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Fastener Torque for T200 Thrusters

For attaching the T200 thrusters to a mount, there are four M3 threaded holes on the cowling of the thruster at 4 points on a 19 mm diameter circle. It appears like they are a kind of nutsert in the polycarbonate cowling.

What torque is recommended for tightening an M3 fastener in these threaded nutserts?

The T200 page also talks about threadlocker: “Most threadlockers are not chemically compatible with with polycarbonate, and will damage the thrusters if used on any of the screws. Refer to the documentation and chemical compatibility notes of your threadlocker of choice for more information.”

Presumably this is mainly referring to the screws that are fastened directly into polycarbonate, or from applying too much threadlocker to the screws that go into the nutserts, such that it overflows onto the polycarbonate. We are thinking of applying a small amount to the screws into the nutserts, such that there would be no overflow onto the polycarbonate.

Without threadlocker, do thrusters tend to vibrate loose, or is this uncommon?

Hi @MechtEngineer,

There isn’t a particular “recommended torque” because it depends primarily on the material you’re mounting the thrusters to. The end of the screws isn’t intended to touch the base of the hole.

As a reference, our BlueROV2 Assembly Guide specifies 12mm screws for installing thrusters to our 3/8" side panels, and 16mm screws for installing to our 1/2" center panels. Those panels are made of HDPE, and we recommend to

Assuming no indenting a 12mm screw through a 3/8" panel would extend ~2.5mm into the thruster insert, and a 16mm screw through a 1/2" panel would extend ~3.3mm into the thruster insert. Accordingly with a “slight indent” the screws likely end up ~3-4mm into the inserts. Measuring with my callipers, it seems that the T200 mounting holes are roughly 6mm, but the threaded inserts seem to be either 4 or 5mm deep.

We can’t stop you from applying threadlocker to your thruster screws, but we definitely recommend against it if you don’t have one that’s polycarbonate compatible. If it comes in contact with the polycarbonate (be that accidentally during installation, or gets carried in by the screw it’s on), it seeps into the polycarbonate and reacts to make it brittle, which makes it significantly more likely for cracks or stress fractures to occur during operation.

This isn’t something I’ve heard of happening. The four screws close together provide some mounting redundancy that would reduce vibration effects, but the chances of vibration issues are likely also decreased by the use of a HDPE frame, since the screws and the thruster mounting surface are butting up against/slightly inset into a material that’s quite vibration tolerant :slight_smile:

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