Clearances on lead axel

[the fatadder]

Having just revived a delivery of Gibson wheels I am finally trying to get my Mitchell mogul working

I am having real issues getting the lead axel conected to the con rod with sufficient clearance

So far I have tried removing the bush and using the nut so the raised bit is recessed into the wheel but I still haven't enought clearance

What should I be doing?

Thankfully my next couple of locos have no outside cylinders, but given i bought this one built, id rather like to get it running...

[DougN]

Can you get us a photo of the area? I have opened out the front rod so almost the entire "nut"is fully recessed into the face of the rod. You will need to file 2 slots and form a screw driver to get the nut home though. One thing to be aware of is the outer larger rod needs to be large enough for the nut to rotate in...As I found on my Q6 thread if this hold is off centre or the nut not fully round you end up with a tight point on the quartering. Which was fixed very gently with a bur in a Dremel... heart in mouth at the same time! just a little and double check each time!

[Horsetan]

The alternative is to redrill the crankpin holes on your leading drivers to take the Ultrascale recessed crankpins, which are designed to be "inset".

Other things you can look at include whether the leading bosses of the coupling rod have ended up wider than they should be due to too much solder being used in the assembly. It may be an idea to file down the bosses a little to create more room for the recessed crankpins to be seated further in.

[the fatadder]

Thanks for the responses

Unfortunately BT have somehow managed to kill my home internet (and won’t be fixing it until Friday at the earliest) so until then I am unable to upload a photo of the problem area.

After looking again at the loco last night, I think I have worked out what needs to be done.
Tidying up the outer surfaces of the rod to ensure it is perfectly flat (likewise with the crosshead)

Then I intend to drill a hole fractionally larger than the diameter of the crankpin nut, in the outer face of the rod,
This would be approximately half the thickness of the rod (a little less to be safe), enabling the nut to be fully recessed. Once the crankpin has been filed down to match there should be enough clearance for the lead axle to work.

Unfortunately this is not the end of the problems. I then decided to give it a test with just the 3 coupled wheels connected (with a blob of blutack securing the centre wheels rods in place)
While the motor / gearbox worked perfectly when only connected to the rear axle, it now moves very slowly (on max speed) before suddenly jerking forward then slowly moving again (the motor was at this point getting very warm so I switched it off)

Wheels were set using a GW models jig to ensure the quartering, there are no pickups to foul the wheels (as I haven't made them yet!)

Before Christmas I did a test fit putting the chassis on my Avonside jig and ensuing the con rods matched the axles (I have realised now that I only did this for one set of rods, as it was more a case of playing with my new jig than testing to make sure the Mogul was accurate)

There is a fair bit of slop in the hornblocks (not sure if this is correct.

Certainly this steam modelling lark is a pretty steep learning curve in comparison with the D&E models I am used to. I think that my next chassis is certainly going to be built using the CSB principle, so that the wheels can be assembled off model (and removed for painting easier!)

This evening I think I will go back to working on my Hornby 2884 class, and see how much of an issue I have in the same area on this model...

[Will L]

You clearly have a nice stiff chassis there. All sorts of things will do that. Coupling rod to axle mismatch will generally cause a jam when one of the the rods is trying to pass the axle centre line, but if they are stiff over a significant part of a revolution, then you are probably looking for something else that isn't turning freely. Try doing only the driven axle and the centre axle to start with and check that everything is free to turn as you go. Only add the third axle when you've got the first two working OK.

Not sure I like the sound of slop in the horn block department. Vertical movement is OK of course but horizontal movement is something we normally try and avoid. That said I'm not sure why that would make it run stiff, irregularly I'd believe, but slop is what the RTR makes include in the axles and crank pin fit so they don't need to worry about any minor mismatch between rods and axle centres.

Once you've got it turning nicely and your attention returns to the front crank pin nut. Your ideas about recessing the nut is in the right direction. The full deal is as follows. You reverse the crack pin nut and have the rods running on the turned down section of the nut in place of the crack pin bush. So ream out the crank pin hole in the rod to be a nice fit on the turned down section of the crank pin nut (I find is ever so slightly wider than the crank pin bush). As you suggest you then drill out the front face of the rods to take the wide section of the nut, but make this is an easy fit as you want the wide edge of the nut and the rod to run clear of each other. The drilled out hole wont be accurate enough to use as a bearing surface. Then file down the crank pin bush until its nearly just a washer, so that when the inverted crank pin nut it screwed down on it, they leave just enough space for the coupling road to turn freely. In section it will look something like this.

resesed crankpin.jpg

As Doug said you then need to file a slot in the edge of the crank pin nut so you can get a grip on it when it's recessed into the rod.

Results look like the picture on this post. From which you will see we have been here before so it may be worth your while reading some of the other related posts to see other peoples ideas.

Will

[grovenor-2685]

And here is what you are aiming for.

crab-gear.png

Regards
Keith

[martin goodall]

And here is what you are aiming for.

Or not (if you want your model to be able to negotiate a curve of less than 20 foot radius!)

As Mike Sharman has always pointed out, prototype frames were flexible, and track curvature on main lines was generous (certainly by model railway standards). Prototype clearances in a loco's working parts simply can't be scaled down.

Some sort of fudge is unavoidable if you want a working model that will actually get round the sort of curves we use.

[Horsetan]

...Some sort of fudge is unavoidable.....

What, like using EM profile wheels

[David Catton]

I think you'll find that

QUOTE prototype frames were flexible ENDQUOTE is an urban myth.

The "flexibility" of steam loco frames is, I am inclined to think, one of those statements that ends up having credibility that defies logic.

By the end of steam most loco building enterprises were using pretty sophisticated alignment jigs to set up frames and axleboxes to very fine tolerances. I very much doubt if allowing for "flexibility" in the frames was any part of striving for ever-increasing mileages between visits to works for overhauls.

David C

[wally]

I remember a series of conversations with a member of the gang who put the broad gauge back in at Didcot in the early days of the G W S there.

When the first reproduction loco was delivered it had great difficulty staying on the track amd all sort of reasons were advanced to no avail untill one of the "old hands" pointed out that it had been constructed as an aprentice project to modern standards of tolerance whereas the original builders only worked to an accuracy of about one thirty second of an inch at the max.

Once the components had been "loosened" up a bit there was no further problem, this was not widely reported at the time to spare embarrassment of the loco builders.

The moral of this story - there is more than one definition of flexibility.

[grovenor-2685]

Or not (if you want your model to be able to negotiate a curve of less than 20 foot radius!)

The prototype locos as pictured are expected to go round curves of 4.5 chains radius which equates to about 4 ft not 20.
Furthermore we are discussing here the clearance behind the crosshead for the leading crankpin that needs the latter to be recessed, nothing to do with the ability to go round curves.
As David suggests, the designers went to a lot of trouble to design rigidity into the frames, and the more modern they get the more rigid the design, take a look at the frame design for a 9F or Britt. There are good photos available. That said the plate frame concept with rivetted construction will work loose eventually, or if welded, may well crack.
Other places at the end of steam went for one piece cast frames including the cylinders so everything was really rigid. The resulting locos were still able to take curves.
Regards
Keith