Shot peening

After coiling, a spring contains stresses at the wire surface on the inside diameter of the spring. For dynamic loaded springs, these stresses do not allow the material properties to be fully exploited. By shot peening the spring, i.e. bombarding the spring with small, round, steel balls, the following improvements with regard to fatigue strength can be achieved:

-Tension in the surface

- Reduction of notch fatigue factor as any small
surface defects are closed up.

- Harder surface finish due to cold working by
peening.

By shot peening, the life of the spring can be increased by more than 100%. Conversely, an increase in performance of up to 50% can be achieved with the same life. We particularly recommend this method of treatment for compression springs which are exposed to fatigue, where long life is required.

Close coiled extension and torsion springs are not normally shot peened, due to the practical difficulties (limited space for the shot inside the spring). Also, the advantages cannot be realized, compared with compression springs. Generally, compression springs should have a wire diameter of at least 1,5 mm. For thinner wire diameters, the effect is lower and there is a further risk of deformation.

Pre setting

Pre setting is a plastic deformation, which is accomplished by loading the spring beyond the actual working range. In this way, tension in the surface is obtained in the opposite direction to the load tension. This leads to non or strongly reduced setting when the spring is working. We recommend pre setting for highly stressed springs. Normally, pre setting is carried out cold. Springs working in increased temperatures should be pre set warm.

Due to the characteristics of the material it is impossible to make identical springs. Material hardness, dimensions and physical properties can vary, which influences the consistency of the spring. It is therefore important to set tight tolerances when necessary.

Typical tolerances for spring loads are ± 5, ±7 or 10%. For the initial force (F_o) on extension springs, the tolerance is ±15%. The tolerances are normally controlled by spot checks.

When a very tight tolerance is required, a tolerance of ±2% can be maintained by a hundred percent load control.

The tolerances are valid for springs with a relationship of:

For the end coil of the compression springs, the values of the table should be doubled.

Where two load values are stated, tolerances for free length should not be indicated. The tolerances are valid for compression as well as extension springs. Normally, the complete tolerance range is not required e.g. most standard springs are produced within tolerance.

D_m = D_y-D_t = D_i ₊ D_t

Tolerances for angle deviation SS 2386

The deviation A of the generating line from the vertical line must not be larger than 0,05 L_o (2,9°). The deviation from parallelism A1 must not be larger than 0,03 D_y (1,7°).

Tolerances for spring diameter SS 2384

Basic measure
ment, D_mTolerance

	- 2,5	±0,15
(2,5)- 4		±0,2
(4)	- 6,3	±0,25
(6,3)- 10		±0,3
(10)	- 16	±0,35
(16)	- 25	±0,45
(25)	- 32	±0,5
(32)	- 40	±0,6
(40)	- 50	±0,8
(50)	- 63	±1
(63)	- 80	±1,2
(80)	-100	±1,5
(100)	-125	±1,9
(125)	-160	±2,3
(160)	-200	±2,9
(200)	-250	±3,1
(250)	-320	±3,5
(320)	-400	±4

Tolerances for other wire and strip steel formations

Basic measure-
ment, mm	Length	Radii	Angles
<3	±0,2	±0,2	±4°
3-6	±0,3	±0,5	±3°
>6-30	±0,5	±1,0	±2,5°
>30-60	±0,8	±2,0	±2°
>60-120	±0,8	±3,0	±1,5°

4-12 ±5%

(12)-15 ±7,5%

Lowest tolerance for L_o = ±0,3 mm

Tolerances for spring force (F) SS 2384

Ratio Dm/D_t

No of active coils

2-3.5 (3.5)-5.5 (5.5)-8.5 (8.5)—12.5 (12.5)+

4- 5±15	±15%	±12%	±11%	±10%	±1
(51-11	±13%	±11%	±10%	±9%	±1

Allowed shear stress (t) at a static load

The material of springs from wire is normally exposed to shear stress. In designing the springs, the shear strength and modulus of shear of the wire is therefore of great importance. For a spring with certain dimensions, the following applies:

- the higher the allowed shear stress is, (t), the
higher spring force

- the higher modulus of shear (G), the higher spring
force for a given deflection.

The diagram above shows the highest allowed shear stress for a static loaded spring or one where the number of load oscillations during the expected life of the spring don't exceed 10 000.

The strength for extension springs is to a large extent determined by the design of the loops. With a normal loop, bent from the body of the spring, a strength loss of about 10-15% should be considered, as the loop is weaker than the rest of the spring.

The life of a spring is very much negatively influenced by elements such as corrosion, increased working temperature, damages in the surface of the material etc.

Shot peening normally extends the life considerably (see passage about shot peening). The life is also dependant on the stress reversals in application, i.e. long deflection - shorter life and short deflection - longer life.

In order to estimate the life (N_c) of a spring, exposed to a dynamic stress, the following reference values for maximum. allowed shear stress apply:

50 000 load oscillations Table value x 0,9

100 000 load oscillations Table value x 0,85

1 000 000 load oscillations Table value x 0,7

10000000 load oscillations Table value x 0,6

For a more exact determination of fatigue performance, please refer to Lesjöfors Spring handbook

ENGLISH MEASURES AND WEIGHTS

Metre Inches Millimetre

Inches

Millimetre

Inches

39,3701 25,4 0,0393

Newton

Kilopond

Pounds

Gram

Multiplier

Kilopond	0,102
Pounds	0,22467
Gram	102
Newton	9,807
Pounds	2,2046
Gram	1000
Newton	4,448
Kilopond	0,4536
Gram	453,6
Newton	0,009807
Kilopond	0,001
Pounds	0,0022046

From	To	Multiplier	From	To	Multiplier
Kp/mm	lb/in	55,998	Kp mm	lbin	0,086796
Kp/mm	N/mm	9,807	Kp mm	N m	0,009807
Kp/mm	kN/m	9,807	lbin	Kp mm	11,52125
lb/in	Kp/mm	0,017858	lbin	N m	0,1129889
lb/in	N/mm	0,175133	N m	Kp mm	101,968
N/mm	Kp/mm	0,101968	N m	lbin	8,850413
N/mm	lb/in	5,7099