Sunday, 30 November 2014

No Longer a Screw (too) Tight!

Sentinel 7109's new chimney castings, having been machined to have flat bases, need a nice flat boiler top surface to sit on. Sounds simple enough except that lately I'd begun to wonder about the actual flatness of the boiler top plate on which the castings would come to reside. So one day I took along a straight edge and looked a bit more closely.
A Sinking Feeling
It was clear from the straight edge that all was not quite as I'd hoped and that there was probably 1/4" sag in the centre at the point where the superheater was supported.

Even more closely:
Another Sinking Feeling
And the other way:
And Yet Another
As the lowest point seemed to be where the superheater's support was located, I surmised that perhaps the superheater might have been tightened down so much that it was distorting the top plate.

Whether it would be possible for the top plate to be flattened simply by screwing both chimney bases down tightly, I did not want to entertain in case a casting cracked in the process.

I then checked the effect of loosening the superheater support (where the two nuts are). Indeed the sinking could be lessened to 1/8" instead of a 1/4" which I felt would be satisfactory.

It's clear that it will be better to slacken the superheater support and tighten down the chimney castings first to make the top plate flat. Then tighten the superheater with the chimneys in place.

Nice idea but see the picture below:
Space between the old chimney bases
The new castings leave even less space between them than the old castings shown above so it will not be easy to wield a spanner in there.

But where there's a will...

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