January / March 2017
A mini-wheg is a small motorised vehicle that features 'whegs' (wheel - legs).
What is a mini-wheg?
I began by sketching out initial ideas and than using paper cut outs to find an arrangement of components that worked.
Required tractive force:
Required Motor Power:
Maximum achievable tractive force:
Maximum achievable tractive force
Acceleration due to gravity
Height of object
Coefficient of friction
Conclusion: Optimal wheg diameter was 65 mm with a motor of 1.79 W (after applying a 1.75 safety factor).
Iteration 1 took up too much space
Motor speed = 8870 RPM at nominal voltage of 3V
Desired velocity = 5 m/s
Ideal gear ratio = 60.38
Actual gear ratio = 58.4
In hindsight, a worm gear would have been much more suitable than this crown gear
Iteration 2 was more compact
A suitable belt was selected using the ContiTech manual.
A tensioner arm kept the longer belt in tension. A gear ratio of 1:1 was used for the pulleys, to minimise the space taken up by the belts.
The steering mechanism involved a motor actuating a rack and pinion attached to a linkage and eventually the whegs.
A servo motor was used to actuate the steering mechanism, providing the required accuracy and range of motion.
Minimum required servo torque:
Ackerman angles were calculated to provide the necessary turning circle radius.
A cup and ball style, simplified universal joint allowing the whegs to steer whilst still rotating about the axle
A torsion spring allows the whegs, usually at 60 degrees out of phase with one another to come into phase when an obstacle is encountered, doubling the available torque to overcome it.
A flexible polymer barrier would complete the casing around the whegs at the attachment points shown in red.
Iterative redesign of the whegs utilising FEA to identify failure load and stress raisers.
The stress experienced by the shaft was calculated to select appropriate material and diameter
Free body force diagrams
Torsional shear stress: