UPVC Window Lock Emergency Repair

thingiverse

I had a slight problem when I opened my window yesterday. Something felt very wrong with the operation of the handle, and it would no longer return to the locked position. On removing the mechanism, I found it was made up of three parts: a pair of identical locking bars, and a gearbox mechanism driven by the handle. The gearbox is just an enclosed double rack and pinion that pushes the locking bars outwards to engage with the various keeps on the window frame. Mine had siezed, so something drastic had ocurred inside. This is a common fault, and is usually a case of trotting to a window company, grabbing a matching new unit, and then whacking it into the window. Despite Lockdown (at the the time of posting: May 2020), it was possible to contact them by phone, and they asked me to send them pictures of the old unit. Needless to say; my gearbox was out of the ordinary, and they couldn't match it. So it moved towards evening with no means of closing my window for the night. Murphy's law dictated that this particular window sits right next to the building's main communal entrance, is particularly easy to clamber through, and right next to an external building lamp, so requires no additional illumination to spot that it's open. I had to improvise, and quickly! Of course, these units aren't meant to be dismantled; the split case is held together by two peened-over flanges where the handle securing bolts attach, and by a small pin on one case half, also peened-over, so creating an integral rivet. Drilling the rivet and removing the flanges isn't hard, and the case halves split nicely without any springs or clips waiting to launch themselves into low Earth orbit without warning! Inside I found two racks, which I'll call master and slave for the purposes of this description. These are separated by a small idler pinion to transfer the motion of the master to the slave. The master itself is driven by a separate quadrant pinion, which we'll call the handle pinion, driven directly through a square hole by the handle's matching shaft. For some unknown reason the handle pinion drives only the master rack, with the slave rack relying on the idler pinion to transfer the motion across. I didn't think to take any pictures of any of this at the time, so you'll just have to imagine the layout (hopefully assisted by my wildly inaccurate drawing) for yourselves. It's a dumb design: Whilst the racks are built like battleships, the entire load of these and the two locking bars is driven by just one tooth on the handle pinion at any given moment. Being made of a cast lightweight alloy, it was little wonder that my pinion had lost two of its four teeth, leaving nothing to engage with the master rack. I recovered all the broken parts of the handle pinion, made approximate measurements, and quickly draughted a close copy in SketchUp 8. I anticipated several revisions to get things right, but I knew I had a good starting point. I printed it out in PLA+, with a 100% fill (and support on the bed surface). The finished print appeared, in all its PLA glory, in a mere 18 minutes (which illustrates how small this vital pinion actually is). For an absolute wonder, the printed part worked first time, once the supports were removed. But how was I going to hold the case together now that I'd destroyed the flanges and rivet securing the case halves? It turned out to be easy peasy: I was able to drill and tap the remains of the rivet to accept a short, 2mm, countersunk bolt found in my computer case spares. This made a great job of keeping the two halves together by itself, and I quickly realised that the window handle bolts would automatically hold the case halves together securely once I fitted it all back in the window. I gave it a little oil before final reassembly, chucked it back into the casement, and the mechanism worked perfectly first time out! It's delicate compared to the original, and I doubt it will stand repeated opening/closing, but it meant I could finally shut my window securely for the night! I have no intention of this being a permanent repair. The mere fact that the original alloy handle pinion failed in the way it did tells me that there is a lot of strain on the teeth. Next time it fails I might not be so lucky as to have it happen when the window's open. But It WILL provide the security of a locking window, yet allow me to open it in an emergency. Anyway, that's all I have for you today. Sorry if I've been a pane, and I hope my decription hasn't left you all glassy-eyed. Take care, and if I don't see you through the week, I might see you through the window! Happy 3D printing!