cad-Integration/SimpleCADAPI-master/examples/20_integrated_bldc_joint_actuator/DESIGN.md

Case 20: Integrated 50 mm BLDC Joint Actuator

Classification

Product-level kinematic assembly. The actuator contains separately manufactured electrical, magnetic, bearing, gear, housing, and output parts. The fixed housing is the ground; the rotor, two carriers, and six planets are rotating components.

Design Intent

• Fit a real inner-rotor brushless motor, a 20:1 reducer, a circular controller PCB, and serviceable wiring terminals into one coaxial 50 mm package.
• Eliminate the separate motor-to-reducer coupler. The rotor shaft and stage-1 sun are one steel solid.
• Preserve a short load path from the output flange through two adjacent output bearings into a separate front bearing cap.
• Keep the stator and both ring gears as replaceable press-fit inserts instead of hiding them inside an impossible one-piece enclosure.
• Reserve rear-facing connector apertures and PCB mounting holes so electronics are not represented by an empty cosmetic volume.

Envelope And Performance

Item	Value
Motor / housing outside diameter	50.0 mm
Structural axial envelope	75.8 mm
Terminal protrusion included	77.3 mm
Motor topology	12-slot / 14-pole inner rotor
Stator active length	16.0 mm
Rotor magnetic length	17.5 mm
Radial air gap	0.30 mm
Stage 1	15/15/45 teeth, 4:1
Stage 2	18/27/72 teeth, 5:1
Total reduction	20:1
Reducer housing minimum cylindrical wall	2.20 mm

The two stages intentionally use different modules. Stage 1 uses module 0.80 to leave enough root section around the 8 mm direct-drive shaft. Stage 2 uses module 0.55 to fit the 72-tooth, 5:1 ring inside the 50 mm housing. Both tooth sets obey the three-planet equal-spacing condition (sun teeth + ring teeth) mod 3 = 0.

Bill Of Materials

Part / subassembly	Manufacturing intent	Material / connection
Main reducer housing	One machined fixed part	6061-T6 aluminum
Motor shell	One machined fixed part	6061-T6, six M3 front screws
Rear bearing spider	Separate machined part	7075-T6, four M2.5 screws
Rear electronics cover	Separate machined part with PCB bosses	6061-T6, four M2.5 screws
Output bearing cap	Separate machined part	7075-T6, six M3 screws
Stator core	12-slot laminated stack	Electrical steel, thermal press fit
Windings	Twelve separately represented coil packs	Copper, varnish/potting retained
Rotor shaft + stage-1 sun	One integrated machined solid	Hardened alloy steel
Rotor magnets	Fourteen bonded inserts	NdFeB
PCB	Circular controller board with real holes/notches	FR-4/copper
Phase terminal	Three-position rear-access terminal	High-temperature polymer
Power/CAN terminal	Four-position rear-access terminal	High-temperature polymer
Fixed ring gears	Two replaceable press-fit inserts	Case-hardened steel
Planet gears	Six gears with standard bearing seats	Case-hardened steel
Stage-1 carrier + stage-2 sun	One integrated interstage part	7075 carrier / modeled as steel-duty part
Output carrier + flange	One integrated output part	7075-T6 aluminum
Motor bearings	8x16x5 and 8x19x6	Standard ball bearings
Interstage bearing	5x10x3	Thin radial ball bearing in fixed divider
Planet bearings	Six 3x6x3 bearings	Standard ball bearings
Output bearings	Two 16x24x5 bearings	Standard ball bearings

Fasteners are represented by matching holes and documented interfaces rather than individual screw solids. This keeps the graph focused while retaining manufacturable attachment geometry.

Interface Table

Constraint / interface	Motion meaning	Real connection	Geometry and clearance
motor_shell_to_reducer_housing	Fixed	Six M3 screws	43.0 mm PCD, 3.2 mm holes
rear_spider_to_motor_shell	Fixed	Four M2.5 screws	40.6 mm PCD, 2.7 mm holes; face-to-face column/spider joint
rear_cover_to_motor_shell	Fixed	Four M2.5 screws	Shared rear columns and holes
stator_to_motor_shell	Fixed	Thermal press fit + potting	Nominal line-to-line CAD fit; tolerance sets interference
electronics_to_rear_cover	Fixed	Four M2 PCB screws	33.0 mm PCD and integrated standoffs
rotor_revolute	Motor input rotation	Front/rear radial bearings	8 mm shaft, 0.30 mm magnetic air gap
stage1_ring_fixed	Fixed ring	Interference fit and axial clamp	0.04 mm diametral modeled interference
stage1_carrier_revolute	First reduction output	5x10x3 radial bearing	Integrated 5 mm stage-2 sun shaft
stage2_ring_fixed	Fixed ring	Interference fit and axial clamp	0.04 mm diametral modeled interference
output_carrier_revolute	Joint output rotation	Paired output bearings	16 mm shaft in two 16x24x5 bearings
Planet revolutes	Planet spin	3x6x3 bearings on 3 mm pins	0.05 mm radial gear-seat clearance
Output cap to housing	Fixed	Six M3 screws	43.0 mm PCD, 3.2 mm holes
Output link	External fixed attachment	Six M3 screws	34.0 mm PCD tapped holes, Ø15.96 locating pilot

Electronics Packaging

The controller PCB is a 44.4 mm circular board behind the rear motor bearing. It has a 10 mm center service bore, four M2 mounting holes, four large edge notches for the rear structural columns, three phase-terminal pin holes, and four power/CAN pin holes. Two rear-cover apertures expose the terminal bodies without removing the controller. The board remains removable after the rear cover and terminal screws are released.

Assembly Order

1. Press the stator stack into the motor shell and pot the twelve winding packs.
2. Install the rear bearing into the four-arm spider and bolt the spider to the shell's rear columns.
3. Insert the rotor/shaft from the reducer side and support it with the front motor bearing in the reducer bulkhead.
4. Bolt the motor shell to the reducer housing with the six-hole front interface.
5. Insert the first fixed ring and planetary stage, the 5x10x3 interstage bearing, then the second fixed ring and planetary stage from the open front.
6. Install the paired output bearings in the removable output cap, then bolt the cap to the reducer housing.
7. Install the PCB and terminals on the rear-cover standoffs, connect phases and sensors, and attach the rear cover.

This sequence avoids the trapped 43 mm ring-gear problem in Case 16: both ring inserts and carriers enter through the open reducer front before the bearing cap is installed.

Strength And Thermal Notes

• The motor shell retains 1.80 mm radial wall around the stator and the reducer shell retains 2.20 mm around the steel ring inserts.
• The 43 mm PCD case holes pass through 18.5 mm-radius end lands, retaining 1.4 mm of continuous aluminum ligament on the bore side of each M3 clearance hole instead of clipping only the thin cylindrical shell.
• The front reducer bulkhead is 8 mm long around the 19 mm motor bearing.
• Two adjacent output bearings form a 10 mm stack with 5 mm center spacing to distribute overturning load.
• The output flange leaves 4.15 mm radial ligament beyond the M3 tapped-hole edges.
• The output face carries a 1.5 mm-high, Ø15.96 locating pilot. Mating links use the pilot for concentric location and six Ø3.3 clearance holes with Ø6 socket- head counterbores; the screws provide clamp load rather than radial location.
• External robot structure clamps the continuous Ø50 reducer sleeve at the case_clamp_axis datum (Z = 20.0) instead of sharing the internal output-cap retention screws.
• The stator yoke contacts the aluminum shell over its full active length for a direct thermal path; controller heat can flow through the rear standoffs and cover.
• Detailed tooth stress, bearing life, winding thermal limits, rotor retention, and fastener preload still require engineering calculation and prototype test.