I have an idea to move an arm that has one fixed point up and down with a threaded rod. The movement would be an arc shape. But I feel it would secure the arm when in it’s set position. Threaded rods usually move in a linear motion. Does anyone have any ideas on how a threaded rod can move an arm with 1 fixed side up and down to different angles?
Thanks
Yes, the drive/motor end must pivot as well as the nut end.
awesome! thank you for sharing. I was trying to search for this concept but I don’t probably use the correct jargon
FYI 2881 Lady Cans used a threaded rod to pivot their arm last year
If you do this I want to point out that normal threaded rod is very mechanically inefficient, around 10-15% of your energy will go towards useful work. Acme threaded rods get about 50%.
Thanks for the advice.
Thanks for the link, it helps
It should be noted that in this example the lead screw is large diameter relative to the throw (and presumably weight) but the screw sees compressive and tension forces.
OP,
At odd angles 4 bar mechanisms can develop a LOT of mechanical advantage. Please make sure that the rod is in tension (ideally all of the time, but make super sure it is in tension when the nut is a long ways down the rod). Compression forces and thin cross section rods don’t mix well.
Also be careful with coupling a screw directly to a motor, ideally isolate it with a thrust bearing/washer. Motors and most FRC gearboxes are not meant to be loaded with forces traveling along the axis of the shaft.
It’s absolutely possible to use a screw to rotate an arm about an axis, but it has some downsides. Most screws of all types are solid steel, making them quite heavy for the strength/stiffness required compared to alternatives like belts, chains, or gears. This problem is largely solved if the screws are always in tension, never flexure or compression, but long thin screws can whip if spun too fast. Screws typically have very high friction, this is why they can stay in place without a motor braking them, as the OP seems to want, but it also leads to wasted power, and in some cases high wear. Those screws with low friction (ball screws) will not stay put without control, and are pretty expensive. Screws create linear motion which is great if you want linear motion, but if you are driving a rotational mechanism, your screw will need to be able to tilt along with the rotating arm, and the output speed and torque will vary along the range of motion. Turning the screw 1 revolution will result different angular changes at different arm positions. A screw can’t be used for more than 180 degrees of arm rotation, and the practical limit is usually much smaller. Using belts, chains, or gears for large, high load rotary joints is not as easy to get right as driving a 1/2" hex and mounting your arm to the hexshaft, but it does get you constant speed and torque and a full 360 degrees or rotation. There are also countless good examples of how to drive a rotary joint. (look at turrets as well as arms for how to drive large rotary joints)
Your best bet is to get one of Rev’s Linear Actuators and figure out how to pivot the top and bottom. It really looks like a great package for a lead screw, and hooks into a gearbox quite nicely to drive the whole thing.
Here’s a photo I took at the Palmetto Regional in 2020. It was used to adjust the hood for different shooting angles. Unfortunately, I don’t recall the team.
If those banners in the background are from that team’s pit, 3140.
I blame TBA for the ability to find teams based on awards banners…
Definitely in the background of the pit with the curved lead screw. I know I was standing near the front of the pit.
Probably could correlate with match videos.
I’ll give you a confirmed, first video in the link, bottom left robot at the start of the match, with the hood assembly clearly visible at the end of auto.
The Poros had a pretty simple implementation in 2020
Thanks for the tip. We actually ordered some of these. Are excited to try them out.
That looks like a really cool idea.
looks like they used 2 for their scissor lift??
Woah, never seen a curved lead screw before.
I imagine the threads only actually make contact on the inner side…
This is really cool. I hate that I like it.
