In the Spotlight Technical

Manual Transmissions … why and how


Many of us just jump into a car, fire it up, select a gear and head off. But have you ever given a thought about what that gear stick actually does, or how that gearbox works? Most of us don’t really care either as long as we don’t get the whining or grinding sounds coming from beneath the floor. But as a 4×4 driver, especially with the manual boxes we need to know a little more than the average Joe driving on the black top, so here are the basics of how and why with a couple of tutorial videos at the bottom of the page.
The first recognised manual transmission system was introduced by Louis-Rene Panhard and Emile Levassor in 1894. The manual gearbox basically includes: clutch, gearbox (transmission), drive shaft or propeller shaft, universal joints and a differential. The basic design developed back then is still used today as the model of every modern transmission system.



If you drive a manual “stick-shift” gear changer, then you may have had several questions that have floated around your head over the years…
1, How does the “H” pattern that I am moving this lever through have any relation to the gears inside the transmission?
2, What is moving inside the transmission when I move the shifter?
3, When I get it wrong and hear that horrible grinding sound, what is actually grinding?
4, What would happen if I were to accidentally shift into reverse while I am     speeding down the road?



Cars need transmissions because of the physics of the internal combustion engine. First, every engine has a redline, or the maximum rpm value above which the engine cannot go without exploding, which would be way too fast for normal driving speeds, so a reduction by gearing is required to deliver power to the road wheels that turn at a much slower speed.
Second, you should know that an engine has narrow rpm range where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower at 5,500 rpm. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You select the gears so the engine can stay below the redline and near the rpm band of its best performance for torque and fuel efficiency.
Ideally, the transmission would be so flexible in its ratios that the engine could always run at its single, best-performance rpm value. That is the idea behind the continual variable transmission or CVT.
The gearbox also provides a selection of gears for the differing environments, such as hill climbing, descents and level ground.
The gear lever operated by the driver, is connected to a series of selector rods in the top or side of the gearbox. The selector rods lie parallel with shafts carrying the gears.
The most popular design is the constant-mesh gearbox. It has three shafts: the input shaft, the lay-shaft and the main-shaft, all which run in bearings within the gearbox casing.
There is also a shaft on which the reverse gear idler pinion rotates.
The engine drives the input shaft, which in turn drives the lay-shaft. The lay-shaft rotates the gears on the main-shaft, but these rotate freely until they are locked by means of the synchromesh device, which is splined to the shaft.
It is the synchromesh device that is actually operated by the driver, through a selector rod with a fork on it that moves the synchromesh to engage the gear.
The baulk ring, is a delaying device in the synchromesh, that is the final refinement in the modern gearbox. It prevents engagement of a gear until the shaft speeds are synchronised.
On some cars an additional gear, called overdrive, is fitted. It is higher than top gear and so gives economic driving at cruising speeds, many modern vehicles are now six speed transmissions, the sixth gear has now lost the name of overdrive.



Synchronised Gears
The synchromesh device is a ring with teeth on the inside that is mounted on a toothed hub that is splined to the shaft.
When the driver selects a gear, matching cone-shaped friction surfaces on the hub and the gear transmit drive, from the turning gear through the hub to the shaft, synchronising the rotation speeds of the two shafts.
With further movement of the gear lever, the ring moves along the hub for a short distance, until its teeth mesh with bevelled dog-teeth on the side of the gear, so that splined hub and gear are now locked together.
Modern designs also include a baulk ring, interposed between the friction surfaces. The baulk ring also has dog-teeth; it is made of softer metal and is a looser fit on the shaft than the hub.
The baulk ring must be located precisely on the side of the hub, by means of lugs or ‘fingers’, before its teeth will line up with those on the ring.
In the time it takes to locate itself, the speeds of the shafts have been synchronised, so that the driver cannot make any teeth clash, (the horrible grinding noise).
The synchromesh is said to be ‘unbeatable’ by human input.
Most modern cars have synchromesh on all forward gears, but on earlier cars it is not provided on first gear.



Crash Gearboxes
Crash gearboxes are pretty much extinct nowadays, only to be found in vintage cars and trucks. The work “crash” means just that, if you got it wrong you crashed the gears due to no synchromesh between the gears, and straight cut gear cogs.
Engaging gear was a skill of driving, to match the transmission speed coming from the wheels to the engine speed and input shaft. This was achieved by double clutching and a blip on the throttle, if you got it wrong it was noisy and embarrassing as you chased the gears down the box.
Any one remember them days? … some of us do, a clip round the ear off dad when you got it wrong when learning to drive.
Gear boxes do need a little TLC occasionally, with things like oil changes, or just checking they have the correct oil to the required level. If neglected they can be very expensive to fix.
Most of the images shown are from lack of oil, incorrect oil or bearing failure, with continual use until destruction from floating pieces of metal within the casing and oil.


Images of internal gearbox damage;

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sync3 sync4









Thanks to Syncro Gearboxes for the internal damage images of what can go wrong within your gearbox.
If you are looking for gearbox advice or parts visit Syncro Gearboxes.
Or call in to see them on stand B33 – 36 at the Donington 4×4 Show on Sunday Feb 14th 2016.
Here are a couple of videos to explain the operational process …
The first video is still one of the best tutorial videos on the web today, enjoy …









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