All this really does is allow the drive to change direction 90 deg. without idlers.
The simplest way to mount these was turned spigot which goes right through the sprocket and is bolted to the back beam with a washer just as a safety precaution (although its all sort of self-aligning unless something is mounted out of skew).
The beams here are only 3mm wall thickness which is adequate for the strength required of the structure but is not a lot of metal into which to tap a thread when there is likely to be forces of 50+ kg in two direction sideways.
Enter the "rivnut".
These are, as the name suggests, a combination rivet and nut.
The ones in the photo here are for 8mm bolts.
An 11mm hole is drilled and the rivnut inserted and then tightened with a special tool
which causes part of the serrated body to expand and the area immediately behind that to collapse outwards as in a "pop-rivet".
The tool is removed and the rivnut can then have a bolt inserted capable of significant loads.
Of course it's easier if you have one of these.
But unless you have a lot of use for it the extra mortgage probably makes it not worth while.
So now the system looks like this. The extra small gear at the top between the two black ones is a temporary arrangement merely to take the load off the motor by jamming the two gears together.
About ready for a trial run.
The first run up was a great relief.
Travelled smoothly with just a little motor noise and not much else.
Whhheeeee !!!
Then I thought I better load test it.
It was quite OK for me to climb all over it (and bounce a little) while it was static.
Not that it was ever intended to support anybody's weight while elevated or moving.
It's intended to lower it right down onto the seat units for actual use.
However it will have a cosmetic valence, mattress, bedding etc. all adding weight.
The whole idea is to NOT have to deconstruct the bed during the day!
So some load testing seemed appropriate while in motion.
About 50 kg of additional weight was added and tried again.
The system worked fine but the motor gave indication of being under quite a high load.
After debating whether to pay somewhere between $400 and $800 for a new higher torque motor it occurred to me that I can just add a second motor of exactly the same type and run it in parallel.
It's like carrying a spare and using it all the time just to make sure it's not really needed - if you can see what I mean ???
The only other issue the loading brought to light was accentuating the slack on the non-working side of the sprockets. If this gets too much it can cause the chain to jump a tooth one one sprocket and all the alignment gets out of whack. Believe me (it did happen at one point) it is a pain to fix !!
So in the spirit of the KISS principle (and throwing out hours of thinking about complex tension adjusters) the simplest possible method I could come up with was an eccentric! Seeing as this all moves quite slowly (about 1 minute for full down to full up) it doesn't have to be super flash.
Works a treat.
Tomorrow off to the wreckers for another $60 motor and it should all be working well.
Now all I have to resist doing is tearing out the guide rails and modifying them to be bolted in (instead of welded as they are now) so they can then be powdercoated instead of painted.
They would look rather nice then.
(And I could fix that rather scrappy grinding out of the upper section where the pulley had to be relocated).
I just have to keep saying to myself -
"If it ain't broke - don't fix it!"
"If it ain't broke - don't fix it!"
"If it ain't broke - don't fix it!" . . .
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