On 2/3/2008 5:53 PM P. Roehling spake thus:
OK; I thought about this after posting, and decided I was maybe a bit hasty with my dismissal of this possibility.
So here's an idea for dealing with this problem: how about adding extra weight to the locos whose wheels slip to increase traction? (Oops, sorry: I mean "tractive effort"). Would that help?
"David Nebenzahl" wrote
Doesn't seem to.
Some nameless optimist poured the boiler of my Balboa GS-2 full of lead at some point, no doubt hoping to gain some tractive effort, and despite being significantly heavier than my Westside GS-4 -which carries only the factory weight- the GS-2 will spin it's drivers when the two are coupled in tandem. The GS-2's gearbox seems to overcome the extra weight with no trouble at all.
Which brings up a question that could support another complete thread: *Why* do some very similar locos run at widely differing speeds?
Example: I have five KTM Japanese-built S.P. GS-series locos from different importers. They all sport the exact same gearbox and motor from the same era, they're all well broken-in and maintained, and they all have similar driver diameters (70" on the GS-8, 73" on the GS-1, GS-2, and GS-6, and 80" on the GS-4) yet the smaller-drivered and overweighted GS-2 runs away and hides from all the others, while the GS-1 and the GS-6 run together perfectly, and the GS-4 lags well behind them with the GS-8 bringing up the rear. (You can run the 1 and the 6 together, or the 4 and the 8, but no other combinations will work without the faster loco spinning it's drivers.)
Do you suppose that the old KTM open-framed motors were so inconsistent that it could account for this seemingly unlikely speed differential?
Speculation by all hands is welcomed.
Pete
The likely factors involved are:
- motor power.
- Mechanism friction.
- weight.
- balance.
- weight carried by drivers.
As far as motor power is concerned, as you have the same motor in several locomotives an interesting experiment would be to map speed and tractive effort at different voltages and then swap motors between locos and retest. Mechanisim friction can be tested in a basic fashion by removing motor and worm and then simply pushing the mechanisim by hand, checking for excessive resistance. The last three are interconnected; the spring tension on leading and trailing bogies can significantly reduce tractive effort by minimising weight carried by the drivers. The weight might be carried to far from the centre of the driving wheelbase. Weight counter-balanced outside the drivers can actually reduce the weight carried on the drivers if there is not an aproximately equal weight at the other end. This negative effect is easy to achieve on 4-6-0s and 4-4-0s - lots of empty space at the front of the boiler and very little at the rear where the motor fills the fire box. The result is the front bogie takes all the weight and takes it from the rear drivers.
Greg.P.
Those open frame motors were a mess. I wonder if the magnets varied a lot in strength, or it's friction of the drivetrains just pulling them down. Did they pull the same loads?
BDK
"BDK" wrote
Never tried to find out, but one of our club members has a very accurate machinist's gauge that measures pressure in grams and fractions thereof.
Might be rewarding to test all five 4-8-4s for comparative tractive effort and then compare their weights to see what sort of correlation exists.
Pete
It's pretty common actually. And when it isn't bad enough to get slip, sometimes you get annoying groaning sounds from the motors laboring.
In a slow vs. fast moving motor that's geared like most trains are, it may not be all that tiny, and there's not lots of friction to spare. Unless you have tires on drive wheels, slippage is definitely an issue.
I'm not sure how much you could do about it on a DC layout - maybe toss in some diodes in series with the faster motor to lower the voltage? On DCC, you can tinker with CV settings to harmonize engines run together, but it's a bit of a black art requiring a lot of experimenting. *
pv+ snipped-for-privacy@pobox.com (PV) wrote in news: snipped-for-privacy@news.supernews.com:
*snip*
Adding diodes in might work, to a point. However, all the diode would do is change the starting voltage. Your speed curves would still be the same, so as you sped up the locomotives would start moving at different speeds.
Do most decoders do anything on DC regarding speed curves? I'm guessing not. (I haven't played with speed settings on my DCC locos yet.)
Puckdropper
Just using a home brewed "scale" that probably was inaccurate, we found that the two pairs of FA-1's (slower and faster ones) had widely varying pulling power. The slower ones, identical except for speed, pulled better than the faster ones. Others seemed very close in pulling power, but speed was totally different. I had another Rapido loco, I don't remember what it was, with as far as I could tell, an identical mechanism as the Geeps had, was much slower. It pulled almost identically, but the Geeps were just insanely fast. In a "drag race" on the longest straight track run I had, the Geeps would run away from any other loco, until it was loaded to the point the wheels spun, or spun when the throttle was increased too quickly. Those two Geeps could get a big train, if you slowly worked the throttle, moving to an insane speed, where a derailment was a 100% certainty. I had big radius curves, especially the outer main, and the Geeps would "fly" if I wanted them too. I had a hunk of foam rubber to catch them before they went off the bench and hit the floor. I removed my garage that I had built there too, as the Geeps nearly destroyed it when they hit it one day when they decided to fly.
BDK
"Greg Procter" wrote
Swap motors between 5 locos and test each one multiple times? Interesting perhaps, but impractical in terms of time.
You can't remove the worms from these gearboxes without breaking the locos down to individual pieces. Again impractical.
None of the locos have any spring tension on the trailing trucks: they rely on the weight of the trucks themselves to keep them on the track, and it works fine in that they never derail. The leading trucks *are* all spring loaded, but only enough to keep them on the track; I.E. less than 1 oz each: an inconsiderable percentage of the loco's total weight on the drivers. (Roughly 26 oz.)
Southern Pacific GS-Class steam locomotives were all large 4-8-4s, and carrying the factory weights these all balance right between the center two pairs of drivers. Right where you want the CG to be located. The Balboa GS-2 that was poured full of lead balances slightly forward of that, but not as far forward as the second pair of drivers, and not enough to cause the front springs to sag measurably.
While I could -and probably will- easily test each loco's tractive effort for comparison purposes, it's the large speed differential between mechanically identical units that interests me the most.
I'm wondering if I could get consistent speeds out of them by switching to modern can motors -which would mean I could run any of them together while double-heading- but since they all run just fine right now I hesitate to swap motors only to find that they *still* run at different speeds!
Pete
Ok, so pick the fastest and the slowest. That would show if motor differences were the entire problem.
It's obviously going to depend on how much you want to solve your problem or if you prefer the problem. Exchanging motors might well eliminate the need to explore further problems.
It was a suggestion.
Agreed. That is presuming you've tested the weight taken by the bogies to a reasonable degree. You can get a fair impression just by picking up the front of the bogie (with say a screwdriver) and seeing how firmly it snaps back to the rails.
Sure, I suggested a common problem - you've got it covered.
If you're certain that they don't have disperate gear ratios the problem is clearly either motors or mechanical resistance.
From others comments it's clear that the motors are quite variable in qualities. A good beefy Sagami (NWSL) in each should give you reasonably consistant operating speeds. They're not perfect but they should be close enough in performance to operate together.
I'm not in favour of coreless motors but I have had excellent results from Sagami and Mashima motors. If you're running analogue then add a big flywheel, DCC, no flywheel.
Regards, Greg.P.
I have never tried it. Given that the decoders want a fixed voltage to operate, I'm guessing the decoder is bypassed when on a DC system. *
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