|Ragnar's HO Model Trains Weeb Site|
|Scherzer Rolling Lift Bridge|
|Drawn for MODEL RAILROADER MAGAZINE by
AL B. ARMITAGE AND ROBERT WEGNER
Magazine purchaser may have photocopies of these drawings made locally as an aid to his personal rnodeirnaking or tool designing, but purchaser does not hcive the right to distribute copies of the drawings to others.
Go to my Scherzer Bridge Page
Scherzer rolling lift span of girder construction fits model railroad situations with no compromising; can be made operable By AI B. Armitage
The two pinions that ride the racks at each side of the bridge must turn together; yet they are at too low a level to be connected by a direct shaft-there would be no clearance for trains. The transverse shaft is, instead, in the framework at the top of the bridge, and two pairs of bevel gears and one pair of simple gears are provided at each side to transmit the motion and provide three steps of gear reduction. This view looks upward, in the direction of the waterwcty. At the center of the transverse shaft is ci large gear for a motor drive, but the motor is not installed. To the left of it is a roller-chain drive to a windlass: to open the bridge, the operator unhooks an endless chain from the top of the ladder at the left cind walks along the deck of the bridge as he pulls the chain to raise the span. He must walk while doing this because the entire gearing and windlass mechanism recedes from the waterway at the same time, and comes a little closer to the ground.
AT North Station, the Boston & Maine A terminal in Beantown, some 20 or more tracks funnel into 8 tracks to cross a narrow channel on 4 massive Scherzer rolling lift bridges that are famous in the bridge-building world. Many times in the past I've watched them heave a few feet to let a small boat pass under, or raise to full open position to let a tug and barge go through. The effortless ease with which these ponderous spans did their work always fascinated me. What an interesting feature they'd be on a model railroad!
I was an 0 scale modeler at the time, and that nasty little circumstance called "available space" always shattered my daydreams; but it is said that "everything comes to him who waits"-and so it has. After all these years I found a prototype Scherzer bridge of such modest size as to be entirely practical to model, even in 0 scale. It was quite by chance that I met the lovely creature-a sort of blind date. I was searching for an entirely different kind of drawbridge located at Galinas, near San Rafael, Calif., but men in the Northwestern Pacific engineering depart- rnent told me the bridge had been removed. They did tell me, however, of another counterweighted structure at a place called Wingo, some miles to the north on their branch line to Schellville. They obligingly dug out several maps to show me how to get there. It was good they did; I never would have found it otherwise. Wingo turned out to be a fish camp of a dozen or so shacks far into the marshes of San Pablo Bay. The marshes are laced with tidal creeks, or sloughs, and this beautiful little through girder Scherzer span, gleaming in the sun like a jewel, carries the railroad over one of these sloughs. The prototype structure is planked over the ties to enable automobiles and horseback riders to reach a nearby ranch.
The bridge was built by the Toledo Bridge & Crane Co. in 1920. It is typical of that period, before welding came into general use. The bridge comprises three main portions: the movable lift span with the quadrants upon which it rolls; the stationary section supporting the quadrant track and, higher up, the rack; and the approaches from either side. The approaches can be varied to suit local conditions and will not be described in any great detail here. Either concrete or stone abutments could be substituted for the caissons should you so desire.
Comments along with the prototype photos explain how the bridge operates. They will help you interpret the model construction.
The hollow concrete counterweight provides space for smaller concrete weights used to more exactly correct the balance. When the bridge opens, this counterweight swings down almost to rail level. The laced columns at the extreme sides of this view are not parts of the moving span; they support the racks upon which the pinion gear rolls while the bridge opens.
The method of operating this lift span beats anything I've seen yet. There is provision for a motor drive, but that is not used. Instead, a hand-powered chain-hoist mechanism drives the pinions along the rack to raise and lower the bridge. Apparently the slough is so seldom used that this simple arrangement suffices. There were no evidences of a bridgetender, so I presume the service must he performed by boatmen who use the waterway. I have no idea what sort of craft navigate the slough, but they must be fairly small. The lift span is only 50 feet long, and the channel didn't appear to be very deep.
From the modeler's point of view the staggering array of rivets in the bridge is likely to be a discouraging feature, especially when you realize the things are duplicated on the reverse side of each girder and plate! Nevertheless, without this impressive detail the bridge model would lose much of its appealing character, so, tedious though they are to form, rivets are a must.
For small scales - HO, TT, N - the sensible solution is to simplify the rivet pattern and widen the spacing. You can't make exactly scale-size rivets in HO anyway, so spacing them as close as they are on the model would be nearly impossible. I'd suggest making your rivets as small as you can and spacing them as closely as the material and method will allow. Nobody counts the rivets in a model, and as long as there are enough of them to create the general impression of authenticity, their purpose is accomplished. I built both 0 scale and HO scale versions of the span, using the same rivet- forming punch and die for both. The larger model looks better by comparison, but by itself the HO bridge is also quite impressive.
For those who enjoy building a model as close to the prototype as possible, we will discuss the subject in full detail with some suggestions on how to make it a working model. If you are satisfied to have the model nonworking, you will save doing some complex and exacting work: Novernber 1967 the quadrants need not be so carefully made, and the bridge floor system-which cannot be easily seen except when the bridge is open-can be reproduced with a simple board.
You can use sheet metal such as brass, nickel silver, or iron (the so-called tin), or you can use Strathmore bristol board or styrene. I do not recommend stripwood, as this material is too thick to give the authentic "feel" of bridge girders. Much of the realism of a bridge model depends upon thin-appearing materials.
I like to work in styrene. It is easy to cut, finish, shape, and join to other pieces. It takes painting well. Bonding is very easy when using styrene. A trace of liquid solvent such as ketone, MEK, or even the liquid-type "cement for plastics" is touched to the edges of two pieces held in position. The fluid runs into the joint by capillary action and results in an almost immediate weld. It is one of the reasons styrene is considered the easiest material to use by the modelers who favor it.
I used the same thicknesses of sheet materials in both my O and MO models, but this is not as noticeable in the HO model as the relative scarcity of rivets.
It is expedient to break the modelwork into subassemblies which are completed and painted before final erection. Start with the chords, or span girders, following the assembly method of fig. 1. It goes without saying that all parts should be cut square and true. Right here is a good place to point out a trick that will prove invaluable in making joints such as occur in girders, beams, and angles.
No matter how you cut styrene-and this applies to other materials in varying degrees - the sheared or severed edges do not come out squared across. Any attempt to join parts not yet squared across may produce girders and other members with flanges tilted as in fig. 2. With metal you can often avoid this tendency by soldering in a jig, as in fig. 3. With cemented materials the warpage may occur after the work is removed from the assembly jig, so it is important to have the materials square across their edges before joining. A piece of tool steel, such as a lathe tool bit, can be ground as in fig. 4 and used for this purpose. Lay your material on a flat surface over a spacer piece (figs. 5A and 5B) and make one or two passes of the tool along the edge. The spacer prevents leaving a burr.
When you are going to cut a new piece from your large piece of stock material, first true the stock as just shown. Then, after breaking away the strip, true the newly formed edge as in figs. 5C and 5D. Here a hold-down piece has been added, and a backstop to prevent your strip from slipping.
Did the bridge painter sign his masterpiece? Name "Bob" and letters "B. B." (which might stand for Bridges & Buildings Department) have been spicisheci on underside of counterweight.
In laying out the pieces, be especially sure that the ends of the chords are cut 90 degrees from their tops and bottoms. Otherwise the vertical columns that hold the pinion to the bridge, and the rack to the quadrant track girder, will lean. The bridge will neither look right nor operate properly.
Next, make the quadrants. If your model is to work properly these must be made and bonded to the colunms with great care. It is imperative that the quadrants he identical in radius and in the 22-degree segment angles. Lay them out accurately as in fig. 6. Build up all flanges and add rivet strips before bonding them to their columns. The fulcrum pockets should also be applied at this time as in fig. 7. Space them equally on alternate sides of the rib, the curved rim upon which the quadrants roll. Use some means, such as a vernier caliper, to cheek that all pockets are identical in size. As in the prototype, these work something like gear teeth to keep the bridge from slipping out of position.
Bond the finished quadrants in position on the columns and cheek them carefully. Both subassemblies must match in profile as closely as possible. When you are satisfied that this has been achieved, add the remaining braces, gussets, and plates to stiffen the unit.
You will notice that I made a slight change in the design of the quadrants by extending their bearing ribs and adding a mating groove on the quadrant-track girder. This tongue-and-groove arrangement keeps the quadrant and girder in proper alignment during the activating cycle, a feature necessitated by the extreme light weight of the model compared to that of the prototype. While the real bridge depends entirely upon the fulcrum blocks and pocket to guide the quadrant during its travel, the model needs the added security of the groove.
Make the quadrant-track girders as before, with the groove a snug -but not tight - fit for the quadrant rib: fig. 7.Attached to the sectors of the quadrants you can see castings with a central rib and side pockets, the pockets located alternately on each side of the rib. The rib rolls on a simple track at the center of the top of the quadrant-track girder across the bottom of this view. Fulcrum blocks on each side of this track engage the pockets so the moving span of the bridge will not wander out of position as it rolls along the rail. On the model, AI Armitage made the centrcii rib proportionately deeper and provided a slot in the track girder to more firmly guide the rolling span.
Fulcrum blocks are next. These too are a critical job. Here's how I did it: Lay one of the quadrant assembles flat on the bench with a track girder in position against the column, and with the rib in its groove. Roll the quadrant back and forth to make sure the fulcrum pockets clear the top of the girder on both sides. With the two parts in closed position, mark the location of the first fulcrum block. Tack the block in place with a dab of ketone. Be careful not to get the ketone in the pocket. Separate the two units after a few seconds to cheek this. Set this assembly aside while you repeat the process on the second set of assemblies. Now go back to the first set and install the second block in the same manner. Proceed thus until all blocks are in place on both sets. As each block is applied you will find that the quadrant can be rolled back to position the next one: fig. 8. It is best to proceed alternately on both sides of the quadrant rather than finish one side first.
The finished quadrant should - nay must -traverse the track smoothly with-out binding, lifting, or slipping in either direction. To achieve this, each block must fit the corresponding pocket fairly snugly, with no play in either direction, and must be as high as it can be without striking the flanges on the quadrant. You may have to do a bit of custom fitting on one or two. Since it is highly improbable that the two units will be interchangeable, I sug- gest marking the pairs so they won't get switched around during subsequent assembly.
At this point we can assemble the :Roor system, comprised of floor beams, stringers, and diagonals as shown on the plan. A typical bay is shown in fig. 9.
Important: note that the stringers are short pieces that go between the floor beams. I might mention, too, that the spacing of the stringers depends on the gauge of your track. If you run narrow gauge, your stringers will be closer than those on the standard-gauge drawing. My own bridge carries 0n3 track laid with code 70 rail.
I suggest using a simple cutoff fixture for trimming the crossbeams to length. A fixture to hold the major parts in position during assembly will also make the job turn out better. This fixture can be made of blocks of wood cemented to a board, or it can be just pins drivian into a board upon which the floor system layout has been drawn.
After the floor system is completed you will need to add ffiler pieces on top of the stringers as shown in fig. 10. They should be flush with the top of the floor beams. Although not shown in fig. 9, the ties rest on these pieces.
As I mentioned earlier, the floor system may be simplified in a nonworking model. If you prefer something just a little more sophisticated than a plain board, a method is suggested in fig. 11. This looks better with individual ties.
It is much easier to paint the floor system at this stage rather than later. I found Pactra chrome silver 'Namel to be an excellent match for the prototype aluminum paint finish. Use an airbrush or spray gun if at all possible. Set it. to deliver a fairly dry first coat, to give ade- quate coverage without danger of sags or runs. (You have to be careful with enamel.) A second, "wetter" coat can be applied to the outer surfaces if a brighter, shinier appearance is desired. I rather like the aluminum finish as a change from the usual dull black. It shows off the rivets and other detail nicely. It would not conceal sloppy workrnanship.
Allow sufficient time for the paint to dry thoroughly; then bond the girder and quadrant units to the floor system. Be careful to see that the assembly is square and true in all respects. When it has set firmly, fit the bottom and back of the counterweight and bond thern in place. Remember, the counterweight must be square and aligned. Fabricate the front and "pocket" of the weight, but leave the top open until the bridge is completed. Then the proper weight can be determined and installed.
Fabricating the crossbeam is a rather tricky proposition that will make yoir wish you had three hands. To assure duplication of parts and corresponding accuracy of assembly, use jigs wherever possible. Details of the chain-hoist mechanism can be seen in the prototype photos. To the best of my knowledge there are no commercially available bevel or miter gears in sizes small enough for even 0 scale. I believe there are small, high-precision gears of this type made for special industrial purposes, but I'm inclined to think their cost would put them out of consideration for most modelers. I machined dummies from styrene rod with 3/16' brass rod for shafting. Teeth can be simulated well enough with a file or razor saw. Chain with 36 links is available from ship model supply houses.
Carry the assembly, including the dummy drive mechanism, as far as you can as a separate unit. Paint it; then bond it in place and add the remaining struts and plates. Set the structure aside while we consider the drive pinions, next on the agenda.
A bit of discussion on these is in order for those who wish to make a working model. The easiest-and probably the most practical-way to install the pinions, even in 0 scale, is to make them dummy as in fig. 12. Trying to make them full working is begging for a bucketful of trouble, believe me! In the prototype, the power-driven pinion advances along the fixed track to activate the span; on a model the pinion cannot be directly driven because there is no place to mount a motor and still preserve the looks of the bridge. With any other arrangement the gear goes along just for the ride, and under such circumstances the pinion teeth set up tremendous resistance against the movement of the gear along the rack.
On my 0 scale model I thought to drive the pinion directly, so I installed it in mesh with the rack. I used Boston Gear pinion wire and brass rack, 64-pitch, 20-degree pressure angle. A foot of each cost in the neighborhood of 3 bucks. When the idea didn't work out in fact as well as it had in my mind, I reverted to the system which I had already employed successfully on the HO model: fig. 13. Since the big model has not been placed in service, I have not changed the pinions nor built a motor unit for it, but I can guarantee the arrangement in fig. 13 will work equally well in 0 scale.
In any event it is essential that the pinions, working or not, travel in an exactly horizontal line as the bridge opens. They must be located at the exact center of the curvature of the quadrants. If the rack is not to engage the pinion, here is where the cheating must be done. The rack must be lowered just enough that the pinion does not touch it.
This completes the assembly of the moving portion of the bridge except for floor planking and rails. We'll take these up later.
The next phase of construction covers the fixed portion of the steelwork, and starts with the track girders already fabricated. Technically the floor system for this unit is similar to that of the lift span, but since it is not visible in the model, the method shown in fig. 11 is adequate.
The important thing to remember is that both girders must be exactly parallel and level so the quadrant will track properly. Custom-fit this assembly to your span rather than to the published drawings. Test frequently during construction. Be sure to match the girders to the proper quadrants!
The completed section should now be set up on whatever you use for a base. The prototype rests on steel caissons filled with concrete. I imitated these with styrene turnings. Paint the assembly before mounting it permanently.
The columns for the rack beam are made, slightly long on the lower end to allow for trimming to length to suit the height of the pinions. Fabricate the beams, install the rack, and assemble to the support colunms. Add the horizontal struts and sway braces and the steelwork is done.
Paint all unfinished parts. Rig the actuating cords and test the operation of the model. It should roll back easily and smoothly when you turn the drum by hand. Less effort will be needed later, when you have added the counterweight.
Although the prototype structure has a solidly planked floor to suit local conditions, I prefer the more usual open floor, so I left the planking off except for a walkway along one side.
All timbers are of pine ripped on a circular saw. Cut the ties to length while you're at it, and be sure to include enough timber and tiestock for the approaches if you include them in your model. Stain all material a dark gray-brown before assembly.
Notice on the end elevation that. while ordinary planks laid across the ties are used for the bridge flooring, there are two slightly higher cap strips along the ends of the ties. Cut two cap strips slightly longer than you need, and pin or tape them to a board at the correct spacing for your tie length. Using a spacer block, cement the ties across the cap strips. This is an inverted assembly at the moment. if you wish, you can add thin stripwood strips along the length of this assembly coinciding with the center lines of the stringers. Although not a necessity, these strips do stiffen the assembly during rail-laying, especially if you use spikes. Perhaps their greatest value is the fact that they absorb some of the spike length, which otherwise can become a problem in HO scale bridgework. I used Kemtron X-195 spikes, with tie plates, in 0 scale and still had a few poke through here and there. If this occurs, grind the protruding spike points flush so your track section will lie flat on the bridge stringer filler strips. Another beneficial service performed by the strips is to provide a larger surface for bonding the tie assembly to the stringers. I used tube-type styrene cement to fasten ties to the plastic floor system on my model.
Let the tie assembly set long enough to he firm and rigid; then flip it over and pin it to the board right side up. Lay the rails by whatever method you prefer, but he careful to get them in the proper location with respect to the bridge center line. If you paint your rails, do it now. Also do whatever touchup may be required on the timbers. Men all is to your satisfaction, the section can be fastened in position permanently.
Important! I must stress the fact that track maintenance is almost impossible after the track section is installed, so be certain that the rails are accurately gauged and fastened in place. If spiked, my suggestion is to use two spikes in every third tie as a minimum.
Possibly because of the planking, this bridge does not have guardrails between the running rails. However, since it is general practice on most bridges and trestles to provide steel guardrails, I included them on my model, running them out almost to the ends of the approaches. Such guardrails look something like a narrow-gauge track centered between the regular running rails. Beyond each end of the bridgework the guardrails bend inward to a point near track center: fig. 14. The guardrails may be of the same weight as mainline rails, but are often of lighter weight: old rail from somewhere else. The spacing between guardrails is often standardized for a given road, but at any rate is such that a car cannot wander so far from track center as to sideswipe any part of the bridge framework.
The short track section over the quadrant span is made the same way as the main span; and, needless to say, its rails must be in perfect alignment with those of the lift span. And they must be at the same elevation. This may take some shifting and shi=ing, but it is imperative if you are to enjoy trouble-free operation. Leave as small a gap as possible between the sections. Some sort of electric contacts should be provided at each end of the lift span so that rails will be powered when the span is down: these can be simple affairs of brass shirnstock wired to the running rails of each section.
This pretty well completes the bridge except for the counterweight. Now that the track sections are in place, the amount of weight needed to balance the span section can be determined. (On the prototype, incidentally, the weight is so located that a straight line drawn from the center of gravity of the counterweight through the axis of the pinion centers would pass through the center of gravity of the rest of the movable structure, almost exactly balancing the forces of gravity at all times.) My 0 scale model required almost 5 ounces, but the figure will vary depending on the scale of the model and the material used in its construction. The simplest means of providing the necessary weight is to pour lead shot into the cavity of the dummy counterweight on the bridge. When the proper amount has been obtained, mix a bit of fairly thin plaster and pour it over the shot to keep it from rattling around when the model is actuated; after this dries hard, the top of the dununy weight can be added. The small removable weights in the "pocket" are little blocks of wood cemented in place. These and the counterweight are painted to represent concrete.
Any small motor can be used for power, since the load is negligible due to the counterbalancing. A 11/2-volt imported motor and a few plastic gears are adequate. One photo illustrates a similar power unit used to operate a turntable: the gear train in this came from Revell's Slant-Six engine kit. A somewhat quieter arrangement can be made using a double set of worrns and gears instead of the spur-gear train. A very high reduction is needed to achieve the slow motion of the real bridge-the slower the better. Remember, this is a hand-operated bridge! Wire the motor to a reversing switch on your control panel or any convenient location. Leave some means of access to the power unit for servicing.
If you like, paint the name of your road on the span girders, add a couple of No TRESPASSING signs-perhaps warning blinker lights atop the structure as a final touch.
| © Ragnar Torfason|
2006 March 28