Before I would go through the trouble of redesigning and rewinding a 
coil, I would try to see if some mechanical problem is the trouble.  
In my Lionel Gabe, the pawls that held the locking gear in place were 
sticking (probably riveted a little too tight).  Higher voltage 
helped, but only because of the impact of the extra force tended to 
rattle the pesky pawls loose.  In the end, I glued small weights on 
each pawl & it works fine now even on a small transformer.

Regarding coil force -  In theory, the force is proportional to the 
square of the current times the coil turns.  Said another way, if you 
double the current, the force increases by four.  Same for the turns.

The simplest way to increase force is to just increase the voltage.  
By going from 15 volts to 18 volts (a 20% increase in both voltage & 
current), you could increase the force by 44%.  This is why going to 
an 18B or 30B transformer with 18 volt output can really "rock".  

My guess is that the Gilbert factory designed the coils by trial and 
error rather than by some formula*.  If they got the coils to operate 
the accessory without burning up during normal use, they were happy.  
If you try making your own coil, just remember this simple design 
rule - "Engines smoke, accessories don't!".

* see this link for why they might have avoided using a theorical 
approach!

http://www.ee.surrey.ac.uk/Workshop/advice/coils/force.html

John Gasal


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