistema de suspenso dos veculos, por exemplo, esto sujeitas afadiga. Um nmero excessivo de ciclos de tenso originar a falha da mola porfadiga.

As tenses de corte mxima e mnima, , na face interior do enrolamento damola so proporcionais s foras actuantes na mola, Fmax e Fmin,

,

em que D o dimetro mdio do enrolamento (medido entre os centros daseco transversal do arame, i.e. dimetro exterior do enrolamento menos odimetro do arame), W o factor de correco de Wahl que tem em conta oefeito da curvatura da mola nas tenses, e C o ndice da mola

, ,

A tenso de corte mdia na mola mean, e a tenso de corte alternada, alt, sodadas por

,

Critrio de Soderberg

Os componentes sujeitos a carregamentos alternados (mean = 0) falham quandoo nvel de tenses atinge a tenso limite de fadiga do material, fatigue, que seobtm atravs dos ensaios de fadiga.

Quando os componentes esto sujeitos a uma dada combinao de tensesmdias mean e tenses alternadas alt , o critrio de Soderberg permite prevera falha em fadiga. No grfico abaixo, tenso mdiavs. tenso alternada, estrepresentada a linha correspondente ao limite imposto pelo critrio de falha porfadiga de Soderberg, que representado pela recta que une os pontos, mean =yield e alt= fatigue:


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Se no grfico acima, o estado de tenso corresponder a um ponto abaixo darecta de Soderberg (linha a azul) o critrio de Soderberg fadiga verifica-se. Seo estado de tenso corresponder a um ponto acima da recta de Soderberg, ento provvel a rotura por fadiga.

O critrio de Soderberg pode ser verificado analiticamente, atravs da equao,

Ocorrer falha por fadiga se a tenso alternada for superior tenso limiteimposta pelo critrio de Soderberg, ie.


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Frequncia natural fundamental das molas helicoidais

Quando as molas helicoidais so utilizadas em mecanismos com movimento, o

seu comportamento dinmico tem de ser considerado.A primeira frequncia natural (ressonncia) de uma mola helicoidal dada por,

em que d o dimetro do arame, D o dimetro mdio nominal doenrolamento, nt o nmero total de espiras, G o mdulo de elasticidadetransversal e a massa especfica do material da mola.

Deduo da expresso anterior por analogia

Uma forma fcil de obter a equao anterior consiste em usar a analogia entreuma barra sujeita a uma fora axial e a mola helicoidal. A analogia vlida umavez que ambos os objectos possuem uma rigidez e uma massa uniforme aolongo do comprimento L.

Tanto a mola como a barra obedecem Lei de Hooke, em aplicaes estticas,

em que L a variao no comprimento da mola ou da barra. A rigidez kpara abarra dada por,

em que E o mdulo de Young do material,A e L so, respectivamente, a

seco e o comprimento livre da barra.


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No caso dinmico, a variao do comprimento da barra pode ser dada pelaexpresso,

em que o nmero de onda n dado por,

e f a frequncia excitadora (em Hz). Para verificar a validade da equaoobtida, notemos os seguintes aspectos: A equao dinmica para L satisfaz aequao diferencial do movimento para a barra e no limite para o caso esttico(i.e. quando ftende para zero) a equao para L no mais do que a expresso

da Lei de Hooke,

Para obter as frequncias naturais da barra procuramos as condies para asquais a variao de comprimento da barra tende para infinito. Isto ocorrequando nL no denominador igual a: {, 2, 3, ...}. A primeira frequncianatural ocorre quando,

Resolvendo em ordem a fres e substituindo na expresso de krod e notando que ovolume da barra (A*L), vem,

A massa especfica multiplicada pelo volume igual massa da barra. Assimpodemos simplificar a formula da frequncia natural, obtendo-se,

Por analogia, a primeira frequncia natural da mola, ter a mesma equao,

em que k agora a rigidez da mola, e M a massa da mola.


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Frequncia natural da mola em funo de parmetros geomtricos

Podemos exprimir a frequncia natural da mola em termos dos parmetros de

carcter geomtrico e do mdulo de elasticidade transversal (em vez da suarigidez global ke da sua massa).

O volume material da mola dado por,

e notando que a rigidez da mola em termos da sua geometria, mdulo deelasticidade transversal G e nmero de espiras activas na,

Substituindo estas duas equaes na formula da frequncia natural, fres , obtm-se,

Se a mola for constituda por um nmero elevado de espiras, podemos assumirque o nmero de espiras activas igual ao nmero total de espiras (desprezandoas espiras terminais). Podemos tambm assumir a seguinte aproximaonumrica,

Estas duas aproximaes permitem obter a expresso para a frequncia de

ressonncia da mola,

Para utilizar esta formula necessitamos de conhecer o modulo de elasticidadetransversal da mola e a sua geometria. muito mais fcil utilizar a frmula daseco anterior que apenas requer o conhecimento da rigidez e da massa damola, especialmente quando se trata de molas das quais se desconhece omaterial.


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Referncias:

J. Shigley, C. Mischke Mechanical Engineering Design, McGraw-Hill 6th ed.

Anexos

Materiais para molas


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Spring wire

Sandvik 12R10/gusab T302

Sandvik 12R10/gusab T302 are general purpose steel grades which meet most requirements with regard to mechanicalproperties and corrosion resistance.

Service temperature.............................-200 to 250 C (-330 to 480F)

Chemical composition (nominal) %

Steelgrade

C Si Mn Pmax

Smax

Cr Ni

12R10T302

0.080.07

0.60.5

1.21.3

0.0300.035

0.0150.015

1818.5

98

Standards

Sandvik Grade: 12R10/gusab T302ASTM: 302ISO: X9 CrNi 18-8 Grade 1 NSEN: 1.4310 NSEN Name: X 10 CrNi 18-8 NSW Nr.: 1.4310JIS: SUS 302/304-WPBProduct standardsEN 10270-3ISO 6931-1ASTM A 313/A 313MJIS G 4314

Mechanical properties

Mechanical properties in delivered conditionTensile strength and proof strength, MPa (ksi)

Wire diameter Nominal, Rm1 Nominal Rp0.2mm inch MPa ksi MPa ksi

0.15 0.20 0.0059 - 0.0079 2365 343 1890 274>0.20 0.30 >0.0079 - 0.012 2310 335 1850 268>0.30 0.40 >0.012 - 0.016 2260 328 1810 262>0.40 0.50 >0.016 - 0.020 2200 319 1760 255>0.50 0.65 >0.020 - 0.026 2150 312 1720 249>0.65 0.80 >0.026 - 0.031 2095 304 1680 244>0.80 1.00 >0.031 - 0.039 2045 297 1635 237>1.00 1.25 >0.039 - 0.049 1990 289 1590 231>1.25 1.50 >0.049 - 0.059 1935 281 1550 225>1,50 1,75 >0.059 - 0.069 1880 273 1505 218>1.75 2.00 >0.069 - 0.079 1830 265 1465 212>2.00 2.50 >0.079 - 0.098 1775 257 1420 206>2.50 3.00 >0.098 - 0.118 1720 249 1375 199>3.00 3.50 >0.118 - 0.138 1665 241 1330 193>3.50 4.25 >0.138 - 0.167 1615 234 1290 187>4.25 5.00 >0.167 - 0.197 1560 232 1250 181>5.00 6.00 >0.197 - 0.236 1505 218 1205 175>6.00 7.00 >0.236 - 0.276 1450 210 1160 168>7.00 8.50 >0.276 - 0.335 1400 203 1120 162

>8.50 10.00 >0.335 - 0,394 1345 195 1075 156Flat wire 800-2200 116 - 319 0.85*Rm 0,85 * ksi

Other strength levels On request1)tolerance on tensile strength + /- 7.0 % in accordance with EN 10 270-3 (ISO 6931-1).

By tempering the tensile strength can be increased by 150250 MPa (22 - 36 ksi). The tensile strength variationbetween spools/coils within the same production lot is maximum 50 MPa (7ksi). The proof strength in tempered

condition is approx. 85 % of the tempered tensile strength. The tensile strength values are guaranteed and aremeasured directly after production. At storing the strength will increase somewhat due to ageing. Depending on storingcondition the ageing can increase the stength with 0 - 50 MPa (0 - 7 ksi)


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Shear modulus, MPa (ksi)as delivered ............................................... approx 71 000 (10 295)tempered ................................................... approx 73 000 (10 585)Modulus of elasticity, MPa (ksi)as delivered ............................................. approx 185 000 (26 825)tempered ................................................. approx 190 000 (27 550)

The strength will decrease by 34% per 100C (184oF) increase of servicetemperature.Straightened lengthsAfter straightening the strength is approx. 7% lower.

Fatigue strength - tempered and pre-stressed cylindrical helical springs

Whler diagram, mean stress 450 MPaThe curve is valid for springs coiled from wire 1 mm indiameter and represents 90% security against failure.Shear stress range = double the stress amplitude.To reach 99.9% security against failure the curvemust be lowered to about 80 % of present values.

Stress range for different wire diameters, mean stress 450 MPa

Shear stress range at 107 load cycles as a function of thewire diameter.

At elevated temperatures the fatigue strength decreases at:

100C (210F) by about 5 %200C (390F) by about 10%

Heat treatment

By tempering the springs at 350C (660F)/0.53 h, the tensile strength will increase by about 100-250 MPa (15 - 35ksi). If a shorter tempering time is used the tempering effect will be lower. In continuous conveyor furnaces, where theholding time at temperature is very short (min. 3 minutes), the temperature can be increased to about 425C (780F).

In the as-delivered condition the ratio proof strength/tensile strength is about 0.80. After tempering the ratio will beabout 0.85.

Please note that tension springs coiled with initial tension must not be tempered at the same high temperature asother types of springs. We recommend batch annealing at 200C (390F)/0.53 h, or continuous tempering in a

conveyor furnace with a holding time of 320 minutes at about 250C (480F).


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Spring wire

Sandvik 11R51

General description

Sandvik 11R51 in comparison with standard grades Sandvik 12R10/gusab T302 these grades have:

higher tensile strength and tempering effecthigher relaxation resistance, especially at elevated temperatureshigher fatigue strengthbetter corrosion resistance thanks to the molybdenum addition


C Si Mn Pmax

Smax

Cr Ni Mo

0.08 1.50 1.80 0.025 0.015 17.0 7.5 0.70

Standards

Sandvik Grade: 11R51ASTM: 302ISO:EN: 1.4310 HSEN Name: X10 CrNi 18-8 HSW Nr.: 1.4310 HSJIS: SUS 302 Mod.Product standardsEN 10270-3ISO 6931-1JIS G 4314

ASTM A 313/A 313M - 98Mechanical properties


Wire diameter Nominal, Rm1 Nominal Rp0,2mm inch MPa ksi MPa ksi0.15 0.20 0.0059 - 0.0079 2530 367 2150 312

>0.20 0.30 >0.0079 - 0.012 2470 358 2100 305>0.30 0.40 >0.012 - 0.016 2420 351 2060 299>0.40 0.50 >0.016 - 0.020 2365 343 2010 292>0.50 0.65 >0.020 - 0.026 2310 335 1960 284>0.65 0.80 >0.026 - 0.031 2260 328 1920 278>0.80 1.00 >0.031 - 0.039 2200 319 1870 271>1.00 1.25 >0.039 - 0.049 2150 312 1830 265

>1.25 1.50 >0.049 - 0.059 2100 305 1785 259>1.50 1.75 >0.059 - 0.069 2040 296 1730 251>1.75 2.00 >0.069 - 0.079 1990 289 1690 245>2.00 2.50 >0.079 - 0.098 1880 273 1600 232>2.50 3.00 >0.098 - 0.118 1830 265 1555 225>3.00 3.50 >0.118 - 0.138 1775 257 1510 219>3.50 4.25 >0.138 - 0.167 1720 249 1460 212>4.25 5.00 >0.167 - 0.197 1670 242 1420 206>5.00 6.00 >0.197 - 0.236 1610 233 1370 199>6.00 7.00 >0.236 - 0.276 1560 226 1330 193>7.00 8.50 >0.276 - 0.335 1505 218 1280 186

Flat wire 850-2400 123 - 348 0.85 * Rm 0,85 * ksi

Other strength levels On request1) tolerance on tensile strength + / - 7.0 % in accordance with En 10 270-3 grade 1.4310HS.

By tempering the tensile strength can be increased by 150300 MPa ( 22 - 44 ksi). The tensile strength variationbetween spools/coils within the same production lot is maximum 50 MPa (7 ksi). The proof strength in temperedcondition is approx. 90 % of the tempered tensile strength.


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Shear modulus, MPa(ksi)as delivered ................................................. approx. 71 000 (10 295)tempered ..................................................... approx. 73 000 (10 585)

Modulus of elasticity, MPa (ksi)as delivered ............................................... approx.185 000 ( 26 825)tempered ................................................... approx. 190 000 (27 550)

The strength will decrease by 34% per 100C increase of service temperature.

Straightened lengthsAfter straightening the strength is approx. 7% lower.

Fatigue strength - tempered and pre-stressed cylindrical helical springs

Whler diagram, mean stess 450 MPaThe curve is valid for springs coiled from wirein size 1.00 mm and represents 90 %security against failure.

The shear stress range = double the stress

amplitude. To 99.9 % seccurity againstfailure the curve must be lowered to about80 % of present values.

Stress range for different wire diameters, mean stress 450 MPa

Shear stress range at 107

load cycles as afunction of the wire diameter.

Heat treatment

By tempering the springs at 425C (780F)/0.5 - 4 h, the tensile strength will increase by about 150-300 MPa (20 - 45ksi). If a shorter tempering time is used the tempering effect will be lower. In continuous conveyor furnaces, where theholding time at temperature is very short (min. 3 minutes), the temperature can be increased to about 475 (780F).

In the as-delivered condition the ratio proof strength/tensile strength is about 0.85. After tempering the ratio will beabout 0.90.

Please note that tension springs coiled with initial tension must not be tempered at the same high temperature asother types of springs. We recommend batch tempering at 250C (480F)/0.53 h, or continuous tempering in aconveyor furnace with a holding time of 35 minutes at about 300C (570F).


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Spring wire

Sandvik 13RM19

Sandvik 13RM19 combines high mechanical strength with a non-magnetic structure. This combination of properties haspreviously been found mainly in expensive Co-Ni-base or Cu-Be-alloys. The steel has very good corrosion resistancecomparable to that ofAISI 302.

Sandvik 13RM19 is characterised bynon-magnetic structure in all conditions

very high mechanical strength in the cold drawn condition. The strength can be further increased without any effecton the non-magnetic structure by a simple tempering operationhigh elastic limit and energy storing capacity in the cold drawn and tempered condition

Sandvik 13RM19 also possesses good fatigue properties and exellent ductility, which makes it a most suitable choicefor springs and other high strength applications where ferromagnetic materials cannot be used.

Service temperature ...............................up to 250C (480F)

Standards

Sandvik Grade: 13RM19EN: 1.4369For Sandvik 13RM19 is the standard EN 10270-3 valid excluding chemical composition and mechanical properties.


C Si Mn Pmax

SMax

Cr Ni Mo N

0.11 0.8 6.0 0.030 0.015 18.5 7 - 0.25

Magnetic permeability

From a magnetic point of view materials can be divided into three groups, para-, dia- and ferromagnetic materials. Formany practical cases para- and diamagnetic materials will however strongly interact with the magnetic fields. In somecases the ferromagnetic properties are desired while in other situations no interaction with a magnetic field can be

accepted. The magnetic properties of a metrial is expressed as the magnetic susceptibility, , or often as the magnetic

permeability = 1 + . By definition the magnetic susceptibility is put to 0 for vacuum from which is follows that

vacuum=1.

The magnetic permeability for a certain material is expressed as , which is its relative permeability versus vacuum.

Further, as , may vary with the magnetic field strength the maximum value ofmax is often given as a representativevalue of the material.

Most types of high strength steel are ferromagnetic in spring hard conditions. The spring properties are achieved byhardening, e.g. carbon and chrominum steels or by cold drawing as e.g. for AISI 302/304 (W.Nr 1.4310). The origin ofthe properties is the martensitic structure. Higher alloyed steels e.g. AISI 316 suffer, side from being more expensive,from the difficulties to reach a high strength by cold working. If high strength is needed together with a non-magnetic(para-magnetic) material the option has traditionally been expensive Copper-Beryllium or Cobalt base alloys.

Sandvik 13RM19 is alloyed in a way that the structure is very stable against a martensitic transformation but stillallowing a strong work hardening effect at deformation. Thus it is possible to obtain mechanical properties similar tothe ones of AISI 302 but maintaining a non-magnetic structure. The following diagram shows typical values for themaximal relative magnetic permeability for different stainless steels.


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Wire diameter Nominal, Rm Nominal Rp0.2mm inch +/- 100 MPa +/- 15 ksi MPa ksi0.15 0.20 0.0059 - 0.0079 2200 319 1760 255

>0.20 0.30 >0.0079 - 0.012 2150 312 1720 249>0.30 0.40 >0.012 - 0.016 2100 305 1680 244>0.40 0.50 >0.016 - 0.020 2100 305 1680 244>0.50 0.65 >0.020 - 0.026 2000 290 1600 232>0.65 0.80 >0.026 - 0.031 2000 290 1600 232>0.80 1.00 >0.031 - 0.039 1900 276 1520 220>1.00 1.25 >0.039 - 0.049 1900 276 1520 220

>1.25 1.50 >0.049 - 0.059 1800 261 1440 209>1,50 2,00 >0.059 - 0.078 1800 261 1440 209>2.00 2.50 >0.078 - 0.098 1650 239 1320 191>2.50 3.00 >0.098 - 0.118 1650 239 1320 191>3.00 3.50 >0.118 - 0.138 1500 218 1200 174>3.50 4.00 >0.138 - 0.157 1500 218 1200 174

By tempering the tensile strength can be increased by up to 300 MPa (44 ksi) without deterioration of the magneticproperties. The tensile strength variation between spools/coils within the same production lot is maximum 50 MPa(7 ksi). The proof strength in tempered condition is approx. 85 % of the tempered tensile strength. The tensilestrength values are guaranteed and are measured directly after production. At storing the strength will increasesomewhat due to ageing. Depending on storing condition the ageing can increase the stength with 0 - 50 MPa (0 - 7ksi).

Shear modulus, MPa (ksi)

as delivered ...............................................approx. 69 000 (10 005)tempered ...................................................approx. 73 000 (10 585)

Modulus of elasticity, MPa(ksi)as delivered ............................................approx. 180 000 (26 100)tempered ................................................approx. 190 000 (27 550)

The strength will decrease by 34% per 100C (184oF) increase of service temperature.


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Fatigue strength

The Whler diagram is valid for spings coiled from wire 0.5 mm in diameter and represents 90 % of security againstfailure. Mean stress = 450 MPa

Stress range = double the stress amplitudeTo reach 99.9 % security aganist failure the curve must be lowered to about 80 % of present values.

At elevated temperatures the fatigue strength decreases at100oC by about 5 %200oC by about 10 %

Cryogenic properties

13RM19 has excellent properties by means of magnetic and mechanical properties at low temperatures. The diagramshows the magnetic permeability down to 4.2 K (-268.95C) for a tensile strength of approx. 800 MPa (116 ksi) at20C (70F).

Heat treatment

By tempering the springs, the tensile strength will increase up to 300 MPa (45 ksi). We recommend 350C(660F)/0.53 h for batch tempering. To obtain best results when tempering in a continuous conveyer furnace, whereholding times at full temperature are very short, the temperature can preferably be increased to about 425C (780F).The holding time should be at least 3 minutes as shorter times might result in uneven tempering.

In the as-delivered condition the ratio 0,2 % offset proof stress/tensile strength is about 0.80. After tempering theratio will be about 0.85.

Please note that tension springs coiled with initial tension must not be tempered at the same high temperature as

other types of springs. We recommend batch annealing at 200C (390F)/0.53 h, or continuous tempering in aconveyor furnace with a holding time of 35 minutes at about 250C (480F).

Tensile strength values at different

temperatures and material conditions.


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Strip steel

Sandvik 15LM and 20C

General description

Sandvik 15LM and 20C are characterised by good properties in respect of:

Fatigue strength and wear resistance Hardness combined with ductility Dimensional tolerances Surface and edge finishes Shape

The materials also have good blanking and forming properties with retaining shape of the parts after the blankingoperation.


Sandvik C Si Mn15LM20C

0.751.00

0.200.25

0.750.45

Specifications

Sandvik AISI W.-Nr. SS15LM20C

10741095

1.12481.1274

17701870

Dimensions

Sandvik 15LM and 20C are available in a wide range of sizes. The following chart indicates the approximate stand-ardsize range.

Figure 1 Standard size range


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Nominal values at 20C.

Thickness Tensile strength,sRm*

Proof strength,Rp0.2

mm.......... inch Sandvik 15LMMPa

Sandvik 20CMPa

Sandvik 15LMMPa

Sandvik 20CMPa


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Applications

Sandvik 15LM

Springs in general Spring washers in cars Scraper blades for the pulp and paper industry

Sandvik 20C

Washers in automatic transmissions Lapping carriers and cutter blades for the semiconductor industry Coater and scraper blades for the pulp and paper industry Springs in general Doctor blades for printing processes

KnivesA document from the Sandvik Materials Technology web-site.


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Org Maq Molas-helicoidais 2011

Documents

Transcript of Org Maq Molas-helicoidais 2011