Slacking Dimensions?

Hi,

Just a thought. When me and my grandfather were talking about the due steaming and test running of Tornado, we ended up talking about the subject of locomotive drawings and such. Now he said, with good logic, that when building a steam locomotive, dimensions given on a drawing may not be used as exact, but instead, a little slackness would be given, spacing between con-rods would be increased slightly beyond what was shown in diagrams and such. I can see the reason behind this, as a little slack would mean that the newly built locomotive would not be "too-tight" when running, e.g when con-rods are moving at a high speed, as to avoid fracturing or possibly collision between any of the running gear. However, this is just what we suspect. Can anybody shed any light on this theory, and if correct, how does one go about changing the sizes to their own requirement. I can imagine it would be tricky either increasing too much or too little.

Thanks

Jonathon

 

that may well have been true in the heyday of locomotive building, but i doubt it holds so true today. in what i would call 'my field' (that of ships) dimensions are given to the millimetre, with tolerances in the thousandth of the millimtre. today the designs will be drawn, then built on a computer where the tolerances can be specified, then the components will be separated and redrawn, then where necessary sent to the subcontractor for manufacture or sent to the shop floor. any final adjustments will be made by the fitters, but anything major will be reported to the engineers in the design office as a major intolerance may have reprecussions elsewhere in the design. in some situations this may be roughly 1mm per metre, depending on the component.

this method means that in a batch of a hundred, a thousand or a million components, every component is capable of being used in the application is is needed with a minimum of adjustment. before computer aided design, CAD, such accuracy was not possible so the degree of tolerance was increased and the level of work required by fitters was also greater. this is part of the reason why shop floors had more workers in those days, because more work was required between the construction of components and the assembly of the product, whereas today final assembly involves just slotting the bits together. i would be interested to know if the A1 had all its components built in CAD or relied solely on the original drawings, as the 5AT group have started showing some of their pre-designed components

 

what you speak of is tolerenceing normaly+- 00.010mm more or less everything has a tolerance they can be any size but there are a few different types too

 

The slack for major components would be built into the origional drwaings, so the sizes would be the ones required. Gresley discovered on borrowing a Castle of the GWR the usefullness of locos actually being a little tight, and based on this, he transformed the performance of his locomotives.
However, a lot of the less major parts of the loco (Cladding Sheets, Pipe work, etc) were often made to fit in situ, following an approximate layout rather than exactly to a drawing, as the Severn Valley have found when swapping the boilers on 7802 and 7812.

 

this is just tolerencing i will post examples later on when i get home

 

Once locomotives were built to basically general arrangement drawings and the craftsmen who built them used their years of knowledge to build in such things as clearances. Components would be 'fitted' together and, as someone has already said the same component on two nominally identical locos were not interchangeable. With improvements in design came the idea of tolerances and components had to be built to these, thus ensuring interchangeability. Even then, some components were still 'fitted', notably bearings. Right to the very end, Hudswell Clarke continued to build locos to basic general arrangements and there was much variation between nominally identical locos (steam & diesel). The pipework was subcontracted to a family business and they use to pipe locos as they thought fit, as long as the two ends of the pipes connected the right things! You could never get a replacement exhaust pipe (say) that was ready to install. You got a length of pipe and two end fittings which you had to make up yourself.

 

Thanks for the information guys, just I didn't know if tolerences were incorperated into the drawings or if it was added during the manufacturing stage to the fabricators preference. Unfortunately I don't have any highly detailed steam locomotive drawings ( only basic Elevations, and some with front elevations and plan setups ), so I couldn't get the answer for myself.

It's also interesting to learn that with some companies that smaller items such as pipework were made "to fit" when the main locomotive body had been put together. Can anyone clarify if any of the works for the LNER, SR or LMS did this also? Thinking in terms of the current boiler swap for No. 4472 Flying Scotsman and such

 

Also, when manufacturing parts you need to think of the finnished article...


http://www.gw-svr-a.org.uk/bridgnorth_pattern_shop_2007.html

So many factors, no wonder a hammer is often the most useful tool to get stuff to fit!

 

The restoration of 6023 King Edward II is a good example of this. Some years ago, in order to speed things up, it was decided to borrow the pipework off 6000, and use it as a pattern for the new pipework on 6023. The benefit of course, is that the new pipework can be made up away from the engine, and so you don't have people getting in each others way. As well as being static in a museum, 6000 also had the benefit of having the same arrangement as 6023, whereas the pipework layout on 6024 is slightly different.

Anyhow, the pipework was made up off-site, and all delivered together at the same time. It would just be a straightforward job of putting it on the engine. But, of course, it didn't fit. And a long time was spent modifying it all before it actually did go on.

And if that wasn't enough, further problems were experienced with the new motion. Made from the original drawings, it all went together fine - until it came to setting the valves... So yet more modification was required.

You have to remember that while they may use standard components, these are at the end of the day, hand built machines.

 

engineers donot use hammers as methods of persuasion if it does not fit there is a reason so take it off and investigate you then incounter standard ISO fits and tolerencing which is allsocovering types of fit from interference fits,clearence fits ect its a little bit of a minefield as im finding seeing as ive just finished a unit on them

 

as for tolerences on drawings normally there is a general workshop tolerence for example in our workshop its +- 0.5 unless stated i tend to work to the bone and go bob on its down to whoever did the works drawings and the works staff

 

This picture is of a pattern and subsquent casting, as metal contracts when it cools the pattern will be a % larger than the required finished item, the metal cools and contracts to make a part the correct size. Because of this you can not use an old part as a pattern for a new part. Sadly the skill of a pattern maker is disappearing. I know the Digital readout on my Mill has a & correction function to compensate for contraction rates if you wanted to machine up a pattern...

 

Half an inch! That's some tolerance!!! :-k

 

And the patternmaker uses a special rule calibrated for different metals/alloys.

He would be able to measure the real thing or take measurements from drawings then make his pattern using the approriate rule without having to add whatever %age is needed for contraction.

It goes without saying that you shouldn't use a patternmakers rule for ordinary measuring jobs.

 

since 1975 all mesurements on drawings should be metric this is a law as should all drawings conform to BS8888 so it isnot half an inch it is halp a millimeter

 

Steve, roughshunter is young. The .5 refered to is probably the dreaded metric millimetre.

 

Have heard it said that the GWR scrapped parts when clearances reached the point other companies wer using with brand new parts

 

it is 0.5 mm the metric mesureents are requierd by law to be used in any engineering operation
the gwr will of scrapped parts as they had high tolerencing if i can find my limits of fits and tolerences ill scan it and post it up

 

Was having a discussion with a doiuble glazing surveyor one day, over some work I'd had done.
When I asked if I was being too fussy, he said,
"Well you're an engineer.
Engineers work to the nearest thou (Thousandth of an inch).
Carpenters work to the nearest 1/2 inch
Builders work to the nearest field!"

 

In the case of this hornstay which is steel with a contraction of a little more than 2.5% or 5/16"/ft.