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July 1998

By far, the biggest debate to date has been what to call the new crane. The leading candidate is "649", the factory order number under which the crane was constructed. The next crucial decision will be paint, as at least 3 different schemes were found under the current battleship grey.

This might very well be the first time that a railway museum has taken delivery of a car straight from the car builder (if anyone knows otherwise, please write), and Master Mechanic Ted E. and his gang of shop volunteers took quickly to the new arrival. With our crane we also received boxes of paperwork, including complete drawings for the car, and shop records which shed some valuable light on the car's history.

All Differential cranes are essentially alike. A round open-top enclosure at the "back end" serves as the mounting location for the 4 crane motors, as well as an operating cab. Because of its appearance, this enclosure is known as the "tub". The boom and tub can rotate, as one assembly, 360 °. In fact, they can keep swinging in circles indefinitely, much to the peristaltic delight of the tub operator.

While such a feature is a tremendous asset to the operating versatility of the crane, it introduces some design challenges. Not only must 600-volt power be fed into the tub to power the crane motors, but it must also be possible to control the traction motors and brakes from the tub, to allow the car to be "spotted" as necessary.

Within the core of the tub, there are two concentric hollow cylinders. Inside of the innermost cylinder, exactly on the axis of rotation, runs a solid rod. This rod projects upwards out of the top of the cylinder, where it is coupled via a ball joint to a lever. The tub operator's up and down movement of this lever is thus transmitted to the rod. The rod remains stationary as the tub rotates around it, the ball joint providing the necessary de-coupling. The rod descends below the floor, where a system of levers translates the vertical motion to horizontal, coupling to a long rod that is suspended beneath the floor and runs the length of the car.

In the case of Branford's Montreal Tramways DifCo Crane, W-3, there is a cab on the "front" end of the car. The horizontal motion of the brake control rod is transmitted under the cab to a rotating rod which pokes up from the floor and terminates in a fitting which can slip over the operator's brake handle. Therefore, moving the lever in the tub down pushes the brake handle towards apply, and likewise moving it up releases.

649, on the other hand, never had a cab or any kind of controls on the front end of the car. The brake control rod on 649 terminates on a small straight-air brake valve mounted under the car, on the front end.

In order to pass electrical current between rotating objects, slip rings are commonly used. In the case of a Differential crane, 13 slip rings are provided. The slip rings are mounted on the outside surface of the innermost tub core cylinder, and are fixed with respect to the car body. Wires are attached to the back (inside surface) of the slip rings and brought down to a junction box under the car body.

A set of brushes (fingers) is mounted on the inside surface of the next enclosing cylinder. This cylinder is attached to and rotates with the tub. Wires attached to the fingers are therefore stationary with respect to the tub and can be run to their various connection points there without fear of tangling. The fingers revolve around the slip rings and make continuous contact with them. Three of the finger/ring pairs are heavier than the other 10, and are meant to be used in parallel to deliver 600-volt power from the car body to the crane motors in the tub. The 10 smaller fingers and rings are used for the master controller.

The crane motors draw perhaps 30 amperes total. In contrast, the traction motors that propel the crane car draw several hundred amperes. To mount an ordinary K-type controller in the tub would require at least half a dozen slip rings capable of handling that level of current. This is not practical. Therefore, the use of a master controller and a switch group combination ("power-operated control") is indicated.

As a rule, Differential supplied to the purchaser a complete crane car body, with the crane motors and their grid resistors and drum-type controllers mounted and wired, but lacking other significant components; it was up to the purchaser to obtain running gear, brakes, power collection apparatus and traction motor controls.

In the case of W-3, Montreal Tramways used Canadian Car and Foundry arch-bar trucks, 4 GE80 traction motors and a D1EG air compressor. Controls and brakes were borrowed from a system Montreal had been using on their two-car streetcar trains. The brake system is straight-air with an emergency feature, and the controls are Westinghouse PK-35. The PK-35 system operates from a 14 volt storage battery. The master controller energizes wires, from battery voltage, which then run to and control a K-35 controller mounted horizontally under the car. Advancing the master controller operates magnet valves which control the supply of air to a piston. This piston is mechanically coupled to the shaft of the K-35 controller, and the K-35 advances in-sync with the master controller.

This system is similar in design to the original multiple-unit controls that Frank Sprague pioneered in 1897. Both GE and Westinghouse made products based on this technology. While a complete history of controls is beyond the scope of this article, it is interesting to note that both companies abandoned this design concept and went to a unit-switch design, and that both companies subsequently returned, in part, to the original Sprague drum design.

Westinghouse for many years made a unit-switch control system known generically as "HL". The "H" indicates "Hand" acceleration, and "L" denotes that "Line" voltage (600 volts D.C., in this case) is used on the control wires rather than battery. With HL control, individual electro-pneumatic contactors ("unit-switches") switch traction power to the grids and motors. HL was an extremely popular design, and was used extensively on interurbans and electric locomotives. Examples of HL control in Branford's collection are Chicago, N. Shore & Milwaukee interurban #709, Utah & Idaho Central/Cornwall locomotive #12, and Fairmont & Clarksburg interurban #250.

From the shop records that came with the car, we learned that 649 was equipped by Differential mostly with second-hand gear. The arch-bar truck frames were fabricated in-house, but journal boxes, brake rigging and wheel/axle sets were purchased used, most likely from a nearby interurban line. The 4 WH93A traction motors and an HL-264 switch group likely came from the same place.

649 never had a trolley pole. DifCo ran the car on a long, heavy "extension cord" from the nearest source of 600 volt power. The bus jumper plug is mounted under the car, near the side for easy accessibility. At some point in the late 1930s, for reasons that we are still researching, the source of power dried up. Differential converted the crane to gas-electric operation. In 1942, a Ford-Hercules gasoline engine, used in gas-electric buses of the time, was purchased new. This straight-six motor displaces 529 cu. in. and develops a whopping 125 horsepower. Attached to it is a 300 volt DC generator. A compartment was welded to the outside of the tub to hold the 20 gallon gas tank and 12 volt battery (what, you expected crank-start??). The motor and generator were mounted on the side of the tub, opposite the controls, causing the car to list slightly from uneven loading. Most of the functions of the HL switch group, such as resistance notching and series-parallel control, were no longer needed, as the acceleration of the car is controllable via the engine throttle. Two of the car's four original WH93A motors disappeared, and the remaining two were wired permanently in parallel (appropriate for the 300 volt maximum voltage output of the generator).

A mechanical inspection of 649 has revealed the car to be in excellent condition, much better than W-3. There is almost no structural metal loss, and all of the crane gears are in good shape. Electrically, however, the car did not fare as well. The switch group had been self-cannibalized as burned-out unit switches were replaced with others, no longer needed in the gas-electric configuration, from the same group. The reverser's piston cups had been replaced with a homebrew widget of not quite the right dimensions, causing the reverser fingers to make marginal contact with the drum segments, and leading to zorched out segments and fingers. Again, this had been self-cannibalized, with "extra" fingers and segments (this is a 4-motor model 272 reverser being used for 2 motors, therefore half of it is redundant) used to replace the bad ones. Water had accumulated in a conduit and damaged the insulation of some of the motor circuit wires; the resultant ground-out was probably the last time the car took power.

DifCo had "turned around" the slip rings so that traction power was fed from the tub-mounted generator back down to the motors. Unfortunately, the slip rings were not designed to handle such large currents, and over time this burned out the leads to those slip rings. DifCo was forced to run a wire from the generator down to the motors, preventing the tub from swinging all the way around. The original air compressor was long gone. Air was being pumped by a really cute little bus compressor driven by the engine, and fed down to the tanks under the car via a rubber hose dropped down the inside of the slip ring column. This didn't really generate enough pressure to safely stop the car, so DifCo would charge up the air system from a shop air hose.

Work immediately commenced to make the self-propelled crane movable. The usual gasoline engine things were done to the 55 year old motor to make it run better. A sorely-needed muffler was added. The itsy-bitsy compressor was removed and a D2F was taken from inventory and placed on the deck. Temporary piping and a compressor governor were installed. The compressor runs slowly on 300 volts, but it does pump up the car in a few minutes. The shorted-out motor leads were pulled back, re-routed, trimmed and re-attached to the motors and protected with canvas hose. The reverser was made functional, but only with a hand-throw lever. The switch group and resistor grids remain detached and reside on the deck. A trolley car circuit breaker was mounted in the tub to switch generator power to the traction motors, as the voltage generated even at idle is enough to creep the car on level track.

By mid-April, only a few weeks after delivery, the crane was functioning well, towing and lifting. On Member's Day, May 2, it made its public debut, posing for photographers tub-to-tub with W-3 (as featured in the May Tripper).

The long-term plan for 649 is to make it a fully functional electric crane equivalent to W-3, and probably to remove the engine/generator set, the presence of which de-rates the load capacity of the crane from 5 tons down to perhaps 2. This is not a simple project. Work is already underway gathering and rebuilding the parts need to restore the HL control group. The plan calls for either a cab or a control stand and trolley tower to be built on the open end. Of course, body and painting work is also needed and has been started. Once all this is accomplished, we can finally afford "downtime" on W-3 to give it some sorely needed attention. Your donations of time and/or money to help meet this goal are most welcome.


The Shore Line Trolley Museum
17 River Street
East Haven, CT 06512
(203) 467-6927

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