Damper systems - RA27057229220X(27-9220)

A principal task of the hydraulic system consists in controlling the working pressure pattern during the gear change. This means that the tractive force of the vehicle must be transferred continuously from the state of the engaged gear into the state of the subsequent gear within a specified period (shift time) for each gearshift.

During this transition, engine speed increases (during a downshift), or decreases (during an upshift) by the respective transmission ratio difference. The same applies for the engaging operation.

Four independent damper systems with the following assignment are provided for this purpose:

Pressure pattern during engaging of a shift element

Fig 1: Pressure Pattern Graph

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig 2: Engaging Damper System Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

"Engaging" damper system, illustrated shortly after engaging drive position "R"

The "engaging" damper (53) controls the working pressure pattern after the drive positions "R", "D", "3", and "2" are engaged.

Operating principle after engaging drive position "R" 

In its starting position, the damper (53) is at the left. The spring chamber is filled with oil. In position "R", the selector valve (1) opens the working pressure outlet and the working pressure flows to the multiple-plate brake B3 and through the kickdown shift valve (40), the ball valve (21) and restrictor (d) to the end face of the damper (53).

The filling pressure at the multiple-plate brake B3 pushes the damper (53) to the right. The oil in the spring chamber flows off pressureless via VL1 at idle throttle. At part throttle, this oil is pushed out against the shift pressure "SD" through the ball check valve (23) into the shift pressure system.

During the filling operation, different pressures exist upstream and downstream of the restrictor (d), which act on the end faces of the control valve (58). As a result, the valve (58) is pushed to the left into the control position. The working pressure inlet is restricted at the control edge (f) and at the same time the running speed of the damper (53) reduced.

Fig 3: Engaging Damper System Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Shown in drive position "R" after completion of shift sequence

Once the pressure at the multiple-plate brake B3 has reached approx. 5 bar, the working pressure at the annular surface (e) presses the kickdown shift valve (40) to the right. The oil in front of the end face of the damper is drained through the ball valve (22) at the zero outlet of the valve (40), and the damper shifts to the left into the starting position.

Parallel to operation of the multiple-plate brake B3, working pressure passes through the ball valve (20) to the annular surface (g) of the shift valve K2 (59) and pushes this valve against the spring force to the right. The clutch K2 is engaged. The damper (53) is again operational for a forward drive position.

Function after engaging drive positions "D", "3" or "2" 

If a forward drive position is engaged, the damper (53) operates in a similar way as for engaging drive position "R".

The differences are:

• The working pressure inlet is now via AD-2 at the two-way ball valve (21). The valve (21) closes upward and separates the connection to the kickdown shift valve (40). The damper remains at the righthand stop, even after the engaging operation is completed.
• In 1st gear, working pressure "A1" at the ball valve (20) passes to the annular surface (g) at the valve (59) in drive position "2". In 1st gear the spring chamber of the shift valve K2 (59) is pressureless. The valve (59) shifts to the right and clutch K2 is engaged.

  A damper system consists of:

• Damper piston with spring set
• Damper control valve with spring
• Operating time restrictor

Fig 4: Damper Systems Clutch Diagram - K1, K2 And Brake Band B1

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The description below deals with the damper system of clutch K1. The operating principle of damper systems K2 and B1 is basically the same. Shift pressure "SD" acts in the spring chamber of the damper (48) and at the end faces of the damper control valve (50). The valve (50) is on the right in the open position. The valve (7) is making contact at the top. Working pressure flows from the command valve through the control valve (50) and locking valve (7) to clutch K1 and through the ball check valve (6) to the end face of the damper (48). The working pressure first of all overcomes the force of the back-pressure springs at clutch K1 and reaches the level of the contact pressure. As soon as the clutch plates are resting against each other free of play, the working pressure rises further and reaches a level with which it moves the damper piston to the right against the force of its spring and the momentarily prevailing shift pressure "SD". As a result, the oil is displaced from the spring chamber of the damper piston and flows off via the operating time restrictor (a) to the shift pressure control valve. This results in different pressures "UPSTREAM" and "DOWNSTREAM" of the restrictor (a), which pass along lines to the control valve (50). The higher pressure "UPSTREAM" of the restrictor (a) overcomes the lower pressure "DOWNSTREAM" of the restrictor and moves the valve against the force of the spring to the left into the control position. The working pressure inlet to clutch K1 is restricted at the control edge (d) and at the same time the operating speed of the damper piston (48) is reduced. Consequently, the pressure rise at clutch K1 is influenced in a suitable manner and the specified damper operating time maintained.

As soon as the damper (48) has reached the righthand stop, the working pressure at clutch K1 increases to its full value. A pressure balance exists at the end faces of the control valve (50), which is pushed back into its starting position. During this engaging, the command valve opens a zero outlet. The connection to the restrictor (c) becomes pressureless. The locking valve (7) and the ball check valve (6) close downward. Clutch K1 is slowly emptied via the restrictor (c). The oil in front of the end face of the damper (48) flows off via the restrictor (b). The emptying process is influenced by the cross-section of the restrictors (b) and (c).

During a braking shift without the throttle being open, the shift pressure acting on the spring side of the damper piston is very slight, so that the locking valve (7) can be pushed upward by its spring. As a result, the clutch K1 does not empty gradually through the restrictor (c), but spontaneously through the large opening below the locking valve (7).