Wheel bosses, hornblocks and side play - practical matters

SFB
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Wheel bosses, hornblocks and side play - practical matters

Postby SFB » Tue May 08, 2012 1:53 pm

I am fairly new to building chassis. I have tried and failed to scratch build one a couple of times for my Mainline Jubilee. So to improve my chances of success, my latest attempt uses the Comet Chassis pack and Alan Gibson wheels. Having absorbed much information from this forum and elsewhere about the relative merits of solid chassis versus compensation versus springing I chose to go with Comet's sprung hornblocks, which are Brassmasters I believe.

Well, it's finished but is not a success. Off track and on DC it seems to perform satisfactorily. On track, however, it is mostly OK on my scenic, large radius, curves but on the tighter (c. 90 cms) curves behind the scenes it will not stay on the track - even if I run it as an 0-6-0 and with plenty of weight on it - (roughly 200 gm and balanced fore and aft) . I have checked the basics such as quartering and clearances on the valve gear and they are OK.

I have a digital caliper gauge and I have checked and verified the wheelsets' back to back measurements. When I check for sideplay on the axles there is none and I think this explains the problem. My calculations show that with the standard Comet frame spacers 14.88 mm, 0.5 mm frame material, and 0.5mm wheel bosses on the back of the Gibson wheels, there should be just enough sideplay – but that seems to be lost because the hornblocks protrude 0.5mm outside the frames.

My I ask the more knowledgeable members of this forum for your advice as to how I might proceed:-

1. Should I be looking to verify something else rather than sideplay as the reason for the chassis not staying on track? I have checked and verified the track gauge and my two other vehicles (a Class 101 DMU and ViTrains Class 37 with Ultrascale p4 conversion work completely satisfactorily.
2. Could the sprung hornblocks be the problem? I have sprung all 3 axles and I'm using a High Level Roadrunner gearbox and Mashima motor to drive it.
3. I am considering purchasing some High Level hornblocks, which do not protrude as much as the sprung ones and rebuilding the chassis with CSB suspension. I have read and reread the threads on CSB theory and how to deal with bogies and pony trucks and I think that I could make it work.
4. On which axles should I be allowing sideplay anyway? The motor and gearbox are on the rear axle, the connecting rod is on the centre one of course, and the front one is behind the cylinders. It seems that there is a good case for not allowing any of them to have sideplay!
5. Should I rebuild the chassis using narrower frame spacers in the first place.

As you can probably tell, my major dilemma is choosing the most sensible way forward. I don't mind spending the time on this, it is quite enjoyable finding, diagnosing and fixing problems, but in the end I would like a working chassis!

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Russ Elliott
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Russ Elliott » Tue May 08, 2012 2:43 pm

Sounds like lack of sideplay could be the problem. You need it only on your centre axle. You'll need approx 0.5mm each side for that kind of radius, so suggest drop the centre axle and take off 0.25mm from each rear wheel boss. You will probably find the slot in the Brassmasters' hornblock is slightly asymmetric (see drawing), so the centre axle blocks can be 'reversed', and that will give you an extra 0.25mm sideplay per side. If the blocks have got an annular ring on the axlebore perimeter, rub that off as well, and that will give you a bit extra. These mods will add up to the 0.5mm each side you need. The only snag with reversing the blocks in their current slots is that the wheelbase conformance might not be quite as good as what you've got now, so instead of reversing them, suggest swap the blocks, i.e. taking block A and putting it in block B's slot, but keep the orientation (i.e. don't turn it over), if you see what I mean - with a bit of luck that should preserve your wheelbase accuracy.

While you've got the middle axle out, you might also want to ease the middle axle springrate a little if you feel the chassis is prone to pitching slightly on the middle axle. Don't try fiddling with the spring, just file off 0.25mm from its upper seat in the frame (or from the top of the hornblock). 0.25mm should be plenty enough to slacken the axle sufficiently. (This is a John Brighton regular bodge, but I think he files a whopping 0.5mm out to safeguard against the slight length inconsistency of the coils.)

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Tim V
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Tim V » Tue May 08, 2012 7:14 pm

I would concur with all that Russ says. I've used the similar Brassmaster's hornblocks with success on 3' curves, using 15mm spaced frames. I would also check that the blocks slide freely in their guides, making sure that they can tip relative to each other.
Tim V
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Will L
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Will L » Tue May 08, 2012 10:15 pm

SFB wrote:1. Should I be looking to verify something else rather than sideplay as the reason for the chassis not staying on track?


No, until you have some side play I wouldn't bother looking elsewhere. Without enough side play, I would expect the leading wheel to climb the outside rail as you enter the curve, although it is possible the rear outside wheel might climb the outside rail instead. It depends on the weight distribution.

Will

SFB
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby SFB » Wed May 09, 2012 8:15 am

Thanks for the swift replies. I'll let you know how it works out as soon as I can.

SFB
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby SFB » Wed May 09, 2012 6:04 pm

This afternoon I took out the central axle, made sure that the hornblocks were orientated with the slim side to the outside of the frames, trimmed off the annular ring and reassembled the chassis. I am pleased to report that there is now a reasonable amount of sideplay on this axle and the chassis, running as on 0-6-0 for the time being, now holds the track in the main. The difference is like chalk and cheese. So thanks for the suggestions.

When I say "in the main", there are a couple of locations on my non-scenic sections where it still derails but I'm confident that this is probably due to track curvature/gauge, which I shall investigate and fix. There are other issues with the chassis - return cranks coming undone, jerky motion etc but I suppose this is all in a day's work for the chassis builder, and I'll persevere!

thanks again,

Stuart

craig_whilding
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby craig_whilding » Wed May 09, 2012 6:48 pm

A railway curve to your minimum radii is invaluable for sliding around and checking for issues in track laying i've found.

SFB
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby SFB » Mon Aug 13, 2012 1:32 pm

The reason that I built this Jubilee chassis was to give me some experience before I tackle a Dave Bradwell B1 chassis kit. Arising from the questions of trackholding, above, I am fairly happy that I can now build a working chassis. [Although I still have a problem fitting the front bogie at the right height and with the correct springing for the chassis to work correctly.}

But I have a still have questions regarding sideplay and I hope someone can assist.

After I created some curved templates of various radii I found that at its tightest radius my track is curved to only about 80cm or 2' 8" and it is making me wary about building the B1 kit in case it won't go round. So here are the questions:-

How do I calculate the sideplay that I need in order for the B1 to go round this curvature successfully and how then do I build the kit to make sure that this sideplay exists? I would prefer to leave the wheels untouched so does that mean I have to use narrower frame spacers? With the Jubilee kit I had no sideplay until I trimmed the wheel bosses off the inside of the centre wheel set. Do frame spacers come in different widths from different suppliers?

I would like to use Ultrascale wheels on the B1 --are they a different width over the bosses than the Gibson ones I used for the Jubilee?

Is it safe to put some sideplay on the front axle behind the cylinders as well as on the centre axle, and if so, how does the combination of front axle and centre axle sideplay contribute to the overall sideplay. Is it simply the case that if I put 0.5mm on those two axles then I'll get 1.0mm in total?

Finally, with that curvature, is it a non-starter to even contemplate building the chassis to P4 standards at all?

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John Bateson
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby John Bateson » Mon Aug 13, 2012 2:42 pm

Side play. In P4, thanks to the gauge widening specified in the standard, I have not found it necessary to provide side play on the axles for the model to negotiate curves. In fact if you use Ultrascale wheels, there will be no play, even without washers. If you are using other, less well defined standards, then you will have to make up your own mind

I must emphasise again that in P4 the wheels must have no protruding boss

These are quotes from Dave B's instructions.

The actual distance across the frames is 16.25 mm on my model of the Thompson B1 (spacers are 15.72 mm) and some may feel that this is too little for smaller radius curves. If you use Gibson wheels then the rear boss should be removed. I think the Exactoscale wheels may be good because they do not have a rear boss. I think the Ultrascle wheels have about a 10' boss.
However this excellent kit does have the EM option and you may wish to consider building the frames with these parts and use washers if needed to reduce the side play.

Calculation of side play is covered in the Digests and is something I found difficult to understand at first, the documentation is written and produced in quite an old style. At this point I will hope that somebody else has done this for a Thompson B1 ...

John

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Russ Elliott
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Russ Elliott » Mon Aug 13, 2012 2:54 pm

SFB wrote:How do I calculate the sideplay that I need


Image

c is the coupled wheelbase, say 70mm. r is the curve radius, say 800mm. So the v required on each side of the chassis is 0.75mm.

In practice, you will have a little bit of sideplay on the front axle (constrained by the crankpin to valve gear clearances, or is there not a problem in this area on the B1? - I don't have a good sideview), and possibly a bit more on the rear axle, plus some inherent clearance of wheelset on track gauge, so the problem is not quite as bad as it seems, but nevertheless, a 0.75mm objective on each side is a lot.

Even with conventional steps like ensuring hornblocks do not protrude from chassis faces, and possibly thinning off of wheel rearbosses, drastic measures might be called for. You could gain an extra 0.25mm (or more) per side by squeezing the frames:

squeezed-frames.png
squeezed-frames.png (3.85 KiB) Viewed 7199 times

No one but you will be able to notice it.

I would like to use Ultrascale wheels on the B1 --are they a different width over the bosses than the Gibson ones I used for the Jubilee?


Don't know. Quite possibly they will not be the same.

Is it safe to put some sideplay on the front axle behind the cylinders as well as on the centre axle, and if so, how does the combination of front axle and centre axle sideplay contribute to the overall sideplay. Is it simply the case that if I put 0.5mm on those two axles then I'll get 1.0mm in total?


Generally, yes - the coupled axle sideplays tend to add up. Where I suspect you will have problems is not so much on the sideplay requirement over the coupled wheelbase, but on the bogie wheels striking the cylinders. If you don't fancy hacking the backs of your cylinders, the only solution would be to lengthen the bogie wheelbase by 2mm or 3mm. Heresy to the "getting everything right" brigade, maybe, but no one will notice the difference.

Finally, with that curvature, is it a non-starter to even contemplate building the chassis to P4 standards at all?


800mm radius is definitely pushing the envelope for 4-6-0s. Good luck.

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grovenor-2685
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby grovenor-2685 » Mon Aug 13, 2012 3:10 pm

You might find this of some use.
http://www.norgrove.me.uk/history_files/Mar73/Mar-73.htm
Regards
Keith

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grovenor-2685
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby grovenor-2685 » Mon Aug 13, 2012 3:22 pm

Do frame spacers come in different widths from different suppliers?

Oh yes, the original P4 offerings used to come in two versions 15mm and 15.5mm. The former intended for tighter radii. Since then there have been lots of variations. Some kit suppliers will offer 3 versions, 00, EM and P4, or just two, EM and P4, some expect you to use the same for EM and P4. And I doubt you will find any two suppliers using identical dimensions for either, there is no defined standard and it also depends on the frame thickness used, and as you discovered the design of hornblock. All adds to the fun ;)
Its easier to washer out a narrow chassis than thin down a wide one but you do lose a bit in the appearance.
Regards
Keith

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Horsetan
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Horsetan » Wed Aug 22, 2012 10:04 pm

SFB wrote:....I would like to use Ultrascale wheels on the B1 --are they a different width over the bosses than the Gibson ones I used for the Jubilee?


You may have a dififculty there as Ultrascale don't actually produce the LNER 6'2" wheel. Your choice is AGW, Exactoscale, or nowt.
That would be an ecumenical matter.

SFB
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby SFB » Mon Aug 27, 2012 12:29 pm

Well "nowt" would severely hamper the running and I have so far used Alan Gibson so I'll investigate Exactoscale. With the Alan Gibson wheels on my Jubilee I have had little success fixing the return crank and its crankpin in place and I'm looking for an alternative/better/reliable solution to that problem.

(For the moment I have turned my attention to other matters so that I can come back to it with renewed enthusiasm and determination later in the year!)

Philip Hall
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Philip Hall » Mon Aug 27, 2012 1:17 pm

I have had little success fixing the return crank and its crankpin in place...


My method is to drill through the slot of the crankpin screw head, once it's firmly screwed in (I put some Hafixs on the thread just before finally tightening). Then I drill through the head of the screw at about 45 degrees with a 0.5mm drill and into the wheel, and insert a brass wire pin coated with Hafixs or other superglue. That has always stopped the crankpin moving. After that, you can secure the return crank by screwing down tightly or by soldering. I often solder, and in this case I find it easier to replace the crankpin with a brass 14BA screw as it is much easier to get the solder to take quickly on brass. It's not too difficult and usually the return crank can be got off again if you need to, but it's best not to do it too many times.

I have recently experimented with Loctite 278 Threadlocker, this is much stronger than 242 we usually use and is sold as a permanent bond. I have, however, found it possible to break the joint (with heat, says their data sheet, but we have a plastic wheel so be careful as with soldering) but I have managed it just by twisting - it is a very small joint after all. I'm going to use the 247 again as it does seem to be very strong indeed.

Philip

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Flymo748
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Flymo748 » Thu Aug 30, 2012 7:13 am

grovenor-2685 wrote:
Do frame spacers come in different widths from different suppliers?

Oh yes, the original P4 offerings used to come in two versions 15mm and 15.5mm. The former intended for tighter radii. Since then there have been lots of variations. Some kit suppliers will offer 3 versions, 00, EM and P4, or just two, EM and P4, some expect you to use the same for EM and P4. And I doubt you will find any two suppliers using identical dimensions for either, there is no defined standard and it also depends on the frame thickness used, and as you discovered the design of hornblock. All adds to the fun ;)


That's a brilliant explanation from Keith of why if anyone tries to say that there is "One True Way" of modelling in P4, they must be barking mad!

Apart from the distance between the rails (and there are modellers in my Area Group that model P87, Iberian Gauge and SAR that would dispute even this...) being 18.83mm in a straight line, there isn't much else that you can hold as a constant :-)

Do whatever works for you - and enjoy doing it...

Flymo
Beware of Trains - occasional modelling in progress!
www.5522models.co.uk

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Paul Townsend
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Paul Townsend » Sun Sep 02, 2012 4:15 am

Flymo748 wrote:Apart from the distance between the rails (and there are modellers in my Area Group that model P87, Iberian Gauge and SAR that would dispute even this...)
Flymo


And there are some 100 enlightened bods who model Brunel's Broad Gauge to P4 standards :thumb

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Flymo748
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Re: Wheel bosses, hornblocks and side play - practical matte

Postby Flymo748 » Sun Sep 02, 2012 8:29 am

paultownsend wrote:
Flymo748 wrote:Apart from the distance between the rails (and there are modellers in my Area Group that model P87, Iberian Gauge and SAR that would dispute even this...)
Flymo


And there are some 100 enlightened bods who model Brunel's Broad Gauge to P4 standards :thumb


And Dorchester Junction should be showing exactly that at this year's Scaleforum.

Only four weeks to go :-)

Flymo
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