Roxey Return Crank

[Hardwicke]

I find it hugely frustrating that Markits invested a gobsmacking amount into a very sophisticated machine tool, produce a bewildering range of items which could make our lives easier, but for the fact that they are almost 'freelance' designs. "Inspired by" their prototypes I suppose.

As his range grew, I had wondered how he managed to do the prototypical research over such a broad canvas. That's left up to us, after which we may decide what we've bought isn't suitable after all.

I also find it strange that businesses make things up at the design stage, both major players and cottage industry levels. It's unessessary added work and when it isn't right you lose sales. I can understand changes due to scaling or material strength but to just get things wrong...

[Horsetan]

I find it hugely frustrating that Markits invested a gobsmacking amount into a very sophisticated machine tool, produce a bewildering range of items which could make our lives easier, but for the fact that they are almost 'freelance' designs. "Inspired by" their prototypes I suppose.

For a very long time, I haven't bought anything by Markits other than safety valves, smokebox door handles, and those curious WD handrail knobs, because it was plain to see that most of the other parts offered were way overscale. The machining may be clean but it's also thick and heavy. The dimensions of ostensibly 4mm scale parts like Wakefield and Silvertown lubricators were (and probably still are) suitable for "S" scale, and possibly even 7mm scale. They are simply too big for a 4mm scale model.

As his range grew, I had wondered how he managed to do the prototypical research over such a broad canvas.....

The question is was it done at all, other than a superficial glance at a photo or two? It's almost like going back to the days of Colin Massingham, but in machined metal rather than whitemetal.

[Hardwicke]

I'm having another attempt at the return crank. The theory behind the Markits item is great but it is far too long, presumably to accomodate 00. I've seen the wheels of the Patriot at Swanwick Junction and the spigot looks like it sticks out a little over 6 inches. It's the same design as the 4P. So my question is how long should they be? My plan it to leave the front with the niche cosmetic bolts and turn the rear and re-thread it to 14BA so it fits straight into the wheel. I have a Myford but i'm a real beginner on the lathe...

[davebradwell]

I'm sure you've asked this before and recently - go to a model engineering site on valve gear design and that will explain all. As for how much it "sticks out", it has to roughly line up with the tail of the expansion link - sometimes the eccentric rod has a slight joggle. Get a proper drg!

As for your proposed fixing, securing it with 14BA tapped into soft plastic seems a bad idea, the standard crankpin has a screw head at the rear to prevent it pulling out. I can't recall anyone having a good word for this return crank.

DaveB

[Hardwicke]

Dave, I agree but will try my method first and if it fails I can drill and tap it and use the crank pin from the wheel. I wonder if any 00 modellers have problems with these cranks coming loose?

[Hardwicke]

Goodies arrived...

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[Will L]

Just go careful it very easy to snap a 14BA tap

[Hardwicke]

Having treated myself to the Wild Swan book on the Fowler class 4 tanks I see the pin that the return eccentric crank fits on should be 10" 1/8 long, 6" 1/4 diameter for the coupling rods and 6" diameter for the connecting rod. The Markits one is 5 mm long (15 ") and 3mm (9") diameter. I measured the return crank eccentric which is almost 1.5mm (4" 1/2) thick narrowing to 1mm(3") when it should be 1 1/2 inches.
Correct me if I'm wrong.
Work to do but at least I now know.

[davebradwell]

I think folk have been trying to warn you all through the thread.

The usefulness of your prototype numbers is dependent on the outside face of the wheel being in the right place - something you might want to look into when considering the alignment of your gear.

Model Engineer have been running a series on valve gears and current issue (4729) is part 1 of designing Walschaerts gear.

DaveB

[Hardwicke]

I think folk have been trying to warn you all through the thread.

The usefulness of your prototype numbers is dependent on the outside face of the wheel being in the right place - something you might want to look into when considering the alignment of your gear.

Model Engineer have been running a series on valve gears and current issue (4729) is part 1 of designing Walschaerts gear.

DaveB

I know but the learning process is quite fun.