Comet 3F Jinty Chassis Issue

[Barry Ten]

Ian

 

I sorted out a friends chassis which was much the same even with Markit wheels

 

We stripped the chassis down to taking out the wheel bearings, there were re-fitted using London Road Models hornblock alignment jigs (rods) with the coupling rods. The bearings were soldered in place then tested so the axles rotated freely

 

Next the wheels and coupling rods were fitted, again testing the rolling chassis on a slight incline so it rolls freely without any binding

 

Check the gear meshing next with an axle in the motor mount before going to the next stage

 

Now fitt the motor and gears then test again 

 

Finally fitting pickups

 

The approach I tried teaching my friend was to test at every stage and not to move on till 100% with stage. I am just building a Branchlines 0 16.5 chassis and initially the meshing was awful, far too tight, had I not tested it prior to fitting I might have wasted hours

 

 

I've managed to damage a couple of motors by running them through a Branchlines gearbox with too-tight meshing, so it's another thing to look for.

[ianLMS]

The meshing seems fine as when I dissengaged the gear on the driving axle, it turned quite nicely. Now I have checked and re-aligned everything, the wheels turn. Its now an issue of getting power to the motor efficiently enough for DCC operation. 

 

The issue is three fold - wheels contacting the rail all the time, good pick-ups with constant clean contact on the wheels and a decent DCC chip. I have heard Zimo or Digitrax are the best chips, but not sure. It could also be an issue with the motor.

 

I am hedging my bets on the chassis causing the wheels to lose contact with the rails. 

 

Ian

[Captain Kernow]

You can sort the 'wheels contacting the rail' issue by installing simple 3-point compensation, so you'd have hornblocks for the leading and middle axles and the rear axle fixed, with the gearbox driving that axle.

 

Is there a problem with the gearbox, if the motor is running OK when disengaged?

 

Less drastic than a replacement chassis might be a replacement gearbox (I'd definitely recommend one by High Level, if that were the case).

 

One thing I always do, when at the relevant part of a chassis build, is to test the motor driving the driven axle via the gearbox, all installed in the chassis. I expect nice, smooth running at a range of speeds, including a slow crawl. If that's OK, then you should be OK to fit the rods and continue from there.

 

I personally use 0.35mm p/b wire for pick ups, I find it more flexible and more yielding than the thicker stuff and you are consequently less likely to import any unwanted friction, due to the pick ups being too stiff against the wheels.

[ianLMS]

Thanks for that - not much progress last night while I wait for the phospher bronze wire to turn up. I did paint the buffer beam though and tidied up a bit!

 

As you say, I might build the new chassis with the sprung hornblocks and try a new gearbox from High Level.

 

Ian

[John_Hughes]

Zimo chips are fine; I've settled on them as my standard for OO Gauge - but never - I repeat NEVER - install DCC unless and until the model is running perfectly under DC, or you'll probably fry the chip.

 

How do you think I learned that lesson? No prize for the right answer 

[Izzy]

 

I have never used a Comet chassis, and I just can’t get my head around the idea that for a fixed axle chassis of 0-6-0 configuration (or greater I presume) the middle axle(s) should be raised above the centre line, which has been seen as a ‘good idea’. Are coupling rods ‘bodged’ like this to match? It’s one thing to have axles allowed to rise and fall in relation to each other in a ‘flexible’ chassis - via springing/compensation, etc, quite another when the axles are fixed in position. If this is indeed the case it explains why some have so much trouble getting decent running.

 

Izzy

[Tullygrainey]

I have never used a Comet chassis, and I just can’t get my head around the idea that for a fixed axle chassis of 0-6-0 configuration (or greater I presume) the middle axle(s) should be raised above the centre line, which has been seen as a ‘good idea’. Are coupling rods ‘bodged’ like this to match? It’s one thing to have axles allowed to rise and fall in relation to each other in a ‘flexible’ chassis - via springing/compensation, etc, quite another when the axles are fixed in position. If this is indeed the case it explains why some have so much trouble getting decent running.

 

Izzy

 

 

In "Locomotive Kit Chassis Construction in 4mm" (Wild Swan Publications), Iain Rice describes this as "Strictly speaking .... bad practice" but suggests that a high centre axle on a 0-6-0 is better than a low one which would leave the chassis teetering on its centre wheel set. Setting it slightly high is playing safe. However, I think your observation about coupling rods is a sound one Izzy. The chassis would be fighting the rods. To work properly, a high centre axle arrangement would surely need rods articulated in the middle.

 

Alan

[Barry Ten]

I have never used a Comet chassis, and I just can’t get my head around the idea that for a fixed axle chassis of 0-6-0 configuration (or greater I presume) the middle axle(s) should be raised above the centre line, which has been seen as a ‘good idea’. Are coupling rods ‘bodged’ like this to match? It’s one thing to have axles allowed to rise and fall in relation to each other in a ‘flexible’ chassis - via springing/compensation, etc, quite another when the axles are fixed in position. If this is indeed the case it explains why some have so much trouble getting decent running.

 

Izzy

 

It's really a tiny offset, easily taken care of by the normal clearances for the crankpins. I've put together quite a few Comet frames and without exception each has required next to no work to get a really smooth running chassis, with absolutely no bodging necessary.

[Izzy]

It's really a tiny offset, easily taken care of by the normal clearances for the crankpins. I've put together quite a few Comet frames and without exception each has required next to no work to get a really smooth running chassis, with absolutely no bodging necessary.

Yes, I realise that it can be made to work, it just seems, as the previous post quoting Iain Rice suggests, bad general practice. Not only that but in effect the chassis becomes just mainly 4-coupled in respect of traction and pickup. I haven’t built a chassis without some kind of axle movement/slop since the late ‘70’s, and I understand the idea behind it. Just seems the wrong way to go about it. But then, each to their own.

 

Izzy

[Barry Ten]

Yes, I realise that it can be made to work, it just seems, as the previous post quoting Iain Rice suggests, bad general practice. Not only that but in effect the chassis becomes just mainly 4-coupled in respect of traction and pickup. I haven’t built a chassis without some kind of axle movement/slop since the late ‘70’s, and I understand the idea behind it. Just seems the wrong way to go about it. But then, each to their own.

 

Izzy

 

My experience is that, once the basic frames are assembled in a 6-coupled chassis, the tread on the central wheelset just "skims" along the railtop but the flanges are in nearly full contact so there's still ample pickup. Whether there's useful adhesion or not I wouldn't know, or whether it even matters. It just seems to work without any fiddling!

 

Proof of the pudding?

 

[ianLMS]

Thank you for disucssion this issue. It threw me too for a while. New pick up wire arrived yesterday so will try again with the existing chassis. If I still cant get it to run well in DC, I will build the new comet chassis and replace the gearbox with a high level one.

[ianLMS]

Almost there!!!!

 

Thank you everyone for helping me with this issue. Last night I stripped the gearbox, replaced the Mitsumi motor with a Mishima 1620 with 2mm worm drive, and re-assembled it, making sure everything was nice and square, and the axles rotated smoothly. I replaced the pick-ups with .30mm phospher bronze wire. I have two axles with pick ups on top of the wheels and the front axle with a pick up at the bottom. The middle "floating" axle has pick-ups top and bottom.

 

Tested everything at each stage making sure the motor ran nice without gears, with gears, with one axle, then all three axels. Everything worked as it should so taped over the motor and gearbox to eliminate shorts, made sure the pick-ups werent shorting and mounted the chassis into the body. Tested the loco on the rolling road and on a short section of track.

 

On the rolling road it was still a little eratic, but on the test track it ran smooth in both directions. The weight of the white metal body helped with holding the wheels onto the track. Now I know its working as it should with the analogue controller I can go ahead with converting it to DCC.

 

I have purchased a new Zimo MX623 decoder and over the weekend I will fit that and programme it. I will hopefully have enough time to also finish the body off with decals, couplings, weathering, crew and fit the brake rodding.

 

I will take a few more pics over the weekend and will post Monday.

 

Thank you all again. 

 

Ian

[Captain Kernow]

Glad it seems to be working out now!

 

It seems to me, that the amount of satisfaction at finally deriving a sufficiently well-running chassis is in direct proportion to the amount of blood, sweat, tears and bad language expended on the project.

[5050]

On a more historical note, doesn't the 'high axle' idea possibly go back to John Ahern or even Edward Beal?  If using this idea (which to me seems to be the complete antithesis of the usual 'plate glass' method of rigid chassis construction) then shouldn't the centre rod holes be slightly elongated vertically?

 

I'm afraid to say that I have never built a rigid chassis.  Even in my early 00 days of 40+ years ago I took on board the Mike Sharman/Iain Rice Flexichas concept and still use it - very succesfully.  Coil springs are for mattresses.

Edited December 7, 2018 by 5050

[ianLMS]

I have just purchased the book by Iain Rice on 4mm Chassis building so hopefully the secrets of the compensated chassis will be revelaed to me and I can then attempt to build one for my next project!

[MarkC]

Most of my locomotives are built using the Sharman Flexichas system, but some recent etched kits I have built are designed with CSBs, so I have followed the destructions provided...

[John_Hughes]

On a more historical note, doesn't the 'high axle' idea possibly go back to John Ahern or even Edward Beal?  If using this idea (which to me seems to be the complete antithesis of the usual 'plate glass' method of rigid chassis construction) then shouldn't the centre rod holes be slightly elongated vertically?

 

I'm afraid to say that I have never built a rigid chassis.  Even in my early 00 days of 40+ years ago I took on board the Mike Sharman/Iain Rice Flexichas concept and still use it - very succesfully.  Coil springs are for mattresses.

 

Not Ahern, I'm pretty sure, so quite possibly 'The Padre' as he sometimes bylined himself.

 

I lilke (and still use) Ahern's Miniature Locomotive Construction, but his advice was pretty poor on the essentials of chassis building. He doesn't even mention the need for the holes in the coupling rods to precisely match the axle spacing, without which little detail nothing at all will run.

[ianLMS]

Here are a few pictures on the finished Cotswold Jinty. Zimo MX623 chip fitted and programmed over the weekend and it runs quite well considering the issues earlier on. Thank you to everyone for the help and guidance and encouragement.

 

Weathering and fitting of tools is all thats left now.

 

 

[Buhar]

Although black shaded insignia was more common for freight locos post 1928, I do prefer the red version you've used it looks more thought-about.  I like the short-###d driver too.

[ianLMS]

Thx Alan - Although I broke the rule that you shouldn't model a model, I followed the Bachmann version on the insignia. The driver is tired after working so hard pulling the regulator, he decided to sit down. He clearly wasnt happy someone was photographing him while on a break though!

 

His legs interfered with the wheels and pick ups below him so unfortunately, they had to be amputated!

[ianLMS]

Here are a couple of pics following my novice attempts at weathering

[hayfield]

Thx Alan - Although I broke the rule that you shouldn't model a model, I followed the Bachmann version on the insignia. The driver is tired after working so hard pulling the regulator, he decided to sit down. He clearly wasnt happy someone was photographing him while on a break though!

 

His legs interfered with the wheels and pick ups below him so unfortunately, they had to be amputated!

 

Ian

 

I had to remove part of the lower led for a driver in one of my 0 16.5 locos, no one can see it and it looks so much better with a driver than without

 

Loco looking very good and its great to see you overcoming issues when building items, not only a good learning curve for yourself, but others also

[ianLMS]

Definately a steep learning curve but with the help of others on RMWeb, it is a lot easier than working blind. I enjoy working problems out for myself and sharing the experience to help others, but sometimes its cheaper, quicker and simpler to ask the question a dpick the repsonses which best suit the situation, ability and budget.

 

Several suggestions here helped me resolve the running issue resulting in a nice little loco which runs relatively well. 

 

next project - a used pre-started Model Loco (DJH) Black 5. The builder used Super-glue/Epoxy so currently breaking the glue down and pulling it apart to re-build. Chassis was built well though with all the motion gear!!!!

[Barry O]

stick the Black 5 in a bowl of warm water..helps to make it easier to get the glue apart..of use a bit of polystrippa/nitromors

 

Baz

[hayfield]

Definately a steep learning curve but with the help of others on RMWeb, it is a lot easier than working blind. I enjoy working problems out for myself and sharing the experience to help others, but sometimes its cheaper, quicker and simpler to ask the question a dpick the repsonses which best suit the situation, ability and budget.

 

Several suggestions here helped me resolve the running issue resulting in a nice little loco which runs relatively well. 

 

next project - a used pre-started Model Loco (DJH) Black 5. The builder used Super-glue/Epoxy so currently breaking the glue down and pulling it apart to re-build. Chassis was built well though with all the motion gear!!!!

 

 

A man after my own heart, and having a working chassis is definitely a big bonus