Gear synchro

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. 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.

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 1^st gear to 2^nd 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:

n_IN [rpm] – input shaft speed
n_OUT [rpm] – output shaft speed
i₁ [-] – gear ratio, 1^st gear
i₂ [-] – gear ratio, 2^nd gear
i₀ [-] – gear ratio, final drive (differential)

The start gear is the 1^st gear. When the driver wants to engage the 2^nd 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 1^st gear to 2^nd 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 1^st and 2^nd gears. For each gear we are going to calculate the speed of the input and output shafts.

When the 1^st 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 2^nd 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.

How 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

1. gear wheel
2. synchronizer ring
3. ring spring
4. locking element (strut)
5. synchronizer hub (body)
6. sliding sleeve

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.

The 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.

The 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°.

The 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.

Gear 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
T_f [Nm] – friction torque between the synchronizer ring and friction cone
T_i [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.

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:

1. 1. Synchronizer approach
   2. Synchronization
   3. Gear engagement
   4. Gear hold
   5. Gear relax

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 F_a [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
n_c [-] – number of cones
μ [-] – coefficient of friction of the friction cone
d_m [m] – mean friction cone diameter
T_F [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 1^st and 2^nd 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.

1. gear wheel
2. locking toothing
3. needle roller bearing
4. inner cone
5. double cone
6. synchronizer ring
7. gear hub
8. sliding sleeve
9. locking elements

Manual gearbox example with different synchronization mechanisms

Getrag Manualshift 6MTI550 transmission.

Key benefits:

• Modular system for middle and high torque applications, optional 7^th speed possible
• High torque capacity at low weight
• Ready for start-stop system (gear detection)
• Flexible gear ratio spread

Key features:

Parameter	Value	Observation
Maximum input torque [Nm]	550	higher torque possible
Weight [kg]	44	dry, without dual-mass flywheel (DMF)
Installation length [mm]	630	for a clutch length of 156 mm
Gear spread ratio [-]	5.5 – 6.9	> 7 also possible
Center distance [mm]	88
Synchronization mechanism
1st and 2nd gear	triple-cone
3rd gear	dual-cone
4th to 6th and reverse gear	single-cone
Others	concept constant gear on output shaft all-wheel drive application possible 7th speed possible

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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