All of the electric traction equipment that I have come in contact with have been in steel mills, where we use two rails for conducting current and no current is passed through the running rails. On normal electric traction equipment how is the connection made to the running rails? Does current pass through the journals or are there wipers to pass current to the wheels/rail without passing through the journals?

Through the journals and wheels to the rails.

In cases where return currents are particularly high and especially for roller bearing journals the use of brushes or wipers to bypass current around the journal bearings was common practice. PCC car trucks for example have a thick copper/graphite brush spring loaded against the axle for the current bypass. Roller bearings can easily be destroyed by return currents.

OK thanks. The effect that electric currents have on roller bearings is what prompted my question.

Rollers weren't normal for most of electric streetcar and railroad history. Sorry I answered the wrong question.

your industrial traction might use lower voltage as exposed rails with high voltage may be dangerous. The Washington DC system had a conduit where pickup power was totally in the conduit, no overhead or thru the rails..

The conduit was due to an unfunded Congressional mandate that the streetcar company could not use unsightly overhead wires in the downtown area (I'm not sure of the precise limits, Brother Mitchell?). The suburban lines used a conventional trolley wire. What had to be the worst job on the system - maybe in the entire industry was to be the poor fellow down in the pit who had to mount and unmount the contact plows where the system transitioned.

GME

P.S. If one of you has way too much time on your hands, spend some time researching why the mandate does not apply to the new DC Trolley.

The conduit boundary was defined basically by Florida Ave., an east-west artery that was the northern boundary of the developed City circa 1890. The conduit requirement was written into each franchise, and the DC Transit franchise lapsed. Thus, in the case of the new DC Streetcar, there is no restriction.

Wesley

Standard DC voltage is 230-250 in industry.

In electric traction, the old standard was 600 volts, DC, with the overhead being positive and the ground, through the rails, was the negative return. Today, voltages range much higher starting at 750 VDC and often 1,200 VDC. Trolley buses, and some street railway lines, did use a double wire overhead system where the second wire was the return circuit. Older streetcars did use the frame and wheel return with no problems - unless there was a derailment and the car truck and wheels were completely off the track. In such cases, the car was hot to ground and one could not just leave the car without the danger of electric shock. The standard instructions to the motorman were to jump from the car and then pull down the trolley pole with the retriever rope thus deactivating the current.

Don't ever ever ever allow a current or significant electric potential difference across a rolling-element bearing. That includes TM bearings.

Note that back EMF in motors can also put current across rolling-element bearings unless there is a 'third brush' running on a clean contact area. Don't ask me how I had this confirmed to me.

I'm not sure plain-bearing equipment needs careful attention to tribology to prevent the equivalent of spark erosion or accelerated corrosion due to current. On general purposes I don't like the idea of high-potential current and hydrocarbon lubricants sharing the same contact surface -- but that may just be excessive caution, and there certainly is restricted air for 'combustion' and maybe even for partial coking or oxidation in there.

When in doubt - arrange a polished contact surface that can be kept clean, and a good brush with positive application oressure...

The car currently under heavy restoration at Pennsylvania Trolley Museum, West Penn Railways 832, was built in 1929 with conventional plain bearings. During the 1930s WP converted the 12 cars of this group to roller bearings. In doing so they added a home-brew ground return brush that pressed a graphite/metallic brush against the center of each axle. Practicality dictated that we restore the car in roller bearing form, so those ground return brushes were also retained.

I mentioned previously that where heavy currents were involved even plain bearings occasionally needed help in bypassing return currents. I found the reference I was seeking that demonstrates this application. The US Steel Edgar Thomson Works in the late 1920s bought an 85 ton Baldwin/Westinghouse steeple cab loco that served on a stretch of elevated track under the ore bridge. Rather than the common 600VDC loco E-4 ran off the 250VDC source for all the rest of the cranes, etc. that ran on DC. This combined with the weight of the loco and it's heavy duty application required the use of ground return brushes on each axle to carry the heavy DC current around the journal and traction motor bearings. I don't believe this arrangement was used on any other B/W steeple cab loco.

Silverliner IV's have ground straps from the car body and trucks that go to a box on the gearboxes with a big brush riding against the axles.

David,
I saw a photo of that steeple cab recently, and wondered where ET would have needed it at. If it was running into the ore yard then maybe it was shoving cars to a car dumper. There was a similar electric loco at Carrie at one time, and it was used to shove hoppers to the stationary car dumper. Later on a diesel 80 tonner was used instead, and that is the loco currently in derelict condition over by the dumper.