Replacing an asterisk on a chainsaw

Chainsaws produced by well-known manufacturers of special tools are characterized by high reliability and long service life. This is due to the well thought-out design and the use of special grades of tool steel, from which all moving and rubbing parts. To them in the first place should include:

• chain with cutting edges;
• guide rail;
• chain sprocket driven into rotation by the energy of a single-cylinder two-stroke gasoline engine.

All of them work in intensive mode and eventually wear out. Even a slight change in the geometric dimensions of these parts affects the performance and smoothness of the saw. If you needed to remove the sprocket from the chainsaw to replace it, you need to understand the principle of the clutch system, with which it has a direct connection.

Chainsaw coupling system

On chainsaws Husqvarna, Stiel, Partner and most models of other well-known brands installed centrifugal clutch. It turns on automatically when the engine sets a certain speed.

Clutch basket has the form of a disk. It has movable friction linings that can move in the radial direction. In the normal state, they are pressed with the built-in springs to the center, without touching the drum of the drive chain sprocket.

When the engine goes to normal mode with a predetermined number of revolutions of the shaft on which the clutch basket is mounted, centrifugal forces arise that exceed the spring force. Items with friction linings are shifted to the sides, clinging to the inner surfaces of the drum sprocket, which begins to rotate with the required frequency. Such a system can save the engine from serious damage in case of chain jamming.

Depending on the model, the sprocket is located outside or inside the clutch unit.For safety reasons, this entire unit with part of the guide rail is covered with a protective cap.

Reasons for replacing the sprocket

Replacing the sprocket on the chainsaw with opening and revision of the entire clutch assembly is performed in the following cases:

• chain jamming;
• physical wear of the sprocket to unacceptable limits.

The deviation of the geometrical dimensions of the teeth of the sprocket from the nominal should not exceed 0.5 mm. Otherwise, the work of the saw will be accompanied by increased vibration, which will lead to premature failure of the crankshaft bearings or the entire tool.

Usually asterisk work resource approximately twice the life of one circuit. In practice, they often use alternately 2-3 chains until their cutting edges are abraded, after which they acquire a new set and change the sprocket.

Sequence of operations when replacing an asterisk

In the process of repairing any working node of the chainsaw, you can change the rejected part only on a similar. Skilled craftsmen purchase the necessary parts in advance.

If you doubt your competence in this matter, you should dismantle the clutch and show it to a specialist in the store,which will easily select a replacement for worn parts.

To open and repair chainsaws you will need:

• universal wrench, sold with a saw;
• piston retainer, which can be replaced by a dense cord with a thickness of 6-8 mm;
• special puller.

An example would be Husqvarna 137, a Japanese or Chinese chainsaw with an asterisk behind the clutch. First, the protective cover of the clutch unit is removed. To do this, you have to unscrew the two nuts with a universal wrench. After that, the guide rail and chain can be easily dismantled.

Since during normal operation all the moving parts of the working body of the chainsaw have right rotation, the clutch basket is rolled onto the shaft with the help of the left thread. To remove the overrunning clutch, it is necessary to rotate it using a puller clockwise. But for this you need to first fix the crankshaft. This is done in the following sequence:

• the air filter is removed;
• the spark plug is removed to provide access to the engine cylinder;
• rotating the shaft clockwise raises the piston of the cylinder to a vertical position, after which it is moved back a few millimeters;
• the inner space of the cylinder above the piston is filled with a tight cord for accurate fixing of the piston, and with it the crankshaft.

There are regular piston clips, which are mounted on the cylinder like a spark plug.

After fixing the crankshaft, the puller is inserted into the holes of the coupling and rotates clockwise until it is completely dismantled.

Behind the coupling there is an asterisk and a needle bearing ensuring smooth rotation. If unacceptable wear is detected, they should be replaced with new ones. Revision and repair of the clutch is to inspect and replace the springs and cams.

All parts are installed in the reverse order. At the same time to install the clutch will also have to fix the piston, but it must be done with left rotation crankshaft a few millimeters to the top point.

The sequence of actions is typical for most chainsaws, with small deviations caused by the specific features of their design.

Work must be carried out carefully and carefully to avoid breakage of the mechanism or its individual parts.Without the presence of at least elementary knowledge of the construction of chainsaws, you should not take on replacing the asterisk - it is better to entrust this task to professionals.