bridge bearings
#1
Has anyone ever scratchbuilt their own bridge bearings? Gary, did you make you own for your many bridges, or just use commercial castings? Perhaps I should just stop by the LHS for these rather cheap detail parts, but the "Dumpster" thread has encouraged me! Some styles of bearings look rather simple.
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Kevin
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#2
I used the parts that came with the CV and ME girder kits. The ones from CV kit, once cut in half, were a very close match to the real ones on the Mason Park bridge. For the 69th Street bridge, the shoes really looked like nothing more than stacked up steel plates. The ME shoes matched fairly well, so I used them. The concrete bridges didn't have shoes, the concrete just set on the supports as far as I could tell.

Now that I think of it, some of the shoes may have been leftovers from a Walther's truss bridge. I also remember that I have a set of metal shoes, maybe those came in the ME bridge kit?

It shouldn't be too difficult to make some.
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#3
I made a mold of a Walthers bridge shoe and then cast them in resin. But, like Gary said, it should not be difficult to scratchbuild them.

   
Jens
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#4
OK, I have to ask,
How do you get those small castings done without air bubbles, that come from mixing the two part resins?
Are you using a slow cure resin?, and if so, what manufacturer ?

Really, those are some extremely fine looking castings ! Thumbsup

It CAN be done, and I'd like to know how?
We always learn far more from our own mistakes, than we will ever learn from another's advice.
The greatest place to live life, is on the sharp leading edge of a learning curve.
Lead me not into temptation.....I can find it myself!
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#5
Sumpter250 Wrote:OK, I have to ask

Yes please, questions are always welcome.

Quote:How do you get those small castings done without air bubbles, that come from mixing the two part resins?
Are you using a slow cure resin?, and if so, what manufacturer ?

I am not sure, what brand I was using back then. Currently I am using a german brand: "Sika". But thats not important, the stuff from Micro-Mark is very good and available in the US.
You are already on the right path. I am using slow cure resin (around 6min pot life - but depends on room temperate). I mix fast, pour fast und try to pick out the bubbles manually with a tooth pick in the remaining time. You can extend the pot life a little bit by cooling down the resin (the un-mixed components of course!) in your fridge before casting.
What also helps is dusting the mold with talcuum (spelling?) powder. It also extends the lifetime of your mold.

The first few casting with a new mold will always have more bubbles. I am not sure why - must have something to do with the silicone.

Last, but not least, there is a way to eliminate almost all bubbles. Its called pressure pot. I will buy one as soon as I win the lottery....
Jens
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#6
SP1 Wrote:I am not sure, what brand I was using back then. Currently I am using a german brand: "Sika". But thats not important, the stuff from Micro-Mark is very good and available in the US.
You are already on the right path. I am using slow cure resin (around 6min pot life - but depends on room temperate). I mix fast, pour fast und try to pick out the bubbles manually with a tooth pick in the remaining time. You can extend the pot life a little bit by cooling down the resin in your fridge before casting.
What also helps is dusting the mold with talcum powder. It also extends the lifetime of your mold.

The first few casting with a new mold will always have more bubbles. I am not sure why - must have something to do with the silicone.

Last, but not least, there is a way to eliminate almost all bubbles. Its called pressure pot. I will buy one as soon as I win the lottery....

I tried using vacuum, and all it did was allow the bubbles to expand, and ruin the casting. Only one more question, Just how much pressure is needed ?
If it is under thirty pounds/square inch, an old pressure cooker pot, and an air compressor ( air brush ) should work well enough.
We always learn far more from our own mistakes, than we will ever learn from another's advice.
The greatest place to live life, is on the sharp leading edge of a learning curve.
Lead me not into temptation.....I can find it myself!
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#7
Sumpter250 Wrote:I tried using vacuum, and all it did was allow the bubbles to expand, and ruin the casting. Only one more question, Just how much pressure is needed ?
If it is under thirty pounds/square inch, an old pressure cooker pot, and an air compressor ( air brush ) should work well enough.

I have to admit, that I dont know how much pressure is needed. I made a few attempts with with an old pressure cooking pot and an airbrush compressor. They all failed. Maybe I should investigate that further - but there is so much to do and so little time.
Jens
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#8
talcum with one u; vacuum with 2. Thumbsup
David
Moderato ma non troppo
Perth & Exeter Railway Company
Esquesing & Chinguacousy Radial Railway
In model railroading, there are between six and two hundred ways of performing a given task.
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#9
Those are some beautiful castings! SP, you are steadily raising the bar. Keep them coming! Inquiring minds want to know! Thumbsup
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#10
I seem to recall back in the second ice age, in one of the first consultant design offices I worked in right out of school, where we spent as much time in the shop building things as on the board designing them, that when we needed multiples of a critical part, we made an RTV mold and cast the parts in resin. Then there was this small "vibration table" that one of the guys had built that we put "fresh pours" on to bring any bubbles to the "surface," which was, of course, the unseen bottom of the part! Misngth

A generous application of "tincture of green soap" (procured from the local apothecary,) once dry was an excellent mold release ... just remember to wash the part thoroughly prior to applying any paint! Big Grin
biL

Lehigh Susquehanna & Western 

"America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves." ~~Abraham Lincoln
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#11
Shifting the thread back toward the subject <ah-hem>, bridge bearings or shoes aren't too hard to scratchbuild if you go with some of the simpler types. Essentially one end or both must be able to move as the metal expands and contracts with temperature changes. One end may be as Gary mentioned, a simple stack of metal plates, while the other may be a set of 4 or 5 metal rods laid on their side perpendicular to the rails. This is easily modeled with styrene sheet and rod, or brass, or cardstock and toothpicks, depending on the scale.

The more complex shoes are best built up in styrene. Yes, you can cast multiples if you like, but I'd only recommend that if you intend to sell or share the castings, or are in need of MANY for your own layout. You can also heavily rust these castings and use them in a junk or scrap pile/load even if you don't use them for a bridge, or if they come out with too many air bubbles. I made a mold for several metal parts that came with a structure kit, for a couple reasons - one, to have a place to dump extra casting resin, and two, in case I needed the extra parts down the road for kitbashing. They weren't highly specialized parts and I have no intention of mass producing them to sell, etc. so I'm not worried about copyright infringement or the like. The first reason is the best, however, since I can make the most of the casting liquid I have on hand when casting any other parts, then just paint and weather the metal plates for a scrap load or junk pile.

This is good to remember when making specialty molds for duplicating parts. If there are large corner areas or blank zones on a mold, find a few detail parts like a stump or some crates or a pile of tie plates and fill in those corners. Might as well make a few detail parts in addition to the part you need for whatever the current project may be.


Now just hold on a minute...how did I get back to casting parts! It must be contagious.
Wallbang
Galen
I may not be a rivet counter, but I sure do like rivets!
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#12
Hi guys. Just a few thoughts in relation to this topic.

Bridge Bearings are described by Engineers [Civil or Structural] as fixed or rolling, and no matter if steel or concrete they are made with allowances for casting errors in the abutment formwork as well as expansion and contraction due to temperature fluctuations and different rates of movement between materials.

Formwork is often [only half ] jokingly described as the stuff used to slow concrete down. When I was an apprentice Carpenter [back in '85] I witnessed firsthand the result of formwork failing under the pressure of wet [also decribed as plastic] concrete and the resultant mess. Often the bottom edges or other awkward parts of the formwork shuttering [the face against the wet concrete] can bow outward under the immense pressures involved. The wet concrete can also spread over [partially or totally ] bearing or anchoring plates and push or submerge Holding Down Bolts used to connect items to the finished concrete structure.

Bridge Beams and often sub-road support panels are now days Pre-Cast in factory settings either at the site in a temporary factory or at an established Pre-Cast factory. These components can then be either pre or post tensioned as specified by the Structural Engineer, where by the steel reinforcing inside the beam is tensioned before the loading develops to reduce sagging and cracking under load.

I would expect that where the concrete bridge beams rest on the abutments, that the actual surface of the beam and also the abutment will have a [probably galvanised] steel facing plate to deal with the enormous surface stresses and friction forces imposed on the contact points.

Here endeth the Bridge Bearing Primer 101

Mark
Fake It till you Make It, then Fake It some More
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#13
Pouring Resin;

I seem to recall a tip from Model Railroader with regard to pouring resin in this case to make a water feature, and to avoid including air bubbles the tip was to pour the resin down a stick and use the stick to direct the flow of resin.

If you wish to make a shaker table then mount your mould[s] on a board or an old table and then run a drill with an off centred bit in the chuck and the resulting vibrations from the motor and the shaking from the chuck will help to work any bubbles to the surface of the mould.

Concrete fact; Concrete weighs @ 2.2 tonnes per cubic metre ie 2.2T per M3

Formwork can also be thought of as like how a cake tin is used in cooking.

Whatever shape or texture the formwork is in will be reversed in the finished product.

The bridge abutment model in the photo above would be described as off the form board finish and was formed using rough sawn form planks of 6 or 8 inches in width. The patchy parts of the surface were caused by insufficient compaction/vibration of the concrete during placement somewhat akin to the air bubble problem in the hobby resin pour.

Mark
Fake It till you Make It, then Fake It some More
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#14
ocalicreek Wrote:Shifting the thread back toward the subject <ah-hem>, bridge bearings or shoes aren't too hard to scratchbuild if you go with some of the simpler types. ... Now just hold on a minute...how did I get back to casting parts! It must be contagious. Wallbang Galen

I don't know ... I guess I just thought that, reading the thread up as far as my own, a few tidbits re: making quick, simple castings might be helpful, along with an inexpensive mold release for getting castings out of the mold ... :?

And since you managed to get to the same place I considered, Galen ...

... maybe I wasn't too far off base. Icon_lol Big Grin
biL

Lehigh Susquehanna & Western 

"America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves." ~~Abraham Lincoln
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