How do gearboxes and differentials work?

By the Vrulum team

Gearboxes and differentials are parts of a vehicle's drivetrain that carry power from the engine to the wheels. The gearbox changes the ratio between engine speed and wheel speed. The differential lets the two wheels on one axle turn at different speeds while still sharing the drive. Both ideas have a long history, and the basic designs are well established.

How they work

A clutch is the link that lets the drive be engaged and released. Wikipedia describes a clutch as "a mechanical device that allows an output shaft to be disconnected from a rotating input shaft", and in a motor vehicle it acts as a mechanical linkage between the engine and the transmission (Wikipedia, Clutch).

The differential solves a separate problem. During cornering, the outer wheels of a vehicle must travel farther than the inner wheels (Wikipedia, Differential). A differential allows both wheels to be driven as they rotate at different speeds. Wikipedia says many differentials in motor vehicles give a gearing reduction by having fewer teeth on the pinion than on the ring gear (same source). Some vehicles, such as go-karts and trams, use axles without a differential and rely on wheel slip when cornering (same source).

What has been documented

Patent record. US patent 1,435,430 was filed on 9 March 1918 and granted on 14 November 1922 to Earl A. Thompson of Portland, Oregon (Google Patents). The record describes an automatic gear-shifting mechanism for a sliding-gear transmission. Its first claim, as summarised in the record, covers moving an operating element to release the clutch, shift the gears and synchronise their speeds, then returning it to re-engage the clutch. The synchronising parts are a planetary gear cluster, and brake mechanisms that hold one of three drums stationary to select a speed. The word synchromesh does not appear in the document.

Museum record. Ingenium holds a catalogue record for a cutaway Hydra-Matic transmission display, dated 1951, with the manufacturer field given as General Motors Corp. Oldsmobile Div. (Ingenium).

What is only reported

These points come from encyclopaedia summaries or secondary accounts. They have not been checked against primary records in this review.

Onesiphore Pecqueur is listed under 1827 as having a differential on the driven rear axle in his patent for a steam wagon. The cited article gives no patent number or patent date (Wikipedia, Differential).
Richard Roberts patented a differential for use in a steam carriage in 1832. The article gives no patent number (same source).
James Starley patented a differential for a two-person side-by-side quadricycle in 1876. The article gives no patent number (same source).
The 1891 Panhard et Levassor car is described as having a front-mounted engine, rear-wheel drive and a crude sliding-gear transmission (Wikipedia, Panhard).
The Hydra-Matic is described as the first mass-produced fully automatic transmission for passenger cars. That statement has no inline citation in the article (Wikipedia, Hydramatic).
Wikipedia says Oldsmobile introduced the Hydra-Matic in 1939 for the 1940 model year, with the first cars so equipped shipped in October 1939. It is described as giving four forward speeds plus reverse, using three planetary gearsets and a two-element fluid coupling (Wikipedia, Hydramatic).
Wikipedia describes synchromesh as automatically matching the speed of the input shaft with that of the gear being selected, using cone-shaped synchroniser rings on each gear (commonly brass) and blocker rings that prevent engagement while the speeds differ (Wikipedia, Synchromesh).

Disputed dates

Sources disagree on two dates. Wikipedia gives 1928 for Cadillac's first production use of synchromesh and 1929 for the first Cadillac car with it, without reconciling them (Wikipedia, Synchromesh). For the Hydra-Matic, the museum record states that it "debuted in 1938", while Wikipedia gives 1939 for the introduction and 1940 for the model year (Ingenium; Wikipedia, Hydramatic).

Limits and open problems

The sources reviewed give no measured efficiency figures for gearboxes or differentials. The sources also give no figures for production cost or for reliability over time. This article does not cover wear, noise or lubrication in detail, because the sources reviewed do not address them. Design details of early automatic transmissions rest on secondary sources. Nothing in the sources reviewed tests how electric drivetrains might change the need for multi-speed gearboxes or mechanical differentials, so this article makes no claim on that point.

What to watch next

Primary patent records for the Pecqueur and Roberts claims would show whether the dates hold. Primary production records would settle whether Cadillac's synchromesh first appeared in 1928 or 1929, and when the Hydra-Matic first went on sale. Measured efficiency data for conventional and electric drivetrains would be the next useful evidence to add.