MASCOT January-February 2003

How Does it Work? - No 1 - The Gearbox

Tool Chest

The classic answer to "What is the Gearbox for?" is "To keep the oil in", which is fair enough unless you want to know how it works! In this series we will dip into the tool chest to see if there is anything to help us understand some of the fundamentals of the motor car's innards. After all, if we have a problem, we stand a much better chance of working out what has gone wrong, and what to do about it, if we know how the thing should work. So what, basically, is inside this particular oil can?

The following diagrams show the simplest form of sliding mesh gear box with three forward speeds and reverse, the function of which is to change the speed of the transmission shaft relative to the primary/input power shaft from the engine. Diagram 1 shows the gears in Neutral. The sliding Gear can be moved along the Splined Mainshaft by the gear Lever. The Primary shaft is in 'constant mesh' with the Lay Shaft, which has a larger gear so that it turns slower. In Neutral the layshaft turns freely and is not connected to the transmission shaft.

To engage 1st Speed, the sliding gear is moved to the rear until its larger gear meshes with the smaller,1st Speed gear on the layshaft. The connection to the transmission shaft is now through two sets of reduction gears as shown in Diagram 2, so the transmission shaft turns at its lowest speed.

To engage 2nd Speed, the sliding gear is moved forward until its smaller gear meshes with the 2nd Speed gear on the layshaft as in Diagram 3. These gears are normally the same size, so the drive path is now though only one set of reduction gears and the transmission shaft turns faster.

To engage Top Speed, the sliding gear is moved further forward until the dog clutch engages, as in Diagram 4, the drive path now being straight through to the transmission shaft, which is now driven at the same speed as the input shaft from the engine. To engage Reverse, the sliding gear is moved fully rearwards until it meshes with the reverse idler pinion as in Diagram 5, the idler having the effect of making the transmission shaft turn in the opposite direction, ie, the car will go backwards. To make room for the idler, the reverse gear was often smaller than the 1st speed gear, so the overall reverse ratio was lower than 1st speed. This is why some old cars could go up hills backwards that they couldn't climb in bottom gear going forwards!

In later gearboxes, the sliding gear was made up of two separate gears, the front gear for top and second, the rear for first and reverse, each moved by its own gear selector fork. The forks are controlled by a gate and interlock mechanism which prevents two gears being engaged at the same time. Diagram 6 shows an early type of selector and 'H' gate mechanism. A latch prevents inadvertent engagement of reverse gear.

Driving with these sliding gears requires great skill and is often very noisy, with double de-clutching being an absolute must! For Connie's benefit, double de-clutching brings the two gears to the same speed so that they can be easily and quietly slid into engagement. The sequence is:

1. Depress clutch and move gear selector to neutral. (The mainshaft gear is rotating relative to road speed.)

2. Release clutch and adjust, engine revs to bring layshaft gear to a speed appropriate to the road speed and mainshaft gear to be selected. (Higher revs when changing down, iower when changing up.)

3. Engage next gear, simultaneously depressing clutch slightly to allow slippage in case gear speeds were not fully synchronised. (Nobody said it was easy!)

In our next dip into the tool chest we will try to find how they made things easier for the driver - and quieter.

T C