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2B: Dead Axle Pivot

Rotary Mechanisms

Rotary mechanisms enable the pivoting of mechanisms through rotational movement. These mechanisms come in many shapes and sizes but there are a couple good practices that should be followed when designing powered pivots.

6036's 2023 Arm and 2910's 2023 Arm

Take a look at Team 2910's 2023 robot reveal video to see their pivoting arm in action.

Pivot Drive Systems

Two types of pivot drive systems exist: Dead Axles and Live Axles. The following table compares the pros and cons of the two types.

Axle Type Description Pros Cons Picture
Dead Axle The axle remains stationary, and the mechanism pivots around it. Stronger, larger axle, no risk of axle twisting. Requires a separate method to transfer power. May run into packaging issues dead axle
Live Axle The axle rotates with the mechanism. Commonly used with hex axles and hubs. Simple setup. Direct power transfer. Risk of twisting the axle. Commonly done with 1/2 hex which may be too weak for the application live axle

Out of these, dead axles (and coaxial designs as a result) are the best choice for pivots, especially ones that need to handle a significant amount of load.

Some key points when designing pivoting mechanisms are:

  1. Keep the center of mass of the pivoting "arm" as close to the pivot as possible.
  2. Ensure the axle is fully supported, using either box tube or nested plates.
  3. Maintain a rigid support structure.
  4. If the structure is wide, power both sides for additional rigidity.

Applications & Examples

Mechanism Type Description Axle Type Friction Management Images
Wrist Short, light rotary mechanisms. These are commonly seen to allow for ground pickup or pitch adjustments. You'll find coaxial, live, and dead axles for wrists. Both bearings and bushings are common.
Large Pivots Include pink arms, pivoting elevators, and large arms. Dead axle is preferred; consider using 35 chain for heavy loads. Bushings are more common, though thin x contact bearings are used occasionally.
High Load Short Pivot Pivots full mechanisms such as a shooter to adjust shot angle. Same as for large pivots. Both bearings and bushings are used.

Other mechanism examples and deep dives for pivots can be found on the pivots page. The mechanism fundamentals page is yet to be made but will also be a helpful resource.