Boston Mountains Sub-Division

I keep forgetting this is N scale. You do good work!

 

You’re a machine! Looking good.

 

Joe,

Bridge 410.6 over the Arkansas River rigging consists of 2 independent basic systems. These systems are divided between the counterbalance and lifting operation.

Counterbalance System​

The simplest and most basic rigging system is the counterbalance system. This system consists of 48 fixed length 2” diameter main counterweight wire rope cables. The system is quartered along the length and width of the vertical lift span. Therefore, near each corner of the movable span are 12 fixed length cables. Quartered, this makes the 48 cables (12 cables x 4 corners).

The wire ropes are 6 x 19 class, specifically 6 x 25 filler wire construction, with a fiber core. This indicates there are six (6) strand bundles each, with nineteen (19) main wires (6 x 19 class) and 6 smaller filler wires (6 x 25 filler wire construction), wrapped around the central fiber core. Once twisted together from individual main wires, the strand bundles are spun together with filler wires around the fiber core to produce the finished wire rope that is 2” in diameter.

From fixed attachment point castings at the top of the bridge lift span the cables run up, drape over the large counterweight sheaves at the top of the end lift towers, and down to fixed attachment point castings at the top of the concrete counterweights. The attachment point castings are perpendicular to the length of the bridge. The cables are in line and parallel to one another.

A sheave is a wheel with a “U” shaped grove to guide a line (cable, rope, wire, etc.). In this case, the large counterbalance sheave castings near each corner of the lift towers are ganged 12 parallel-groves (cables) wide. Each sheave casting has 8 transverse teardrop shaped spoke openings radiating outward from the center. On the bridge there are inverted “U” shaped sheet metal moisture / debris covers above each sheave group.

Each of the 4 counterbalance sheaves measure 12’ 2 ¾” in diameter x 2’ 4 ¾” wide. Each sheave has a trunnion (shaft) that is 5’ 11 ¼” long x 1’ 6” diameter. Each trunnion is supported by and rotate on a pair of plain bronze bearings. The 4 trunnions support the dead weight of the lift span, counterbalance sheave, cables and counterweight.

The counterbalance system cables terminate in an anchor fixture atop the lift span and the counterweights. The fixtures are centered under the two plumb fall lines from each counterweight sheave – one anchor fixture on the bridge lift span near each corner and on the counterweights, centered along the top side edge on each outer side.

Each corner splay anchor fixture holds 6 cables per side, front to back, for the 12 cables per anchor. Odd numbered cables are on one side, with even numbered cables on the other. The stub end of each cable is inserted vertically down into a socket in the anchor fixture clamping it in place.

As the lift span moves, the counter balance system cables move in a simple motion see-saw path – up - 180 degrees over - down. The length of travel of the lift span and the 2 counterbalances are in direct proportion, counter to one another.

The weight of the lift span is 1,796,000 pounds (898 tons).

Lifting Operation System​

The lift operating system is also simple and easy to understand. It can be subdivided into the span drive, operating sheaves and cable rigging.

Span Drive​

The drive system is in the equipment house located above the track, at the center of and within the framework of the lift span. The normal operating machinery consists of a General Electric (GE) 75 HP, 690 RPM electric motor, a motor brake, a machinery brake, gears and transmission. In case of electrical utility power failure there is a back-up emergency drive consisting of a John Deere, model 4045TF290, 4 cylinder, 74 HP diesel engine with power take-off (PTO) coupled to a reversing gearbox.

The power systems are interlocked such that only one can be engaged at a time. They drive a series of gears that power an axle projecting from the equipment house on the upstream (west) and downstream (east) side of the structure. Attached on each end of the pinon axle, outboard of the lift span superstructure, is a 6’ diameter 8 position sheave drive drum. Powered by the common pinion axle the two drive drums rotate in the same direction, clockwise or counterclockwise, to raise or lower the lift span.

The span is equipped with electrically interlocked limit switches to control the maximum height of a lift and determine when the span is fully seated in the lowered position. Up is the “normal” position for the lift section to permit river traffic to pass. It is typically lowered only to permit train passage, it is then returned to the “normal” up position.

Operating Sheaves​

Again thinking quartered, along the upper lift span cord, just beyond half way down its arch are 2 single position idler support sheaves. Each of the idler sheaves supports the length of horizontal operating cables only on the very top of their grove.

Further down the top cord, near the lift span outer ends are 2 single position deflector sheaves. Each of these sheaves causes a 90 degree change in direction of the operating cable – horizontal to vertical. These are also quartered with a pair at each corner of the lift span.

At the deflector sheaves the uphaul cable passes under the bottom of its grove, making its 90 degree turn upward. The downhaul cable passes over the top of its deflector sheave grove, making its 90 degree turn downward.

The idler and deflector sheaves are independent and counter rotate relative to one another when in operation. Each of the trunnions or axles for these sheave pairs are mounted to the outer edge of the lift span top cord. Like the trunnions for the counterbalance sheaves these do not cross or span the width of the bridge.

Each of these is a single position sheave over which passes a single cable. The lift span idler and deflector sheaves each have six rectangular in cross section spokes. The sheaves are also 6’ in diameter x 3” wide (6” each pair).

Also quartered within the lift towers, largely hidden by the superstructure, are two much smaller diameter single position return deflector sheaves. There is 1 at the top and 1 at the bottom, stacked vertically on each side of each lift tower. Each of these sheaves causes a 90 degree change in direction of the operating cable – vertical to again horizontal.

Cable Rigging​

There are 8 fixed length 1 ½” drive cables – 4 uphual operating ropes and 4 downhaul operating ropes. These are also 6 x 19 class construction, 6 x 25 filler wire construction, with a fiber core. The operating ropes are counter wrapped at the operating drums. One cable leaves the top and one the bottom of the operating drum toward each end.

Again quartered - 2 cables (1 uphaul and 1 downhaul) from the center drive drum run horizontally toward each end, on each side of the lift span. Each drive cable takes a continuous route in this manor: lift span drive drum sheave - idler support sheave - deflector sheave, then vertically to respective tower uphaul or tower downhaul deflector sheave - then horizontally to the respective tower attachment point anchor.

For each rotation of the drive drums, clockwise or counterclockwise, it simultaneously winds the direction of travel (raise or lower) and unwinds the opposite direction of travel of the drive cables. Allowing for the take up of slack in the fixed length drive cables, the drive drums need only rotate a couple of turns to move the bridge from lifted to lowered or vice versa position. The normal vertical operating movement of the lift span is 30’.

The direction of the lift span drive drum rotation, clockwise or counterclockwise, determines which direction, uphaul or downhaul, the bridge moves. In this case, using basic geometry for each rotation of the drive drum the drive cables travel about 18’ 10” (18.85’). (C = or Circumference = 2 x Pi x radius).

When coupled with the counterbalance system, the lifting operating system cables are much fewer in number and smaller in diameter. This is the result of only needing to overcome the friction from rotating the counterweight sheaves and that introduced by any contact along the lift span guide rails.

Operation Economy and Simplicity​

The 2 independent basic systems, counterbalance and lifting operation, are the essence of the bridge’s operation simplicity.

The counterbalance system minimizes the relative force needed to move (raise or lower) the lift span. In turn, the lift operating system equipment and component requirements are minimized as they do not need to overcome the deadweight of the lift span.

Unlike a crane or derrick there are no visible wrapped spools or reels of bulk cable that are played out (lengthened) or played in (shortened). As noted above both the counterbalance and operating systems use fixed lengths of cables. Only very minimal changes in cable length occur due to temperature changes (extremes) and long term limited stretching over time.

Hope this helps.

Thanks!

Mark

 

A good choice to model the lift span idler support and deflector sheaves is available from Tichy Train Group.

Strongly consider using their Structure Parts item number 8310. This is a six spoke single line sheave with a 1/2” diameter. That scales out to 6’ 8” in diameter for N-scale. They come in packages of two sheaves and four pillow block supports. They are cored for 0.032 wire, Tichy part number 1105.

In pairs these would be a great addition for the lift span top cord intermediate idler support and end deflector sheaves. These open six spoked, single line sheaves would very closely represent those on the bridge.

Recommend ordering eight packages of part number 8310 and one package of part number 1105. Tichy has a good reputation for having items in stock, promptly filling orders and they can be delivered directly to your home.

Please see page 45 at https://www.tichytraingroup.com/Portals/0/Instructions/HO_catalog_2019.pdf?ver=2019-05-22-132010-000

For additional realism the 1.5” drive cable rigging scales out in N-scale to 0.0094.

Common sewing thread is a good choice for these cables. It is available in multiple colors and readily available in retail stores or on line. For nylon thread this equates to commercial size 46, government size B, Tex size 45, tensile strength 7.4, needle size 14-16 for 0.0094 diameter.

Polyester thread measures slightly different and falls between commercial size 46, government size B, Tex size 45, tensile strength 7.09 needle size 14-16 for 0.0080 diameter and commercial size 69, government size E, Tex size 70, tensile strength 10.13, needle size 16-18 for 0.0107 diameter.

The 2” counterweight system rigging cables scale out in N-scale to 0.0125.

Common sewing thread is also a good choice for these cables. Again it is available in multiple colors and readily available in retail stores or on line. For nylon thread it falls between commercial size 69, government size E, Tex size 70, tensile strength 11.31 needle size 16-18 for 0.0115 diameter and commercial size 92, government size F, Tex size 90, tensile strength 15.16, needle size 18-20 for 0.0133 diameter.

Polyester thread measures slightly different. It equates to commercial size 92, government size F, Tex size 90, tensile strength 7.09 needle size 14-16 for 0.0080 diameter and commercial size 69, government size E, Tex size 70, tensile strength 13.48, needle size 18-20 for 0.0124 diameter.

Ship modelers when building sailing craft often draw thread across the edge of paraffin or bees wax to protect rigging lines from changes in humidity It only takes a couple of passes to “seal” the thread against moisture. This is an inexpensive and effective modeling tip.

Hope this helps.

Thanks!

Mark

 

Navigation aids are another possibility to enhance the appearance of the bridge.

There are 4 red navigation lights on the north and south fenders – one on each side of the channel, 2 upstream, 2 downstream. There is a red-green center channel navigation light mounted on the east and west side sides on the lower cord of the center lift span. This light is red when the bridge is down or moving. When the center lift span is in the full up position, the center channel light changes to green.

There are also 8 white lights on sides of the machinery house on the lift span, 4 on the upstream and 4 on the downstream sides. These flash on and off when the bridge is raising or lowering to warn river marine traffic the lift span is in motion.

There are also two navigation horns, one each upstream and downstream. They are powered from an air compressor located in the machinery room. They sound to warn of pending movement of the bridge lift span. There is a 10-minute delay before bridge lowering or lift to warn marine traffic. They sound again before the 8-minute cycle to lower or raise the bridge begins.

Also, do not forget the electrical utility power pole spikes on each end of the bridge. They support the electrical utility swag lines for the lift span. There is a power pole spike on the outboard corner on each lift tower and on each end of the lift span top arch cord outboard of the center equipment shed. All of these 4 poles are on the downstream or east side of the bridge.

Hope this helps.

Thanks!

Mark

 

The ice blocks that were most common were rectangular in shape.