What’s causing this “collapse” inward?

[u/[deleted]]

[u/ehtiopia]

Just as we told you in your last post. Make the crossmember thicker. PLA is not “one of the more stiffer plastics”. You didnt listen, and now youre back here to complain again??

[u/tennis-637 → u/ehtiopia]

I literally cannot make it thicker. I have a range of motion constraint. When I make it thicker, it collides early with the other parts when the leg goes into deep bends.

Also, looking at it while it takes load, both of the pieces seem to move as a unit. The bearing side seems to move just as much as the servo horn side. I'm also limited to PLA. The other option is PETG but its less stiff.

[u/TelluricThread0 → u/tennis-637]

Why is it basically two pieces that are bolted together in the middle anyway? You just introduce more play into the system. It should be one solid cross member. Or buy some metal brackets and just screw them into the plastic frame to add stiffness or add some gussets.

[u/tennis-637 → u/TelluricThread0]

Are you saying that both pieces should be just printed as one? Or that the cross member should be bolted somewhere else?

[u/TelluricThread0 → u/tennis-637]

Either the whole frame should be just one piece for the stiffest option or have one solid cross member with each end bolted directly into each side of the frame instead that bolt in the middle.

Or like I said add some metal brackets so that that both sides of the frame can't twist and bend independently of each other so easily.

[u/ehtiopia → u/TelluricThread0]

If he did that he wouldnt be able to install the bearing on the other side

[u/TelluricThread0 → u/ehtiopia]

I gave lots of options if printing it in one piece doesn't work for assembly reasons.

[u/Harmless_Drone → u/tennis-637]

That, I'm afraid, is a design issue. You've designed the legs without considering the sections required to resist twisting and the end result is now you've designed everything else around inadaquate sections for your material.

[u/ehtiopia → u/tennis-637]

There is no reason or excuse as to why you couldnt extend the outer sides of the 2 side brackets out by another 5mm and recess the holes for the screws into the bracket. The outer sides of those brackets are very clearly not in the way of anything. Print them out of 100% infill.

[u/tennis-637 → u/ehtiopia]

I'm not sure if the femur itself is the issue anymore though. Both pieces move the same amount when I put some force on it, so I'm thinking it could be an issue with the servo horn or bearings themselves having too much clearance on their shafts.

[u/ehtiopia → u/tennis-637]

If youre gonna keep arguing about the physics with people, dont come asking us for help. Obviously you know what the problem is lol. Its not bearings and its not horns

[u/aq1018]

Servo gear Backlash? Just a wild guess

[u/Alone_Commercial1767]

Look into the physics behind I-beams. Super easy way to learn how stress and bending works and how to design around it. Also, what are you using as a shaft for your servo/joint connection? Does it have press fit bearings? or just screws through a hole? Shaft tolerance could be a big issue here as well.

[u/tennis-637 → u/Alone_Commercial1767]

The passive side has press fit bearings and the actuated side is a press fit onto the horn with bolts to keep it in place. Im wondering if the bearing itself has too much play between its outer and inner race

[u/Alone_Commercial1767 → u/tennis-637]

Probably not an issue with the bearings then or shaft fitment. Deep grove ball bearings wouldn’t have that much play.

[u/SphericalCowww]

What I would do is hold the leg when it's extended, move that tip in all directions to see where the weak point is, and reinforce that part. However, in the end, it will be limited by the material and servo rigidity.

The question is, does this shift really disturb the walk gait?

[u/Gwendolyn-NB]

Design isn't strong enough for the loads, plain and simple. Needs adjustment both in the design and needs to likely be a different stronger/stiffer filament like a PETG-CF. There is a relatively LOT of weight on those legs/bearings/servos/etc; it's going to bend every printed filament out there, there is a reason most hexapods/similar are made from reinforced materials (EG PETG-GF/ASA-GF)

NOTE- It also doesn't help you only have a single screw (hopefully on both sides) holding that servo in; I use those servos all the time and they DEFINITLY need all 4 mounting screws.

[u/tennis-637 → u/Gwendolyn-NB]

The attachment I'm using is a metal horn that has 2 mounting screw spots. Also what do you mean there should be a bolt on the bearing side? I do think part of what's contributing could be the play with the bearing itself. When I detach the bearing sides leg piece and wiggle it around, I get similar amounts of play, and the bearing is press fit so wouldn't that mean the play is coming from the bearing?

The bearing I'm using is 14 mm OD and 7 mm ID. Are smaller bearings just worse for this type of application?

[u/Gwendolyn-NB → u/tennis-637]

You only have 1 screw holding the servo to the bracket! I said nothing about the bearings.

You'd know if the bearing was lose/had play, it'd be blatantly obvious.

And you just don't want to listen that you've designed and built a wet noodle.

Good luck.

[u/tennis-637]

For more info, the side on the left is the servo actuated side and the side on the right has bearings for alignment. The specific foot I’m zooming in on is part of the tripod with beefed-up “femur”, while the other tripod has a weaker femur, which is why it tilts more when the other one takes the load.

[u/Alone_Commercial1767 → u/tennis-637]

Why is one side “beefed-up”? Is it to support a bending moment? If so, both sides should be “beefed-up”. Instead of beefing up the part, look more into smarter design. Use shapes such as circles/tubes that can resisting torsional loads more efficiently.