The manual transmission
by François Dovat
(© François Dovat)
Located downstream of a friction clutch, a manual transmission comprises a number of forward ratios (5 or 6 on a modern car and between 10 to 16 on a 40 tons truck) plus a reverse.
These ratios are obtained by gears which constitute systems of rotary levers. A small gear of 10 teeth driving a bigger one of 40 teeth generates a reduction ratio (underdrive) of 4, i.e. the large cog will revolve 4 times slower than the small one. As the power is the product of the torque multiplied by the revs, it results that the torque of the 40 teeth gear will be, (if one neglects the friction losses) 4 times greater than the torque applied to the 10 teeth gear.
To clarify, let the driving wheels convert this torque into a tractive force. This force is proportional to the torque exerted on the driving wheels. Therefore, all other conditions being equal, the tractive force is proportional to the transmission ratio. It can be deduced that for the same engine rpm the speed of the vehicle is inversely proportional to the transmission ratio.
A high tractive force and therefore an important reduction ratio (known as a "low ratio") is necessary to start a heavy vehicle on a grade. At the opposite, the force of traction required being quite small when the vehicle is at cruising speed, a high gearbox ratio – which may be an overdrive – is desirable in order for the engine not to rev too fast at cruise.
Between these two extremes, all kinds of conditions occur and each one of them would require a different ratio. In practice, one is satisfied with a compromise because it is impossible to multiply the number of gears of a transmission ad infinitum, would this be only due to the fact that the numbers of teeth must necessarily be whole.
The automotive gearboxes have constant mesh gears, one of each gear pair being assembled free running on its shaft. In neutral, none of the gear pairs is coupled with both shafts. Collars are driven by their shaft while being able to slide axially on them to engage a gear. The gear shift lever actuates the rods (1) on which the forks (2, 3, 4) are fixed and in which the collars rotate. Each fork is actuated by a different rod; when at neutral the shift lever is transversely moved, it goes from a rod to the other and is thus put in position to command another fork. On the picture, collars of forks 2 and 4 are in neutral position whereas the collar inside fork 3 has been engaged by moving the lever longitudinally. Before engaging a collar on a gear it is necessary to disengage the other, therefore to bring back the shift lever to the neutral position.
All the gears on shaft 5 are an integral part of that shaft, which could, on this figure, be either the input shaft if it were a transmission without a direct drive or the layshaft if it is a gearbox with a direct drive, like the 5-speed Alfa-Romeo of the 1960's and 1970's. (below) .
In this case, the box comprises 3 shafts:
- the input shaft which fits in the clutch disk by its splined front end.
- the layshaft, which is a one piece unit integral with 5 helical gears and a spur gear for reverse.
- the output shaft on which all the collars are assembled.
(Picture Alpha Auto)
The free running gears (in white) rotate at different speed. In order to engage any dog clutch, the revs should be synchronized and it is the role of the synchronizers, kinds of small cone clutches (1 and 2, picture below). If the box is deprived of synchronizers or if they are worn out, it belongs to the driver to synchronize the revs by means of a double clutching.
2. Synchronized dog clutches collar sleeve.
3. Non synchronized dog clutches collar sleeve.
4. Reverse and crawler spur gears.
5. Helical gears of 1st and 2nd.
The helical gears reduce the noise quite well but generate an axial thrust and increase the friction losses. Spur gears are nowadays used only for the reverse and sometimes for a crawler which is fitted to some heavy trucks. These gears are narrower because they are not supposed to be frequently used under full engine torque. For reverse, an idler gear is engaged by sliding it to mesh with a pair of gears which otherwise are not in contact.
The efficiency of a properly sized pair of gears is approximately 98%. A transmission without direct drive such as in front-wheels drive cars has a similar efficiency on all its ratios because it comprises always only one pair of gears under load, whatever the engaged ratio is .
On the other hand, a gearbox with a direct drive (1:1) uses two pairs of gears on its other ratios and thus has a lower efficiency on those, but nearly 100% efficiency in direct drive. The direct drive may be the top ratio, which is advantageous because it is the most used on highways, or either the one before the top, the top gear being then an overdrive. Whether one or the other solution is selected does not change the final transmission ratio if the axle ratio is adjusted consequently. Indeed the final transmission ratio is the product of the gearbox ratios by the axle ratio. The latter is always a reduction because the engine revs much faster than the wheels even at high speed in cruising.
Most heavy trucks transmissions comprise downstream of the main gearbox a planetary gear train which doubles the range and widen the ratio span up to about 16. This Scania transmission has 2 x 5 = 10 ratios. The clutch disc and diaphragm pressure plate are in plain view. The pressure plate revolves permanently with the flywheel whereas the disc is coupled to the gearbox's splined input shaft which enables it to slide slightly when in declutched position (see our file about the clutch).