Analysis and Troubleshooting Methods for Main Clutch Slippage of Crawler Bulldozers

1. Phenomenon description

The working speed (travel) of a bulldozer cannot increase along with the rise in engine speed. When the resistance to pushing earth is high, the bulldozer does not move forward.

2. Cause analysis

From the structure and working principle of the bulldozer clutch, it can be known that after the clutch is engaged, the driving disc and the driven disc should become an integrated whole, that is, there should be no speed difference between the two. If there is a speed difference between the driving disc and the driven disc after engagement, it indicates that the clutch is slipping. The greater the speed difference between the driving and driven discs, the more severe the slippage will be.

A friction clutch transmits torque by means of frictional torque. The frictional torque is the product of the frictional force and the coefficient of friction. When the average radius of the friction plate is constant, a decrease in the frictional torque must be due to a reduction in the frictional force. Frictional force is the product of the clamping force N(normal pressure) exerted by the driven disc on the driving disc and the friction coefficient f of its working surface. If either the clamping force of the rear pressure plate and the coefficient of friction decrease or both decrease, the frictional torque of the clutch will decrease. When it is less than the torque output by the engine, the clutch will slip.

The main reasons for the reduction of normal pressure and friction coefficient are:

(1) The influence of the clamping mechanism

The main clutch driven disc is pressed mainly by the elastic force of the butterfly spring, which lifts one end of the pressing lever. The other end will then press against the rear disc of the driven disc, generating a positive force. As a result, the clutch engages and generates friction to transmit torque. The magnitude of its clamping force depends on the distance from the fulcrum of the clamping lever to the force application point at the driven disc. Its distance is large and the clamping force is small. Conversely, the smaller the distance, the greater the clamping force. During the operation of the clutch, the friction surfaces are often worn. The pressure plate of the driven disc gets closer and closer to the driving disc, and the fulcrum of the compression lever gets farther and farther from its force application point. If not adjusted in time, the pressure exerted by the compression lever on the driven disc will decrease accordingly, resulting in a reduction in the normal pressure and causing the clutch to slip.

In addition, the elastic force of the disc spring has a direct impact on the clamping force of the clamping mechanism. If the disc spring loses its elasticity due to fatigue or improper material selection or poor manufacturing quality, the pressure on the rear disc of the pressing lever will also decrease accordingly, thereby causing the clutch to slip.

(2) The influence of the coefficient of friction

Since the frictional force is the product of the normal pressure and the coefficient of friction, when the coefficient of friction on the clutch plate decreases, the frictional force also decreases accordingly, causing the clutch to slip.

3. Diagnosis and Exclusion

When a bulldozer is in operation, if the above symptoms occur, it indicates that the clutch is slipping. If the clutch control lever feels very light when pulled (the general operating force should be 147 to 243N), it suggests that the clutch slipping is mostly caused by a reduction in the clamping force of the clamping mechanism. If the clutch slips due to prolonged use, it should be adjusted. If the clutch control lever feels to be operated with a force of 147 to 243N, then the main clutch slipping is mostly caused by a too small friction coefficient.

(1) The adjustment method is

① Place the gearshift lever in neutral position.

② Pull the control lever to disengage the clutch.

③ Remove the inspection hole cover of the clutch upper cover.

④ Turn the clutch to press the lever frame and place the clamping bolts in a position that is prone to loosening.

⑤ Place the gearshift lever in any gear to prevent the clutch shaft from rotating.

⑥ Rotate the compression lever bracket and screw it in to enhance the compression force on the rear pressure plate. If it is screwed out, the compression force will decrease. When adjusting the clutch to the full-load working state of the bulldozer, the clutch should not slip. At the same time, the force reflected on the clutch lever should be 147 to 243N, and a distinct "dead point" should be felt.

After adjustment, the pressing screws on the pressing lever frame should be tightened to prevent them from loosening.

⑧ Install the inspection hole cover on the clutch cover.

If the clutch slippage fault is eliminated after the above adjustments, it indicates that the clutch slippage is caused by excessive wear of the friction elements or even a reduction in the elasticity of the disc spring due to fatigue, etc. If the clutch slips after maintenance, it indicates that it is caused by improper adjustment.