High Level 'Pug' Chassis

[Sapper]

I have partly constructed a High Level Pug chassis and on fitting the wheels and coupling rods found there was quite tight binding when moving it along. I inspected the chassis and all appeared to be sound except when I measured the spacing of the axles with a micrometer there appeared to be a slight variation on about 0.001mm when measured at the bearings. I would rather not unsolder and refix the horn plate(s) if possible and was wondering if it would be viable to enlarge the coupling rod bearing holes. On reading various articles there seems to be a divergence of opinion on whether one should have coupling rod bearings with some slop (and if so how much) or as tight a clearance as possible but allow for free movement. Certainly on some proprietary chassis I have had there seemed to be a lot of slop but they ran quite well.

I attach a couple of pictures to show the chassis as at present.

Any advice would be welcome.

Sapper

Chassis 2.jpg

Chassis 1.jpg

[grovenor-2685]

a slight variation on about 0.001mm when measured at the bearings

Do you really mean a thousandth of a mm? I would suspect the measurement accuracy.
Have you tried swapping or turning the square bearings? I would expect that they vary by more than that amount.

But very slight easing of the coupling rod holes will probably solve the problem, enough clearance to cover a variation of .001mm is negligible, hardly counts as slop!
Regards

[Philip Hall]

I think enlarging the holes in the coupling rods would be your solution, notwithstanding the accuracy or otherwise of your earlier measurements! A crankpin bush is nominally 1.5 mm and I routinely start with a hole in the rods of 1.6 mm, maybe a fraction less. A little bit of clearance helps.

Philip

[Mark Tatlow]

Sapper,

Have you also measured the coupling rod dimensions across the crankpin holes? It is important that they match the axle spacings on the frames with those on the rods.

I routinely measure both the axle spacing and the spacing of the holes on the coupling rods with a pair of digital callipers - I measure the inside dimension and the outside dimension for each and divide by 2 to get the actual dimension! I reckon to be able to measure with a tolerance of a couple of 100ths of mm and I find that if you can get the spacings consistent across both sides of both the axle spacings and the coupling rods give or take 5/100ths you will get a very smooth chassis.

Since I have started doing this I have found getting a smooth running chassis much simpler to achieve and no longer fear making chassis!

Thus, I would say that if you have got the axle spacing anywhere near a thousandth of a mm, you have achieved practical perfection! Thus do not touch the hornblock spacings. Before you open up anything, check the coupling rod crankpin hole spacings and if these are similarly good then that only leaves one of the holes as being too tight as an option so open up the axle and crankpin holes to a small degree (only a small degree though). Even if you do not think that you have removed enough at the first attempt, you should find the binding easing, so you will know you on the right course.

Good luck!

[nberrington]

I've taken to using the Avonside Jig. Pricey, but makes life soooooo much easier with getting the rods right.

[Mark Tatlow]

I've taken to using the Avonside Jig. Pricey, but makes life soooooo much easier with getting the rods right.

I use this too but I still measure after!

.....and find I need to tweet things as a result.

[grovenor-2685]

The photos do show the rods sitting very snugly on the jig axles, however the plan view seems to show the rods splayed out which suggests they are slightly longer than the axle spacing. This could be an illusion because of the camera angle, or a result of the jig axle tapers not being identical. Either way it needs investigating.
I would suggest swapping the jig axles around and seeing if the skew on the rods changes as a result.

If this shows the rods as being a tad long the cure is probably just the same, open them out just a touch.
Regards

[Sapper]

Gentlemen
Thank you for your suggestions regarding my problems with the above. I completely misread my micrometer readings and they now show a variation of .02mm between the axles when measured internally. I have slightly enlarged the holes for the bearings in one of the rods and this seems to work, unfortunately when reaming out the other rod the end broke off. I have contacted Chris Gibbons who may be able to help me out with some new etchings, if so, all well and good.

I did note that when setting up the connecting rods there was little or no clearance between the rivets and the slide bars using a bearing and brass nut as supplied by Alan Gibson. I have got some threaded nuts with a flange which match the thread on the coupling rod pins but require the holes in the rods to be reamed out slightly to fit. If this works then there should be reasonable clearance between the slide bars and the connecting rods. Hope this is of interest.

Regards

Sapper

[nberrington]

Chris is a real gent with stuff like this. I botched my High Level Manning Wardles, and he helped out with replacement rods.

[steve howe]

Having built two of these beauties in tandem, I have to take my hat off to Chris's ingenuity as a designer. Whilst these are not 'fall together' kits, they do go together extremely well and everything is so well thought out. I particularly liked the clever way the gearbox sits inside the firebox and comes apart in two halves held together by the layshaft. My only observation would be the front axle pivot which is shown as a short length of rod soldered to the chassis bulkhead. Its worth making sure this joint is absolutely solid, one of mine came adrift with the result that the loco gradually became nose-heavy and the pivot ceased to pivot. Getting it back level and soldered up was a devil of a job in such close quarters. If I were doing it again I'd use a longer bit of rod and solder it at both ends. That said, both chassis run beautifully (although the AG wheels were another story) and have made for really characterful little locos.

Now...I hear Hornby are producing a Peckett this year...can we expect a suitable undercarriage from the High Level Wizard?

Steve

[David Knight]

[quote=
Now...I hear Hornby are producing a Peckett this year...can we expect a suitable undercarriage from the High Level Wizard?

Steve[/quote]
Apparently yes. There has been a fair amount of chatter on RMweb about the Hornby Peckett and Chris indicated an interest;
http://www.rmweb.co.uk/community/index. ... ?p=2040027 So all we have to do is wait until August when the model comes out.

Cheers,

David