25.4.
Types of Gearboxes
These are two primary groups of gearboxes such as manual change and automatic gearboxes.
Fig. 25.10. Simple reverse gear train.
25.4.1.
Manual Change Gearboxes
In these gearboxes the driver has complete control of the gear changing process and can select a gear ratio appropriate to the driving conditions by means of the manual control lever. Generally, these are four to five gear ratio options, apart from the reverse gear. There are three basic types of gearboxes :
• Sliding-mesh • Constant-mesh • Synchro-mesh
Of these, the synchro-mesh type is most prevalent today. It is essentially a combination of the other two types. Although sliding-mesh type gear box is obsolete at present, it has been considered in the text for initial study.
25.4.2.
Automatic Gearboxes
This gear box system includes several sub-systems so that the gear ratio is automatically altered. The driver merely selects the direction of movement, andzthe gear ratio range. In the US, an automatic gearbox is often called an automatic transmission unit as it is synonymous with transmission.
Most current automatic gearboxes use an epicyclic gear system, in which the required gear is obtained by holding or driving a part, or parts of the gear train by means of a friction clutch or brake. A hydraulic system controls the brakes and clutches. It incorporates its own sensing system, or uses an electronic sensor to monitor engine and vehicle operating conditions. Most automatic gear box systems use a torque converter between the engine and gearbox, in addition to the 3 or 4 speed epicyclic gearbox. The torque converter is generally a third clutch that replaces the conventional friction clutch. The two functions provided by the converter include
(a) automatic disengagement of the engine from the transmission when the engine speed
is less than 1000 rpm ; and
(6) provision of an infinitely variable torque and speed ratio to bridge the steps between the discrete epicyclic gearbox ratios.
In the UK the combination of an automatic gearbox and torque converter is called an automatic transmission system.
