1. Transmission, Reduction Gear and Differential

1.1 General Description

The drive unit of clutch type truck is a one-body construction consisting of the transmission, reduction gear and differential (Fig.3-1). The transmission is provided with a synchromesh mechanism that synchronizes the rotation of gears which are about to be meshed, ensuring smooth gear shifting. The transmission of this type avoids clashing gears and reduces noise arising when shifting, especially shifting from forward to reverse or vice versa.

1.2 Transmission

The transmission consists mainly of a driving shaft, an output shaft, a main shaft and an idler shaft, each having gear(s) of different sizes on it. The gear(s) can be shifted with the aid of the synchromesh mechanism installed on the main shaft by operation of the shift handle. The power from the output shaft is transmitted through the reduction gear differential and half shafts to the drive shaft.

1.2.1 Driving shaft and slide lead screw

Driving shaft’s end toward the clutch is held by the ball bearing located in flywheel, another end fitted with input gear is held in transmission case by the ball bearing and its middle portion is held in the bearing retainer which is fixed in transmission case with the slide lead screw. When the replacement of the friction piece is required, the driving shaft along with the bearing retainer is moved axially through turning the slide lead screw until the shaft end toward the clutch returns inside the transmission case.

1.2.2 Output shaft

The cluster gear is installed on the output shaft through two needle bearings and a spacer. Also the output gear is splined to the output shaft through a spacer. The output shaft is held in the transmission case with two tapered roller bearings and several shims are used to adjust the backlash between the output gear and the bearing. The bigger gear of the cluster gear normally meshes with the input gear and high speed gear while the smaller gear with the low speed gear. The output gear normally meshes with the forward gear or reverse idler gear.

1.2.3 Main shaft

The high and low speed reverse and forward gears are all installed on the main shaft through needle bearings. As they normally mesh with the cluster gear, idler gear and input gear respectively, it’s easy to shift for changing speed or direction synchronizer by operating the synchromesh mechanism.

1.2.4 Idler shaft

Both ends of the idler shaft are supported by the transmission case and its rear end is positioned by a steel ball. The idler gear is installed on the idler shaft through needle bearings and normally meshes with the reverse gear and output gear.

1.2.5 Rotating rod and shift forks (See Fig.3-1 and Fig.3-2)

Two rotating rods are used for performing the changeover in travel speed and direction respectively. The shift forks are supported on the shift rods. The ball is designed to rest in the notch of the shift rod to secure gearshifting position.

1.2.6 Synchromesh mechanism (See Fig. 3-3)

The synchromesh mechanism consists primarily of synchromesh cones, block rings and inserts.

  • Synchromesh cone

The gear (11) or (13) has a male cone, i.e. synchromesh cone mating with the block ring (2) through respective cone friction surface, and an involute spline (3) engaged with mesh sleeve spline (6).

  • Block ring

The block ring has a female cone friction surface mating with the male cone’s of the synchromesh cone and three notches on its circumference to align the spline of mesh sleeve with block ring’s so that the mesh sleeve spline (6) is to be pressed toward the block ring spline (1).

  • Inserter

There are three inserters included. Their center projections are fitted in the inner annular groove of the mesh sleeve spline, respective two ends in three notches of the block ring. These inserters are pressed against the top of mesh sleeve spline by two springs (8) to keep the block ring in position.

The operation of synchromesh mechanism is completed in six steps below (take the gear (11) for example).

1st Step (See Fig.3-4)

When the force is applied on the shift lever, it is transmitted to the mesh sleeve (5) through the shift fork and then makes the mesh sleeve (5) and inserters (7) axially move toward the gear (11) by  X1 and X2 respectively. In this time, the center projections of inserters (7) are still in the groove of mesh sleeve spline.

2nd Step (See Fig.3-5)

After the elimination of the clearance X1 and X2, the force above acts on the inserters (7) and synchromesh cone (4) through respective friction surface and makes the inserters inclined by an angle against the spring force to contact with synchromesh cone. At this time the mesh sleeve moves by a distance of Z.

3rd Step (See Fig.3-6)
Fig.3-6 to Fig. 3-10 are all vertical views.
The force acting on the block ring creates a friction moment between synchromesh cone and block ring and in turn makes the block ring turn an angle and the side of the notches of the block ring contact with the side of inserters. The mesh sleeve and the block ring turn an angle and the side on the notches of the block ring keeps in position at this time.

4th Step (See Fig.3-7)
While the completing the 3rd step, the mesh sleeve shifts over a distance of Z and the chamfer (15) of the block ring comes into contact with the chamfer of the mesh sleeve spline (6) and the friction torque between the synchromesh cone and block ring gradually increases and the inertial moment of the gear (11) gradually decreases until the former’s value is bigger than the latter’s, i.e. Tc>Ti, driving the gear.

5th Step (See Fig.3-8)
When the relative speed between the gear (11) and the mesh sleeve (5) becomes zero, the inertial torque Ti becomes zero too and the speed of the gear (11) is equal to the main shaft’s. At this time, the block ring shifts in peripheral direction to allow every mesh sleeve spline tooth to place between the spline teeth of the gear (11) and, in the case of the block ring floated by foreign force, the mesh sleeve to pass through the block ring smoothly.

6th Step (See Fig.3-9 and Fig. 3-10)

While passing through the block ring, the mesh sleeve shifts by a distance of Y, shown in Fig.7 and the chamfers of the mesh sleeve spline (6) come into contact with the chamfer of the spline (6) (See Fig.3-9). Due to the contact of chamfers, the torque Tc turns the gear (11) over an angle relative to the mesh sleeve and meshes the mesh sleeve spline with the spline (6).

Until now the complete synchronization course is over and then the power is output through the main shaft, clutch hub, mesh sleeve and gear (11).

Working principle of forklift transmission

In neutral position —

The power from the driving shaft (1) is transmitted through the input gear, the cluster gear (3) & (4) to the high speed gear (6) or low speed gear (11). Due to the mesh sleeve is in the neutral position, the main shaft, output gear and output shaft are not rotated so the power is not transmitted to the high speed or low speed gear,

Gear shifting —

When the shifting lever is operated, the shift fork moves the mesh sleeve to allow relative gears to mesh through the synchromesh mechanism. Power is transmitted in the following order:

Driving shaft-Input gear-Cluster gear-High (or low) speed gear-Synchromesh mechanism-Main shaft-Synchromesh mechanism-Reverse (or forward) gear-Output gear- Output shaft.

Power flow in forward 1st speed gear position: 1-2-3-4-11-10-8-9-12-16-15-17-18-5-21

Power flow in forward 2nd speed gear position: 1-2-3-6-7-8-9-12-16-15-17-18-5-21

Power flow in reverse 1st speed gear position:


Power flow in reverse 2nd speed gear position:


1.3 Reduction Gear (See Fig.3-12)

The reduction gear located in the front of the transmission is used to reduce the speed and increase the torque from the output shaft of the transmission and impart them to the differential. It consists primarily of a small spiral bevel gear on the output shaft and a pinion shaft splined with a big spiral bevel gear. Both ends of the pinion shaft are supported by tapered roller bearing. Several shims are installed between the case and bearing covers to adjust the clearances between them.

1.4 Differential (See fig. 3-12)

The differential is housed in the front portion of the case of the differential the front end of which is connected with the axle housing. The differential case is of splitting type. The differential includes two half shaft gears and four planet gears. The thrust washers are installed between the differential case and each gear and between gear pairs to keep a proper clearances between them. The planet gears are supported by planet gear shaft Ⅰ and Ⅱ . The shaft Ⅰ and ring gear (1) are fixed to the differential case respectively with knock pin and bolt.

The power from the transmission is transmitted through the reduction gear, differential, half shaft gear and half shaft to driving wheels.

1.5 Removal of Shift Forks

The figures from 3-13 to 3-20 show the removal procedures of the shift forks in the drive unit which has been removed from the truck. The procedures are also applicable to the removal of the shift

forks under the condition that the drive unit is in the truck.

  • First remove the mounting bolts on the shaft arm positioned at the rear end of the side shift rod (Fig. 3- 13).
  • Draw out the shaft arm little (Fig.3-14).
  • Remove the mounting bolts on the transmission case cover (Fig. 3-15).
  • Speedly shift the speed gear from forward gear to backward gear or vide versa to let the front end of the shift rod leave the transmission housing (Fig.3-16).
  • Remove the shaft arm (See Fig. 3-17).
  • Remove the clip ring placed at the outer end of the rotating rod with a pair of pliers (SeeFig.3-18).
  • Knock at the end of the rotating rod slightly but not heavily and remove the rod (See Fig. 3-19).
  • Remove the shift forks together with the shift rod.

Note: Take down the position of the shift forks then. (See Fig.3-20)

Working principle of forklift transmission

1.6 Remounting of Shift Fork

The remounting of the shift forks is contrary to the removal of them in operation order. Besides these, the followings should be noticed in the course of the remounting:

  1. The remounting should be done in a clean place to prevent dust and foreign matters from going into the transmission.
  2. Check all parts for wear and damage and replace the parts excessively worn or damaged.
  3. As a rule, each of O-ring and other seals removed should be replaced with new ones.

The remounting procedures of the shift forks are as follows:

a) Place the spring and the steel ball into the hole of the shift forks and put the shift lever in and then slightly knock at the lever to fit them properly (See Fig.3-21).

Note: The shift forks position should be the same as the position before the removal above.

  • The steel ball in the shift fork

should fall into relative midchannel of the shift lever.

b) Align the shift forks with the channel of mesh sleeve and put the shift forks and the direction shift lever into the transmission. (See Fig.3-22)

c) Properly fix the shaft arm (See Fig.3-23) and notice the position of the shift rod. (See Fig.3-24)

Working principle of forklift transmission

d) two mounting bolts should be tightened to a torque of 28.4-44N.m (2.9-4.5kgm) before tightening the stop screw at the end of the shaft arm (See Fig.3-25).

Working principle of forklift transmission

e) After the front end of the shift lever is put into the transmission, tighten the stop bolt to a torque of 7.8-17.6N.m (0.8-1.8kgm) and the lock nut to a torque of 7- 23.5N.m (1.4-2.4kgm) (See Fig.3-26, Fig.3-27).

Working principle of forklift transmission

f) Place the rotating rod and O-ring into the transmission case (See Fig.3-28) and fix them with a clip ring (See Fig.3-29).

g) Put the transmission cover and its gasket on the case and tighten all bolts to a torque of 20.6-34.3N.m (2.1-3.5kgm) (See Fig. 3-30).

forklift transmission