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Motorised swing bridge


PMW
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It seems that layouts nowadays need animation, and mine will be a reasonable facsimile of a swing bridge which carries the Bittern Line over the river Waveney near the village and estate of Somerleyton, on the Norfolk - Suffolk border. Scratch building the model doesn't bother me too much, I may well make the basic mechanism from meccano and then attach it to a piece of board and add the girder work from commercially available plastic beams. The construction of the bridge means that when closed the weight of the bridge and it's load is carried by three beams under the track bed and when swung the bridge mass is supported by a pair of overhead truss girders. in 1:76 I think the board which forms he track bed should be self supporting when swung, and will sit on three piers when closed so the weight of any load will go through those.

 

My issue is with motorising. Getting the bridge to swing isn't difficult, and there will be definite stops on each pier to locate the bridge in the closed position. The issues are getting the motor to stop once the bridge hits the closed position, and keeping it locked so that it doesn't "drift" open with the obvious consequences. 

 

I have one or two ideas. A non latching push to break switch on the stop would isolate the motor but would need to be wired in parallel with a push to make switch to provide power to open the bridge. I could perhaps use magnetic latches to keep it closed.

 

Any other suggestions are welcome. 

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I'm also building a bridge based on this one and reedham,  I did think of making it work but in the end space prevented me doing it

I look forward to seeing how yours comes on. To be totally prototypical it should raise slightly before swinging 

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7 minutes ago, PMW said:

It seems that layouts nowadays need animation, and mine will be a reasonable facsimile of a swing bridge which carries the Bittern Line over the river Waveney near the village and estate of Somerleyton, on the Norfolk - Suffolk border. Scratch building the model doesn't bother me too much, I may well make the basic mechanism from meccano and then attach it to a piece of board and add the girder work from commercially available plastic beams. The construction of the bridge means that when closed the weight of the bridge and it's load is carried by three beams under the track bed and when swung the bridge mass is supported by a pair of overhead truss girders. in 1:76 I think the board which forms he track bed should be self supporting when swung, and will sit on three piers when closed so the weight of any load will go through those.

 

My issue is with motorising. Getting the bridge to swing isn't difficult, and there will be definite stops on each pier to locate the bridge in the closed position. The issues are getting the motor to stop once the bridge hits the closed position, and keeping it locked so that it doesn't "drift" open with the obvious consequences. 

 

I have one or two ideas. A non latching push to break switch on the stop would isolate the motor but would need to be wired in parallel with a push to make switch to provide power to open the bridge. I could perhaps use magnetic latches to keep it closed.

 

Any other suggestions are welcome. 

Why not drive it with a stepper motor as with some TT projects on here?

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Sounds like a  big worm drive as a final stage reduction would stop the mechanism backing off while a micro switch operating when the bridge reaches full travel should stop the motor. A flywheel on the motor shaft might help.  Worm drives are not normally reversible,  Pre WW2 lorries used worm drives instead of crown wheel and pinion to provide retardation down hills and locos with worms wont normally push along, (yes I know some Wrenn ones do. )

With a diode set up the motor will stop but still reverse when the micro switch is depressed  it should be possible to power it with a simple open or closed polarity changing DPDT  switch, ideally centre off but not necessarily.

Otherwise a mechanical or electric lock, solenoid operated, could lock the bridge in position, a tapered pin in a hole will centre the deck.

Edited by DavidCBroad
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I did think about using a belt, or band drive on the basis if it were to over rotate it would not strip gears but melmerby has got my interest peaked in steppers and arduinos, that might be the way to go. 

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Presumably it only ever needs to rotate through the same 90 degrees and it never rotates while loaded with a train. That makes it quite a bit simpler than a loco turntable.

 

You could use a motorized winch winding up a cord to open it. A weight at the end of a cord over a pulley would bias it back to the closed position when you reverse the motor and unspool the winch.

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3 hours ago, AndyID said:

Presumably it only ever needs to rotate through the same 90 degrees and it never rotates while loaded with a train. That makes it quite a bit simpler than a loco turntable.

 

You could use a motorized winch winding up a cord to open it. A weight at the end of a cord over a pulley would bias it back to the closed position when you reverse the motor and unspool the winch.

You would need some sort of mechanical regulator to stop it slamming shut instantly

 

Personally I would make it automatic using electronics, which could be integrated into DCC later if wanted.

Edited by melmerby
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32 minutes ago, melmerby said:

You would need some sort of mechanical regulator to stop it slamming shut instantly

 

 

The winch has a large mechanical advantage over the arm (or sector plate) attached to the bridge's axle. It's won't be possible for the bridge to slam shut. A sector plate would actually be better as it would maintain a constant mechanical advantage (lever length).

 

Here's a sketch. I've shown the cords acting on a pulley but it would only need to be one quarter of a pulley.

Swingbridge.png.cda659779f81401d626c254366c13733.png

 

 

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My plan has always been to make it as automated as possible and I like the idea of using an arduino controlled stepper motor to allow push button control. 

 

Since reading melmerby's suggestion yesterday I have been doing "arduino for dummies". It is not something which should be beyond me. From a software and programming basis it is not that dissimilar  to things I have done in the past, in fact it seems very similar to modding a 3d model on something like farming simulator. 

 

I had thought of building a dedicated controller using 4 pwm motor controllers to power the 4 main track sections and adding the bridge control to that, but my voyages into the world of arduino have me dreaming of DCC++ via raspberry pi, arduino, motor shield and one of the commonly available interfaces such as JMRI. I have lots of videos on my you tube list still to get through!

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53 minutes ago, PMW said:

 

I had thought of building a dedicated controller using 4 pwm motor controllers to power the 4 main track sections

I did that years ago using 4 home built motor controllers driven by a digital-analog convertor from a BBC micro.

I had a 4 line PBX panel with about 15-20 extensions as the main switch panel.

I had all the various bits working but not installed when DCC and it's forerunners appeared and that's as far as it went.

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One thing that is difficult to replicate in miniature is the flexibility of most swing bridges. The side girders support the transomes. The transome to side girder joint must be suffiently flexible to not influence the resting position of the side girder. This is much like compensation for a four wheeled wagon and keeping all four wheels on the rails.

 

In model form this may need a helping hand. My experiments some years ago involved driving 'bullets' from the bridge ends into the bridge itself. They were spring loaded into the withdrawn position, and in the case of my experiments, driven in by hand. For each 'bullet' six indentical plates were made  and a hole drilled through the three whilst sandwiched together. This ensured they were in line. Two were used landward in each assembly and one on the movable bridge. Four such assemblies were used. A rod sat in the landward holes that was sharpened to a blunt point, hence my reference to the bullet. When driven from the landward structure into the bridge they forced alignment. In addition to this the movable structure was spring loaded upwards. Withdrawing the 'bullets' allowed the moveable bridge structure to rise around 1mm to clear the landward bearing plates. Driving in the 'bullets' pulled it down again, mimicking some of the actions of a real bridge.

 

The problems i encountered were many. Most would have been sorted by more robust construction. I used quite thin plasticard. If i were to try again i maybe consider brass with due regard to the earlier mention of flexibility. I used one spring to lift the bridge. It didn't always lift squarely. Further refinements were planned including spring loaded 'shutters' obscuring landward holes where the 'bullets' emerge from. the plan was they were to be driven open by the bridge. There were two reasons for this. Firstly to hide the mechanism and secondly to provide and interlock so the 'bullets' could not be driven without the bridge being somewhere near the right position.

 

The whole thing was a lash up to prove ideas. I gave up in the end as there wasn't anywhere suitable on the layout for it.

Selby open.jpg

Edited by LNERGE
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