Points driver

[RAF96]

I have gotten fed up with solenoid tech and decided to have a go at a compact motor driven solution. Compact as in not a great big plastic box that has to go under the layout if your framework is deep enough.

 

The starting point is one of these..

 

 

 

The shaft shown is 55mm long and the motor is 16mm long, the gearbox being 9mm long. 

It will obviously need a limit switching system similar to Fulgurex motors and also a bellcrank multiplier arrangement to change the screw drive to a side to side motion at the point tie-bar.

 

Motor as supplied is 12v DC and output drive is 400 RPM.

 

Initial calcs indicate 3.5mm tie bar movement, which if doubled by the bell-crank gives us 14 turns of the M4 x 0.7mm pitch output shaft in around 2 seconds.

 

So far it drives and it stops at the microswitches switches. Just need to refine the limits to avoid destroying the fragile point mechanism.

 

Future developement may include a current sensing cut off to bring the size of the ‘point motor’ assembly down to a size suited to overboard mounting.

 

I will report back with pictures when the prototype is more presentable to the public.

Edited December 29, 2020 by RAF96
Topping up info.

[AndyID]

 

You could use a single microswitch to limit the travel, like this:

The trick is to take advantage of the hysteresis in the microswitch (the cheaper ones have greater hysteresis) so that the button is at its mid point while the motor is running and allowed full travel at the end points. You can also do it with a "loose cam" that only moves when the end points are reached.

 

I've also included is an optional relay to control frog polarity. If it's a two pole relay you could use one pole to feed the frog and the other pole to replace the control switch. To change the point direction you'd only need to turn the relay coil on or off.

 

 

[ikcdab]

Looks interesting. Why don't you just use ordinary servos which are much the same size this answer, very easy, effective and already developed. Of course, you might like creating new solutions which is great!

[AndyID]

52 minutes ago, ikcdab said:

Looks interesting. Why don't you just use ordinary servos which are much the same size this answer, very easy, effective and already developed. Of course, you might like creating new solutions which is great!

 

Unfortunately servos have their own set of problems but you can always dump the internal electronics and just use the motor and gear train.

[AdamsRadial]

An omega loop between the points tie-bar and whatever runs on the thread should absorb any shocks and stop disintegrations. 

 

Servos seem like a perfect solution but they need a driver that puts out a repetitive pulse to specify where the actuator is to move to, you can get multiplexers for running a bank of servos off an arduino but it's all adding to the complexity.

Edited December 29, 2020 by AdamsRadial

[ikcdab]

Servo drivers are very cheap. So are servos and the whole setup is quite easy. 

The OP's proposal looks imaginative and interesting and looks similar to how a real point machine works. 

I didn't see any provision for polarity switching.  With servos I have used miniature relays very effectively. Micro switches are always tricky to setup and maintain in adjustment.

[RAF96]

1st picture - Rough prototype using a wooden nut runner which acts as an actuator and microswitch striker.

 

2nd picture - Second iteration will use cable conduit as the motor housing and the cover strip as the moving element. Actuating parts to be developed.

 

Seeing the other post using the N20 motor-gearbox employing a cam system has given me ideas to alter the tie bar actuation to ditch the bell-crank and go for a direct push pull with Omega link, which would affect the limit switch arrangement as well.

 

 

Edited January 2, 2021 by RAF96
Tidy up

[AdamsRadial]

51 minutes ago, RAF96 said:

direct push pull with Omega link

My thoughts here are to have the limit-switches operated by the traversing nut, but connect the traversing nut to the tie-bar by a flexible piece of wire or an omega loop.  I assume you are not modifying the points in any way, they will still have the spring that goes over-centre and holds the blades in one or the other position?

 

The cable-conduit idea is a neat trick.

Edited January 2, 2021 by AdamsRadial

[RAF96]

3 hours ago, AdamsRadial said:

My thoughts here are to have the limit-switches operated by the traversing nut, but connect the traversing nut to the tie-bar by a flexible piece of wire or an omega loop.  I assume you are not modifying the points in any way, they will still have the spring that goes over-centre and holds the blades in one or the other position?

 

The cable-conduit idea is a neat trick.

 

 

The nut runner will attach to the back of the slider, which will have a ramp on it exactly the same thickness as the point tie-bar travel. Off ramp the point is one way, on ramp it is the other way. The link to the tie-bar will be rigid with a return spring so when the slider moves to off ramp the spring pushes the point back. The flip-over spring fitted to the point remains in-situ. Finite travel limit switches will be actuated by the nut runner and a shaft free end  support block will be fitted to the conduit, which will be provided with mounting lugs and laid on its side with the slider facing but parallel to the point. This direct transverse actuation gets rid of the previous bell-crank.