Team 3015 Lead Screw Driven Telescoping Arm

Posting this for someone else to use someday if they are looking for a relatively simple solution to drive a telescoping arm.

Early in the build season we identified that a telescoping arm would potentially be beneficial for the 2023 game challenge. I had some concerns. First, to pick up cones off the floor, or more importantly, the shelf, I was concerned at the tight tolerance required to complete the task. Too high, and you’d go over the cone, too low, and you would push the cone away. This is all being done 50 feet away with the robot between the driver and game object.

If we wanted to go with a Telescoping Arm, it had to have 3 stages, almost no backlash, and as simple as possible. Taylor, one of our most talented mechanical mentors led a group of students through the design and construction of this arm. Their design was the most simple solution to a telescoping arm I had seen in FRC, so I am sharing it here for all to use.

1st Stage-Driving the Arm:
Driving the arm with as little backlash as possible meant as few gears, belts, or chain as possible. The Lead Screw was the unconventional solution they came up with and I was blown away during the season with it’s effectiveness and robustness. We had absolutely zero issues with it all season.

The arm extension is driven with a single Falcon motor, 1:1 to the lead screw. We had the option to adjust the pulleys to go faster or slower but it was 1:1 all season. The Lead Screw is a 4:1 Fast Travel 3/8-8 Steel Alloy from McMaster-Carr, Part# 98940A607. The driven end of the lead screw was turned down and keyed to match a CIM Output which mated perfectly with the belt pulley. The red part is a plastic Stop Block used as a hard stop for arm extension.

The other end of the 1st stage has machined pockets for double bearings and holes for custom precision shafts, tapped on both ends, to hold and retain the bearings that support the second stage.
2nd Stage:
This is the bottom of the 2nd Stage.

The dark grey plate is welded to the tubing of the second stage. This plate holds the nut that rides along the lead screw to allow the 2nd (and 3rd) stages to move. The nut is used to secure the travel nut and for added security we pinned the nut to the plate so that it could not turn. We quickly understood that with any failure in this area it would be easier to replace the entire arm rather than replace components and access to that nut after assembly was limited. The smaller pin was drilled in the threads of both components and then the larger pin was used to pin the outer nut to the plate. The McMaster Part# for the fast travel nut is 95072a125.

The black bearing block was printed out of Onyx and held the bearings that rode on the inside of the 1st stage. These lasted all season with no failures. The small black printed part allows for the belt that runs the 3rd stage to move easily around that tight radius.

This is the other end of the 2nd stage. As you can see we used the same method for bearing placement and retention as the first stage. And again, a 3D printed clip to facilitate the webbing running on a tight radius.

3rd Stage:

This is the bearing end of the 3rd stage. The same bearing block is used. The slots you see are for belt anchoring into the 3rd stage.

Belt Path:
The last piece of magic for this mechanism was the belt path which was how the 3rd stage was extended and retracted in phase with the 2nd stage. The belt, which is nylon webbing, 1" wide, forms a loop that starts and ends on the 1st Stage. The webbing is fed through the small gaps in the 2nd stage pictured above with the 3D printed clips. To anchor to the 3rd stage, we weave the belt through those slots seen above. The belt then continues out of the bottom of the 2nd stage and back to the 1st stage. In the CAD model below you see a blue plate where the belt was twice anchored to the 1st stage. In actuality, we used a Ratchet Strap Mechanism so we could apply (and potentially re-apply) tension when needed. If you’re still confused about the belt path (As I was for most of the season) see the beautiful Paint sketch below.

The stage tube sizes are 4", 3", and 2".

Any other questions, please ask! If you’re interested in the CAD Model please DM me.

Some of Taylor’s Favorite FILM Photos:


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