Direct and overdrive controlled gear change action

(Fig. 3.29) When direct drive is selected, hydraulic pressure is steadily increased and this gradually releases the double-sided cone clutch member from the cone brake fixed to the casing. The release of the cone clutch frees the sun gear and removes the load from the engine. The engine speed increases immediately until it catches up with the output shaft, at which point the unidirectional clutch rollers climb up their respective ramps and jam. The input shaft's power coming from the engine is now permitted to drive the output shaft, which in turn transmits drive to the propellor shaft. At the same time the double-sided cone clutch completes its movement and engages the annular ring cone.

Overdrive is engaged when the double-sided cone clutch moves away from the annulus gear cone and makes contact with the stationary cone brake, thus bringing the sliding cone clutch member and sun gear to rest. As a result of the sun gear being held stationary, the gears now operate as an epicyclic step up gear ratio transmission. During the time the double-sided cone clutch member moves from the annulus cone to the brake cone the clutch will slip. This now permits the unidirectional roller clutch to transmit the drive. Whilst the input ramp member rotates as fast as the output ramp member the roller clutch drives. However, as the annular ring gear speed rises above that of the input shaft, the rollers will disengage themselves from their respective ramps thereby diverting the drive to the epicyclic gear train.

Electrical system (Fig. 3.29) Overdrive or direct drive gear ratio selection is controlled by an electrical circuit which includes an overdrive on/off switch, inhibitor switch and a relay switch. An inhibitor switch is incorporated in the circuit to prevent the engagement of overdrive in reverse and some or all of the indirect gears. A relay switch is also included in the circuit so that the overdrive on/off switch current rating may be small compared to the current draw requirements of the control solenoid. The overdrive may be designed to operate only in top gear, but sometimes the overdrive is permitted to be used in third or even second gear. Selection and engagement of overdrive by the driver is obtained by a steering column or fascia panel switch. When the driver selects overdrive in top gear or one of the permitted indirect gears, say third, the on/off switch is closed and the selected gearbox gear ratio selector rod will have pushed the inhibitor switch button into the closed switch position. Current will now flow from the battery to the relay switch, magnetizing the relay winding so that as the relay contacts close, a larger current will immediately energize the solenoid and open the control valve so that overdrive will be engaged.

Hydraulic system (Fig. 3.29) A plunger type pump driven by an eccentric formed on the input shaft supplies the hydraulic pressure to actuate the slave pistons and thereby operates the clutch. The pump draws oil from the sump through a filter (not shown). It is then pressurized by the plunger and delivered through a non-return valve to both slave cylinders (only one shown) and also to the solenoid controlled valve and dashpot regulator relief valve. The dashpot pressure regulator ensures a smooth overdrive engagement and disengagement under differing operating conditions. When in direct drive the pump to slave cylinder's line pressure is determined by the regulator relief valve spring tension which controls the blow-off pressure of the oil escaping to the lubrication system. This line residual pressure in direct drive is normally maintained at about 2.8 bar, but when engaging overdrive it is considerably raised by the action of the dashpot to about 20-40 bar.

Overdrive engagement Energizing the solenoid draws down the armature, thereby opening the inlet valve and closing the outlet valve. Oil at residual line pressure will now pass through the control orifice to the base of the dashpot regulator relief valve causing the dashpot to rise and compress both the dashpot spring and relief valve spring. Consequently, the pump to slave cylinder pressure circuit will gradually build up as the dashpot spring shortens and increases in stiffness until the dashpot piston has reached its stop, at which point the operating pressure will be at a maximum. It is this gradual increase in line pressure which provides the progressive compression of the clutch thrust springs and the engagement of the cone clutch with the fixed cone brake.

Direct drive engagement De-energizing the solenoid closes the inlet valve and opens the outlet valve. This prevents fresh oil entering the dashpot cylinder and allows the existing oil under the dash-pot to exhaust by way of the control orifice and the outlet valve back to the sump. The control orifice restricts the flow of escaping oil so that the pressure drop is progressive. This enables the clutch thrust springs to shift the cone clutch very gradually into contact with the annulus cone.

3.7.5 Lay cock compound gear train overdrive

Overdrive When overdrive is selected, the double-sided cone clutch contacts the brake cone which forms part of the casing. This brings the sun gear which is attached to the sliding clutch member to a standstill.

The input drive passes from the pinion carrier to the annulus ring and hence to the output shaft through the small planet gear. At the same time, the

Fig. 3.30 Laycock double epicycle overdrive

large planet gear absorbs the driving torque reaction and in the process is made to revolve around the braked sun gear. The overdrive condition is created by the large planet gears being forced to roll 'walk' about the sun gear, while at the same time revolving on their own axes. As a result, the small planet gears, also revolving on the same carrier pins as the large planet gears, drive forward the annular ring gear at a faster speed relative to that of the input.

The overall gear ratio step up is achieved by having two stages of meshing gear teeth; one between the large pinion and sun gear and the other between the small pinion and annulus ring gear. By using this compound epicyclic gear train, a relatively large step up gear ratio can be obtained for a given diameter of annulus ring gear compared to a single stage epicyclic gear train.

Direct drive (Fig. 3.30) Direct drive is attained by releasing the double-sided cone clutch member from the stationary conical brake and shifting it over so that it contacts and engages the conical frictional surface of the annulus ring gear. The power flow from the input shaft and planet carrier now divides into two paths — the small planet gear to annulus ring gear route and the large planet gear, sun gear and double-sided clutch member route, again finishing up at the annulus ring gear. With such a closed loop power flow arrangement, where the gears cannot revolve independently to each other, the gears jam so that the whole gear train combination rotates as one about the input to output shaft axes. It thereby provides a straight through direct drive. It should be observed that the action of the unidirectional roller clutch is similar to that described for the single stage epicyclic overdrive.

Clutch operating (Fig. 3.30) Engagement of direct drive and overdrive is achieved in a similar manner to that explained under single stage epicyc-lic overdrive unit.

Direct drive is provided by four powerful springs holding the double-sided conical clutch member in frictional contact with the annulus ring gear. Conversely, overdrive is obtained by a pair of hydraulic slave pistons which overcome and compress the clutch thrust springs, pulling the floating conical clutch member away from the annulus and into engagement with the stationary conical brake.

Hydraulic system (Fig. 3.30) Pressure supplied by the hydraulic plunger type pump draws oil from the sump and forces it past the non-return ball valve to both the slave cylinders and to the solenoid valve and the relief valve.

Direct drive engagement When direct drive is engaged, the solenoid valve opens due to the solenoid being de-energized. Oil therefore flows not only to the slave cylinders but also through the solenoid ball valve to the overdrive lubrication system where it then spills and returns to the sump. A relatively low residual pressure will now be maintained within the hydraulic system. Should the oil pressure rise due to high engine speed or blockage, the low pressure ball valve will open and relieve the excess pressure. Under these conditions the axial load exerted by the clutch thrust springs will clamp the double-sided floating conical clutch member to the external conical shaped annulus ring gear.

Overdrive engagement To select overdrive the solenoid is energized. This closes the solenoid ball valve, preventing oil escaping via the lubrication system back to the sump. Oil pressure will now build up to about 26-30 bar, depending on vehicle application, until sufficient thrust acts on both slave pistons to compress the clutch thrust springs, thereby permitting the double-sided clutch member to shift over and engage the conical surface of the stationary brake. To enable the engagement action to overdrive to progress smoothly and to limit the maximum hydraulic pressure, a high pressure valve jumper is made to be pushed back and progressively open. This controls and relieves the pressure rise which would otherwise cause a rough, and possibly sudden, clutch engagement.

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