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Vehicles fitted with manual transmissions (MT), automated manual transmissions (AMT) and double clutch transmissions (DCT) need gear synchronizers in order to perform a gearshift (upshift or downshift). The purpose of a gear synchronizer is to synchronize the speeds of the input and output shafts of a gearbox. during a gearshift, before the engagement of the upcomig gear.
Within a gearbox, the synchronizers are located between two adjacent gears. For example, gears 1-2 share the same synchronization mechanism, 3-4 another one and the same for 5-6.
Within a gearbox, the synchronizers are located between two adjacent gears. For example, gears 1-2 share the same synchronization mechanism, 3-4 another one and the same for 5-6. It is not mandatory to fit a gear synchronizer for the reverse (R) gear because, in order to engage R, the vehicle must be stopped (if moving) and the speed of the output shaft will be zero. Nevertheless, there are manual transmissions which have gear synchronizers also for the reverse gear.
Image: Synchronizers in a manual transmission (gearbox)- How synchromesh gearbox works? Now for the engagement of the gear D, the synchromesh device has to be slid towards left with the help of. When the Main shaft A rotates, the power will be transferred to the gear U2 of the layshaft which rotates.
- From the launch, the MGB was fitted with a 3-synchro 4spd gearbox. Overdrive was an optional extra. Later, a 4 synchro box was fitted and again, o/d was an option. On the later 1977 onwards UK cars, o/d became standard fit.
Credit: Getrag
For a better understanding of the main components of a transmission and how they work, read the article How a manual transmission works.
Why do we need gear synchronizers ?
For a given manual transmission, let’s imagine that we want to shift from 1st gear to 2nd gear. The parameters of the transmission are as follows:
[ begin{split}n_{IN} = 3500 text{ rpm}
i_{1} = 3.4
i_{2} = 2.5
i_{0} = 3.1
n_{OUT} = text{?}
end{split} ]
where:
nIN [rpm] – input shaft speed
nOUT [rpm] – output shaft speed
i1 [-] – gear ratio, 1st gear
i2 [-] – gear ratio, 2nd gear
i0 [-] – gear ratio, final drive (differential)
The start gear is the 1st gear. When the driver wants to engage the 2nd gear, first, he needs to disconnect the engine from the transmission, using the clutch pedal. This is required because the change of a gear in a transmission with simple gear mechanisms, which are continuously meshed (engaged), can not be performed while engine torque is being transmitted through the gears, so that is why the clutch needs to be open.
To transition from 1st gear to 2nd gear, the transmission must go through neutral for a short period of time.
In the image below we can visualize the engine power flow through the 1st and 2nd gears. For each gear we are going to calculate the speed of the input and output shafts.
Image: Gearshift (1-2) processWhen the 1st gear is engaged, the speed of the output shaft is:
[n_{OUT} = frac{n_{IN}}{i_{1} cdot i_{0}} = 332 text{ rpm}]If we want to engage the 2nd gear, the speed of the input shaft must become:
[n_{IN} = n_{OUT} cdot i_{2} cdot i_{0} = 2573 text{ rpm}]This means that the input shaft must be decelerated from 3500 rpm to 2573 rpm. If a 2-1 downshift had to be performed, the input shaft had to be accelerated from 2573 rpm to 3500 rpm. This is when the synchronizers come into play.
The synchronizer acts like a friction clutch and decelerates (upshift) or accelerates (downshift) the input shaft, in order to match the speed for the upcoming gear.
Image: Gearbox schematic with component namesHow a gear synchronizer works ?
Synchronizers are essential for gear shifting in manual transmissions. Their purpose is to match (adjust) the speed of the input shaft (gears and secondary mass of the clutch) to the output shaft (wheel).
There are several types of synchronizers used for manual transmissions. The most commons way of classification is function of the number of friction elements (friction cones). Therefore, we have:
- single-cone synchronizer
- dual-cone synchronizer
- triple-cone synchronizer
Credit: VW
- gear wheel
- synchronizer ring
- ring spring
- locking element (strut)
- synchronizer hub (body)
- sliding sleeve
Credit: VW
The gear wheel (1) is mounted on the output shaft of the gearbox. It can rotate relative to the shaft (radial motion) but it can not have an axial movement along the shaft. Between the gear wheel and the shaft there are usually needle roller bearings which facilitate rotation.
The gear wheel has an integrated “clutch gear” with friction cone. The clutch gear is made up from the locking toothing and the friction cone. It’s called a clutch gear because it has the role of a clutch, to engage smoothly the upcoming gear wheel.
The clutch gear matches the speed of the gear wheel with the speed of the synchronizer hub. The mounting on the gear wheel is done by press fitted or laser welding. When the gear is engaged, the external teeth (with chamfer on both sides of the teeth) will interlock with the chamfer on the internal teeth of shift sleeve.
Image: Gear wheelThe synchronizer ring (2) also called blocking ring, balk ring or friction ring, has a conical surface which comes into contact with the friction cone of the gear wheel. The purpose of the synchronizer ring is to produce friction torque in order to decelerate/accelerate the input shaft during a gearshift.
The synchronizer ring, together with the friction cone of the gear wheel, form a “conical clutch” which can be engaged and disengaged through sliding.
The inside surface of the synchronizer ring has threads or groove patterns, in order to prevent the forming of any hydrodynamic oil film. If an oil film is created between the synchonizer ring and the friction cone of the gear wheel, it will take higher pushing forces and longer time to synchronize the speeds of the shafts.
Image: Synchronizer ringThe locking elements (4), also called synchronizer keys, central mechanism, strut keys or winged struts are arranged on the circumference of the synchronizer body, in specific grooves, between the synchronizer sleeve and synchrnozer hub.
The locking elements rotate together with the synchronizer hub (5) and can move axially, relative to the sliding sleeve (6). The struts are used for preliminary synchronization, which means that they generate the load on synchronizer ring to perform the synchronization process.
When in neutral position (no gear selected), the locking elements maintain the sliding sleeve in a central position on the synchronizer hub, between both gear wheels. Usually, the synchronizer assembly has 3 locking elements, distributed at an angle of 120° . In the case of large synchronizers, there might be 4 locking elements distributed at 90°.
Image: Synchronizer hubThe synchronizer hub (5) is mounted on the output shaft, rigidly connected by a spline. It can move on the axial direction but it an not rotate relative to the shaft. It contains specific grooves which will contain the locking elements.
The ring springs (3) are placed on each side of the synchronizer hub and are meant to keep the strut keys in the designated grooves.
The sliding sleeve (6), also called gearshift sleeve, synchronizer sleeve or coupling sleeve, has a radial groove on the external side for the gears shift fork. The interior has splines that are in constant mesh with the external splines of the synchronizer hub. The sliding sleeve can only move on the axial direction (left-right), from a neutral position to an engaged position.
Image: Sliding sleeveGear synchronization phases
The synchronization process, with the sliding sleeve starting from a neutral position (central) and ending with a full gear engagement, can be described in five steps, as depicted in the picture below.
The synchronization process is going to be described using the parameters:
F [N] – gearshift force
Δω [rad/s] – speed difference between gear wheel and synchronizer hub
Tf [Nm] – friction torque between the synchronizer ring and friction cone
Ti [Nm] – inertia torque of the input shaft, gears and clutch secondary mass
Phase 1: Asynchronizing
Before the gearshift process starts, the sliding sleeve is held in the middle position by the locking elements. The gearshift force generates the axial movement of the sliding sleeve, which pushes forward the synchronizer ring against the friction cone gear wheel. The speed difference between the gear wheel and the synchronizer ring causes the rotation of the synchronizer ring.
Phase 2: Synchronizing (locking)
This is the main phase of the speed synchronization. The sliding sleeve is pushed further, which brings the internal splines (teeth) of the sliding sleeve and the teeth of the synchronizer ring into contact. In this phase, the friction torque starts to counteract the inertia torque and the speed difference starts to decrease.
Phase 3: Unlocking (turn back synchronizer ring)
The gearshift force is kept on the synchronizer ring through the locking elements and the sliding sleeve. When speed synchronization has been achieved, the friction force is reduced to zero and the synchronizer ring is turn back slightly.
Phase 4: Meshing (turn synchronizer hub)
The sliding sleeve passes through the teeth of the synchronizer ring and comes into contact with the locking toothing of the gear wheel.
Phase 5: Engaging (gear lock)
The sliding sleeve has completely moved into the locking toothing of the gear wheel. Back tapers at the teeth of the sliding sleeve and the gear wheel locking toothing avoid decoupling under load.
Gear engagement position control
In automated manual transmissions (AMT) and double clutch transmissions (DCT), the position of the shift fork (sliding sleeve) in controlled with position sensors.
Cost To Replace Transmission Synchro
In the image below we can see how the position of the sliding sleeve is changing through the gearshift process. The position is split in five phases:
In the Synchronizer approach (A) state, the shift fork (sliding sleeve) starts from a central position and starts to move towards the synchronizer ring. When the position of the shift fork remains constant (P1), after moving, it means that the synchronizer ring has hit the friction cone of the gear wheel.
In this phase, the position (speed) of the shift fork is controlled and not the gearshift force (pushing force). The shift force is usually around 60 – 120 N.
After the contact between the synchronizer ring and friction cone has been detected, the Synchrnozation (B) phase begins. In this phase the position of the shift fork is constant and the pushing force gradually increased. Due to the friction torque, the input shaft starts to decelerate. The end of this phase is when the speed of the input and output shafts are synchronized (P2).
The Gear engagement (C) phase begins when the shift fork starts to move again. In this phase the sliding sleeve went through the synchronizer ring and starts to engage with the locking toothing of the gear wheel. The phase ends when the sliding sleeve reaches the end position and can not move forward anymore.
In this phase is critical to have a precise position (speed) control of the shift fork. If it moves to fast, at the end of the stroke it will smash in the gear wheel causing gear engagement noise and possible mechanical damage.
After the shift fork has reached the end position, the Gear hold (D) phase begins. In this phase a high pushing force is maintained on the shift fork for a particular amount of time, in order to ensure that the engagement of the gear is complete.
In the Gear relax (E) phase, there is no more force actuation on the shift fork and the gear is maintained in place due to mechanical locking of the sliding sleeve with the gear wheel.
The total travel length of the shift fork can be around 8 – 12 mm, with the synchronization point starting at 3 – 6 mm.
Gearshift force (credit: Hoerbiger)
The size and calculation of synchronizer mechanism has to take into account various parameters, like:
- installation space
- mechanical inertia to be synchronized
- shaft speed difference to be synchronized
- torque to be transmitted
- transmission oil properties
- gearshift quality parameters
- synchronizing time
- shift fork travel length
- maximum shift force
- drag torque
- load cycles
- interfaces
- spline data
- clearance of gear wheels
- sleeve groove size
The capacity of a synchronizer is limited by
- torque capacity of sliding sleeve, gear hub and gear wheel locking toothing
- capacity of friction material (sliding speed, surface pressure, friction power, friction work)
- heat dissipation through the oil, the synchronization ring and the friction cone
- transmission oil (viscosity and thermal stability)
The shift force at the sliding sleeve Fa [N] is calculated with the formula (source: Hoerbiger):
[F_{a} = frac{2 cdot sin{alpha} cdot J cdot Delta omega}{n_{c} cdot mu cdot d_{m} cdot T_{F}}]where:
α [rad] – friction cone angle
J [kg·m2] – input shaft, gears and secondary clutch mass inertia
Δω [rad/s] – synchronization speed difference
nc [-] – number of cones
μ [-] – coefficient of friction of the friction cone
dm [m] – mean friction cone diameter
TF [Nm] – friction torque
The reduction of the shift force at sleeve can be done by:
- increasing the diameter of the mean friction cone
- increasing the number of friction cones (using double-cone or triple-cone synchronizers)
- increasing the friction coefficient
- reducing the friction cone angle
Gearshift times
The gearshift process is the same for upshift and downshift, but the shifting times are different. During a gear upshift, the speed of the input shaft should be reduced. Since there are friction losses between the moving parts, the deceleration of the shaft will be quicker.
On the other side, when a downshift is performed, the input shaft needs to be accelerated. The same friction losses will act in the same way, which is trying to slow down the shaft. Therefore, a higher friction torque and a longer synchronization time are required to synchronize the shafts during a downshift.
The total shift time for a manual transmission depends mainly on the driver and can be anywhere around 0.5 – 2.0 s. Some high performance double clutch transmissions (DCT) can achieve shift times of around 10 ms.
Double-cone synchronizer
A double-cone synchronizer mechanism is usually used for the 1st and 2nd gears. The double-cone synchronizer mechanism is a compact device capable of heavy duty meshing. The synchronizer mechanism reduces meshing (gearshift) time and improves operation (less force required to engage the gear). The double-cone synchronization mechanism includes a synchronizer ring, double cone, and an inner cone.
Image: Double cone synchronizer (complete set)- gear wheel
- locking toothing
- needle roller bearing
- inner cone
- double cone
- synchronizer ring
- gear hub
- sliding sleeve
- locking elements
Manual gearbox example with different synchronization mechanisms
Getrag Manualshift 6MTI550 transmission.
Image: Manual transmission Getrag 6MTI550Key benefits:
- Modular system for middle and high torque applications, optional 7th speed possible
- High torque capacity at low weight
- Ready for start-stop system (gear detection)
- Flexible gear ratio spread
Key features:
- concept constant gear on output shaft
- all-wheel drive application possible
- 7th speed possible
| Maximum input torque [Nm] | higher torque possible |
| Weight [kg] | dry, without dual-mass flywheel (DMF) |
| Installation length [mm] | for a clutch length of 156 mm |
| Gear spread ratio [-] | > 7 also possible |
| Center distance [mm] | |
| 1st and 2nd gear | |
| 3rd gear | |
| 4th to 6th and reverse gear | |
| Others |
Source: Getrag
Video – gearshift synchronization process
In the video below you can clearly see the synchronization and shift fork position phases.
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Conrad Kafunda
Dear Sir/Madam,
I urgently need locking elements for a Hino Truck Gearbox, MF06 Gearbox. Please advise if you have the locking elements that you can sale to me.
SIBUSISO
CAN I ORDER THE LOCKING ELEMENT (9)
georgy
what is the gear wheel standard number of teeth?
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Synchronized manual transmissions are widely used globally in both commercial vehicles and passenger cars, although they are less common in North America. These transmissions contain a complex array of components and materials that deliver longer service and better performance when the correct, dedicated lubricant is used.
Manual transmissions come in two main types: synchronized and unsynchronized. Unsynchronized transmissions require manual synchronizing, which depends on the skill of the driver at each shift event to synchronize gear speeds, particularly on the downshift. Unsynchronized transmissions are usually only found in motorsport applications or heavy-duty commercial vehicles. North American heavy trucks are typically equipped with unsynchronized manual transmissions, whereas European truck manufacturers tend to favor synchronized manual transmissions.
A synchronizer does exactly as the name suggests. It equalizes its speed with that of the next gear to be engaged, allowing a smooth, crunch-free selection. Modern synchronized manual transmissions are of the “constant mesh” variety. This means that idling (free spinning) gears on a main shaft are in constant mesh with a corresponding set of gears, machined as one single component and forming a second “lay shaft.”
The most common synchronizer design is the “cone clutch” or “blocker ring” type. Typically, gears are arranged on the main shaft in pairs; for example, first and second gears are adjacent, as are third and fourth. In between each pair is a synchronizer unit fixed to the shaft. The two key components in the synchronizer unit are the sleeve and the “blocker,” or “synchronizer,” ring. Gears are selected by the sleeve, which can be moved in either direction by the gearshift mechanism. When the driver selects first gear, the sleeve will move to the first gear and lock onto its gear engagement teeth (also known as “dogs”). The gear is then effectively locked to the main shaft and drive is taken up. When the driver de-clutches and selects second gear, the sleeve moves the other way, de-selecting first gear and selecting second in the same way.
Before the sleeve can lock on to each gear, however, the speed of both sleeve and gear must be synchronized. This is accomplished by a blocker (synchronizer) ring, one of which sits between the synchronizer and each gear. The inner face of the ring is conical and this locates over a cone on the face of the hardened steel gear with a gripping action, as the shift event is taking place. As the surfaces of this “cone clutch” grip, the rotational speed of the gear becomes synchronized with that of the synchronizer sleeve and gear selection can be completed.
These blocker rings were traditionally made of brass; the internal conical surface was faced with fine grooves in order to provide better grip on the surface of the gear cone. In an older transmission, synchronization begins to fail (leading to crunching gears) when the internal surface of these blocker rings becomes significantly worn and their ability to grip the gear is reduced.
Earlier or more basic synchronized manual transmissions are equipped with one blocker, or “synchro,” ring per gear. However, the latest generation transmissions now feature double or triple cone synchronizers on the lower gears to facilitate smoother shifting and shorten the synchronization phase. Materials technology has improved, too. Brass is being replaced by molybdenum-based materials in commercial vehicles, sinter compositions, phenolics in Japan, and carbon materials. Each is chosen for its wear and friction performance.
Commercial vehicle and passenger car synchronizers follow similar principles, but the choice of materials reflects the much higher torque commercial vehicle transmissions must transmit. A typical heavy duty synchronization ring can be made from steel coated with molybdenum or carbon, with torque capacities as high as 18,000 Nm (13,276 lb ft).
B16 Synchros
Although the process of synchronization might seem simple, in engineering terms it is defined by nine different stages. These are:
1. Disengagement
2. Neutral
3. Neutral détente
4. Pre-synchronization
5. Synchronizing
6. Synchronization
7. Blocking release
8. Engagement tooth contact
9. Full engagement
Lubricating synchronizers is a complex proposition. Clearly, there is a need to prevent wear, but the synchronizer blocker rings still need to generate sufficient friction to perform the synchronization. That same lubricant also has to protect bearings and seals and resist degradation in the face of increasingly extended drain periods. It must also survive higher temperatures caused by reduced airflow due to improved vehicle aerodynamics and the increased energy density typical of modern, high performance powertrains.
Synchro Gearbox Replacement Parts
Considering the long and hard life of synchronizers and their mechanical complexity, it becomes easier to understand the importance of using the correct fluid. Maintenance mistakes that shorten the life of a manual transmission include filling with engine oil or even automatic transmission fluid (ATF).
Synchro Gear Drive
Dedicated manual transmission fluids (MTF) offer far better protection against wear and pitting. They combine high temperature resistance with high levels of gear and bearing protection, and they are individually designed to adapt to the behaviors of various synchronizor materials. Additive and viscosity modifier technology can be tailored during the design process to meet individual OEM specifications, so as to provide a fluid that functions as an integral component of the transmission.
Synchro Gearbox Problems
The trend is toward lower viscosity MTFs that reduce churning losses and improve fuel efficiency, without compromising protection. This is achieved through the use of robust additive and sophisticated viscosity modifier technologies. In North America, the trend is toward SAE 75W-80 and 75W-90 viscosity grades. In emerging markets like China and India, the trend favors SAE 80W-90.
Using dedicated fluids has a major impact on the cost of equipment ownership, reducing service costs and fuel consumption, and delivering improved reliability. There’s also an environmental benefit, thanks to extended drain intervals. And, from the drivability point of view, shift quality is also improved. Using a dedicated MTF to protect manual transmissions does not represent a significant additional cost compared to using an inappropriate fluid, but it does have major benefits for both owners and drivers.