A synchroniser can be defined as a crucial internal shifting element present in manual, automated manual and dual clutch transmissions. Its objective is to match the circumferential speeds of parts to be connected in short periods of time with the application of minimal force, in the same way preventing premature locking by blocking the shift movement.
A gearwheel transmission with multi gears can be synchronised in the following ways:
1. Synchronising mechanism for each individual gear
2. Central synchroniser for the whole transmission
3. Speed synchronisation by the prime mover
The main functions of synchronisers are:
- Adapt speed, accelerate or decelerate masses
- Measure speed difference and determine synchronous speed
- Lock the positive engagement until speeds are synchronised
- Establish positive engagement and power flow
The synchroniser unit exploded diagram is shown in Figure 1:
1 – Idler gear
2 – Synchroniser hub with selector teeth and friction cone
3 – Main functional element or baulk ring
4 – Synchroniser body with internal and external toothing
5 – Compression spring
6 – Ball pin
7 – Thrust piece
8 – Gearshift sleeve with internal dog gearing
It is divided in 5 different phases as shown in Figure 2
Phase 1 – Asynchronising
Before the shifting process starts, the gearshift sleeve is held in the middle position by a detent. The gearshift force F triggers the axial movement of the gearshift sleeve (8), which causes the ball pins (6) to act on the thrust pieces (7) to press the synchroniser ring (3) with its counter-cone against the friction cone of the synchroniser hub (2).
Phase 2 – Synchronising Locking
The gearshift sleeve is moved further. This brings the bevels of the internal dog gearing of the gearshift sleeve (8) and the external dog gearing of the synchroniser ring (3) into contact. The gearshift force is applied to the synchroniser ring via the thrust pieces (7) and the dogs (8), the force being divided between them.
Phase 3 – Unlocking
When speed synchronisation has been achieved, the friction torque tends towards zero and the unlocking process starts. The gearing torque becomes greater than the friction torque, and acts via the bevels to turn back the synchroniser ring. The gearshift force decreases rapidly in this phase.
Phase 4 – Meshing
The synchroniser ring is pressed against the friction cone of the synchroniser hub only by residual pressure (coming from the friction between the moving gearshift sleeve and the thrust pieces) via the thrust pieces.
Phase 5 – Engaging
The gearshift sleeve toothing twists the synchroniser hub relative to the synchroniser ring. Later, the gearshift sleeve positively engages the power flow between the gear pair and the transmission shaft.
SYNCHRONISERS & DYMOLA
Dymola as an engineering software can be used to design, test and validate multi-domain systems including their components such as synchroniser units.
In this paper two synchronisers from the library VeSyMA – Powertrain were studied in order to show the differences between an ideal and a detailed model.
Figure 3 shows a dual ideal synchroniser.
Below, Figure 4 shows a dual detailed synchroniser.
As can be appreciated, the main difference between the ideal and the detailed model is the lack of a baulk ring and the use of different types of dog clutches. While the ideal model employs an ideal dog clutch focused on a relative speed threshold value to allow engagement between the dogs, the detailed one utilises a dog clutch which considers the tooth geometry during both engagement and disengagement.
Likewise, the dual detailed synchroniser includes thrust pieces and a baulk ring backlash model. The trust piece has the task of recreating a hard stop in order to prevent two elements (hub and baulk cone) moving beyond a particular relative position. On the other side, the backlash model does basically the same but instead of acting as a longitudinal constraint it acts as a rotational one with the objective to limit the angular movement of the baulk ring teeth relative to the sleeve.
RATTLE ON SYNCHRONIZERS
The presence of rattle and its consequences for the different components of the car is a topic of interest around the automobile industry for both NVH and durability assessments. With that in mind, an analysis of this phenomenon was carried out on a dual detailed synchroniser (Fig. 5).
To study the effects of synchro rattle a 4 cylinder 1800cc turbocharged engine model was taken from the VeSyMA – Engines library to get the corresponding rotational speed profile for the gearbox input shaft operating at 3500rpm at wide open throttle (WOT).
The sampled speed signal was then used to drive a synchroniser model hub shaft in an experiment. The synchroniser model hub is held in neutral position to study the rattle effects of the non-engaged gear synchros. The model simulation results are shown in Figure 6. Clearly we can couple a crank angle resolved engine model to an entire gearbox but the analysis example focusses on an individual synchroniser.
The plots in general show a considerable non-uniform rattling behaviour which is expected. The correct parameterisation of the inertias and hydrodynamic friction models which relate to oil temperature and viscosity allow predictive analysis and optimisation of such systems. In this case, the total lash in the anti-rotation lug is approximately 5.4 degrees. The lash region damping as well as the end stop damping will depend on the oil and sliding friction characteristics which can be predicted and tuned where required.
Synchronisers are a relatively long-standing invention that radically changed the way transmissions work. Although synchromesh units increase the price and complexity of transmissions, they ensure their correct operation and also protect gears from premature wear due to mis-engagement.
As described previously, rattle is an undesired event that compromises the components’ durability and can generate undesired NVH.
Moreover, the results obtained can be further used to develop a series of different analyses such as finite element analysis, frequency analysis, fatigue analysis in order to prevent the presence of cracks or any other type of failure that could reduce the components lifetime and potentially cause catastrophic component failure.
FUTURE DIRECTIONS: THE END OF SYNCHRONISERS?
Last March a new model of the considerable new but highly respected Swedish brand “Koenigsegg” was released at the 2019 Geneva auto show (Fig. 7). A car with more than 1600 hp, a weight of only 1420 kg and a ton of downforce are just some of its impressive facts; however, the story does not finish here.
As part of its drivetrain, a new transmission called “LST or Light Speed Transmission” was also presented (Fig. 8). A 7-clutch and 9-speed automatic transmission that instead of using synchronisers uses clutches to shift between gears, decreasing the shifting time even more than a dual-clutch transmission and therefore, setting new standards to what is about to come in the near future, at least whilst EVs have not yet got the majority of the market share and the need for multi-speed transmissions dramatically reduces.
Written by: Jose Miguel Ortiz Sanchez, Project Engineer and Alessandro Picarelli, Engineering Director