Crab Comet conversion

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Will L
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Re: Crab Comet conversion

Postby Will L » Thu Sep 28, 2017 3:58 pm

Julian Roberts wrote:I wonder Will if the reason your preference for the 1024 over the 1020 can be perceived from the EMGS data sheets that give torque etc, or whether your preference is for less definable reasons.


I didn't get it from the EMGs data sheet but, but my preference is down to the skew wound armature which makes the 1024 is quite powerful enough for almost all 4mm applications, and ensures a very smooth even power output. That and the 10mm size making it easy to get in to the smaller prototypes I favour in by preferred motor in the boiler layout. I'm going to miss it.

And I do like the way good CSB advice comes from all sorts of directions these days. I saw Ted Scannel at Scaleforum and we were getting a bit CSB self congratulatory. But no doubt somebody will bring us down again soon.

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Jol Wilkinson
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Re: Crab Comet conversion

Postby Jol Wilkinson » Thu Sep 28, 2017 4:18 pm

The inertia in a flywheel is proportional to the square of its rotational speed. Therefore its effect at low speed is relatively small.

Likewise the greater its effective mass means that a flywheel of greater mass and larger diameter is much better than a lighter, smaller one.

My own experience is that a mechanically free mechanism, coupled to a good motor through an efficient gear train and with effective pickups (including sprung/compensated wheels) provides a very sound basis and one which gives me what I need. It helps to have a decent controller, but it is unwise to rely on "add ons" such as DCC or flywheels to overcome basic deficiencies.

David Knight
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Re: Crab Comet conversion

Postby David Knight » Thu Sep 28, 2017 5:26 pm

PeteT wrote:
David Knight wrote:Going back to the suspension for a moment, are there suitable CSB carriers for the Brassmasters hornblocks or do conventional hornblocks have to be used? I ask because I have a Crab frame lurking in my “eventually” pile.

Cheers,

David


Well yes! I designed some, which are available from Rumney Models. These include a tail from the bottom to allow the springs to be attached to it. They are covered on my workbench thread.

For completeness Jeremy has also done some which are available via the Scalefour stores, and I think are covered on his steam railmotor topic.


Thanks Pete. I had a look in the Stores but could not find them, perhaps they are OOS at the moment. I would get them from Rumney but, unless there has been a change of late, those of us over the seas and far away can’t use our plastic or PayPal. Still, there are a few items in the queque ahead of the Crab so no need for panic.

Cheers,

David

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MarkS
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Re: Crab Comet conversion

Postby MarkS » Thu Sep 28, 2017 6:09 pm

Hey Dave,
there is also this approach from the CLAG site - http://www.clag.org.uk/pics/csb-gallery ... -4-4-0.jpg

Probably the approach I will take with a certain whitemetal 4-4-2 that is too heavy for the Brassmaster springs...
Cheers,

Mark.
"In the end, when all is said and done, more will have been said than done..."

David Knight
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Re: Crab Comet conversion

Postby David Knight » Thu Sep 28, 2017 8:23 pm

Thanks Mark. That has a certain simplicity to it that is appealing especially with a jig to get all the holes in the same place. That and the fact I already have handrail knobs galore :D

Cheers,

Dave

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PeteT
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Re: Crab Comet conversion

Postby PeteT » Thu Sep 28, 2017 8:41 pm

Yep that method certainly works Mark, I did my first one like that without any issues.

The reasons I developed an etch were:
- saves time. To fit the handrail knobs, the ring needs filing off one side of the block. Then a drilling jig to align the hole, then the handrail knob hole aligning while soldering. Nothing complex, but still.
- lettering. The etch has a letter (or number, as the fat & thin sides of the blocks are handed) on each one so that when they are paired up to a slot, a marker pen line isnt lost as they go through the ultrasonic bath.
- springs. A tail at the bottom easily allows the spring detail to be incorporated into the block.

Obviously horses for courses, but I felt it worth drawing them up!

Apologies for the diversion Julian...

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MarkS
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Re: Crab Comet conversion

Postby MarkS » Thu Sep 28, 2017 9:38 pm

Pete, don't get me wrong, I think the etch is brilliant.
Like Dave I'm in southern Ontario, so if you happen to be coming to Toronto in the next little while... :lol:
Cheers,

Mark.
"In the end, when all is said and done, more will have been said than done..."

David Knight
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Re: Crab Comet conversion

Postby David Knight » Fri Sep 29, 2017 1:41 am

David Knight wrote:Thanks Pete. I had a look in the Stores but could not find them, perhaps they are OOS at the moment. I would get them from Rumney but, unless there has been a change of late, those of us over the seas and far away can’t use our plastic or PayPal.


:oops: I stand corrected. Having had a closer look at the Rumney website I discovered that they do take PayPal. An order is in the offing ;)

Cheers,

David

Mark Tatlow
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Re: Crab Comet conversion

Postby Mark Tatlow » Fri Sep 29, 2017 8:50 am

Jol Wilkinson wrote:The inertia in a flywheel is proportional to the square of its rotational speed. Therefore its effect at low speed is relatively small.

Likewise the greater its effective mass means that a flywheel of greater mass and larger diameter is much better than a lighter, smaller one.

My own experience is that a mechanically free mechanism, coupled to a good motor through an efficient gear train and with effective pickups (including sprung/compensated wheels) provides a very sound basis and one which gives me what I need. It helps to have a decent controller, but it is unwise to rely on "add ons" such as DCC or flywheels to overcome basic deficiencies.


May I also add that the armature of a motor has a flywheel effect too. Thus, if you get a slightly larger motor in you also have a slightly larger flywheel effect just from the motor.

I think I ended up using a 1620 mashima and it neatly slips through the frames, so I am seeking to never need to dismantle it off the axle. There there is a fair amount of room above the motor in the firebox, largely because it sits down nice and low between the frames. Thus a larger motor would have been possible.
Mark Tatlow

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Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Fri Sep 29, 2017 7:02 pm

No problem Pete!
So does a skew wound motor, whatever the size and torque, in general have the advantage Will?

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Will L
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Re: Crab Comet conversion

Postby Will L » Fri Sep 29, 2017 10:18 pm

Julian Roberts wrote:So does a skew wound motor, whatever the size and torque, in general have the advantage Will?

Yes because a skew wound motor will run smother and the power delivery will be more even, at low speed,. Once a motor is spinning resonably quickly i doubt you'll see much difference. But slow speed performance is important if your want stuff to stop and start nicely in public. I don't think it makes them any more powerful as such, although good armature design must be a factor.

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Knuckles
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Re: Crab Comet conversion

Postby Knuckles » Fri Sep 29, 2017 11:02 pm

Could you please explain to this bonehead (me) what Skew Wound means in comparison to others?

Is it a worm?

I only have experience building worm drive and don't like it, mainly because the shafts in some motors are useless and have too much play resulting in mesh issues for starting and stopping smoothly.
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Jol Wilkinson
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Re: Crab Comet conversion

Postby Jol Wilkinson » Sat Sep 30, 2017 6:38 am

The armature windings are at an angle to the centreline of the armature. This reduces "cogging", gives a smoother output and better starting, if my memory of my engineering course is correct.

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Knuckles
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Re: Crab Comet conversion

Postby Knuckles » Sat Sep 30, 2017 7:48 am

Um...any chance of a sketch?

If not no worries.

I've got worm drive to work well a lot of the time, it's just the odd motor seems to let the set up down if the shaft shunts in and out too much. I say I don't like worm drive yet more accurate would be for me to say I don't like this particular issue with some motors. Haven't tried another method yet so wondering what the options are.

I have seen the helical gear set up and thought that might be a solution although I really do not know.
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Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sat Sep 30, 2017 8:13 am

Thanks for that info Will and Jol, that (about skew wound motors) was something I had never learnt in 20 years of modelling and trying to get as near as I can to creating that sense of inertia when trains stop and, particularly with a heavy train, start.

Another thing I still haven't really understood is, what is "cogging"? I have assumed it to be when at the lowest speed and not connected to any power train, the motor tends to move in a series of very small jerks.

Regarding the helpful replies on flywheels, my question was really on the premise, that, having got everything else as good as it can be in terms of a freerunning chassis and power train, as many pickups as possible, that do their job properly, and of course clean track and wheels, does a flywheel add anything more than the larger motor which could fit into the space it takes.

It would be interesting to know how difficult it would be to make this chassis with CSBs given they are not "designed in".

Meanwhile my intention was to make this kit "as is" in the first place, and basically this thread was to highlight any problems I had where a departure from the in instructions had to be made. I will edit one earlier post to explain why I made up the rods John Brighton style rather than just using them as they came.

In the last couple of days modelling time I have been experimenting with bits of lead to see what force the springs exert. The springs sit like this:
2017-08-20 18.03.18-1 (Small).jpg

This is the hornblock bearing as etched. Having removed the etched bearings the bearing sits with the spring uncompressed as pictured on the previous page 1.4mm from the top of its travel. To compress it to 1mm down from the top of its travel takes 30g. To compress it a further 1/2mm, to 1/2mm down from the top of its travel, takes 60g, and to compress it to the top of its travel takes 80g. (The spring is not fully compressed at this point of course.)

These are "ball park" figures, probably not terribly accurate, but give me a handle or general idea on what is needed weight-wise.

I think the design is that the bearing should ride 1/2mm below the top of its movement, and that the required movement is 1mm - half a mm each way from a centre of travel. So that infers a six wheel loco must weigh 360g to be at the ride height as designed.

I have established my ride height needs to be 1/2mm further down, so 1mm from the top of the travel, and have modified the bearings so that they cannot rise any higher. A 12BA nut is just over 1mm thick and fits over the spring spigot, so the spring can act in just the same way as designed, but my intention is to "bottom out" the front and rear axle springs, and to lighten the centre axle force and give it more upward travel. How I do this will depend on how heavy I can make the loco at the front. The rear will be weighted down by the tender hanging off the back Mike Sharman style so should not be problematic getting enough weight.
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Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Sat Sep 30, 2017 8:57 am

Cogging is where the armature segments pass from the influence of one magnet to the influence of the other one. There is a reduction in this influence as the segment moves into air, as it were, which is regained when it reaches the other magnet. Thus a slight jump (cogging) as the armature moves round. This can be reduced by having more poles to the armature, five is common, seven has been known, and this means that one armature segment at least is always under the complete influence of one or other magnet, which results in less jumping.

Conventional motors have the armature segments aligned parallel with the shaft. Skew wound motors have them angled, so that part of one segment comes under the influence of one magnet almost before the other end of it leaves the other magnet. Difficult to photograph without taking a motor apart!

Either of these two, or a combination of both, reduces or in many cases eliminates the jump from one pole to another, so that the motor merely starts to turn smoothly as the very lowest speed.

Philip

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Le Corbusier
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Re: Crab Comet conversion

Postby Le Corbusier » Sat Sep 30, 2017 9:26 am

I do like these lessons :thumb What kind are mashima motors?

Tim
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John Palmer
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Re: Crab Comet conversion

Postby John Palmer » Sat Sep 30, 2017 10:09 am

Knuckles wrote:Um...any chance of a sketch?

The illustrations here: http://www.sumidacrossing.org/LayoutElectricity/ModelTrainPower/DCTrainMotors/DCMotorTechHistory/ may help, as they include a couple of pictures of skew-wound armatures.

In a skew-wound motor, the trade-off for the reduction in cogging is some loss of torque.

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Knuckles
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Re: Crab Comet conversion

Postby Knuckles » Sat Sep 30, 2017 10:58 am

Ohhhh.

http://www.sumidacrossing.org/LayoutEle ... w-2620.jpg

And there was me thinking it was some alternate gearbox configuration.

Ok thanks for the pointers.
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https://www.sparkshotcustomcreations.com/
Mostly 3D Printed Loco kits etc.

SCC Price list (7/4/22)
https://www.sparkshotcustomcreations.co ... e77d42.pdf

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PeteT
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Re: Crab Comet conversion

Postby PeteT » Sat Sep 30, 2017 12:32 pm

Julian Roberts wrote:It would be interesting to know how difficult it would be to make this chassis with CSBs given they are not "designed in"


As said I've yet to tackle a crab, but have the Ivatt tank to a rolling coupled chassis (ie all the deviation required for CSB is done) covered in my workbench & that went smoothly.

I guess the main thing other than working out the anchor points is to potentially reposition a few frame spacers to prevent collisions.

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Jol Wilkinson
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Re: Crab Comet conversion

Postby Jol Wilkinson » Sat Sep 30, 2017 12:41 pm

Knuckles,

the clue is in the word "wound", referring to the electric winding on the armature.

Skew is also a description of a type of gear profile (also called helical gears), so confusion is quite understandable. In that case the teeth are angled across the centreline of the gear axis. It's common in high load applications to reduce noise although there is more friction. Gearboxes for racing and vintage cars use "straight" cut gears, recognizable by the whine under load.

HL and Portescap spur gears are straight cut gears (which explains why my LNWR Watford Tank sounds like it is turbocharged). Some others are skew cut, usually to match a corresponding worm gear (which is designed to match the worm angle) and create a longer gear train (e.g. Branchlines Multi-box).

Any errors in this statement can be attributed to the fact I last dealt with all this stuff on a professional basis about fifty years ago.

Jol

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Will L
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Re: Crab Comet conversion

Postby Will L » Sat Sep 30, 2017 5:00 pm

Julian Roberts wrote:It would be interesting to know how difficult it would be to make this chassis with CSBs given they are not "designed in".


The basic routine to put CSB onto a chassis not designed for it is given in this post using the Highlevel CSB Drilling Jig which makes it remarkable simplest process. Highlevels own tutorial is here. It helps if the chassis has standard 6mm square horn block holes designed in like the comet chassis. Works with either the Highlevel hornblocks or the Brassmaster style bearing blocks with the add on Rumney models CSB fulcrums etch. If using the later you'll need to know that the offset between the axle centre and the fulcrum point is 3.5mm.

After you've sorted out the hornblocks, you then need to ensure that the CSB can pass through any chassis spacers which get in the way, which requires a 1.5 or 2mm hole drilled 1mm from the spacer edge on the fixed fulcrum point centre line( the Highlevel Jig will tel you where that is).

Mark Tatlow
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Re: Crab Comet conversion

Postby Mark Tatlow » Sat Sep 30, 2017 7:30 pm

Julian Roberts wrote:So that infers a six wheel loco must weigh 360g to be at the ride height as designed.



I am in the process of weighting mine this weekend, having given it a fairly good running at Scaleforum. It was much too light (155g) but was not prone to derailment even though I agree with you that the springs were probably not correctly depressed (or possibly at all!). Its maximum haulage capacity was six reasonably free running coaches but with additional lead balanced on top, this went upto eleven coaches. Here she is with eight on.

https://www.youtube.com/watch?v=TQjwolE_sPA

I anticipate being able to get up to 360g but it will fill a lot of the space and I want to leave enough room for a speaker and stay alive, so I am cramming away!

Picking up on the earlier comments regarding the possibility of using CSBs; I think if I were doing this again (I might one day) I would do it with CSBs using the High Level jig as Will suggests.

And going back to one of my earlier posts, this is what I meant by the extension part to the Comet chassis to allow it to be affixed to the Bachmann body.

IMG_4601.JPG


And this is where my chassis is at present; I intend to change the brake hanger arrangement of which more anon.

IMG_4602.JPG
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Mark Tatlow

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Will L
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Re: Crab Comet conversion

Postby Will L » Sun Oct 01, 2017 8:59 am

Will L wrote:
Julian Roberts wrote:It would be interesting to know how difficult it would be to make this chassis with CSBs given they are not "designed in".


The basic routine to put CSB onto a chassis not designed for it is given in this post using the Highlevel CSB Drilling Jig which makes it remarkable simplest process. Highlevels own tutorial is here.


And I was forgetting, there is another good article with lots of pictures on the CLAG website

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Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Oct 01, 2017 9:13 am

Quick thought Mark - lots of room in the tender surely for speaker? If weighting the tender like me the weight has much more effect at the front so still room for speaker further back maybe.

A video that came up after your Crab showed a steam loco braking to a stop under coaling tower still under steam i.e. regulator open acting against brakes. Just sometimes in real life to get a precise stop that might be necessary. Or is that a naff kind of sound system that doesn't shut off steam any time the loco is going? (Don't think the video was yours! ;) )

By the way please don't take any figures of mine as having any authority! Though the CLAG link on these bearings mentions the same 80g.


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