Garden Railway Forum

I wonder if anyone can explain an anomaly in the attached drawing. My mate is painstakingly creating a model of a South African Railways Class 15F locomotive using the original drawings and recreating every part on his 3D printer. (His dad was a station master on the SAR and he spent most of his childhood watching trains).

He's now working on recreating every component of the Walschaerts valve gear but has discovered what seems to be an anomaly on the drawing for the reversing linkages - one of the arms appears to be slightly shorter on one side than the other in plan view but not in end elevation.

He's highlighted the differences in red.


Can anyone explain this? Has the draughtsman made a mistake?

Rik

I don't pretend to have a clue abut any of this but I bet Dazza could read and understand it.

Peter Butler wrote: ↑Wed Feb 24, 2021 10:41 am I don't pretend to have a clue abut any of this but I bet Dazza could read and understand it.

Fingers crossed

Rik

ge_rik wrote: ↑Wed Feb 24, 2021 10:35 am Can anyone explain this? Has the draughtsman made a mistake?

First rule of building models from original builders drawings - DON'T scale from builders drawings. If no dimension is shown, find the drawing that shows the necessary dimension.

It's unlikely that the real loco had different length 'shaft arms', which is what the SAR-SAS appear to call what I know as a lifting arm. If they were a different length, the cut-off would be different between the LH and RH cylinders, which wouldn't be desirable.

The drawing elevation showing the shaft arms at different lengths has no dimension given for length. The draughtsman has possibly drawn the assembly that way to fit it on the paper, as it is an arrangement drawing, not a part drawing.

Somewhere in the many drawings used to build a large modern steam loco like the SAR-SAS 15F will have been one with fully dimensioned drawings for both the LH and RH 'shaft arms'. If that hasn't survived, the drawing in the top RHS of the drawing gives a dimension of 2’ between centres for the 'shaft arms', which is a start at least.

Yes, draughtsman make mistakes and still do, which makes life more interesting for the manufacturing shop. One of the problems is keeping track of changes. The steam era drawings that survive are usually the drawing office original tracings, or archive prints, which are not necessarily the same as the final version drawings used by the workshops to build the real loco.

Regards,
Graeme

Thanks Graeme
That's more or less the conclusion we reached (though a lot less eloquently). It looks like the draughtsman (or woman) shortened the arm to accommodate the drawing below it. We also speculated that he's (or she's) aligned the end of the 'short' arm with the hole centre in the other arm - maybe by accident.

Rik

Rik, having difficulty working out exactly which dimension you are talking about, and though I am an engineer, I am not fully conversant with all locomotive part names. However could the issue be in the end elevation the arm length is at an angle, but when viewed in the plan the length does not show as the full length due to the truncated view. i.e. it is not perpendicular to the horizontal.

Jimmyb wrote: ↑Wed Feb 24, 2021 5:45 pm Rik, having difficulty working out exactly which dimension you are talking about, and though I am an engineer, I am not fully conversant with all locomotive part names. However could the issue be in the end elevation the arm length is at an angle, but when viewed in the plan the length does not show as the full length due to the truncated view. i.e. it is not perpendicular to the horizontal.

Hi Jim
That was my thought initially. But then my mate pointed out that the arm would also look truncated in the end elevation - and also we'd see them at different angles in the side elevation.

RIK

The lifting arms are not horizontal and the plan view shows the 'apparent' length of the rhs arm and the 'true' length on the lhs.

The 'true' length between centers is shown at 'J' and the orthogonally correct view at 'F'.

The drawing is an assembly drawing and as such would not be used for the part manufacture, but it would be used by the the fitters assembling the parts prior to welding before the final machining and erection.

If you really want to check, use trigonometry to solve a 6 x 24 triangle with an included angle of 34° 9', that'll give you apparent difference.

I don't think the draftsman's made a mistake.

Hope this helps.
S

Wobbly Wheel wrote: ↑Wed Feb 24, 2021 8:43 pm The lifting arms are not horizontal and the plan view shows the 'apparent' length of the rhs arm and the 'true' length on the lhs.

The 'true' length between centers is shown at 'J' and the orthogonally correct view at 'F'.

The drawing is an assembly drawing and as such would not be used for the part manufacture, but it would be used by the the fitters assembling the parts prior to welding before the final machining and erection.

If you really want to check, use trigonometry to solve a 6 x 24 triangle with an included angle of 34° 9', that'll give you apparent difference.

I don't think the draftsman's made a mistake.

Hope this helps.
S

Ah, that makes sense. I'm surprised this discrepancy isn't explained on the drawing, though.

Rik

Rik,
Remember that when this was drawn everybody who looked at it would of had many years training and be able to understand the function of the part.
If he'd drawn the 'true' or 'apparent' length at both ends in the plan view the lifting arm may of been assembled (or made) incorrectly.

The draftsman used this technique to draw attention to the difference without having draw an additional view - time is money for a commercial locomotive builder.

I'd suggest he was particularly competent (unlikely to be a 'she') as the method of highlighting the issue obviously works even after 90 years, I'd happily offer him a job.

S

I built this one from the original drawings Gerrik, and never hesitated to deviate from them to favour logic, and of course wherever it made sense to do so for 1:8 scale, 5" gauge.

What a beauty, Andrew.
It must be a powerful hauler!

Rik