UPVC Window Lock Emergency Repair

thingiverse

I was opening my window yesterday when I encountered a small problem. The handle wouldn't return to its locked position as it should have. I removed the mechanism and found it consisted of three parts: identical locking bars and a gearbox driven by the handle. The gearbox is essentially an enclosed double rack and pinion that pushes the locking bars outwards to engage with various keeps on the window frame. Mine had seized, indicating something drastic had occurred inside. This is a common issue, usually resolved by contacting a window company for a new unit. During lockdown (May 2020), I was able to contact them by phone and send pictures of the old unit. However, my gearbox wasn't ordinary, so they couldn't match it. With no means of closing my window for the night, Murphy's law dictated that this particular window sits next to the main communal entrance, is easy to climb through, and has an external lamp nearby. I had to improvise quickly! The units aren't meant to be dismantled; the split case is held together by peened-over flanges where the handle securing bolts attach, along with a small pin on one case half. Drilling the rivet and removing the flanges isn't hard, and the case halves split nicely without any springs or clips waiting to launch into space. Inside, I found two racks separated by an idler pinion transferring motion from the master to the slave. The master is driven by a quadrant pinion directly through a square hole by the handle's matching shaft. However, for some reason, the handle pinion drives only the master rack, with the slave relying on the idler pinion to transfer motion across. I didn't think to take pictures at the time, so you'll have to imagine the layout (assisted by my inaccurate drawing). It's a dumb design: while the racks are built like battleships, the entire load is driven by just one tooth on the handle pinion. Being made of lightweight alloy, it was no surprise that my pinion had lost two teeth, leaving nothing to engage with the master rack. I recovered all broken parts, made measurements, and quickly drafted a close copy in SketchUp 8. I anticipated several revisions but knew I had a good starting point. The printed part worked first time once supports were removed. However, holding the case together after destroying the flanges and rivet was easy: I drilled and tapped the remains of the rivet to accept a short countersunk bolt found in my computer case spares. This made a great job of keeping the two halves together securely, and the window handle bolts would hold it all back in place. I gave it a little oil before reassembly and put it back in the casement; the mechanism worked perfectly first time! It's delicate compared to the original but means I can shut my window securely for the night. I have no intention of this being permanent; the original alloy handle pinion failed due to strain on the teeth. Next time it fails, I might not be so lucky as to have it happen when the window is open. But this will provide security and allow me to open it in an emergency. That's all for today. Sorry if I've been a pain, and I hope my description hasn't left you glassy-eyed. Take care!