EoW July 2014

Technical article

Load / N @ 9.15@ 30mm 17.61@ 40mm 21.81@ 45mm 24.33@ 48mm 26.00@ 50mm 27.63@ 52mm 29.23@ 54mm 30.80@ 56mm

Load/ N @ 30mm

Free Length /mm

Loss of load /N @ 30mm

9.15 9.13 9.12 9.10 9.07 9.02 8.91 8.72

25.005 25.010 25.020 25.020 25.020 25.020 25.030 25.030

0

0.02 0.03 0.05 0.08 0.13 0.24 0.43

▲ ▲ Table 1 : Example result for a carbon steel spring heat treated at 250°C

Outside Diameter mm

Initial Tension N

Load at which 0.1N loss

Load at which 0.2N loss

LTHT / °C

% Rm

% Rm

% Rm Spring

Rate N/mm

‐

9.15 6.77 6.16 5.12 3.72 2.16

14.2 10.5

17.6 24.3 26.8 26.6 25.0 24.0

27.3 37.7 41.5 41.2 38.7 37.2

19.6 26.5 28.8 28.6 27.2 26.0

30.4 41.1 44.6 44.3 42.1 40.3

0.810 0.840 0.850 0.850 0.856 0.881

6.13 6.10 6.06 6.03 6.03 6.03

150 200 250 300 350

9.5 7.9 5.8 3.3

▲ ▲ Table 2 : Results for the carbon steel springs

Outside Diameter mm

Initial Tension N

Load at which 0.1N loss

Load at which 0.2N loss

LTHT / °C

% Rm

% Rm

% Rm Spring Rate N/mm

‐

5.78 5.30 4.90 4.65 4.35 4.00 2.85

11.3 10.3

12.7 15.4 16.3 16.6 16.6 15.6

24.7 30.0 31.7 32.3 32.3 30.3 29.8

14.5 17.4 18.2 19.2 19.0 18.5 18.2

28.2 33.9 35.4 37.4 37.0 36.0 35.4

0.740 0.750 0.760 0.767 0.770 0.765 0.770

6.12 6.12 6.13 6.12 6.13 6.14 6.14

200 250 300 350 400 450

9.5 9.1 8.5 7.8 5.5

5.3

▲ ▲ Table 3 : Results for the 302 stainless steel springs

temperature, as shown in the European Standard for stainless steel spring wire, EN 10270‐3. Load/ deflection graphs for a stainless steel spring, heat treated at 350°C, loaded

The spring rate increased slightly as the LTHT temperature was increased despite the outside diameter increasing. This is because the torsional modulus, G, increases with the heat treatment

Not only does theory ignore the elastic deflection of hooks, it also ignores the plastic deformation that occurs before the elastic limit of the body coils is reached. If the elastic limit of body coils is defined at a loss of 0.1N, then part of this loss is due to the plastic deformation of the end hook. Indeed the results show in Table 1 that when 0.1N is lost, the free length has increased by 0.015mm, which equates to more than ten per cent of this load loss. The actual load/deflection graph to the load at which there is a 0.1N loss is shown as Figure 4. The stainless steel springs were processed in a similar manner to the carbon steel except that they lost load at much smaller deflections.

▼ ▼ Figure 4 : Load/Deflection characteristic of a carbon steel extension spring heat treated at 250°C

The results are shown in Table 3 .

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July 2014