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I am building a large around the room standard gauge layout which requires an entry lift bridge.  The bridge is 48" long, 38" wide at the hinges and 24" wide at the other end.  The bridge takes about 10 lbs force to lift and I don't want the hazard of it accidentally falling closed.  The solutions I am considering are:

1.  Counterweight attached to arm at hinged side.  About 25 lbs should suffice - dumbbell fitness weights would work. 

2. One or two gas filled lift struts as used on automobile hoods and hatches.

I would like the bridge to be balanced about 12" above the closed position to reduce the chance of it slamming shut and causing damage or injury.  The biggest problem with lift struts is determining which ones I need.  I am sure there is a common automotive part that would work, but I expect it would be a lot of trouble to figure out which one.  I have seen a page where lift struts were used for this purpose, but my application would require a different strut.  The counterbalance is relatively simple, but lift struts would be more impressive!   Pictures show the bridge closed and a temporary counterbalance.  Your advice is appreciated.

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Interesting topic.  I have also wondered how to calculate gas strut size.  The entry to my cellar is through a lift up door in the floor.  My wife has often asked if it could be made easier to lift.  The door is light but if it were to slam down on your toes, you might take notice.  

On the other hand, since I like simple, counter weights on cables and pulleys is a no brainer.  That is it takes no brains to figure it out.

Thanks for the link to Susan Deat's site.  There is a link to the supplier she used for the gas strut.  The supplier has information on picking struts.  I still wonder if there is a way to find detailed specifications on readily available automotive gas struts?  Dan, you are correct about the simplicity of counterweights - I wasn't even planning on having cables or pulleys!

Just an FYI - the pics show what I did to create by opening into the center of my layout. Approx 3' wide x 4' deep opening. You can see the shiny bolt head just above the "faux" crossing gate counterweight. Notice the black paint can hanging down on a cord - this is the actual counterweight to help me open up, and I can adjust the weight by adding more gravel inside the paint can.

Since pics, have also added some LED clusters on the bottom of the arms, created a 555 timer IC based flasher, and a magnetic proximity switch that turns off the flashers when the in the open position.

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That's awesome MED.

I could see a balsa/bass bascule bridge done on the idea too 

It could be made more contemporary if you wanted to also.

  I like "piano hinges"; often drilling extra holes along the length for the best grip. Thin, but they need to sit topside with hinge pin down on a lift section leaving a stip to disguise, or need to cut 1/16-ish steps for a flush fit; and to go with hinge pin down, the original holes taper is often reversed so you have to drill all new holes and countersink a taper for scew heads to get a true flush.... And this is provided the hinge you chose will open beyond a 90° so the door can rest with the weight shifted away from the opening... check that.   I like them because there is never hinge to hinge alignment issues and the hinge design keeps things, even dust, from dropping into the gap, into the doorseat area.

Whatever you do, leaving ability to trim and adjust the latch edge or footing as the option for a very last step is a good idea.

Roger Wasson posted:

A little off topic, and you can delete this if you want.

But, how and what do you use to center the tracks when they come down? I'm having a little difficulty with my homemade version, even though it has worked for the past 8 years. As I said, just asking.

I've seen brass dowl and sockets used (cabinet and boating hatch hardware) and also  a blade inserted & soldered to the stationary side and the bridge side has its rail's foot and web spread slightly to accept the blade and the span drops onto the footing.

My update:  I have finished my bridge (except for wiring) and successfully used a gas lift strut.  It took several iterations and two types of lift struts.  The first lift strut was for a minivan hatch.  It was much too strong and I eventually broke the plastic ball joints while trying one of multiple configurations.  I found lift struts than fit 1993-97 Chrysler Concordes for only $9.99 a pair - probably a close out.  These were much shorter and just barely long enough to fit my bridge.   I only needed one lift strut and the force on the mounting brackets wasn't as scary as the minivan lift strut.  The bridge takes a bit over 4lbs to start opening compared to over 10 lbs without the strut.  The bridge stays open and can't fall on anyone.  If I had a completely different setup that accommodated a counterbalance, I might have not used the lift strut.  The difficulty with using a lift strut is finding one that fits the application. The advantages are that it takes almost no room and can work really well.

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