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Electrical pickup - On Members Workbenches. - More Practical Help - Your Model Railway Club
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 Posted: Sun Jul 12th, 2015 07:34 pm
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BCDR
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[The following is a continuation of the rewiring of the trucks/bogies of the Cascade Tunnel electric boxcars. The trucks/bogies are milled steel, electric pickup is via a wire pressing on the tread of 2 wheels per truck/bogie on the opposite sides of the 2 trucks/bogies. I am concerned about the wear to the tread surface, and have decided to change the pickups so they bear on the inner side of the rim. For this I'll be using phosphor-bronze wire and some copper clad board that is 1.06mm thick. So how much drag do those pickups on the tread of the wheel actually generate? Read on.......]

It's a quiet day today, the hobby shop is closed, so no pickup soldering. I thought I'd have a look at the effect of no pickup, a pickup on the treads, and pickups on the inner rim surface with respect to rolling resistance. This will provide the datum point for pickups on the rear rim of the wheels - anything less than the tread arrangement will be to the good.

There are easy 2 ways to do this (without a full-blown lab): The first is used to optimize pinpoint bearings, where the truck (bogie) or wagon is rolled down a U-shaped track setup and how far it goes up the other side is determined. Reboxx over here sell a gadget for doing this, I don't have one; The second is to simply determine the slope at which the truck/bogie or wagon will roll. Simple trigonometry (right-angle triangles) is all that's then required to determine the rolling slope.

I stripped one of the trucks/bogies from the electric boxcab of its gears, cleaned off the old oil, washed everything with 99% IPA, and when dry lubed with Labelle 108 oil These trucks/bogies do not have brass bearings, so this is steel axles on a steel "bearing" surface, not the most ideal of arrangements.* I used some Peco code 100 track, and 1/2" and 1" height supports, plus a ruler. I carried out a series of runs with various height/lengths and a precision of 0.5" to arrive at the minimum slope required. I did each run 5 times just to make sure. I looked at no pickup, the tread pickup, and a PB wire pickup (20 thou', 0.5mm) on the inside rims (copper clad tacked in place with CAA).

The truck/bogie weight was 75gm without the gears.

The mean results for the minimum slope to start rolling were as follows:

1. No pickup. 0.5" height, 15" length, slope 1:30 (3.33%).
2. Tread pickup in place. 1" height, 10.5" length, slope 1:10.5 (9.52%).
3. 20 thou' wire pickups on inside rims. 1" height, 19" length, slope 1:19 (5.26%).

I was actually surprised at the results. Spinning the wheels by hand (which is subjective) or giving the truck/bogie a shove along the track gives no indication of resistance for 1, 2 or 3. Having just one pickup on the tread of 2 wheels does apparently make quite a difference to rolling resistance. Presumably it would be even more with tread pickup on both sides (and on both trucks/bogies). Using 20 thou' PB wire on the inside rims of 2 wheels was almost half that of the tread wire on 2 wheels. Doing both sides on the inside rims will be no worse than one side on the treads. If I can get better than this with thinner whisker PB wire pickups on the back of the rims then less rolling resistance, which should be a priority in this hobby. I'll try 15 and 10 thou' next week.

To put this into perspective an 8 wheel freight car properly weighted and with clean pinpoint axles and delrin trucks/bogies runs away with a slope of 1:100.

Nigel

*Micro Mark do a copper plating kit that will put a thin (around 50 microns) layer of copper on steel. If the paddle can get into the axle slot that would provide the soft bearing surface for the steel axle. I believe Bachmann have used this approach in the past on their split chassis designs.




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 Posted: Mon Jul 13th, 2015 02:08 am
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BCDR
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Another surprise this afternoon. Mike (dorset) mentioned that if I had half insulated wheels I could tap the axles on one side. I had initially thought the wheel set was fully insulated. Cleaning the wheels for the PB wire wiper this morning showed the telltale red line of insulating material on one side, nothing on the other. These are all metal wheels, so I had a quick investigate with the multimeter. Turns out they are half-insulated wheel sets with the axle set directly in the wheel hub. Which means the truck/bogie block is live.

This is good news and bad news. Good in that I can simply tap the truck/bogie for power pick-up, bad in that it has to be kept away from the insulated wheels. The clearance is a small red fiber washer on the axle, so pickups that keep the insulated wheels away from the truck/bogie block are going to help. The original design of the pick-up wire on the tread actually pushed the insulated wheel in towards the truck/bogie block.

I may end up only installing pickups on the insulated side, and adding another 15 thou' fiber washer split and slid over the 3mm axle as I don't fancy the risk of pressing the wheels off the axle and not getting them back on. The covering on the cosmetic truck/bogie sides is non-conductive, as is the underside of the carrier plate so they are not short-circuit issues.

I have a feeling that the notorious poor running of this model is partially due to intermittent short-circuits between the insulated wheel and the live truck/bogie. Something that will have to be addressed if I'm going to put a decoder in. If necessary a sheet of 5 thou' black styrene or ABS sheet glued on the insulated wheel side would solve the issue.

Nigel




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 Posted: Mon Jul 13th, 2015 01:07 pm
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Dorsetmike
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Is it possible to arrange things such that one bogie picks up on the left and the other on the right, (put wheels in opposite way round in one bogie) that way you don't need any additional pick ups. Some N gauge locos (e.g. Union Mills and Rivarrossi)  pick up from one side and the tender from the other (probably in other scales as well)






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 Posted: Mon Jul 13th, 2015 05:28 pm
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BCDR
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Hi Mike,

That's the original design, pickup on 2 wheels one side on one truck/bogie, pick on 2 wheels on the opposite side of the other truck/ boogie. The length of the wheelbase is only 39mm/1.6", and I have to allow for meeting Atlas turnout dead frogs that long on the modular club layouts, large gaps between running rails and frogs, or iffy live turnouts that rely on current from the switches (it happens). OK if I have the two boxcabs coupled and linked electrically, or use a very large stay-alive capacitor. Not good if I use just one. I used to run into these issues with the Lima GWR diesel railcar until I wired all 8 wheels.

Because the truck/ boogie is live I can simply tap it for the other pick-up leads and avoid extra pickup wiper drag. Whether I need to have wipers on all 8 wheels with a live truck/bogie is debatable. 

As far as I am concerned the less pickup contacts the better as I don't think it matters to the motor or decoder if current is getting through 2 or 4 or 8*, and extra ones are just going to increase rolling resistance. Something to be avoided. We go to great lengths to have rolling stock that will go down a 1:100 gradient, yet add pick-up wipers willy-nilly in the hope it will improve running without considering the overall effect. I've done it in the past, not anymore, especially after the above results.

Nigel

*Does anybody have any data on this?



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 Posted: Tue Jul 14th, 2015 08:56 am
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My mate wanted some extra pickups on a couple of locos such a Bachamnn 0-6-0 with tender C class units 31-464 so
I used these http://www.dccconcepts.com/flickerfree-fx-products/flickerfree-pickup-springs-pickups/12pr-wiper-pickups but could only easily fit one to the middle tender axle due to wires & plug mounted in the floor - helps most of the time over Insulfrogs but as they come, the really press onto the backs of the wheels near the tyres & stop the wheel/axle turning.



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 Posted: Tue Jul 14th, 2015 04:02 pm
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BCDR
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Hi Sol,

They are great pickups, no drag whatsoever. If I was doing a tender they would be first choice. Means a 1 and 2 arrangement unless the axles are split with insulating middles, and also needs live wheels or jumper sections soldered in from the rim to the axle. This to me is where Romfords score over Gibson's. 

I couldn't get them inside the trucks, the walls are too thick, and as I said I'm loath to take the wheels off. No need anyway since the whole truck is live.

With a wire of 0.01"  the rolling slope is even better. More tomorrow.

Nigel



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 Posted: Fri Jul 17th, 2015 05:03 pm
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Hi All,

Quick update. Tried some 0.01"/0.254mm diameter PB rod for the pickup, rolling slope was 1:27. Not far from having no pickups. Wired up one truck/bogie (PB on insulated wheels, direct wiring to truck/bogie), locomotive ran fine (this is a relative term for old brass, bit like estate agent fiction). No shorts, so wiring for DCC is now the next step. I did identify another potential area for shorts, that will need some insulating material.

Addressed the gear issue, a poke around the innards showed there was a missing thrust washer which allowed the transmission to lock. Lost in some long-ago clean. Turns out the trucks/bogies are a milled hard white metal alloy (on with the gloves), so the bearing surface is actually fine as is. I also addressed the issue of some stripped threads in the screw holes for the screws that hold the base plate on and a screw with half its head missing and held in place with what looked like Loctite or similar. Hmm.

More of the definitive conversion later.


Nigel



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