September 29, 2010

Turntable - Part Three

I continued work on the rebuilt turntable. I removed the ties as my attempt at making the turntable look "homemade" just made it look like I was a bad builder. I trimmed all the ties to the same length and reinstalled them. Then I added the NBWs along the chord. These bolts are what hold the ties to the chord, as there is no wooden support member  under the ties other than the pivot pedestal. I did not add the NBWs on the bottom of the chord as I don't think they will be visible.

Before I added the rail, I measured the proper location and drilled two holes for the turntable feeder wires. Them I soldered the feeders to the bottom of the rail. By running the feeders through the ties I ended up with no visible solder joints. Then I spiked the rail down. The overall length of the table will be 12.5 inches, or 50 scale feet.

Next I have to build a new shaft and pedestal, as well as all the rest of the details. I'll probably laser cut the roller wheels. In careful study of the picture I note that they are not flanged. Also, look at the pit rail it rides on. It doesn't look like rail to me. I suspect that it is a wood base with a strap iron railing.

A mock up of the engine terminal at Falmouth. A wood rick, water tank and engine house are still to be built, as well as the terminal tracks.

Here is a close up of the turntable

I received a question about the bearing plates on top of the trusses from the ACWRRHS Yahoo forum. Yes, there are metal bearing plates on the tops of the posts. These prevent the truss rods from cutting into the wood. They also shed water off the end grain, though that may be a secondary concern as the whole turntable doesn't look like the builders were concerned with longevity. There also appear to be iron end plates at the ends of the chords. I plan to use my laser to cut the plates from 1/16 inch acrylic.

What thickness iron do you all think those truss rods are? The heaviest wrought iron rods specified in Haupt's bridge book is 1.5 inches. That scales to about 0.032 inches nearly perfectly. But to me they look beefier than that.

Finally, I wonder why they mortised the posts into the chords. The mortise joints don't appear to be that tight. The joints would significantly weaken the chords in bending, but not so much in compression, since the upright post would see a compressive load acoss its width. The design of the truss does put the chord into compression, but it would experience bending moments as the live load moves along the turntable. The chords are relatively massive, especially for such a small structure. They help give the turntable a "chunky" look. But I think the iron rods are providing most of the strength of this bridge.

9 comments:

  1. Bernie,
    do you know if any of the original builders were boat builders? That sort of mortised through post construction is very wooden sailing ship hull lookig to me. Perhaps old habits died hard?

    The close-up looks fabulous BTW!

    ReplyDelete
  2. If not boat builders, they were home and barn carpenters and probably very adept at mortise and tenon construction.

    ReplyDelete
  3. Great model! I have been saving the Alexander plan for a future project and am inspired.

    I suspect the mortises are just to locate the posts in one place. Since they are held in compression by the truss rods, the mortises can be shallow and relatively loose and you can maintain a reasonably rigid structure that still has some give in it. Wooden railroad car frames from the latter part of the 19th century seem to be built on similar principals (if the mortise and tenon joints in an end sill were not loose when built, they would certainly be after a few years of wood shrinkage and constant frame flex). I built a couple of wooden snow sledges on similar principles this winter and it is a very effective technique.

    Consider what would happen if you didn't have a mortise or other locating pocket: a little lateral movement and the bottom of the post would rack or kick out, the tension of the truss rods would slacken and the whole thing would collapse.

    Any way, that is a non-engineer's guess.

    Matt Lewis

    ReplyDelete
  4. My second guess is that the chords are so large because they have to bear the dead weight of the turn table and the load of the engine, all of whihc is suspended from the chords, from the central pivot point (it looks like the spider or other central bearing supports the gang of sistered cross ties that lies on top of it in the middle of the bridge, and in turn the chords lie on top of and are supported by that) to each of the 4 truss rod pick up points (I am further guessing that this table is balanced at its central pivot and the roller wheels only support significant weight when an engine is rolling on or off of the table), so on a turntable of this size, the chords have to support that weight for a span of 12 feet or so. If I were knocking together a rough structure using green wood, I would opt for beefy.

    Matt

    ReplyDelete
  5. OK, my last attempt to cause trouble here.

    When I looked at the old photo I saw two other interesting things about the construction:

    1. The two outer posts appear to lean towards the end of the turntable by a few degrees (5-10?). Guess #3 is that this is to make it so the post comes closer to bisecting the angle formed by the truss rods, and to direct the force of the truss rods more in a straight line down the post (the truss rod bends here so I would think that some of the force on the top of the post is downward and some towards the center of the turntable, tending to bend any post put in the apex.

    2. The tops of the chords appear to be chamfered (the top edges relieved or rounded off so that a cross section of the chord would appear as if the top had a slight arch) by a few inches everywhere but where the posts are let in. I have no idea why the chamfering, although I have done that on frame members of outdoor structures (using a 19th century style draw knife!) to discourage water puddling leading to pre mature rot. Leaving the chords square across the top where the posts fit probably makes lay out and cutting easier and maybe leaves a bit more meat for a shallow locating mortise so that the effective carrying depth of the chord is not impacted by the joint at all.

    This is all, of course, just wild guesswork.

    Matt

    ReplyDelete
  6. Matt,
    I agree with just about all that you say. It looks to me that only one upright post is angled. I wonder if that was a design feature or just a deflection from use.

    ReplyDelete
  7. Hi Bernie:

    When I look at the Manassas Jct. photo, the two posts farthest from the camera have a noticable lean to my eyes. Its harder to tell with the two closest, but that may be a shortening effect from the perspective.

    Some of the old Maine two footers (which always seemed to be several decades behind in technology, even at their peak) had similar turntables with more elaborate center posts but the same arrangement at the ends and they look to be leaning outward (I googled "maine 2 foot wooden turntable" and quickly came up with a photo of one at Philips). Again, this is all guesswork but I imagine that if the end posts were straight up and down, you would then want deeper, relatively well fitted (hard to do well with timbers that size with the green wood that I imagine a capital starved Southern or temporary military railroad would resort to), or even pinned, mortises to resist the truss rod forces that otherwise would tend to deflect the top of the posts towards the center, and the logic of the rest of the structure suggests that the actual mortises are not more than a couple of inches deep.

    Awesome modeling. It has inspired me to pick up a couple fo the BTS cars.

    Matt

    ReplyDelete
  8. Don't forget my Alkem Scale Models ACW cars :)

    ReplyDelete
  9. From the web site, it looks like everything is in those dinky scales! Is there a [n O Scale] treat I don't know about?

    Matt

    ReplyDelete