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Memory wire Applications
I thought that this new addition to a modellers electronic arsenal deserved its own page of explanation . Memory wire is an alloy of titanium and nickel called nitinol. By heating Nitinol it can be made, amongst other things, to shorten by 5%. This 5% change in length is constant for any length or diameter of wire. You will see that this has any number of potential uses for the railway modeller. Memory wire is available in many different diameters ranging from 5mm to 50 microns. ( one micron = 1/1000 mm).The 5% shortening produces only small movements ie. A10cm length will contract by only 5mm. This can however be amplified by the use of levers as shown in the example below
The above diagram shows the bottom half of a semaphore signal as you can see a 5mm pull will result in a 10mm push on the signal control wire if the control lever is twice the length of the pull lever.
Memory wire will not however naturally return to its origional length and must be biased or stretched back. The force required to achieve this is much smaller than the pulling force that the wire exerts so a small spring can be used to pull the wire back into shape ready for the next time.
If you are heating the wire by electrical current then you need to know the ammount of current to pass through it without overheating and damaging it. This is done by using Ohm’s Law.
I = V / R
R= V / I
A table of resistance is provided with memory wire for reference but I have recreated it below
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Wire Diameter
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Resistanc e in Ohms/inc h
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Max Pull Force
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Approx Current at Room Temp
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Contracti on Time
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Off Time
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0.0015”
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21
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17gm
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30mA
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1 sec
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0.25 sec
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0.002”
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12
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35gm
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50mA
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“
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0.3 sec
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0.003”
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5
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80gm
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100mA
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“
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0.5sec
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0.004”
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3
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150gm
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180mA
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“
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0.8sec
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0.005”
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1.8
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230gm
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250mA
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“
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1.6 sec
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0.006”
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1.3
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330gm
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400mA
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“
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2 sec
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0.008”
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0.8
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590gm
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610mA
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“
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3.5 sec
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0.010”
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0.5
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930gm
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1000m A
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“
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5.5sec
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0.012”
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0.33
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1250g m
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1750m A
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“
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8 sec
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0.015”
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0.2
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2000g m
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2750m A
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“
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13 sec
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Currents which heat the wire in 1 sec can be left on without overheating it.
Looking at the information in the table we immediatley see that the 0.006” is ideally suited for our needs as the off time is 2 seconds which will simulate slow moving points and signals etc. To find the voltage needed to use a 10cm(3.94) length of 0.006” gauge wire we multiply the current(I) by resistance (R)
10cm = 3.937 007 874 inch
Therefore V = I (0.4) x R (1.3 x 5.90) (remember that the resistance value in the table is per inch of wire).
V = 0.4Amps x 7.67Ohms = 3.06 V
Practical Applications of Memory Wire
Linear Actuation
Memory wire is most easily used to provide linear or straight line movement which is ideal for railway modelling . The example shown uses memory wire to activate a point. Because of its slow action in both contraction and recovery times this makes the point very realistic for the fraction of the cost of a slow point motor
Angular Actuation.
In many practical applications of memory wire a simple lever mechanism is used to amplify the movement of the wire, as shown in the above example of the signal armature.
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