2014-01-2328

AbstractThis paper introduces a new design for the synchromesh gear boxes, which serves dual purpose of economizing the production cost and time of the gears manufacturing and enhancing the shift experience by reducing the shift effort.

This is achieved by making changes in the Entry chamfers which are the angles placed on the shift sleeve and gear dog teeth, and the Back Taper Angle also placed on the shift sleeve and gear dog teeth. In the new design the Entry chamfer angles is reduced from 120 (in the existing design) to 90 and is made Un-symmetric from Symmetric. This help in reducing the shift effort, hence enhancing the shift experience by making it smooth.

Another change is the Back Taper Angle, which is eliminated from the sleeve and the gear dog teeth and is repositioned on the hub, keeping all other important design parameters like strut / insert design, gear cone angle, gear cone finish, number of cones, cone friction material the same.

The new design ensures that the change in back taper angle position does not compromise its function of avoiding the Gear jump out. As per existing design for making the back taper angle on the sleeve and gear dog teeth special machining operations are required which increase the production cost and time. But according in new design for making the same angle (back taper angle) on the hub, only simple machining operations are enough. There is no need of any special machining operation, so this helps in reducing the production cost and time in making gears.

IntroductionToday almost all the manual transmissions used in automobiles are fitted with the synchronizers for smooth gear shift. The function of a synchronizer is to provide friction clutch inside the

transmission. These synchronizers are activated when the driver shifts the gear. The function of the synchronizer is to bring the relative speed of gear, clutch and output shaft to zero during the shift period.

Various types of synchronizers are used in manual transmissions but Strut / Insert type are most commonly used ones, because of their high torque capacity for relatively small size and their extreme durability. Synchronizers are differentiated by the number of cones used which are the single, double and triple cone synchronizers. The synchronization process always follows the same sequence irrespective of the number of cones used. Shift force moves the sleeve towards the gear dog teeth to get engaged. As long as there is a speed difference between sleeve and gear, the sleeve is blocked by a blocker ring. When the relative speed is zero, the sleeve moves further and engages with the dog teeth of the gear.

Shift Effort and Synchronization Time are the most important design considerations after the basic functional relationships are satisfied. To achieve this, equal entry chamfers are provided on both the sleeve teeth and the gear dog teeth. These angles play an important role in the shift force calculation.

Until disengagement force is not applied, the gear and the sleeve will remain engaged and avoid the gear jump out. This is achieved with a locking angle provided on both gear dog teeth and sleeve internal spline. This locking angle is known as Back Taper Angle which is manufactured with the help of special machines.

Nowadays, the cost of the raw materials and machining operations is increasing because of the inflation. So it becomes a necessity to bring out new innovative designs which can help in economizing the use of valuable resources and time.

New Gear Locking Design in Synchromesh Gearbox Which Reduces Gear Shift Effort

2014-01-2328

Published 09/30/2014

Jagjeet SinghNew Holland FIAT (India) Pvt. Ltd.

Gagandeep SinghUniversity Of Technology, Sydney

CITATION: Singh, J. and Singh, G., "New Gear Locking Design in Synchromesh Gearbox Which Reduces Gear Shift Effort," SAE Technical Paper 2014-01-2328, 2014, doi:10.4271/2014-01-2328.

Copyright 2014 SAE International

Downloaded from SAE International by University of Michigan, Wednesday, October 29, 2014

In this paper a new design is presented which will provide the following benefits: 1) Reduction in the Shift Effort. 2) Reduction in the manufacturing cost. 3) Reduction in the production time, without compromising with the efficacy of the function of gear locking.

Synchronizer Working PhasesThe working of a synchronizer from Neutral Phase to Full Engagement Phase consists of steps mentioned below:

Neutral: Synchronizer sleeve is in the center position.

Figure 1. Showing the neutral phase of synchronizer working phases and the main parameters of gear geometry in existing design.

Where:N1- Speed of synchronizer sleeve and blocker ring - RPMN2 - Speed of gear - RPM

Neutral detent: When the fork applies force, the sleeve starts to move from its neutral position, energizes strut/ insert and starts the oil wipe.

Figure 2. Showing the neutral detent phase of synchronizer working phases in existing design.

Pre-Synchronization: Strut / Insert force causes the blocker ring to index and a cone torque is generated. The sleeve teeth come in contact with blocker ring teeth. Cone torque builds up with Strut / Inert axial force resulting in to the reduction of relative speed between input and output gear.

Figure 3. Showing the pre-synchronization phase of synchronizer working phases in existing design.

Synchronizing: The sleeve remains stationary while the speed difference between input and output gear continues to reduce.

Synchronization: The speed difference reaches zero (N1=N2), no more cone torque is generated and now the sleeve begins to move forward.

Blocker ring release: As the sleeve moves forward, the blocker ring move circumferentially with the gear and the sleeve moves past the blocker ring.

Figure 4. Showing the blocker ring release phase of synchronizer working phases in existing design.

Tooth contact engagement: The sleeve continues to move axially until contact is made with the gear dog teeth.

Figure 5. Showing the tooth contact engagement phase of synchronizer working phases in existing design.

Full engagement: The sleeve moves through gear's dog teeth to the final position.

Figure 6. Showing the tooth full engagement phase of synchronizer working phases in existing design.

Existing DesignIn order to understand the new design it is important to give an overview of the existing design. In the existing design:

1. The entry chamfer is symmetric and is in the range of 120 to 125 (Fig - 7a).

2. To avoid the gear jump out equal back taper angle () is provided on both the shifting sleeve and gear dog teeth.

3. The Cone Angle on gear and blocker ring is generally kept in the range of 5 to 7.


Fig - 7 a.

Fig - 7 b.

Fig - 7 c.

Fig - 7 d.

Figure 7 (a, b, c, d). Showing the gear dog teeth, blocker ring, hub and shifting sleeve in the existing design respectively.

New DesignThe new design aims to reduce the shift effort, manufacturing time and cost. This is achieved by making the following changes in the existing design:

1. The entry chamfer is changed from 120 to 90and also from symmetric to unsymmetric, thereby reducing the shift effort.

2. Back taper angle () is eliminated from both the sleeve and gear dog teeth. Instead of generating this angle on the sleeve and gear dog teeth it is machined on the hub () (See Fig - 12). This taper angle is generated on the hub by simple machining operation of hobbing, hence saving the valuable time and cost factors.

Fig - 8 a.

Fig - 8 b.

Fig - 8 c.

Fig - 8 d.

Figure 8 (a, b, c, d). Showing the gear dog teeth, blocker ring, hub and shifting sleeve in the new design respectively.

Figure 9. Showing the main parameters of the sleeve, hub, blocker ring and gear dog teeth in both existing and new design


Following figure gives the comparison between the engagement status of the sleeve with gear dog teeth between the existing and the new design. In existing design, the entry chamfer on the sleeve and gear dog teeth helps in engagement and back taper angle avoids gear jump out. In the new design, the modified entry chamfer helps in reducing shift effort during engagement and the taper angle on the hub avoids gear jump out.

Figure 10. Showing the engagement status of the sleeve and gear dog teeth in both existing and new design

Increase in the trend of the use of bigger engines results in higher shift effort as bigger is the engine the higher is the shift effort required, but the driver still demands for smooth shiftability. This new design helps in reducing the shift effort hence providing a smooth shift experience as compared to the existing design.

Shift EffortShift force required to do an upshift or downshift is one of the first parameter which the driver observes about the vehicle's performance. Hard shifting provides a bad shifting experience to the driver. In the new design, modification of the entry chamfer on sleeve and gear dog teeth and repositioning of the back taper angle not only gives gear locking but also helps in reducing shift effort. Shift effort depends upon the combination of various factors which are listed below: Coefficient of friction between the blocker ring and cone Cone angle Mean cone radius Cone torque Contact pattern at entry chamfers

In this new design all factors mentioned above remain the same except entry chamfer angle and this is one of the parameters which affects shift effort / shift force. Shift force in terms of W and is given by the expression

(1)

Where: = Entry chamfer angle at pitch circle diameterW = Normal force generated by shift force (F)B = Static coefficient of friction between chamfersW Sin /2 = Axial component of the normal forceB W Cos /2 = Axial component of friction force

Figure 11. Showing force diagram at pitch circle diameter of entry chamfer - Free body diagram

From equation (1), it is clear that for any fixed value of shift force (F) and coefficient of friction (B), normal force (W) depends upon entry chamfer (). The following graph gives a comparison between normal forces generated with different entry chamfer angle values assuming coefficient of friction to be 0.17 and axial force to be 10kg.

Graph 1. Showing effect of change in entry chamfer angle on normal force

With the same axial force (Shift force - F), the normal force (W) increases which results in the reduction of shift effort. In other words to generate same the normal force (W) less axial force (F) is required with reduced entry chamfer angle, hence reducing the shift effort. As shown in Graph-1 there is reduction of 12.9% in the shift force when angle is changed from 120 to 90.


Disengagement ForceDisengagement force (FD) is the force required to disengage the gear or bring back the gear to neutral position. In this new design the back taper angle () on gear dog teeth is removed and is repositioned on the hub (). Back taper angle on hub () is of same value as back taper angle on gear dog teeth () in the previous existing design ( = ). So, by replacing with in equation 1, there is no change in disengagement force.

Figure 12. Showing the back taper angle () on sleeve and the taper angle () on the hub.

Coefficient of friction is independent of the amount of surface in contact, so B is same in both the cases.

Stress on Sleeve and the Gear Dog TeethAs gear dog teeth profile gets modified according to the new design, it results in change of stress on the teeth surface. For comparison of the change in stress on the teeth between existing and new design following gear geometry and material properties are taken:

Gear GeometryTable 1. Showing gear geometry taken for comparison

Material PropertiesThe material of the gear is low carbon, hot rolled alloy steel with lower hardenability and good machinability.

Table 2. Showing chemical properties of the gear material

Table 3. Showing mechanical properties of the gear material

Input ParametersTable 4. Showing the input parameters taken for stress calculation on gear dog teeth

Engine power will generate torque which is given by expression:

(2)

Stress on teeth is

(3)

Where:T = Engine torque, Kg-mmP= Engine power, HPNrpm = Engine RPMA = Teeth length in contact per teeth, mmD1 = Spline major Diameter, mmD2 = Spline Minor Diameter, mm

Since all the parameters of the gear dog teeth are same except engagement length, so the surface stress increases. In the existing design there is an overlap area between the external and internal spline of gear dog teeth, and in new design it is a line contact. Substituting the values of engagement length (A) for both existing and new designs in Equation (3) will give following values:


Table 5. Showing stress comparison between existing and new design with the change in engagement length

Stress on gear dog teeth in the new design increases by 4 times but this value is still well below the yield and ultimate tensile stress of the selected material (See table-3).

Significant ParametersFollowing are the significant parameters satisfied by the new design, which are important for proper functioning of the synchronizer.

Cone TorqueWhen shift force is applied, friction develops between the gear cone and blocker ring cone in the direction of cone angle. This friction force gives rise to a torque between clutch cones at cone mean radius and is known as Cone torque. For single cone synchronizer cone torque would be:

(4)

Where:Tc = Cone torque, Kg-mmF = Shift force applied in synchro ring, Kgc = Coefficient of friction between ring and coneRc = Mean cone radius, mm = Cone angle, Degree

Figure 13. Showing Cone torque - Free body diagram

Index TorqueWhen the sleeve's entry chamfer comes in contact with the blocker ring, entry chamfer angle torque is generated which known as Index torque is given by the expression:

(5)

Where:TI = Index torque, Kg-mmRB = Blocking chamfer pitch radius, mmB = Static coefficient of friction between chamfersFt = Tangential indexing force, Kg = Included angle of chamfers at the pitch radius, degree

For proper working of a synchronizer, the lowest cone torque must be greater than the highest index torque. If this condition is not satisfied then there will be a relative speed between gear and sleeve which will result in to malfunction called clash. Clash is a sound which comes when the sleeve teeth hit the gear dog teeth which are still rotating.

Following graph shows that in new design for all entry chamfer angles, cone torque is greater than index torque, taking cone angle as 7.

Graph 2. Showing the effect of change in entry chamfer angle on Cone and Index torque

Comparing the Manufacturing Processes Involved for the Making Gear Dog Teeth in Existing and New DesignAccording to the existing design, the manufacturing processes involved in gear making are Turning, Hobbing (for making external teeth), Shaping (for generating back taper angle on the gear dog teeth where hobbing is not possible due to space constraint), Roofing (for generating entry chamfer on both the internal and external teeth), Broaching (for making internal teeth), Swaging (for generating back taper angle on internal spline), Shaving, Grinding, Honing (for gear finishing).

In the existing design, Taper Shaping is done to generate back taper angle on gear dog teeth and Swaging is done to generate back taper angle on sleeves internal teeth.

In the new design, back taper angle has been repositioned from the gear dog teeth to the hub. The process involved in generating the back taper angle on the hub is just simple hobbing on CNC machine, which is an easier manufacturing process as compared to shaping and swaging. Also the numbers of processes involved have been reduced to half in the manufacturing of back taper angle.


Figure 14. Showing machining operations for back taper angle (locking angle) and entry chamfer according to existing design on sleeve and gear dog teeth.

Figure 15. Showing the machining operations used for making taper angle on the hub according to the new design. Swaging and taper shaping operations have been eliminated from the sleeve and gear.

Comparison of the Machining Cost & Time as per the Existing and New DesignAs the new design reduces the number of manufacturing processes, it economizes the manufacturing by reducing the cost and time involved in the production of gears, thereby improving the productivity. The following table gives us a rough estimate of the cost of swaging process involved in production of gear as per the existing design. The gear considered for production cost comparison has the parameters mentioned in Table-1.

As the swaging operation has been omitted as per the new design hence all the cost and time involved in doing this operation is saved, hence improving the production.

Table 6. Showing process cost for swaging operation

There is a saving of INR 10.8 per piece. In mass production this small amount can lead to huge saving in terms of production time and manufacturing cost. In the above mentioned saving, savings due to machine tool wear, profit and other over heads have not been considered. Taking these factors into consideration there will be a further increase in the savings.

ConclusionIn this paper the new design describes the repositioning of back taper angle from gear dog teeth and sleeve () to the hub () (See Fig. 12). The repositioned angle () still performs its function of gear locking hence avoiding gear jump out in new design also. Apart from avoiding gear jump out this new design has other advantages also like:

Shift force reduction (See Graph. 1) Saving in terms of production time and manufacturing

cost (See Table. 6) No need for any special machine. Since there is a reduction in shift effort so other

components like shifting forks can be reduced in size and weight.

References1. Socin, R. and Walters, L., Manual Transmission

Synchronizers, SAE Technical Paper 680008, 1968, doi:10.4271/680008.

2. Razzacki, S., Synchronizer Design: A Mathematical and Dimensional Treatise, SAE Technical Paper 2004-01-1230, 2004, doi:10.4271/2004-01-1230.

3. Gear Design, Manufacturing and Inspection Manual, Society of Automotive Engineers, Inc., Warrendale, PA, ISBN 978-15609-1006-0, 1990.


The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAEs peer review process under the supervision of the session organizer. The process requires a minimum of three (3) reviews by industry experts.

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2014-01-2328

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