Design of parking mechanism based on CTF25 continuously variable transmission

1 Overview

Due to the importance of the parking mechanism in the automatic transmission, its design, analysis, and calculation are of great significance. Taking the parking mechanism of a continuously variable transmission as an example, this paper analyzes the force of key parking components (parking pawl, parking ratchet, push rod spring, etc.), explains the quantitative relationship between the design parameters of the parking mechanism and the parking performance, and provides a theoretical basis for the parameter design of the parking mechanism.

2 Design input and functional requirements of parking mechanism

2.1 Composition of parking mechanism

The parking mechanism of the CVT mainly includes a fan-shaped plate, a push rod assembly (parking guide rod, push rod spring, pressure block, parking cam, etc.), a return spring, a guide block, a parking pawl, a parking ratchet, etc. According to the matching requirements of the layout space and the docking parts, the specific structure is shown in Figure 1.

Figure 1 Schematic diagram of parking mechanism structure

2.2 Design Input

(1) Fully loaded mass of the vehicle (kg).

(2) Maximum parking slope (30%).

(3) Maximum connection speed (5Km/h).

(4) Maximum force to disengage P gear (Nm).

(5) Main reducer speed ratio.

(6) Tire radius (m).

(7) Slip distance (0.15m).

(8) Distance from center of mass to front axle (m).

(9) Road adhesion coefficient.

(10) Wheelbase (m).

2.3 Functional requirements

(1) When the vehicle speed is not higher than 5 km/h, the parking mechanism can achieve safe parking.

(2) When the vehicle is not in the parking position, the parking mechanism cannot automatically park the vehicle.

(3) After the vehicle is parked, the parking mechanism cannot automatically disengage.

(4) When the car needs to move, the parking mechanism can smoothly shift out of the P gear and leave the parking position.

3. Design and calculation process of parking mechanism sub-parts

3.1 Parking mechanism ratchet design

3.1.1 Calculation of maximum load of parking ratchet

Where m is the fully loaded mass of the vehicle (m);

g--acceleration due to gravity;

α--the angle between the ramp and the horizontal plane (°); as shown in Figure 2

r-tire radius (m);

id--The transmission ratio between the differential and the output shaft where the ratchet is located;

rg--parking ratchet contact radius (m).

Figure 2 Parking on a slope

3.1.2 Calculation of ratchet load in parking abuse condition:

Fmax=2Fg

3.1.3 Calculation of contact stress on ratchet tooth surface (as shown in Figure 3)

Where, σH--contact stress on ratchet tooth surface;

Fn--positive pressure of the pawl on the ratchet (N);

Fn = Fgcosβ;

b--The length of the contact line between the ratchet and the pawl (m);

ρ--Contact fillet curvature radius (m).

Figure 3 The force of the pawl on the ratchet

The tooth surface contact stress of the parking mechanism under abuse conditions is usually not more than 6000MPa, and the tooth surface contact stress under normal parking conditions is usually not more than 4000MPa.

3.1.4 Calculation of ramp rollback:

The ramp rollback amount Sr must be less than the specified sliding distance of 0.15m.

3.1.5 Calculation of internal spline strength

Spline strength check mainly includes bending strength check and contact strength check.

(1) Bending strength

Where Tmax is the maximum torque that the spline can withstand, Tmax=s11T, s11 is the safety factor, the recommended value is 2, T is the actual torque that the spline can withstand, T=Mg;

Db—pitch circle diameter (m);

e—thickness of indexing circle teeth (m);

L—contact length (m);

Ze—actual effective number of teeth;

S1—Meshing load factor, generally taken as 0.25.

(2) Contact strength

S—contact area (m2), its value is equal to the product of contact length L and contact height;

S2—Area load factor, represents the actual contact area, and the recommended value is 1.

Regarding the calculation under the ratchet reversal condition, the process is the same as above.

3.2 Design of parking mechanism pawl

3.2.1 Calculation of parking pawl load at maximum slope;

Where, Fn1 — normal pressure of the ratchet wheel on the pawl (N);

Fn— Positive pressure of the pawl on the ratchet (N), Fn = Fg cosα.

3.2.2 Calculation of pawl load in parking abuse condition

3.2.3 Calculation of contact stress on pawl tooth surface

Where - the positive pressure of the ratchet on the pawl (N);

b—the length of the contact line between the ratchet and the pawl (m);

σ—Contact arc curvature radius (m).

The tooth surface contact stress of the parking mechanism under abuse conditions is usually not more than 6000MPa, and the tooth surface contact stress under normal parking conditions is usually not more than 4000MPa.

The calculation process under the ratchet reversal condition is the same as above.

3.3 Calculation of parking cam self-locking:

3.3.1 Taking the ratchet as the object:

As shown in FIG4 , according to the force balance of the pawl, the torque provided by the push rod assembly is equal to the sum of the return torsion spring torque, the torque of the ratchet on the pawl, and the torque of the cam on the pawl, then:

F N 3--Positive pressure of cam on pawl (N);

R3--arm of force (m);

Figure 4 Pawl force analysis

3.3.2 Taking parking cam as the object:

β--the angle between the contact surface of the parking cam and the parking pawl and the horizontal plane (°);

When Fh≥0, the parking cam can satisfy the self-locking function.

Under the ratchet reversal condition, the calculation process is the same as above.

3.4 Design of parking mechanism pawl return spring

When the P gear is not engaged, the pawl should not be stationary in the ratchet wheel and the following formula should be satisfied:

Where - mass of the pawl (kg);

a—vibration acceleration of the ratchet (m/s2);

b—the distance from the center of gravity of the pawl to the center of gravity of the pawl rotating axis (m);

k—safety factor;

—Torque of the pawl return spring (Nm).

4 Conclusion

By combining the vehicle information and the worst working condition - parking on a 30% slope, the main components of the CVT parking mechanism - parking ratchet, parking pawl, parking cam and return spring are subjected to force analysis, and the calculation results are as follows:

1. When the P gear is not engaged, the pawl will not engage the ratchet wheel;

2. The parking cam has a self-locking function;

3. The contact stress of the pawl tooth surface meets the requirements;

4. The maximum sliding stroke is 0.15m less than the vehicle requirement.

5. The contact stress of the ratchet tooth surface meets the requirements;

Although the CTF 25 continuously variable transmission is used as an example in this paper , the research method can be applied to the design and calculation of parking mechanisms of most automatic transmissions.