Load Analysis & Sizing
This page presents the engineering calculations that size the rotation subsystem, focusing specifically on the structural and drive-side load requirements under worst-case punch scenarios.
Load Philosophy & Structural Loading
The load analysis answers a specific engineering question: Can the yaw stage rotate responsively enough for realistic re-angling while remaining structurally safe under punching? This means the yaw stage must be analysed as both a support structure and a drive system.
Load Philosophy
BoxBunny distinguishes between a characteristic training load (Fchar = 1.8 kN) for normal operation and a conservative structural design load (Fdesign = 2.7 kN) for safety. A service factor (SF) is applied to nominal punch forces to account for dynamic impacts, ensuring components are sized with sufficient margin. The bearing, rotating support structure, and frame are checked against Fdesign, while the motor and transmission are selected primarily from Fchar and motion demands.
Structural Loading of the Yaw Stage
The yaw stage bearing and frame experience three main loads that dictate their design:
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Fa (Axial Load): Total vertical load. This is the primarily the weight of all rotating components above the bearing. Assuming an upper mass of ≈ 25 kg:
Fa ≈ mupper × g = 25 kg × 9.81 m/s² ≈ 0.25 kN
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Fr (Radial Load): Forces acting perpendicular to the axis of rotation. This includes the horizontal component of a worst-case strike (Fdesign). This significant radial loading is why a light turntable bearing was rejected.
Fr ≈ Fdesign = 2.7 kN
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Mk (Tilting Moment): Most critical structural load. It arises from the robot's weight at a vertical offset and punching forces applied at the head/torso height.
Mk = Fdesign × d = 2.7 kN × 1.0 m ≈ 2.7 kNm(Where d ≈ 1.0 m is the estimated perpendicular distance from the bearing centre to the upper punch impact zone). This moment drives the bearing-capacity requirement and the need for outboard edge support. By placing the rotation axis low in the stack, it is best positioned to react to this combined loading.
To validate the 010.10.120 non-geared slewing ring bearing against the manufacturer's load charts, the following peak structural requirements must fall within its safe operating envelope:
| Load Type | Required Capacity (Minimum) |
|---|---|
| Axial Load (Fa) | ≥ 0.25 kN |
| Radial Load (Fr) | ≥ 2.7 kN |
| Tilting Moment (Mk) | ≥ 2.7 kNm |
Note: These represent the Ultimate Limit State (ULS) using the 2.7 kN design load. The bearing's dynamic and static load curves must encompass this point (Fa, Mk) while accounting for the Fr radial contribution.
2. Drive-Side Load Analysis
To achieve the target yaw speed of 150°/s, the motor and transmission must supply enough torque to overcome inertial torque (accelerating the rotating mass), friction, and disturbance torque caused by user strikes. The timing-belt must carry the equivalent tangential force needed to rotate the stage and resist these loads. The motor mount, pulley hub, and shaft interfaces must sustain peak reaction loads associated with startup, reversal, and disturbance rejection, which close back through the fixed welded structure.
Worst-Case Strike Scenario
Punches are rarely perfectly centred or perfectly radial. In a worst-case scenario of a straight strike at a 45° angle, the yaw stage must tolerate a combination of radial load, overturning moment, and torsional disturbance.
The motor is not expected to resist against every extreme strike by brute torque alone. Instead, the bearing, rotating plate, outer cam followers, and welded base carry the bulk of the structural disturbance, while the motor and transmission are sized so they can control the stage under realistic operational disturbance and recover cleanly after impact. This is fully consistent with the wider BoxBunny philosophy of separating structural survival from actuator serviceability.
Remaining Analytical Gaps
- Physical measurement of yaw speed and acceleration.
- Validation of belt compliance, tooth-jump margin, and tension stability under reversals.
- Confirmation of cam-follower contact behaviour under real load to avoid overconstraint caused by fabrication tolerances.