RC15DriveModule
The drive module consists of 3 integral components:
1) A drivetrain plate/mount
2) Motor
3) Omnidirectional wheel (omni wheel)
Background
The 2008 fleet of robots utilize a bull and pinion gear to transmit power to the omnidirectional wheels. Because of this arrangement
Pros:
Cons:
The 2011 fleet of robots utilize an internal ring gear and a spur gear to transmit power to the omnidirectional wheels. A standoff was also welded onto the drive module
Pros:
Cons:
Requirements
Prototypes
When performing injection molding, it is important to scale your CAD appropriately such that the shrink rate of the plastic is accounted for. Doing this will mean that the volume of the cavity of the mold will be larger than that of the desired component in CAD - however, once the part is molded and cools, it will ideally shrink to the correct size.
Thus, the team milled a test mold for the injection molding machine. This mold had a cavity about 1/8" deep, and was approximately 3" x 2", roughly the surface area of a credit card. This mold was used to mold a part and measure the shrink rate by simply looking at the percentage difference of the plastic part compared to the milled mold. Numbers are below:
Cavity width: 1.935"
Plastic part width: 1.871"
Estimated shrink rate: 3.3%
Plastic Processing Information:
From http://www.tangram.co.uk/TI-Polymer-POM.html:
Injection moulding
Pre-drying is not necessary but can be carried out at 110oC for 2 hours if the material has become moist or if the uniformity of the material needs to be improved.
Injection pressures of 1200 to 1500 bar are used depending on the viscosity of the melt, the flow to wall thickness ratio and the type of sprue. Processing temperature is 180 to 220oC and up to 230oC in the event of deep flow and thin walls. The best processing temperature is around 205oC. Thermal damage may occur above 230oC unless the residence time in the cylinder is kept short.
At a mould temperature of 120oC the mouldings are tougher and have greater rigidity and the general range is 50 to 120oC. The material is partially crystalline thermoplastics and the mechanical properties are determined by the degree of crystallisation which increases with the mould temperature. Mould tempering is important in the production of high surface quality parts with low distortion and care needs to be taken in this area. Mouldings produced at 90oC have less post-shrinkage than mouldings produced at lower temperatures. Mould shrinkage is about 2% but is greatly dependent on the processing conditions. The glass fibre reinforced materials types have lower shrinkage but distortion can occur if the shrinkage is not uniform. Moulded parts with a high accuracy in gauge or parts used at high temperatures can be tempered for 24 hours at 110 to 140oC in order to allow for post-shrinkage.
As with all crystalline polymers the follow-up pressure can be influential on the shrinkage of the part and a longer hold time is preferred to reduce shrinkage.
High thermal stress during processing will result in the formation of formaldehyde.
Rationale
Analysis/Performance
Prototype Notes:
