Constant-mesh and Synchromesh Gears
Created | Updated Mar 8, 2002
In the early days of motor transport, changing gear was a delicate operation requiring considerable skill. Straight-cut gears had to be matched in speed before being brought into mesh together. The result was as often as not a horrible grinding noise - crashing the gears, as it was known.
To change up with a crash gearbox you must disengage the clutch, shift to neutral and let the engine run down to a lower speed, re-engage the clutch briefly in neutral to slow the gears down, then select the new, higher gear. To change down you need even more pedal dexterity: you disengage the clutch, shift to neutral, re-engage the clutch, blip the throttle rapidly to spin the gears up, depress the clutch again and select the new, lower gear.
This process, known as double de-clutching, is almost a lost art. Mini owners are often familiar with the technique, as are rally drivers (when done confidently and with skill it allows for very quick downshifts with minimal stress on the gearbox). But for the rest of us, the juggling with clutch, throttle and gearstick (while braking with your third foot!) is blessedly unfamiliar.
The reason we no longer need to trip the light fantastic on our pedals is an invention known as the synchromesh gearbox.
The Constant-Mesh Gearbox
Straight-cut crash boxes were very noisy, wore quickly and tended to shed teeth at awkward moments, so it wasn't long before engineers began to experiment with other methods, most notably helical gears in constant mesh. It is well worth understanding how this process works - it will make you a better driver (assuming you drive a car with manual transmission).
You know what a straight-cut gear looks like: it's the classic cog, as seen on bicycle sprockets, clocks and watches. It's simple to make, but the bearing surface (the area of contact with which gear A actually pushes gear B around) is narrow. This has two effects: it wears quickly (all the force is distributed over a small area) and it is noisy (as it wears the gears fit less well, and each new tooth coming into mesh essentially hits its companion tooth and is braked sharply until it fits into the requisite gap).
Helical gears are cut on the slant, a bit like screw threads. Imagine a long bolt with the head removed; now bend it into a ring. The outer edge of this ring is a basic helical gear. Helical gears have a longer bearing surface, which reduces the wear problem, and - crucially - each tooth comes into mesh while its predecessor is still in contact so there will be two teeth in mesh at any time. Instead of the constant judder of teeth banging into each other we have a silky-smooth progression from one tooth to the next.
A Constant-mesh System
Helical gears are great when they are in mesh, but they are much finer and bringing them together on the fly without damage is not possible. The development of helical gears required the use of a constant mesh system. But how can all the forward gears be in mesh simultaneously without it ending in tears, though? The answer is simple and elegant - easy to draw but fiendishly difficult to describe!
To set the scene: a gearbox has an input shaft which is connected to the clutch. This is how power goes into the gearbox - the engine turns the input shaft when the clutch is engaged. The gearbox performs its magic and power comes out on an output shaft, which can run at a different speed from the input shaft. The reason we do this is so that the engine can stay within its power band; otherwise we would either need an engine which produced enormous torque at 100rpm to move us in slow traffic or one which would rev up to 20,000rpm to move us at speed.
Constant-mesh gearboxes are things of great beauty (if you are an engineering-minded sort). They rely on a strange-looking thing called a lay gear. This is a long gear running on a shaft (the layshaft) parallel to the line of the input shaft. Let us imagine a four-speed system such as that fitted to the Mini.
The laygear will have four gears on it, in ascending sizes from one end to the other. It looks a bit like a child's drawing of a Christmas tree (or a four-armed Cross of Lorraine). The gears are helical, of course. The gears are not evenly spaced. There is one large gear, then a space, then two middle-sized gears fairly close together, then another space, then a fourth, smallest gear. The laygear is a single solid piece of cast steel: it is quite heavy, and the gear edges are sharp - but drop it and you could chip the teeth and cause irreparable damage. The biggest gear goes at the input end, nearest the clutch.
Parallel to the layshaft is the input shaft. There are three gears mounted on it, but unlike the laygear they are independent and run on bearings so they can spin on the shaft. They run in size order opposite to the laygear with the smallest at the input end. Each gear is meshed with a laygear, the smallest obviously matching the biggest end of the laygear. The smallest end of the laygear is meshed with a separate gear on a separate shaft - the output shaft. The axis of output shaft is the same as that of the input shaft - it is effectively an extension of the input shaft, but cut off just inside this final gear. A roller bearing keeps the two shafts in perfect alignment. This helps keep the two shafts in line.
If you rotate the laygear each main gear rotates at a different speed. The output shaft will turn (its gear is not on a separate bearing) but the input shaft need not turn, as all the other gears spin freely on it. Conversely, because top gear is connected directly to the output shaft and is permanently meshed with the laygear, any time the wheels are turning, the laygear will turn.
Now the clever bit. The two large spaces on the laygear match up - obviously - with two large spaces on the input shaft. These lengths of shaft are splined (grooves running along the shaft, as if the shaft itself were a straight-cut gear). On these splines are mounted two hubs, one in each space. These are known as synchro hubs in a synchromesh system.
The synchro hubs can slide along the input shaft. They have a ridge running around them, and have an arrangement to lock them with the gear one side or the other. Imagine a slim, straight cut gear with rounded ends to the teeth, and a matching recess on the mating face of the hub. So, when the synchro hub moves towards the main gear, the teeth slot neatly together and the two parts lock. At this point when the input shaft turns it is locked to one of the main gears, so the laygear will turn and - because it is meshed with the gear attached to the output shaft - the output shaft will turn.
Selecting a Gear
All that remains is to build a mechanism which moves the collars into place to select the relevant gear. The gear lever works a mechanism which moves a pair of selector forks, one fork running on the ridge around each hub.
To select first gear you move the gearstick to the left - which engages the first selector fork - then up, which pushes the first/second hub into mesh with the smallest gear, closest to the input shaft. The input shaft turns, the small gear is being driven, so the laygear turns relatively slowly. The other end of the laygear is small and meshed with the largest main gear, which is attached to the output shaft, so the output shaft is driven slower in turn than the laygear. Voilà - first gear.
Now change to second. Disengage the clutch and move the lever down. The first/second hub moves to the second gear, one of the pair in the middle - the one nearer the input end. This is slightly bigger than the first gear, so when it is coupled to the main shaft the laygear spins rather quicker. The other end of the laygear is still meshed with the gear on the output shaft, so the output shaft runs a bit quicker. Second gear.
Up to third. Disengage the clutch again and move the selector up (to disconnect the first-second hub from second gear), across to engage with the third/fourth selector fork, and up to move the third/fourth hub into position on the third gear. Third is bigger again than second, so the laygear runs nearly as fast as the input shaft is turning. The smallest end of the layshaft is still meshed with the output shaft, so the output shaft turns at a brisk pace only slightly slower than the input shaft. Third gear.
Now fourth. Clutch in and move the lever down, disconnecting third and engaging directly with the fourth gear which is attached to the output shaft. The laygear is still spinning, but drive is going straight through so it is transmitting no power. Power is lost every time you transfer drive from one gear to another, so it makes sense for top gear to bypass the laygear entirely.
Constant-mesh gearboxes are quieter and slicker, but you still have to match the spinning speeds before you could engage the chosen gear. The final improvement, and the one which removes the need for double-de-clutching, is synchromesh.
Synchromesh is a refinement of the part of the constant-mesh gearbox where the trouble happens: matching the little teeth on the inside of the selector hubs with the little teeth on the side of the main drive gears. In a synchromesh gearbox a system is introduced which spins the main gear (and main shaft) up as the hub approaches. In its simplest form there is a tapered section on the hub side of the main gear and a matching, grooved, tapered, sintered (a form of heat treatment) bronze ring called a baulk ring on the gear side of the hub. As the hub approaches the gear the bronze ring rubs against the taper, and friction spins the gear up. Too fast and the selector teeth will crash, too slow and there won't be enough movement between the teeth on the hub and the gear for them to slide into place (they might meet point-to-point).
Synchromesh started to be fitted to cars in the late 1920s and early 1930s, but was by no means universal until the late 1950s. Even then it was not uncommon for first and/or second gears not to be synchromeshed.
And, as anybody who has ever driven a Mini will know, early designs of synchromesh were prone to fail. After about 40,000 miles the bronze rings in the Mini's synchro hubs became smooth, so they no longer worked. Changing up was OK but changing down without embarrassing grating noises required double de-clutching. This was exacerbated by the Mini's engine having the gearbox in the sump. This design, while brilliant from the point of space-efficiency, meant compromise in the use of oil. The oil had to be fluid enough to pump around the engine - but gears need a much thicker (and generally cooler) oil to function effectively. Thus the BMC/Austin/Leyland cars of two decades or more (including the Mini, the Allegro and the revolutionary but flawed Maxi) were brought low by the Achilles Heel of gearbox problems.
The system described above is a four-seed system. Most gearboxes are fundamentally four speed. 'But wait,' you cry, 'every car I've ever driven was a five-speed!' And so it was. Fifth gear is typically an overdrive ratio - the gearbox output shaft goes quicker than the input shaft for relaxed cruising at speed. Overdrives work much the same but have a separate laygear.
So Why Should I Want to Know That?
Simple, dear Researcher. If, as you change gear, you consider what is going on in the gearbox, you'll remember to be polite but firm with your gear stick, not rush the synchro and make it baulk, and not change so slowly that the whole thing grinds to a halt. You will remember that if your engine is labouring up a long hill and you want a really fast change to a lower gear, you need to drop out of gear into neutral, lift the clutch pedal, blip the throttle quickly, down with the clutch and quickly into the lower gear. With practice this will give you faster, slicker changes and prolong the life of your gearbox.