Memory Wire Point Actuator Eddie Price 10592
There is currently a lot of interest amongst members of the Model Electronic Railway Group (MERG) in using ‘muscle wire’ or ‘memory wire’ for point actuators. This wire has the unusual property of contracting when heated to about 70 degrees Centigrade. The change of length is typically 4%, so that the 2mm of movement required for ‘N’ Gauge point blades can be achieved with 50 mm of wire. Because the wire conducts electricity and is resistive, applying an electric current will generate the necessary temperature rise. The resultant actuator is silent and the rate of movement is more prototypical. The wire used in the actuator described here is Flexinol 100 obtained from Milford Instruments (Telephone 01977 681465). It is rather expensive at £11.75 for one metre but this should be enough for ten actuators. The 100 refers to the diameter of 100 micrometers.
55mm of memory wire (allow a little more to ensure sufficient movement) is positioned on the baseboard alongside the point. A Gem angle crank is used to activate the tie bar when the wire contracts. Memory wire requires a return spring to pull the wire back to its as-cold length and to return the point blades to their rest position. The 100 micron memory wire exerts a force of about 150 grams and the return spring must exert a force of about half this value.
Building the Actuator
One end of the memory wire is crimped between a strip of 5thou. phosphor bronze strip. This strip is soldered to a piece of copper clad printed circuit board, which in turn is pinned to the baseboard.
The other end of the memory wire is attached to a short length of brass wire, which is hooked into the angle crank. The angle crank is also attached to a piece of copper clad board and pinned to the baseboard. A piece of thin copper wire (unravelled from a length of flexible cable) is attached to the brass wire and to the copper clad board to provide a flexible electrical connection. The main electrical leads, which provide power to heat the memory wire, are soldered to the copper clad boards rather than directly to the memory wire. A piece of brass wire connects the other end of the angle crank to the tie bar.
The memory wire could be concealed beneath some plastic channelling painted to represent concrete trunking.
Modifying the point
The diagram above relates to an Electrofrog Peco Streamline point and some of the necessary modifications are illustrated.
The toggle spring, which normally ensures that the moving blades are pressed firmly against the stock rails, is removed. Two alternative forms of return spring are shown, an expansion spring and a length of music wire.
The music wire has the advantage of being almost invisible beneath the track. It is formed from 0.5mm music wire about 18 mm long and positioned beneath the sleepers. A right angle crank is formed at one end to engage in the hole in the tie bar formally used by the toggle spring. The other end of the wire is placed in slots cut in the bottom surface of the two end sleepers. A piece of 10thou. Plasticard is glued onto the sleeper bottoms to hold the wire in place. Before gluing, the wire needs to be bent about 5 degrees to provide the necessary force on the tie bar to press the moving blade against the stock rail. Some trial and error testing is necessary to ensure that the moving rail returns fully to the stock rail. It is also necessary to shave off some of the plastic on the bottom of the sleeper between the two end sleepers and the tie bar to allow free side to side movement of the wire. The music wire is neat and unobtrusive, but it is difficult to readjust once the point is fixed to the baseboard
The expansion spring is a simple alternative that is easier to adjust. One end of the spring is attached to the angle crank, while the other end is attached to a pin in the baseboard. The removal of the toggle spring, which exerts considerably more force than the return spring, means that the electrical connection between the moving rails and the stock rails cannot be assured. Thus an external switched connection is necessary using a changeover toggle switch or a relay. Before this can be used with the Electrofrog Peco points it is necessary to remove the wire on the underside of the rails which connects the crossing vee to the moving blades and to connect the moving blades to the stock rails, as shown in the above sketch.
A slight difficulty in using memory wire is making an electrical connection to it. It is an alloy of nickel and titanium which, like nickel-chrome resistance wire, does not take solder very readily. The suppliers of the wire recommend crimping, but several members of MERG have reported that the wire pulls itself free from a simple mechanical crimp. However, solder can be used with a suitable flux, for example, Carrs Brown Label. To attach the memory wire to a brass wire, first clean the memory wire by rubbing the end 3 or 4 mm with 600 grit carbide paper. Then bind the wire to the clean brass wire with a strand of fine copper wire taken from some flexible connection wire (i.e. 10/0.1mm wire). Apply some flux and solder with a small iron. Alternatively, 5thou. brass or phosphor bronze strip may be folded and crimped around the memory wire (cleaned first) and secured with solder, again using the Brown Label flux.
To heat the wire it is simply a matter of applying about 180mA. Since we are only heating the wire the supply can be AC or DC. The simplest solution is to use a low voltage AC supply. Many transformers are available with 6V-0-6V at 1 or 2A per winding. Remember that unlike solenoid operated point motors the power must be maintained for as long as the point setting is required. Thus with perhaps 10 or more points being set at any one time the transformer has to provide 2A or more. The resistance of the 50mm of memory wire is approximately 7ohms, so that if the 6V were to be applied directly the current would be about 850mA. This is too large and therefore, it is necessary to use a resistor in series with the wire. A suitable value with a 6V supply would be 27ohms. This should be wire wound and need to be rated at 1Watt.
There are many possible alternative designs. If the tie bar moves very freely, then a shorter length of memory wire may be used with a mechanical lever of say 1.5:1 or even 2:1. Thus the 1mm of movement produced with 25mm of memory wire could be amplified to 2mm with a 2:1 lever. Additionally the expansion spring can be replaced with a compression spring placed around the memory wire and between the angle crank and the fixed connection of the memory wire. The compression spring presses against the crank to push the rail back to its rest position (the spring must not touch the memory wire). An important requirement of any design is to ensure that the amount of free movement in the linkages is reduced to a minimum.
This article first appeared in N Gauge Journal 3/98. Members can purchase back copies of Journals.