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IS 10878-1 (1984): Flat Form Springs, Part 1: Design andCalculation for Springs Made from Rectangular Cold-RolledStrips [TED 21: Spring]

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UDC 621-272 : 669’14-418’42-122’2 IS : 10878 ( Part 1 ) - 1984

Indian Standard

Is] 1 I

SPECIFICATION FOR FLAT FORM SPRINGS

PART 1 DESIGN AND CALCULATION FOR SPRINGS MADE FROM RECTANGULAR COLD-ROLLED STRIPS

I - Scope - Lays down calculations for design of flat formed springs made from rectangular cold- ,olled strips.

1.1 It applies to cold-formed springs with material thickness up to 3’0 mm.

I.2 It does not cover the design and calculations of disc springs and similar products.

2. Terminology - Following symbols and units shall apply ( see Fig. 1 ).

b = Width of spring mm s = Thickness of spring mm E = Modulus of elasticity N/mm2

f = Deflection of spring mm

fi... fn = Deflection of spring associated with spring foroes Fl...Fn . mm

h = Stroke, difference between two load lengths mm

h, = Unloaded height of spring mm

hl... hn = Heights of loaded spring corresponding to the spring forces Fl...Fn mm

k = Coefficient (bending stress correction factor)

I = Effective length of spring mm

D+ = Total developed/blank dimension mm

F = Spring force N

F I... Fn = Spring forces corresponding to spring deflections hl...h” N

R = Radius assumed by the spring on loading mm

rmin = Minimum radius (see Fig. 6 ) mm

QB = Ultimate tensile strength of the strip N/mm”

=b = Uncorrected bending stress N/mm’

Obk = Corrected bending stress N/mm’

cbl . . . bbn = Uncorrected bending stress corresponding to spring forces ,Fl...Fn N/mm’

oblk...“bnk = Corrected bending stress corresponding to spring forces Fl...Fn N/mm:

bbh = Uncorrected strokestress i.e., the difference between bending stresses-at hl and hz N/mm:

=bnk = Corrected stroke stress i.e., the difference between bending stresses at hl and hz N/mm’

=bH = Permissible stroke stress N/mm:

dbsp = Permissible bending stress N/mm’

Orn = Maximum stress N/mm’

3. Modes of Loading - Before any calculations are made it should, as a rule, be established whethel under the type of loading envisaged the springs will be subjected to static or infrequently varying loac or to alternating or dynamic loading.

3.1 Static or lnfreqoently Varying Load - Springs under static load or infrequently varying load are :

a) all springs which carry only static loads; and

b) all springs to which occasional load variations are applied at fairly long intervals and at les! than 10 000 load cycles during life envisaged.

3.2 Alternating Load -- Flat springs subjected to alternating load are springs that have applied tc them a load which alternates over the height range ‘h’ between spring loads Fl and F2.

l Total developed/blank dimension - approx dimension of strip required to form the spring.

Adopted 10 April 1984 I

Q October 1984, ISI I

Gr 2

INDIAN STANDARDS INSTITUTION MANAK BHAVAN. 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

IS : 10878 ( Part 1 ) - 1984

\\\\\\\\\\\\\\\q .s

7 b

-I t I

FIG. 1 SPRING CHARACTERISTIC DIAGRAM

4. Coefficient ‘k’

4.1 For flat form springs with deflections due to the bending radius additional stresses come into play which are inversely proportional to the ratio of bending radius to the thickness of the spring.

Bending stress considering the deflection of the spring.

=bk = kxot, N/mm2

1.6

1.c

1.2

1.0 12 3 4 5 6

R/s

FIG. 2 COEFFICIENT ‘k’ CORRESPONDING TO R/s

2

IS : 10878 ( Part 1 ) - 1984

5. Bending stress and spring load formula for most frequently loading types.

6.1 Maximum Bending Stress (see Fig. 3)

6xFxi ob =

b x s2 N/mma .- .(I)

F = Exbxs’ xf N

4 x IS . ..(2)

FIG. 3

6.2 Maximum Bending Stress (see Fig. 4)

3xFXl b,, =

2 x b x s’ N/mm’

cbk = k =b

F =I 4 x E x b X s” x f N

I

F

?

t

I I T

c* -- 2

FIG. 4

5.3 Maximum Bending Stress (see Fig, 5)

SxFx/ 06 =

8 x b x sa N/mm’

=bk = k =b N/mm’

F 64 X EXbxs* =

7 x I’ xf N

w(3)

. ..(4)

I

I

. ..(5)

. . . (6)

FIG. 5

Note 1 - For formulae (1) and (6), designer can calculate the proportion ‘6’ and ‘s’ for given value of force ‘F deflection ‘f’ spring length ‘I’ and Young’s modulus ‘I’?.

Not+‘ 2 -The beam formulae (1) to (6) are valid only if the maximum deflection ‘f is not exceeding 25 percent of the length of the spring.

Note 3 - Formulae (1) to (6) have to be modified in those cases where the spring is built up of several leaves.

3

IS : 10878 ( Part 1 ) - 1984

6. Modulus of elasticity for strips for design of flat formed springs is as given below:

Sl No. i)

ii)

iii)

iv)

Material

Cold rolled steel strip for springs as per IS : 2507-1975 ‘Cold-rolled steel strip for spring (first revision )’

Austentic stainless steel strip as per IS : 6011 - 1972 ‘Stainless steel plate sheet and strip’

Phosphor bronze strips as per IS : 7814-1975 ‘Phosphor bronze sheet strip and foil’

Copper ~beryllium strips

Modulus of Elasticity

206 000 N/mm2

190 200 N/mm2

105 000 N/mm2

127 000 N/mm2

6.1 The modulus of elasticity values specified at SI No. (ii), (iii) and (iv) above are for general auidance only. Reference may be made to the material manufacturer for more exact values. - -

7. Calculation and Design

7.1 Permissible Bending Stress

7.1 .l Static or seldom changing loading

obsp = 0’75 0s N/mm2

# 7.1.2 Alternating or dynamic loading - For springs made with natural rolled/dressed round and with no stress raisers in the form of punched holes/sharp bends, the maximum permissible stress %H is 0’6 times the maximum bending stress under static loading c&+.

8. Dimensions - Tolerances on dimensions shall be as specified in relevant in 6.

9. Bending Radii ( see Fig. 6)

9.1 For soft annealed strip

r min = 2s

9.2 For hardened and tempered strip steel

rmin = 8s for 0’10 < s g 0’25 mm r min = 7s for 0’25 < S Q 0’50 mm r min = 6s for s > 0’50 mm

9.3 For copper alloy strips

rmin = S

FIG. 6

10. Finish - As specified in IS : 10878 ( Part 2 )-1984 ‘Flat form springs : springs made from rectangular cold-rolled strips’.

EXPLANATORY NOTE

. ..(7)

edges stroke

specification mentioned

. ..(8)

Part 2 Specification for

This standard is Part 1 of Indian Standard on the design calculation and specification of flat form springs. This part deals with design and calculations, while Part 2 covers the specification.

4 Printed at Printrado, New Delhi. India