No Crisis Management

Warning:  this is one of those techie posts about metal working and stuff.  I can't imagine that everybody out there likes this type, but since you are all too polite to leave any comments, I keep writing them...

Yesterday there was a crisis at the House of Luuurve!  The magnificent three-pane bi-fold door which forms the back of the house completely refused to close.  This would have presented a bit of a security risk, except that Dales was down for a visit and staying there, so he could be on the lookout for marauding ne'er-do-wells bent on mischief until some sort of fix was found, while everybody else went to work as usual.  And it wasn't even raining.  So when Leo called me up and asked me if I wouldn't mind taking a look, it wasn't even a crisis really. 

It turned out that the reason the door wouldn't close was this.  The system relies on nearly all the weight of the moving parts (i.e. everything) being taken by a little truck which runs in rails at the bottom.  The weight is taken by the bottom hinge, and thence goes down through an 8mm bolt which is threaded into the truck.  This thread provides the adjustment needed to make sure the truck at the top of the doors runs inside its track, and all the doors stay clear of the threshold at the bottom.  Alas, the thread in the aluminium body of the bottom truck had stripped, so that the door couldn't be raised enough to adjust it properly.  It must have been sitting on only the last turn or two of thread, and progressively stripped more and more threads under the (fairly huge) weight of it all until the top truck fell out of place and the whole thing became unworkable.

Here's a picture of the bottom truck with its partially stripped thread in the middle at the bottom:

stripped

There are ways to reconstruct a thread, of course, but the fundamental problem would remain - not enough turns engaged to support the weight.  However, the necessity of the bolt being so high up offered the possibility of a better solution - add more threads above.  Fortunately, my M8 nuts collection provided this rather appealing offering:

appealing...

A few minutes work with a hacksaw and grinder resulted in this:

counter-bored and ground to size

i.e. a threaded fitting with threads slightly longer than the thickness of the plate (because the hole is swaged) which drops down the funny-shaped slot in the truck and can't rotate there.  I counter-bored the stripped-thread hole a bit, to make room for the swage. You get full thread engagement when it's adjusted to suit the House of Luuurve's door, and it's made of tough steel, rather than an aluminium alloy that's optimised for casting.

So, stick it all back together again, and it's fully functional again before teatime.  Hoorah!  Another not-crisis averted in the nick of time - or, as we used to say in the 16th century, in pudding time.

However, I was surprised by another aspect of the engineering of the whole thing.  This is the hinge assembly in pieces, showing the bolt (upon which most of the the weight of all the doors sits).  The weight-bearing interface between the hinge and the bolt is that little collar (between the bolt and the allen key), which is retained by a 4mm grub screw!

grubby engineering

The whole weight goes through that collar and its grub screw.  There's no locating step in the bolt or anything, it's just down to the clamping force and the single-sheer strength of the grub screw.  There are two more grub screws which locate into the machined grooves to the right, which clamp the bold to the hinge after adjustment and help a bit - or maybe a lot - but even so, it all seems amazingly optimistic to me.  Anyway, that part seems to work just fine.  So far...

It's just occurred to me that while the hinge fits into a slot in the truck so that it can't rotate, it's not a tight fit (because the hinge needs to be adjusted up and down).  So whenever the door is opened and closed, there is a slight movement between the two parts.  Since the bolt is clamped to the hinge, this movement must take place in the bolt thread.  Maybe that's another reason the thread failed.  And maybe one should lubricate the thread, in anticipation of movement there.  Who'd have thought?  I bet that's not in the installation instructions.  Not that that matters, because the door was installed by a builder, and builders are always far too busy to worry about installation instructions.

Incidentally, our builders didn't do a great job (re)installing our patio doors, either.  I improved the fit quite a lot by adjusting the hinges (they hadn't bothered) but they are still tight to close because the top and bottom rails just aren't far enough apart to provide any clearance.  Oh well.  Maybe one day I'll do something inventive with my big angle grinder!  Fwooorh!