Here's What Goes Into Making A Geared Limited-Slip Differential

Most petrolheads will probably have a vague idea of what a limited-slip differential does, but have you actually wondered what they look like inside, and how they go together?

We have, and with Quaife - one of the most well-known manufacturers of the things - right here on our British Isles, we thought we should pay a visit to its factory, to see lumps of steel get transformed into one tyre-fire killing works of magic.

While Quaife does produce a small amount of plated differentials, its mainstay is its ATB (Automatic Torque Biasing) differential. It’s a geared differential working on a similar principle to a Torsen, and as such isn’t able to fully lock. However, while a plated diff only locks to 50/50 across an axle, an ATB diff can send up 80 per cent of torque to one wheel.

We’ll delve into the differences between a differential like this and a plate-type differentials another time, but as Quaife is constantly making ATB units, it’s possible to rock up to the factory and see the whole production process in one hit. Here’s what we discovered…

Here’s how every Quaife ATB differential starts out: as a chunk of steel billet. This goes onto a CNC lathe, and 10 minutes later, we have something that looks like this:

It’s the right shape, but the inside is smooth and requires shaping so that the gear set fits in nicely. Five housings go into a massive CNC milling machine at a time, which does its work in around 40 minutes.

Each differential has a pair of side or ‘sun’ gears - one for each half shaft. As with the casings, these start off as pieces of steel billet, which are passed through an auto-loading CNC gear cutter.

A conveyor belt feeds the billet into the rear of the machine, with a two-sided robotic arm removing the previously completed gear, before rotating and picking up the fresh billet for the next gear cutting job. It cuts one a side gear a minute, and is rather mesmerising to watch.

The gear isn’t finished at this point though, as it needs to have a spline cut in on the inner circumference. The cog is sent to a massive machine that pulls a long bar through it, cutting in the spline that’s needed for the sprocket to mesh with the end of its corresponding half shaft.

Next up, we have the worm gears. Each ATB unit has 12 helical-cut (as opposed to straight-cut) worm gears, which transfer torque between the two side gears and - as a result - between either half shaft.

These start off as - you guessed it - steel billet. Each piece of billet is put through a hobbing machine to be cut, before being sent off-site for heat treatment.

All finished components are assembled by hand, generally in batches - although we asked to see one differential built up on its own to get a better idea of how it fits together.

After the top part of the casing is bolted down and torqued up (the bolts are the only part of the diff to be manufactured off-site), the assembled unit is sent through a CNC grinder, before having a quality check using a coordinate measuring machine (CMM).

Finally, the ATB diff is finished off with some laser etching, is wrapped up and loaded up with a bunch of other completed units to be shipped away to mainstream manufacturers, distributors, specialist companies and individual customers. Most of the differentials you’ve seen built here are destined for Ford, where they’ll be fitted as part of the Ford Focus RS ‘option pack’.