Load Analysis & Sizing
1. Payload & Service Factor
The estimated payload (the mass of everything above the height-adjustment column: torso, arms, upper frame, and attached components) is approximately 15 kg.
The base gravitational load (W) is calculated as:
A service factor (FoS) of 1.5 is applied to account for modelling uncertainty, future attachments, and minor dynamic effects during height adjustment:
In "kg-equivalent" terms, this represents:
This is the design axial load for screw-jack selection. Lateral loads from punching are carried by the telescopic column, not the screw jack.
2. Lateral Loads & Moments
Punches applied to the head or torso generate lateral forces and overturning moments on the upper carriage. The BoxBunny load philosophy distinguishes between two load cases to size the components against these forces:
- Characteristic training load (Fchar = 1.8 kN): Represents a strong but realistic punch for serviceability and moving-component behaviour.
- Structural design load (Fdesign = 2.7 kN): Represents a conservative upper-bound event (incorporating a 1.5 factor) for ultimate structural survival.
The lateral reaction carried by the rear vertical guides is approximated as the full punch force in the worst-case direction:
The resulting overturning moment about the guide rail supports is:
where e is the horizontal offset between the line of action of the punch and the guide rail plane. These loads size the guide rail cross-section, bearings, and bolt shear interfaces.
3. Separation of Load Paths
The load analysis of this subsystem is fundamentally a load-path analysis. The final design is mechanically robust because the screw jack is explicitly isolated from absorbing punching-induced side loads.
travelling nut → HK2T screw jack → lower support structure
outer welded lift tube → welded plate & lower structure → robot base
The telescopic column expressly behaves as the lateral stabiliser and structural guide, diverting bending moments away from the screw jack.
4. Stroke & Speed Requirement
The required vertical stroke is 400 mm. The engineering target for adjustment time is approximately 32 seconds over the full stroke. This yields a required linear speed:
This speed (equivalent to ~0.75 m/min) is practically sufficient for a setup-time operation between user rounds.
5. Screw Jack Selection (HK2T)
The HK2T travelling-nut manual screw jack was selected as the axial lifting device. In a travelling-nut configuration, the screw rotates while the nut moves linearly; the nut is connected directly to the moving inner member of the telescopic column.
- Design Load Capacity: Commercially available screw jacks are typically rated in the kilonewton range. A jack rated for ≥ 1 kN easily exceeds the 0.22 kN (220.7 N) design load by a large margin, providing immense implicit safety against eccentricities and long-term wear.
- Self-Locking: The screw thread lead angle is below the friction angle. The column inherently holds its position even without motor power, making it non-backdrivable under the 15 kg payload.
- Safety: The self-locking property ensures the column does not bounce or back-drive under downward axial impact forces during training.
6. Motor Input Requirement
Based on the screw-jack lead and the required linear speed of 12.5 mm/s, the back-calculated motor speed requirement at the jack input shaft is approximately:
At 800 rpm, the motor torque requirement is modest because the screw-jack's internal worm gear provides significant torque multiplication. The motor-sizing logic therefore focuses on speed capability rather than raw torque.
Requirement Traceability
These calculations verify that the mechanical design meets system-level requirement RM-3 (Height Adjustability): accommodating users of different heights by providing a vertical adjustment range of at least 400 mm within an acceptable 32-second setup time, utilizing a safe, self-locking mechanism.
Remaining Analytical Gaps
- Quantitative measurement of column deflection under lateral punching loads.
- Repeated-cycle wear testing of the Delrin bearing interfaces.
- Motor-control validation against the aspirational 10-second full-stroke user preference.