CLUB OF CAR FANS
WHAT DOES SHOES BREAK?
WHY SHOULDER NOT ALWAYS DO IT
TEXT / IGOR MIKHEEV
To remove the cover of the gas distribution mechanism on the Niva - not like on the Zhiguli - they suffered … But when they removed it, they realized: not in vain! The chain hung unsightly, completely loose - where can the poor soothing man deal with it! And when they got to the tensioner, then the questions seemed to disappear: here he is, the culprit! The supporting platform, breaking the welding points, broke away from the shoe (see photo) - now there is a "gap" between it and the plunger … 25 mm!
Generally speaking, the destruction of the chain tensioner shoe in the Zhiguli engines is nothing new. But in recent years, in our experience, there have been more such cases. Many believe that the reason is the poor quality of contact welding of the support platform with the shoe. But is only welding always to blame?
The chain drive has its (unpleasant!) Features. On the one hand, the chain, consisting of links and hinges between them, is very flexible - and this even surpasses the timing belt. On the other hand, a chain transmission (such is its nature), even when in perfect condition, works with vibrations caused by the interaction of links with teeth. The hinges and links wear out, for various reasons, unevenly, so even a well-tensioned chain transfers torque to the camshaft unevenly. But these factors, complicating the operation of the chain, are not limited.
Let's turn to fig. 1. On the left is a pie chart of the valve timing of the VAZ 2121 engine 3. The open position of the valve (intake and exhaust) corresponds to a 232 ° angle of rotation of the crankshaft. Points A and B mark its positions at which the valve springs are compressed with maximum force.
And on the right is a diagram of the forces acting on the springs for the 1st, 2nd, 3rd and 4th cylinders. Below is the overall picture. The forces in the chain driving the camshaft pulsate. When the rocker passes over the top of the cam, the force of the compressed spring no longer slows down the camshaft, but, on the contrary, helps its rotation - this is similar to a descent from a hill. Such moments are shown in the diagram by arrows. The stretched chain does not allow the camshaft sprocket 5 to sharply accelerate rotation (Fig. 2b), and then to sharply slow down (Fig. 2c); the last two situations are realized if the chain is not tensioned, and the damper 6, as usual in this case, is broken.
By the way, in this state, the chain can jump over the teeth of the sprocket. What this threatens is hardly necessary to explain again. Here, the repair is unlikely to be limited to replacing the tensioner, shoe and damper. But all the troubles begin with a poorly working tensioner, which was repeatedly mentioned on the pages of the magazine. If the chain is loose, then four times in two turns of the crankshaft it sags as much as possible, and then it stretches with a click, striking the shoe and the damper. And if the work of the capricious "machine" is in doubt - pull the chain by hand, without relying on this mechanism! (As described in ЗР, 1999, No. 7, p. 127.)
Fig. 1. Diagram of the valve timing (a) and the forces in the valve springs (b).