Axles, RDUs & Components

We engineer and manufacture a complete line of manual, semi-automatic, and fully automatic precision measurement and testing equipment for all axle-related assemblies and components

An axle or rear drive unit is not one rotor. It is a high-speed pinion, a low-speed ring gear and carrier, two half shafts with constant-velocity joints at each end, and a housing that ties them together — each member running at a different speed and therefore carrying a different permissible unbalance.

Balance Technology Inc. builds balancing, gauging and test equipment for axle and rear drive unit components — half shafts, pinions, ring gear and carrier assemblies, hubs and complete driveline units, including the electric drive units replacing them.

The Pinion Is the Sensitive One

The pinion runs at driveshaft speed, several times faster than anything else in the unit, and it is overhung — supported on bearings to one side of its mass rather than between them. Both facts work against it. High speed shrinks the permissible unbalance, and an overhung geometry means a residual at the gear end produces a larger bearing reaction than the same residual would between bearings.

Overhung rotors also need care in how the permissible unbalance is allocated between correction planes. It is not simply half to each plane when the planes are unequally spaced from the bearings; the allocation follows the geometry.

Half Shafts Carry Their Joints

A half shaft is a driveshaft with constant-velocity joints, and the joints are part of the measurement. A plunging joint at the inboard end and a fixed joint at the outboard end each contribute their own unbalance and their own compliance, and a joint with excessive lash will not give a repeatable reading at all.

Shaft length, diameter and service speed together decide whether the assembly behaves as a rigid rotor. Long, slender, fast shafts may not, and a tolerance derived on a rigid-rotor assumption is not valid for a shaft that bends at operating speed.

Correction is by adding mass, usually a welded weight at a defined angular position and axial plane, because the tube wall is thin and structural. Weld position repeatability becomes balance repeatability directly.

Ring and Pinion Is a Gear Problem Before It Is a Balance Problem

The dominant noise complaint from an axle is gear whine, and gear whine comes from tooth geometry, contact pattern, backlash, bearing preload and housing stiffness — not from unbalance. Balancing a pinion to a fine grade will not quiet a unit with a poor contact pattern.

That is not an argument for ignoring balance. It is an argument for knowing which variable you are chasing, because the two produce different signatures: unbalance appears at rotational order and rises with the square of speed, while gear mesh appears at tooth-count order and is tonal. Order-resolved measurement separates them; an overall vibration number does not.

Electric Drive Units Raised the Bar

An e-axle combines a traction motor, a reduction gearset and the axle in one housing, and it did two things to this problem at once. It raised the input speed well above what a propshaft ever turned, which tightens the permissible unbalance. And it removed the engine noise that used to mask driveline tones, which means an order that was inaudible in a combustion vehicle is a warranty conversation in an electric one.

The practical consequence is that component balance budgets that were comfortable for years are no longer comfortable, and tooling and drive contributions now consume a meaningful share of them.

Setting the Tolerance

Tolerances follow ISO 21940-11 from a balance quality grade, the rotor mass and the service speed, applied per rotating member rather than once for the unit. BTI publishes a free ISO 21940-11 balance tolerance calculator with the method behind it — grade selection, the permissible unbalance formula, plane allocation for overhung rotors, and a worked example.

Fixturing

Half shafts and pinions are normally held in tooling that has to reproduce the service mounting rather than merely grip the part. Tooling stiffer than the vehicle gives readings the vehicle will not reproduce. An index test settles what belongs to the part and what belongs to the fixture: measure as mounted, rotate 180 degrees relative to the tooling, re-clamp and measure again. Half the vector difference is the part; half the vector sum is the tooling.

What Else Gets Measured

Axle and RDU lines commonly combine balancing with dimensional gauging of bearing journals, splines and bores, runout measurement, gear pattern and backlash checks, leak testing of housings, crack detection on forgings, and end-of-line NVH testing with order analysis.

Equipment Configurations

Layouts range from single manual stations through to fully automatic cells with correction, test and data collection, and vary to accommodate any production rate. For platform selection across the full range, see BTI balancing machines.

Related applications include driveshafts, electric vehicle components and flanges and yokes.