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RELATIVES BRYUCH LEG BELT
One of the oldest types of mechanical gears, which has found application in automobiles, is a belt drive. These days have to deal
with its different types.
And the camshaft, and the generator, and the pump most often rotate thanks to the strap (Fig. 1). Not only that, the modified toothed belt presses the usual metal chain. For example, on some, even heavy and powerful motorcycles, the rear wheel is also driven by a belt. The secret of such widespread use lies in the advantages of the belt: it is cheap, easy to manufacture, works smoothly, without noise. He does not need special devices for damping oscillations, as, for example, in a chain transmission of a “Zhiguli” motor. If a damper “kicks” in it, the chain can easily jump over the teeth of the sprockets - not far from the bent valves.
A toothed belt (Fig. 1, c), designed to transmit great effort, under normal conditions, practically does not lengthen - in contrast to the motor chain. It is rarely necessary to pull it up: the main reason is not the hood, but the wear (abrasion) of the contact surfaces of the belt and gear pulleys. By the way, modern timing belts are very durable: they rarely require replacement before 60–70 thousand kilometers, and sometimes after 100 thousand or more!
Of course, we do not recommend traveling more than expected: there is a high probability of destruction of a belt that has exhausted its resource. Having looked at the teeth of the used belt (it is best to turn them out), you will find cracks near the bases (Fig. 2). These are the first signs of fatigue failure. A little more - and the teeth are likely to cut. Of course, the belt requires accuracy when repairing or servicing the motor. It is easy to damage it with a knife, a screwdriver, even a casing - it is not iron. Keep an eye on the condition of the motor oil seals: leaked oil will certainly fall onto the belt and at once break the reliable connection between it and the gear: an oily belt can easily slip through the pulleys and become completely toothless. Of course, you should carefully adjust the belt tension - hauling or loosening will also shorten its service life.
However, these recommendations apply to other belts, such as multi-ribbed (Fig. 1, b), often combining all mounted units under the hood (see Fig. 5). If everything is in order there, such belts in their durability compete, say, with an engine, gearbox. You ask how this is achieved?
The fact is that a multi-ribbed belt has strong cord material in the “neutral” layer, which, when bent on pulleys, almost does not undergo tension or compression. Many closely spaced small wedges ensure good adhesion of the belt to even the smallest pulleys even at a low tension (in comparison with conventional V-belts), and this increases the mechanical efficiency of the transmission, reduces the load in it, and increases the service life of the belt. It is the multi-ribbed belts that made it possible to create such “picturesque” kinematic schemes of drives as in fig. five.
There are, of course, belt drives and disadvantages. For example, large radial dimensions. In order to transfer forces with which a chain can easily cope with an ordinary V-belt, the dimensions of the pulleys and the belt itself will have to be made large.
For a V-belt (especially as the transmission parts wear), slippage is characteristic, increasing with increasing transmitted force, up to complete slipping. In other words, the gear ratio for a conventional V-belt is a variable parameter. This is acceptable, for example, in the drive of a generator, coolant pump or power steering pump, but completely unacceptable for a gas distribution mechanism, where the shafts are “mathematically” connected: the camshaft must rotate twice as slow as the crankshaft - and nothing else.
The objective disadvantage of any flat or V-belt is the need for such a tension, in which the transmission works reliably. And this overloads the bearings, especially in the engine, where the pulleys are very worn (Fig. 3). With such, even a heavily tightened belt transmits rotation to the aggregate (for example, pump, generator) unreliably. Because of this, in the "classics" of VAZ, there are frequent cases of breakage of the pump drive roller along the front groove for balls.
That is why the belt must be properly tensioned so that it does not slip, but does not overload the bearings. But doing this to an inexperienced person is not as easy as it might seem after reading the instructions. It usually indicates only the deflection of the branch of the belt under the action of a certain effort of the hand. Yes, everyone’s hands are different! Not everyone boasts of having the ability to push exactly two kilograms with effort. In this sense, even “funnier” are some techniques for checking the tension of the toothed belt: try to twist it with your fingers 90 °! On the VAZ 2108, "experienced" found a useful diagnostic sign: when they are slightly tightened, the belt "roars" loudly with an increase in speed - then it is slightly weakened. Unfinished is very vibrating.
The service life of the V-belt decreases sharply when the pulleys connected by it do not work in the same plane or have a large runout - especially axial (Fig. 4, a, b, c). The latter should not exceed 0.1 mm at a radius of 100 mm, which, alas, is not often found on domestic cars. So belts burst in such machines much earlier than the allotted time. One treatment is to pick up new pulleys (or, if possible, correct old ones) and put them on the same plane. And then you can forget about the belt for a long time, although changing it, as a rule, is nothing.
Fig. 1. Belt drives: a - with a V-belt (next is a more flexible version with "corrugations" on the inner surface); b - with polycline; in - with gear.
Fig. 2. Most often, the belt breaks down due to fatigue of the tooth material.
Fig. 3. When the pulley is worn, the V-belt does not work properly.