How it Works: Synchromesh

Introduction

Early cars demanded much more skill from the driver than do modern machines. One of the major skills that the driver had to acquire was that of changing gear silently and smoothly. Originally, gear changes were affected by sliding one gear out of mesh with another and then engaging another pair of gears. The small clearance between the sets of teeth demanded that the speeds of the gears that were to be engaged had to be accurately synchronized if noise and damage were to be avoided.

This difficulty meant that the driver had to master the technique of "double declutching", a skill which is still of value to drivers of cars that have no synchromesh between first and second gears. The point of double declutching is to synchronize the speed of the input shaft and, therefore, on layshaft gearboxes, the layshaft with the speed of the output or mainshaft, so that the gears can be cleanly and quietly engaged.

The technique is to briefly re-engage the clutch when the gear lever has been moved into the neutral position during a gear change. The input shaft rotates faster in the lower gears, so to make an upward gear change, re-engaging the clutch slows down the rotational speed of the input shaft until it is closer to the speed of the output shaft. Alternatively, to make a downward gear change the engine speed can be increased when the clutch is temporarily re-engaged and this will have the effect of raising the speed at which the input shaft is turning until it is synchronized with the speed of the output shaft. It is the timing and the judgement of these gear changes that makes accurate double declutching difficult to master. Following the introduction of dog clutches, the art of double declutching, however, became less important.

By the late 1920's, moreover, the car had ceased to be a machine for the enthusiast alone and was becoming primarily a utilitarian device, with a consequently much wider usage. The new class of car owners were less interested in mastering the finer points of operating their vehicles but the grating of badly synchronized gear changes was still an undesirable occurrence. This created the demand for a significant technological advance that would eliminate the need for double declutching and careful control of the accelerator during gear changes. This advance was called synchromesh and was first developed, in the USA, by General Motors.

The Principles of Synchromesh

The principles of synchromesh are relatively simple. The actual mechanical details of the various systems, however, are a good deal more complex but, basically, synchromesh is really an improvement on the dog clutch. Dog clutches were the next stage in the development of the gearbox, once the problems of sliding pinion engagement had become apparent. Instead of sliding the gears in and out of mesh, the dog clutches locked each gear to the mainshaft when the gear lever was moved in the appropriate direction. Although an improvement, dog clutches also suffered from the need for accurate synchronization and synchromesh systems were designed to automatically adjust the speed of the dogs to permit easy engagement.

In essence, engagement of the dogs is preceded by contact between two friction surfaces, normally an internal and an external cone. This contact, made by the initial movement of the gear lever, tends to synchronize the speeds of the two friction cones, thus allowing easy engagement of the dogs when the gear lever is moved fully home. The adjustment in rotational speed, due to the frictional contact, always occurs on the input shaft because the output shaft, being connected to the driving wheels, revolves at a rate determined by the road speed of the car. The speed of the cone connected to the output shaft is therefore unaffected by the contact. The speed of the cone on the input shaft is consequently raised or lowered to match that of the output shaft.

Once the cones have engaged and are synchronized, the sliding member of the dog clutch can continue its travel and engage the dog teeth on the gear pinion. Originally, synchromesh was fitted only to the two most frequently used ratios in the gearbox, fourth and third gears on four-speed gearboxes, or third and second on the three-speed units that were popular at the time of introduction of synchromesh. In later years, synchromesh was added to the other ratios in the gearbox but first gear was often unsynchronized on a number of cars made as late as the mid-1960's.

The Crash Gearbox

A basic gearbox consists of a number of gearwheels, which transmit power from one shaft to another, and by selection of the correct size of gearwheel, the required shaft speed can be obtained. In changing gear, the gearwheels are moved so that combinations of different-sized gearwheels come into mesh. At one time, cars were fitted with the now-obsolete crash gearbox. In this, the gearwheels simply slid along splines into mesh. The disadvantage of this type of gearbox was that, for a silent gear-change, the gears had to be turning at the same speeds; thus, the driver, having disengaged one gear, had to release the clutch pedal and flip the accelerator to rev the engine to the speed at which it would have been running if the required gears had been in mesh.

The driver had to declutch again and the gears were left rotating at the correct speed under their own impetus. They then slid into mesh quietly and easily. This method of gear-changing is known as 'double declutching'. If the driver failed to double declutch correctly, the gears grated, excessive wear occurred and the gears may even have been damaged. Double declutching requires considerable skill and practice before it can be performed easily and effectively; although it is employed by some who regard driving and its techniques as an art, it is not now widely practised. To overcome this disadvantage of the crash box, a system of automatic synchronisation of the gear speeds has been developed. The system, known as synchromesh, originated in America, where it appeared on Cadillac and La Salle cars in 1928.

Cone Synchromesh

The simplest form of synchromesh is now rarely used but it illustrates the principles on which many of the more sophisticated systems operate. The side of the gear to be engaged has two features. First, it has a hollow cone and second, the cone is surrounded by a ring of dog teeth. The cone and the teeth are the components that the synchromesh mechanism contacts when a gear change is made. The synchromesh mechanism itself comprises two parts. It has a central hub that is located in narrow splines cut into the mainshaft of the gearbox which allows the hub to both turn with and slide along the mainshaft. The second part of the assembly is a ring, with teeth that match those on the side of the gear wheel. The ring is, in turn, splined to the hub but is restrained from sliding along it by a series of spring-loaded balls that locate in recesses set into the ring.

The mechanism is quite straight-forward in operation. Most gear changes are made in what appears to be one smooth movement by the driver but they do, in fact, comprise two stages, moving the gear lever into neutral and then into the desired position in the gear lever "gate". The gear lever is connected to the collar of the synchromesh assembly, so when the lever is moved into neutral the whole assembly, comprising the hub and the collar, moves up the mainshaft. This brings the two cones, one on the gear wheel and one on the synchromesh hub, into contact and the cones turn together. The friction between the cones forces both the gear wheel and the synchromesh assembly to turn at the same speed.

When the driver moves the gear lever into the second stage he consequently puts greater pressure on the synchromesh assembly. The hub cannot travel further as it is already pressed hard against the gear wheel so the extra pressure forces the collar to slide along its splines, compressing the spring-loaded balls. This movement of the collar brings its teeth and those on the side of the gear wheel into contact and they therefore engage. The gear wheel is then locked to the mainshaft through the collar and the splined hub and the gear changing process is completed. This type of synchromesh works well in theory but has proved to be less satisfactory in practice. It depends on the two cones achieving synchronization before the gear lever is moved into its second position but it does not guard against the effects of an impatient driver's gear change.

If the gear lever is moved early, the driver will "beat the synchromesh" and will force the dogs into contact before the speed of the components has been synchronized. If this happens all of the old faults will arise, such as the grating noises, inefficiency and heavy wear on the teeth of the dog clutches. The solution is to incorporate a component that will prevent the dogs from engaging until the cones are running at the same speed. There are, in fact, several types of synchromesh that feature such a device.

The Constant Load Synchroniser

Vauxhall introduced it to the British motoring public in 1931. These cars usually had only three gears and the synchromesh was fitted only between second and top gear. Four-speed gear boxes had synchromesh on second, third and top gears. These days all cars have synchromesh on all forward gears, regardless of the number. Several types of synchromesh are in common use. The constant load synchroniser is one of the oldest types, and operates by means of two conical surfaces, whose engagement is caused by the gear-change; the friction of the surfaces brings the rotating parts to a similar speed. Another member is also moved but delayed by a spring loading arrangement. This component is toothed, the teeth positively engaging the two rotating parts.

Baulk-Ring Synchromesh

Another type of synchromesh is the baulk-ring type. This employs a baulk ring that prevents the gears' engaging before they are rotating at the correct speeds. A particularly interesting baulk-ring type was the Porsche system. It was based on a minimum number of components and, additionally, lightened the gear-change movement by a servo action. The basis of the Porsche synchromesh system was a circular clutch unit with internal teeth. The teeth were chamfered so that, when the unit was shifted by the forked gear-change lever, they contacted a chamfered split ring, which formed part of the gear to be engaged. Friction of the clutch unit against the chamfered split ring caused the gear assembly to rotate at the same speed as the clutch unit. Further movement of the gear lever brought the teeth of the clutch unit into engagement with teeth on the end of the gear assembly, thereby achieving the final positive engagement.

The synchronising components of this system are extremely compact: by the mid 1970s full synchromesh gearboxes could be built no larger in size or weight than the old-fashioned crash boxes. During the adoption of synchromesh most manufacturers had their own variations to suit their particular requirements. They all relied for their operation, however, on the effects of friction to rotate the gearbox components at the correct speed for engagement of the chosen gears. One variant of the original idea used lozenge-shaped blocks, which baulked any gear-meshing until the correct speed had been obtained.

Another method used spring split rings, which sprang outwards and contacted an external ring on the gear assembly. They would bear on this and rotate it until the rotational speeds were identical, at which time the outward pressure would cease and the spring would fall back, so allowing the gears to engage. A means of changing gears without either double declutching or the use of a synchromesh system was available with the freewheel device fitted to some pre-1960 cars. This freewheel system allowed the gearbox and engine to return to idling speed when the car was not being 'driven'. Thus, gears could be disengaged and engaged without the use of either the clutch or synchromesh. By just removing their foot from the accelerator, the driver could achieve completely smooth and noiseless gear changes.

Baulk Pin Synchromesh

The baulk pin synchromesh system has a layout that is similar in some respects to the simple cone type, in that the gear wheel is equipped with a hollow cone and a set of dog teeth but the relative positions of the two are reversed. The teeth are cut inside the hollow section of the cone and the construction of the synchromesh mechanism is similarly reversed. There is again a hub and a thick ring but the hub carries the teeth of the dog gear and the end of the ring is shaped to form the cone. While the hub and the ring are linked, by spring-loaded balls, there are no splines in this type of synchromesh.

The baulk pins themselves take the form of narrow fingers running out from the hub and passing through angled slots cut into the ring. The ends of the pins are grooved to accept the gear selector mechanism. When the gear lever is moved the pins push the hub forward. The pressure exerted by the spring-loaded balls is sufficient to carry the sleeve forward with it. This movement brings the cone on the end of the ring into contact with the cone on the side of the gear wheel. The friction caused by this movement synchronizes the speed of the cones. The problems encountered with cone synchromesh occurred at this point. An impatient driver, by hurrying a gear change could push the dog-toothed component, in this case the hub, into mesh before the cones had reached a synchronized speed. On the baulk pin design, however, it is at this stage that the baulk pins play their part.

When the cones are first engaged, the pins running through to the hub are forced, by the torque of the gear wheel's cone into the angle of the slot. Therefore, if the driver pushes hard on the gear lever he will only succeed in jamming the pins further into the angle and this, as in the baulk ring design described above, will prevent them from sliding the hub into engagement. The extra pressure will, in fact, be transferred to the ring, so the cones will be forced harder together and ease synchronization. When the cones reach the same speed, however, the torque on the cone's ring will disappear and the pins will be free to slide right down the slots. This movement will push the hub further along the mainshaft and will engage the hub's teeth with those on the side of the gear.

The Warren Synchroniser

The Warren synchroniser, a device that had all but disappeared by the 1980s, was fitted between the gearbox and the propeller shaft and was operated by the clutch pedal. When the driver disconnected the gearbox from the engine, the Warren synchroniser disconnected the gearbox from the propeller shaft and the driving wheels. This allowed the gears to stop rotating completely and the gear change could be made quickly and easily. When the driver accelerated, the synchroniser resumed transmission of power to the propeller shaft and the driving wheels. A useful advantage of the Warren synchroniser was that it could be used as a freewheel device. The driver, by depressing and releasing the clutch, disconnected the drive until they accelerated and the drive connected again automatically.

After a long working life, the synchromesh components of a gearbox will become worn. This will lead to a tendency for the driver to over-ride the synchromesh and to engage the gears before they are rotating at the correct speed. This is a noisy operation and it causes excessive wear on the gear teeth. If synchromesh components do become ineffective for this reason, the only means of rectifying the situation is to replace the worn parts. If the gear changes are not rushed, however, the system may well operate satisfactorily. If it does deteriorate to the point where it becomes virtually inoperative, the driver can always resort to double declutching, rather than invest in an expensive repair to what is probably an already ageing car.

One possible danger is the addition of lubricating additives to the gearbox oil. These, while possibly reducing the effects of friction and wear on the gears and bearings, may reduce also the frictional effects on which the synchromesh system depends for its operation. If a gearbox has become noisy as a result of old age, it may be more prudent to consider a different grade of oil - or if ordinary engine oil is specified, to use a hypoid oil-than to run the risk of reducing the effectiveness of an already worn unit. If you are buying an older classic and the gearbox is quiet, but difficult to change, this may well be a sign that the seller has effected a cover-up of the true state of the gearbox.

General Motors' Synchromesh

The General Motors' design was one of the earliest synchromesh mechanisms to incorporate a baulking device and it has been widely used on Vauxhall cars and those produced by other General Motors' subsidiaries, though it has now been largely superseded by the baulk ring design. The design is best understood by reference to the engagement of direct-drive top gear in an orthodox layshaft gearbox. The pinion on the input shaft again has a ringed extension that passes over the mainshaft. This extension has a conical outer surface and teeth are cut on its inner circumference. The other cone ring is carried on the mainshaft by three inwardly-projecting fingers. These fingers are narrower than the mainshaft splines with which they engage, so, although the cone ring has to revolve with the shaft, it does have some freedom of movement.

The second cone is therefore in light but permanent contact with the cone on the gear pinion and it is carried on the projecting fingers. As the shaft and cone run together, the fingers of the cone are brought hard up against the edge of the mainshaft splines. Behind the fingers and splined to the mainshaft is the sliding member of the synchromesh assembly. Integral with the sliding member, at the end nearest the pinion, is a ring of teeth which match those which are inside the pinion's cone. Three open-ended slots are cut into the sliding member. They, too, are narrower than the mainshaft splines but are a little wider than the fingers to allow them to enter the slots when necessary. The entry corners of the slots are bevelled, as are the corners of the fingers.

Completing the assembly is a series of tongue-shaped springs which, when they are installed between the cone ring and the sliding member, locate the former correctly when the synchromesh is in the disengaged position. In this situation the two sets of dog teeth are, of course, clear of each other but the two cones are, as already mentioned, in light rubbing contact. The gear lever then moves the sliding member along the mainshaft towards the constant-mesh pinion on the input shaft. The initial movement of the sliding member puts more pressure on the cones, as the corners of the slots bear against the cone's fingers and force it into firm contact with the pinion's cone. In this condition, the greater the force on the gear lever, the higher the pressure on the cones, and this ensures speedier synchronization. This constitutes the baulking device. As soon as the cones reach the same speed the drag between them ceases. Consequently, the fingers are no longer pressed against the sides of the mainshaft splines. The gear lever's force on the sliding member causes the cone's fingers to enter the slots. The member continues its travel and brings its dogs into engagement with those on the gear pinion.

Porsche Split-Ring Synchromesh

The Porsche system was patented in 1947 by Dr Ferdinand Porsche, a German engineer renowned for his own high-performance cars as well as for having designed the original Volkswagen car. Dr Porsche produced the design specifically for the Cisitalia racing car of that era but the principle has since been applied, with amendments, by numerous other car manufacturers. Apart from its undoubted efficiency, the Porsche layout has the advantage of compactness, allowing the gearbox to be relatively short and the three shafts to be correspondingly shorter and stiffer than those on other gearboxes.

It is simplest to describe the layout of Porsche synchromesh if it is applied to the third and fourth gears of a four-speed gearbox. Between the input shaft pinion and the third-gear pinion on the mainshaft a "spider" is solidly mounted on the mainshaft. The spider has three equally-spaced radial projections. These projections have feet which are a sliding fit in slots cut into an internally toothed ring. Both ends of the teeth have a shallow taper, thus forming two outward-facing cones. On the outside of the ring are ribs that carry the gear selector fork.

The two gear pinions have the usual externally toothed dog clutch member, the teeth of which correspond to those within the sliding ring. Each of the pinions also has an extended hub which carries a gapped or split ring known as the "Porsche ring" and this resembles a large-section piston ring. The Porsche rings have conical outer surfaces which, in turn, correspond to the cones within the sliding ring. Since the rings taper slightly in thickness round their circumference, from the middle to the ends, they cannot be carried directly on the hubs of the dog clutch members since that would make their outer surfaces eccentric. Between the ring and the member is another ring with the appropriate eccentricity between its inner and outer surfaces, thus making the outside diameter of the Porsche ring coaxial with the shaft assembly. A key prevents the intermediate ring from rotating on the hub.

Diametrically opposite this key, on the outside of the intermediate ring, is a projection that fits into the gap of the Porsche ring. The latter is thus compelled to revolve with its pinion and dog clutch member but, however, the gap is significantly wider than the projection, allowing the Porsche ring some rotational freedom on the intermediate ring. Thus, when the gear lever is moved from neutral towards one of the gear positions, the selector fork takes the sliding ring in the appropriate direction, bringing one of its internal cones into contact with the facing cone on the Porsche ring. The first effect of the friction between the cones is to rotate the Porsche ring by the amount necessary to take up the clearance between its gap and the projection on the intermediate ring.

Once one end of the Porsche ring has butted up against the projection, the rotational drag on the ring causes it to expand. Expansion of the ring in this manner has two results; it increases the pressure between the cone faces of the Porsche ring and the sliding ring and it prevents the latter from moving any further towards the dog clutch teeth. The tapering thickness of the Porsche ring makes it exert an equal outward thrust all round its circumference when it is expanded. Increasing the force applied to the gear lever merely raises the pressure between the cones, thus helping synchronization of the speeds. When synchronization is achieved, the frictional drag on the Porsche ring disappears, so that the ring is no longer expanded. The effort on the lever then causes the sliding ring's cone to exert a wedging action on the Porsche ring, thereby forcing down the intermediate ring and reducing the Porsche ring's diameter enough for the sliding ring to pass outside it and engage the teeth of the dog clutch member.