Single-Speed Reduction Gearbox Design for an EV

The problem

Most EVs use a single fixed reduction between a high-speed motor and the wheels. The ratio sets top speed and launch torque, and the gearbox must be compact, quiet and serviceable. A student project that goes from vehicle requirements to a modelled gearbox with real bearings, oil sealing and housing features covers the full design chain.

Objectives

• Choose an overall ratio from example motor speed, wheel radius, top speed and gradeability targets.
• Split the ratio into two stages and size gears by bending and contact stress hand calculations.
• Model shafts, gears, bearings and a split housing with sealing and breather features.
• Check shafts and housing in static studies and verify centre distances and clearances.
• Produce assembly and part drawings with bearing fits and gear data tables.

What you need

Key concepts

Overall ratio

Motor speed at top vehicle speed divided by wheel speed. It is bounded above by the motor's maximum speed and below by the torque needed to climb the target grade.

Lewis bending and Hertz contact stress

Simplified hand methods for tooth root bending and surface contact stress; full ratings use standards such as ISO 6336 or AGMA, which add many factors.

Hunting tooth ratio

Choosing tooth counts with no common factor so each tooth meets every tooth on the mating gear, spreading wear evenly.

Split housing

A housing divided on a plane through the shaft axes, so shafts with bearings can be dropped in. It needs dowels, a sealant flange and bolts.

Step-by-step guide

1. 1

   Derive the ratio

   For an example small car, take motor maximum speed 12000 rpm, peak torque 150 N·m, wheel radius 0.29 m, target top speed 120 km/h and 25 per cent gradeability at an example 1300 kg. Compute the maximum ratio allowed by top speed and the minimum ratio needed for gradeability, including rolling resistance and an efficiency estimate. Pick a ratio between them, for example near 8.5:1, and show both limits in a short spreadsheet.

   Checkpoint: A chosen ratio that satisfies both the top-speed and gradeability limits, with the margin stated.

2. 2

   Split the stages and choose teeth

   Split the overall ratio into two stages of roughly equal ratio for compactness. Choose pinion tooth counts above the undercut limit, for example 17 or more for 20-degree pressure angle spur gears, and use hunting tooth combinations so wear spreads evenly. Pick a module and face width, then compute centre distances for each stage. Record the actual overall ratio after rounding tooth counts and compare it with the target.

   Checkpoint: Tooth counts, module, face width and centre distances in a table, with overall ratio within 2 per cent of target.

3. 3

   Size gears by hand

   Compute tangential tooth load from peak torque at each stage, then Lewis bending stress and Hertz contact stress using simple factors for dynamic load and load distribution. Compare against allowable values for an example case-carburised steel taken from a textbook table. Increase face width or module if needed, and note that a full rating to ISO 6336 or AGMA needs many more factors and is beyond this project.

   Checkpoint: Both stages show bending and contact factors of safety above about 1.5 under peak torque, with sources for allowable stresses.

4. 4

   Lay out gears with Toolbox

   Insert Toolbox spur or helical gears with the chosen module, tooth counts, face width and bore. Place shafts on the computed centre distances using global variables so a change in module moves everything. Add Gear mates with the correct ratio for each mesh and rotate the input shaft to confirm output direction and speed. Remember the Toolbox tooth form is approximate and use it for packaging and kinematics only.

   Checkpoint: Rotating the input by 360 degrees turns the output by 360 divided by the overall ratio, measured in the assembly.

5. 5

   Model shafts and bearings

   Revolve stepped shafts with bearing shoulders, keyways or splines, and circlip grooves. Insert Toolbox deep-groove or tapered roller bearings and oil seals at catalogue sizes. Compute bearing reactions from gear tangential, radial and, for helical gears, axial forces with a free-body diagram of each shaft. Check basic rating life with the catalogue formula for an example duty cycle and record the result for every bearing.

   Checkpoint: Every bearing has a calculated rating life above an example target, and shafts are fully mated with one rotational freedom each.

6. 6

   Design the housing

   Model a split housing around the gear set with a parting plane through the shaft axes. Keep a uniform wall with Shell or Offset Surface, and add bearing bosses, ribs between bosses, a sealant flange with bolt holes and dowels, a breather, a drain plug and a fill-level plug. Add draft if the housing is to be cast and check it with Draft Analysis against the intended pull direction.

   Checkpoint: Interference Detection finds no clashes between gears and housing, and gear tip clearance to the wall exceeds an example 5 mm.

7. 7

   Check shafts in simulation

   For the intermediate shaft, run a Static study with bearing fixtures at the seats and gear forces applied as remote loads at the gear centres. Add mesh controls at the shoulder fillets and converge peak stress. Compare stress with the hand calculation and check shaft deflection and slope at the gear seat, because excessive deflection misaligns gear teeth and raises both tooth stress and noise.

   Checkpoint: Shaft factor of safety above 2 and gear-seat deflection below an example 0.05 mm.

8. 8

   Check the housing

   Run a Static study on the housing with bearing reaction loads applied as bearing loads on the boss faces and fixtures at the mounting points. Look at stress around the bosses and at the relative movement of bearing bores, which changes the working centre distance and gear alignment. Add or deepen ribs where deflection is high, then rerun and compare both versions in a short table.

   Checkpoint: Change in centre distance under load stays below an example 0.05 mm, and housing stress is well below yield.

9. 9

   Check lubrication and assembly

   Set the oil level so the lower gear dips roughly one to three tooth depths into the oil, and show it with a sketch on a section view. Confirm that every shaft and bearing can be dropped into the lower housing half in a sensible order using an exploded view, that seals sit on ground shaft surfaces and that the breather sits above the oil level in all mounting angles.

   Checkpoint: An exploded view with a clear assembly sequence and a section showing the oil level.

10. 10

    Draw and report

    Prepare an assembly section drawing, shaft drawings with bearing fits and surface finish, and gear drawings with a gear data table listing module, number of teeth, pressure angle, helix angle and accuracy grade. Report the ratio derivation, hand calculations, simulation results and limitations, especially the simplified gear rating. Link every main dimension in the drawings back to the calculation that set it.

    Checkpoint: A complete drawing set with gear data tables and a report linking every size back to a calculation.

Testing & validation

• Measured output rotation matches the designed ratio.
• Gear hand calculations give bending and contact factors of safety above the target.
• Bearing rating lives exceed the example duty-cycle target.
• Shaft and housing deflections stay within the stated alignment limits.

Expected results

A two-stage single-speed EV gearbox model with derived ratio, sized gears, bearings with rating lives, a split housing checked for stiffness, and a drawing set with gear data tables.

Take it further

• Model a true involute tooth with an equation-driven curve and analyse tooth stress.
• Add a parking pawl mechanism and check it in a Motion Study.
• Run a Flow Simulation of oil splash or a thermal estimate of gearbox losses.
• Compare a planetary layout for size and mass.

Viva and review questions

Learn it at DIYguru

These DIYguru courses and programmes teach what this project needs.