The problem
Separate motor, gearbox and inverter boxes waste space and need long high-voltage cables and many brackets. Integrated e-axles share housings, coolant and mounts, but the packaging is hard: the differential, bearings, inverter, connector and coolant ports compete for space, and the unit must fit between the wheels with acceptable half-shaft angles. This is a packaging and systems-integration study rather than a single-part design.
Objectives
- Define an envelope and interface list for an example front e-axle from vehicle targets.
- Compare coaxial and parallel-axis layouts of motor, reduction and differential.
- Model a master layout with skeleton sketches, then housings, bearings, inverter and coolant passages.
- Evaluate volume, mass, centre of gravity, half-shaft angles, service access and high-voltage routing.
- Recommend a layout with a scored trade-off and a list of risks.
What you need
Tools and toolboxes
- SOLIDWORKS Assembly with layout sketches — Top-down design with a skeleton part or Layout sketch controlling shaft axes, envelope and mount points.
- Global Variables and Configurations — Coaxial and parallel-axis variants held in one assembly for fair comparison.
- SOLIDWORKS Toolbox — Bearings, seals and fasteners at catalogue sizes.
- Mass Properties and Interference Detection — Mass, centre of gravity, inertia and clash checks for each variant.
- SOLIDWORKS Routing (optional, Premium) — Coolant hose and HV cable routes; 3-D sketches with sweeps work without it.
- SOLIDWORKS Simulation (Static, optional) — Housing stiffness check at mount points under drive torque reaction.
Software
- SOLIDWORKS 2023 or later
- Excel for the trade-off matrix
Extra parts and data
- Benchmark data from published e-axle literature — Volume and mass ranges from papers help set targets without copying a product.
Key concepts
- Top-down design
- A layout sketch or skeleton part defines key axes and envelopes, and components reference it, so a change in centre distance updates every part.
- Coaxial versus parallel-axis e-axle
- In a coaxial unit the motor shaft surrounds a half-shaft and a planetary set is common; parallel-axis units use offset gear stages, which are simpler but wider or taller.
- Half-shaft angle
- The angle of the drive shafts between the e-axle output and wheel hubs at design and full bump. Large angles reduce CV joint life and efficiency.
- Power density
- Peak power divided by unit mass or volume, a common figure for comparing drive units, best computed consistently for your own variants.
Step-by-step guide
- 1
Define envelope and interfaces
For an example small car with a 1450 mm track and 60 kW peak power, define the e-axle envelope between the subframe rails, the wheel centre height, minimum ground clearance and the inner half-shaft joint positions. List every interface: three mounts, two outputs, the HV DC connector, coolant in and out, the low-voltage connector, breather and drain. Record the example overall ratio and the differential type you intend to use.
Checkpoint: An interface table and an envelope body in the vehicle assembly, with all dimensions linked to global variables.
- 2
Build the skeleton
Create a skeleton part containing sketches, reference axes and planes for the motor, intermediate and output shafts, the differential centre, mount points and the envelope. Insert it at the origin of the e-axle assembly and build every component in context from it. Name planes and axes clearly, for example AX_MOTOR and PL_SPLIT, so later variants can swap layouts without breaking references across the assembly.
Checkpoint: Moving the output axis in the skeleton updates the downstream parts without rebuild errors.
- 3
Sketch two layout concepts
Create a parallel-axis concept with two offset gear stages and a conventional bevel differential, and a coaxial concept with a hollow motor shaft, a planetary reduction and one half-shaft passing through the motor. Represent motor, gears, differential and inverter as simple envelope bodies at first, sized from the ratio and torque. Hold the two concepts as configurations of the same assembly so comparisons stay fair.
Checkpoint: Both concepts fit in the envelope with output axes on the wheel centre line, confirmed by Interference Detection.
- 4
Detail the motor and gear housings
For each concept, model the motor housing with a cooling jacket, the gear housing with bearing bosses, the split line and the joint face to the motor. Insert Toolbox bearings and seals on each shaft at catalogue sizes. Add mount bosses on the housing at the skeleton points and size ribs to carry load from the bosses into the mounts, keeping a uniform cast wall thickness where you can.
Checkpoint: Every shaft has supported bearings, the housings have a clear assembly order and no internal clashes remain.
- 5
Integrate the inverter
Place the inverter on top or at the side of the housing with the shortest phase connection to the motor terminals, and model simple busbars between them. Model a coolant path that runs from the inlet through the inverter cold plate to the motor jacket, a common order because power devices usually have lower temperature limits. Reserve space around the HV connector for creepage, clearance and tool access as a placeholder.
Checkpoint: A continuous coolant path in a section view and the phase busbar length recorded for each concept.
- 6
Check half-shaft angles
Add half-shaft sketch lines from the output joints to the wheel hubs and move the wheel centres to full bump and full rebound using configurations or a driven dimension. Measure joint angles at each position for both shafts. Unequal-length shafts in an offset layout can cause torque steer, so record both lengths too, and note whether an intermediate shaft would be needed to equalise them.
Checkpoint: Joint angles at design position and full travel are within an example limit, such as 6 degrees at design height.
- 7
Compare mass and centre of gravity
Assign example materials and use Mass Properties for each concept, overriding mass for bought-in parts such as the inverter and motor active parts with estimates from literature. Record total mass, centre-of-gravity height and position relative to the axle line, and the moment of inertia about the vertical axis. Compute an approximate power density per kilogram and per litre of envelope for each concept.
Checkpoint: A table of mass, CG and power density for both concepts, with assumptions for every overridden mass.
- 8
Check housing stiffness
Run a Static study on the housing with fixtures at the three mounts and the peak drive torque reaction applied through the bearing bosses as bearing loads. Look at stress around the mount bosses and at bearing bore misalignment, which affects gear mesh and noise. Converge the mesh at boss fillets and run the same check on both concepts so the stiffness comparison is like for like.
Checkpoint: Mount factor of safety above 2 and bearing-bore misalignment within an example limit, for both concepts.
- 9
Assess service and assembly
Make exploded views for each concept and write out the assembly sequence. Check that the inverter can be removed without draining the gearbox, that the drain plug can be reached from below the car, that the HV connector can be disconnected first during service and that the unit can be lowered out of the subframe. Score each concept on access using a simple, stated scale.
Checkpoint: An exploded view and a scored service checklist for each concept.
- 10
Score and recommend
Build a weighted trade-off matrix in Excel covering volume, mass, CG height, half-shaft angle, coolant path length, busbar length, part count and service access. Justify each weight in a sentence and run a quick sensitivity check by changing the two largest weights. Recommend one concept and list the main risks, such as NVH, sealing between oil and coolant circuits and thermal coupling between motor and inverter.
Checkpoint: A trade-off matrix with justified weights, a weight sensitivity check and a recommendation that follows from it.
- 11
Document the study
Produce a layout drawing with key dimensions and interfaces, a section through the shafts for each concept and a report covering interfaces, both concepts, mass and CG results, the half-shaft study, the stiffness check, the service assessment and the recommendation. State clearly that losses, NVH, sealing and durability need separate analysis and testing before any layout could be committed. Attach the skeleton part and the trade-off spreadsheet so the study can be repeated.
Checkpoint: A layout drawing and report that another engineer could pick up and continue.
Testing & validation
- Both concepts fit the envelope without interference at all suspension positions.
- Half-shaft angles stay within the stated limits.
- Mass and CG are computed consistently with stated overrides.
- Housing stiffness check, if run, shows converged stresses and acceptable bore misalignment.
Expected results
A top-down SOLIDWORKS model of two e-axle concepts, a comparison of mass, CG, packaging volume, half-shaft angles and service access, and a recommended layout with its risks.
Research angle for your M.Tech thesis
Treat packaging as a multi-objective problem: define metrics such as volumetric power density, CG height, half-shaft angle and busbar length, and study how they trade off across a family of parametric layouts generated from the skeleton. A defensible contribution is a transparent, reproducible layout method with sensitivity of each metric to ratio, motor diameter and inverter position, compared against published e-axle data ranges rather than any single product.
Take it further
- Add a park lock and analyse its engagement in a Motion Study.
- Run Flow Simulation on the coolant path for pressure drop.
- Run a Frequency study on the housing for mount NVH.
- Estimate cost with SOLIDWORKS Costing for both concepts.
Viva and review questions
Why use a skeleton part?
It holds the key axes and envelopes in one place, so changing a centre distance or mount point updates every in-context part consistently and variants stay comparable.
Why route coolant to the inverter first?
Power semiconductors usually have a lower allowable temperature than the motor windings, so giving them the coldest coolant is common, though the order depends on the loss split and limits.
What does the half-shaft angle affect?
CV joint efficiency, heat and wear increase with angle, and unequal shaft lengths can cause torque steer, so both are checked across suspension travel.
Learn it at DIYguru
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