Building Draw Bridges

[GP-9_Man11786]

I've been rolling plans for my future railroad around in my head for a while now. I know that I'd like a water feature. Then I got to thinking "why not have a moveable bridge to cross said water feature?" Draw bridges are very common on real railroads and would add interest, but they do seem incredibly difficult to model.

There are three main types of drawbirdges used on railroads. Swing bridges, which pivot horizontally, would be impossible do to the anchorage required in the middle of stream.

A vertical lift bridge would be difficult because it would require two motors, one on each end, to raise and lower the span. Keeping the motors in sync would be critical so both ends raise and lower at the same speed.

A bascule bride pivots on one end and raises vertically. This seems the easiest to model. These are basically truss bridges with hinges and a counterweight on one end. What would be the best way to model one of these?

Also, is it possible to wire three feet of track on either side of the span to have power cut off while the bridge is raised? Better yet, could you wire the bride to stay open util a train approaches, then close, then re-open when the train passes?

 

[ttrigg]

A vertical lift bridge would be difficult because it would require two motors, one on each end, to raise and lower the span. Keeping the motors in sync would be critical so both ends raise and lower at the same speed.

The bridge has four corners. Put four pulleys on one motor.  Run cables to the other side of the bridge through the overhead superstructure, above the bridge span.

Also, is it possible to wire three feet of track on either side of the span to have power cut off while the bridge is raised? Better yet, could you wire the bride to stay open until a train approaches, then close, then re-open when the train passes?

Think of a two bladed knife switch. Put a spade system under the rails of one end of the bridge so that it connects two contacts.  Bridge is down, power arrives into the first contact, passes through the blade on the bottom of the rail, passes through the second contact and on out to the rail sections needing to be controlled.  Bridge is up, no contact made with the blades no power to ends of rail.

[Capt Bob Johnson]

Much depends on the waterway, frequency of train and vessel traffic.  Some bridges are normally open and close only for the approaching train, and go back up as soon as train clears; while others are normally closed and only open on whistle signal from the approaching vessel and then close again after the vessel passes.   To further confuse the issue, some bridges only move when it is in the wrong position for the traffic of the moment; bridge closes to allow train to cross and stays closed until signalled to open for vessel traffic, then stays in open position until called to close for train.

Kinda like running a model railroad, it's your bridge, run it the way you want to!

[DSchmitt]

On prototype vertical lift bridges the lift span is connected to counterweights in each tower.  The weight of the weights is nearly identical to the weight of the lift span.  A low horsepower motor is mounted in the center of the lift span to raise and lower the bridge.

I don't know whether the weight are heavier or lighter than the lift span.  I suspet they are usually heavier on a railroad bridge so there is a bias toward the up position, and probably usually the opposite on a highway bridge.

On a bascule bridge a counter weight is mounted on the pivot end. outside the pivot.  As with the vertical bridge the weight balances the span.  I've seen photos where of a small SP bridge that was operated by one man using a chain mechanism of the type used on roll up doors.