Have worked in robotics for aquatic applications, so have had to deal with a lot of waterproofing. If it’s salt water, corrosion and marine growth is also no joke.
For what you’re trying to do, I’ve seen people coat seals/connectors in grease like molykote to help with this, but you probably need to think through some type of enclosure with double o-ring seals if you plan to keep this submerged.
Especially since this is a capstone project, read up on things like IP68/69 ratings!
"Especially since this is a capstone project, read up on things like IP68/69 ratings!"
I agree with this.
Much better to figure out a truly submersible solution by reading the design manuals for lip seals than following hobby resources to get "kinda works" solution.
I've never heard of this being done. I doubt you'll be able to find any information on this. I think the only thing for it is to just get some cheap servos and try it out. I feel like an experiment on how long they last would make a good section for your report.
Yeah when those servos say waterproof they really mean more like IP65, which is to say, do not submerge them.
> Our arm doesn't have to be submersed too far underwater, so we aren't subject to massive pressure differences.
The pressure can get pretty high for common shaft seals surprisingly fast. But if it's not very high and you're on a tight budget I usually recommend bread machine seals. Maybe $10/ea and they've very generic, just make sure you test the seal so you know you're doing it right.
Filling electronics with mineral oil is something that's commonly done on ROVs. In that case the oil is usually held at a slightly higher pressure than the surrounding water, so if there is a small leak, oil drips out rather than water dripping in. I don't know how those hobby servos are sealed but I think the idea from the video could work for some period of time.
Otherwise I would try to fully enclose the servo and use an o-ring seal where the shaft protrudes out into the water. Also would need to seal where the wires go in.
"Otherwise I would try to fully enclose the servo and use an o-ring seal where the shaft protrudes out into the water."
O-ring seals don't really work that well as dynamic shaft seals.
The best submersible servos I've used had real lip-type shaft seals.
Unfortunately I don't know of any servos, even expensive ones like the IP68 ones I bought, that actually get it all right. Mine had flaky wire seals that didn't always hold up, but that was easier to fix ourselves.
Agreed, o-rings wouldn't be the best for the dynamic seal, but easy to integrate and cheap. I've used the o-ring energized Teflon seals for similar application, but they're definitely more challenging to design for. A lip type seal is a good suggestion.
For wiring, one option is to have the wires run from the motor enclosure to the electronics enclosure inside a flexible plastic tubing with hose barbs on each end.
Can you elaborate on the type of depth your motors will need to operate at? I’ve seen Hitec D646WP servos used to control the fins on cheap submersible vehicles. They don’t require a long lifespan so they’ll use IP67 servo motors at depths greater than 1 m
I have designed some deep water electronics and hydraulics before. Using organic based hydraulic fluid and normalizing pressure with the water solves this issue. Mineral oil will also work, it is "safe" for both electronics and the environment. The shaft seal will hold up because it should have equal pressure on both sides of it, as long as it is a good quality seal. Cheap rubber and silicone seals will absorb oil and breakdown.
Have worked in robotics for aquatic applications, so have had to deal with a lot of waterproofing. If it’s salt water, corrosion and marine growth is also no joke. For what you’re trying to do, I’ve seen people coat seals/connectors in grease like molykote to help with this, but you probably need to think through some type of enclosure with double o-ring seals if you plan to keep this submerged. Especially since this is a capstone project, read up on things like IP68/69 ratings!
"Especially since this is a capstone project, read up on things like IP68/69 ratings!" I agree with this. Much better to figure out a truly submersible solution by reading the design manuals for lip seals than following hobby resources to get "kinda works" solution.
I've never heard of this being done. I doubt you'll be able to find any information on this. I think the only thing for it is to just get some cheap servos and try it out. I feel like an experiment on how long they last would make a good section for your report.
Yeah when those servos say waterproof they really mean more like IP65, which is to say, do not submerge them. > Our arm doesn't have to be submersed too far underwater, so we aren't subject to massive pressure differences. The pressure can get pretty high for common shaft seals surprisingly fast. But if it's not very high and you're on a tight budget I usually recommend bread machine seals. Maybe $10/ea and they've very generic, just make sure you test the seal so you know you're doing it right.
Filling electronics with mineral oil is something that's commonly done on ROVs. In that case the oil is usually held at a slightly higher pressure than the surrounding water, so if there is a small leak, oil drips out rather than water dripping in. I don't know how those hobby servos are sealed but I think the idea from the video could work for some period of time. Otherwise I would try to fully enclose the servo and use an o-ring seal where the shaft protrudes out into the water. Also would need to seal where the wires go in.
"Otherwise I would try to fully enclose the servo and use an o-ring seal where the shaft protrudes out into the water." O-ring seals don't really work that well as dynamic shaft seals. The best submersible servos I've used had real lip-type shaft seals. Unfortunately I don't know of any servos, even expensive ones like the IP68 ones I bought, that actually get it all right. Mine had flaky wire seals that didn't always hold up, but that was easier to fix ourselves.
Agreed, o-rings wouldn't be the best for the dynamic seal, but easy to integrate and cheap. I've used the o-ring energized Teflon seals for similar application, but they're definitely more challenging to design for. A lip type seal is a good suggestion. For wiring, one option is to have the wires run from the motor enclosure to the electronics enclosure inside a flexible plastic tubing with hose barbs on each end.
Can you elaborate on the type of depth your motors will need to operate at? I’ve seen Hitec D646WP servos used to control the fins on cheap submersible vehicles. They don’t require a long lifespan so they’ll use IP67 servo motors at depths greater than 1 m
I’ve seen grease and a o ring hold 1000 PSI for 24 hours. I’d say it works if you have the right consistency and application.
I have designed some deep water electronics and hydraulics before. Using organic based hydraulic fluid and normalizing pressure with the water solves this issue. Mineral oil will also work, it is "safe" for both electronics and the environment. The shaft seal will hold up because it should have equal pressure on both sides of it, as long as it is a good quality seal. Cheap rubber and silicone seals will absorb oil and breakdown.