The gearbox is mechanical, three-way, has five gears for moving forward and one for moving backward

Fifth gear is direct. The gearbox is equipped with two inertia-type synchronizers for engaging second, third, fourth and fifth gears.

Gear ratios:

  • First gear – 7.44
  • Second gear – 4.10
  • Third gear – 2.29
  • Fourth gear – 1.47
  • Fifth gear – 1.00
  • Reverse – 7.09

The box is attached to the clutch housing on four studs screwed into the body of the clutch housing.

The centering of the box is carried out along the flange of the cover 4 of the rear bearing of the input shaft. -

Input shaft 1 is mounted on two ball bearings.

Front bearing 53 is installed in the bore of the crankshaft flange, rear bearing 2 is installed in the front wall of the gearbox housing.

The rear bearing has a protective washer and is secured against axial movements by a retaining ring 3 installed in the groove of its outer ring.

Gearbox ZIL-131

The front end of the secondary shaft 34 rests on the roller bearing 51.

The rear end of the secondary shaft rests on a ball bearing 22, secured by a retaining ring 29 in the crankcase wall.

The intermediate shaft 44 is mounted on two bearings.

Front bearing 46 roller, without inner ring; the rollers roll directly along the shaft journal.

Retaining ring 48 limits the movement of the outer ring of the bearing.

The hole for the bearing in the crankcase is closed with a plug 47, which is installed on the paint; when installing the box on the clutch housing, this place is additionally sealed with a rubber ring 49.

Rear bearing 31 balls, with a protective washer and fully filled with balls to increase its service life; the bearing is secured with a retaining ring 32.

The reverse gear block 57 rotates on two roller bearings 54 mounted on a fixed axis 56.

Gearbox bearings do not require adjustment.

The input shaft gear 1 and gear 45 of the intermediate shaft drive, the fourth gears 7 and 41, the third 8 and 39 and the second 18 and 35 gears have helical teeth and are in constant mesh with each other, the remaining gears have straight teeth.

The gears of the fourth 7, third 8 and second 18 gears of the secondary shaft rotate freely on the corresponding journals of the secondary shaft.

In this case, the contacting surfaces of the gears of the second and third gears and the shaft, as well as the gears of the fourth gear and bushing 6 work steel on steel.

Sleeve 6 is prevented from turning on the shaft by a pin.

Gearbox ZIL-131 longitudinal section

To prevent jamming and ensure reliable lubrication during operation of parts, steel on steel shaft journals and the outer surface of the bushing have a special shape in the form of alternating protrusions and depressions; the surface of these parts is phosphated, and the phosphate layer is impregnated with a special composition that prevents seizing during the running-in period.

When installing gears on the secondary shaft in this way, it is necessary to strictly comply with the requirements of the oil used in the lubrication chart.

The use of other oils or contaminated oil may cause the gears to seize on the secondary shaft journals and bushing.

The gears on the shaft journals are secured in the axial direction with locking rings 37 and 43. Support washers 36 and 42 of the fourth and second gears have splined connections to the shaft.

For shockless engagement of the second and third, fourth and fifth gears, two inertia-type synchronizers are installed in the gearbox; the gears have cones for working with synchronizers.

The presence of synchronizers makes gear shifting easier and increases the service life of the gearbox.

In the right wall of the crankcase there is a threaded plug 59 for the control and filling hole, through which the gearbox is filled with oil in the absence of a power take-off.

If there is a power take-off, oil is poured through the plug in the power take-off. In both cases, oil is filled to the level of the control filler hole in the gearbox.

In the left wall of the crankcase at the bottom there is a drain hole, closed by a threaded plug 60, which is equipped with a magnet that attracts small particles of metal from the oil.

There are two hatches with flanges on both sides of the crankcase for attaching power take-offs.

Upon special request, a traction winch can be installed on the vehicle. IN In this case, to drive it, a power take-off is installed on the right transmission hatch.

Power is taken from the front rim of the reverse gear block 57.

The gear shift mechanism is located in the gearbox cover 11.

The upper part of the cover 64 with the gear shift lever 68, the intermediate lever 69 for engaging first gear and reverse gear is removable and is installed using bushings 12.

The additional force required to engage synchronized gears is provided by the ratio of the gear lever arms.

The presence of an intermediate lever 69 reduces the travel of the gear shift lever when engaging first gear and reverse, as a result of which the lever travel for engaging all gears is the same.

The intermediate lever 69 is blocked in the neutral position by the safety pin 62 located in the wall of the gearbox cover 64.

To engage first gear or reverse gear, you need to use lever 68 through the intermediate lever pin and the fuse pin to compress the fuse spring until it stops, then move lever 68 to the position corresponding to the position of the lever when engaging first gear or reverse gear.

To remove the upper part of the gearbox cover 64, you must first unscrew the fuse housing 62 by 8-9 turns.

The gear shift rods 71, 72 and 73 are held in the required position by clamps consisting of a ball 14 and a spring 13; the rods have grooves for the ball.

To protect against accidental engagement of two gears at the same time, there is a locking device consisting of a pin 15 and two pairs of balls 16; when moving any rod, the other two are locked with balls that fit into the corresponding grooves on the rods.

Operation of synchronizers

The cone rings 3 of the synchronizer are rigidly connected to each other using three fingers 1, the ends of which are flared.

These fingers in the middle part have conical surfaces that act as blockers.

The holes in the disk of carriage 2, through which the locking pins pass, also have locking surfaces in the form of chamfers on both sides of the hole.

The cone rings 3 do not have a rigid connection with the carriage 2 and can be shifted relative to it; they are connected to the carriage through three fixing fingers 7, inside of which springs 6 and two balls 5 are placed.

The supports 4 for the retainer balls are pressed into the rings 3.

When moving the carriage, for example, with a fork 10 (see Fig. 2), the cone ring 3, moving together with the carriage, is brought to the cone 8 of the gear.

Due to the difference in the speed of the carriage connected to the secondary shaft and the gear connected through the intermediate shaft to the primary shaft, the conical ring 3 shifts relative to the carriage 2 until the locking surfaces of the fingers 1 come into contact with the blocking surfaces of the carriage 2, preventing further advancement of the carriage.

As soon as the rotation speeds of the carriage and gear become equal (synchronization occurs), the blocking surfaces will not impede the movement of the carriage and the gear will engage without noise or impact.

For normal operation of the synchronizers and to prevent premature wear of the rings, it is necessary to correctly and timely adjust the free play of the clutch pedal.

If the clutch drags, changing gears becomes difficult.

If synchronized gears are switched on with noise, you should immediately find out the cause of the malfunction and eliminate it in order to avoid premature failure of the synchronizers.

To prevent oil from leaking out of the gearbox, the outlet of the secondary shaft is sealed with a self-clamping rubber seal 28 (Fig. 1), additionally protected by a reflector, and the primary shaft has an oil removal groove (auger).

To prevent water from entering the gearbox when overcoming fords, the entry point into the shift lever box is sealed with a rubber boot with tension clamps, and the interface between the gearbox housing and the clutch housing, as well as the box cover, hatch covers and bearing covers are sealed special sealing paste.

To prevent an increase in pressure in the gearbox or the appearance of a vacuum in it during temperature fluctuations, the internal cavity of the box communicates with the atmosphere through a ventilation tube 21 installed on the rear wall of the cabin.

Gearbox care

When caring for the gearbox, it is necessary to check the fastening of the box to the clutch housing, as well as the fastening of the power take-off (if equipped), maintain a normal oil level in the gearbox and change it in a timely manner according to the lubrication chart.

Use only the type of oil specified in the lubrication card.

When changing the oil, it is necessary to clean the magnet of the drain plug and rinse the ventilation tube, clogging of which can cause an increase in pressure in the gearbox housing, which leads to oil leakage.

When disassembling e boxes need to check the reliability of locking and tightening of nuts 27, 30 and 52 (see Fig. 1); the tightening torque must be at least 25 kgm.

The locking of these nuts is carried out by pressing the thinned edge of the nut into the groove of the shaft.

The edge of the nut must be pressed into the groove of the shaft using a bit with a rounded end to prevent tearing of the edge of the nut.

Unscrew the nuts using a long-arm wrench without first straightening the depressed edge of the nut.

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