Liquid trapped

Liquids, unlike gases, are practically incompressible. And that determines their interesting behavior if they find themselves in a... trap. So how does the liquid behave in the trap?

So how does the liquid behave in the trap?

For example, if a pipe in which water flows quickly is suddenly blocked, then the energy of moving water can cause trouble. Since the liquid is compressed with great difficulty, the high-pressure water head develops a very high pressure, often breaking pipes. This phenomenon is called water hammer. Therefore, in apartments they make the cranes slowly close so that there are no hydraulic shocks.

But the water hammer can be made to work and benefit. The water fountain rises high enough with a water hammer (Fig. 1). So you can supply water to the top, and without pumps and energy costs. The device that allows this is called a hydraulic ram. This is how a liquid in a trap behaves when it can still escape from it.

And it happens that the liquid is specially driven into traps from which it is not so easy to escape, and in these cases it develops tremendous pressure. Take at least the lubrication of machines when they trap oil.

The first miracle is hydrodynamic lubrication. Imagine a shaft with a clearance sitting in the sleeve. All of this is in liquid oil. Until the shaft rotates, it rests on the sleeve and touches it. But if the shaft begins to rotate, then, as it were, it pops up in the sleeve and stops touching it. An oil wedge forms between the shaft and the sleeve, in other words, oil trapped in a trap.

The fact is that the oil, being captured by the rotating shaft, where its clearance with the sleeve is still large, is actually driven into a trap - to where the gap between the shaft and the sleeve has not yet appeared while the shaft was lying on the sleeve. The oil pressure rises strongly, and it lifts the shaft, forming a small gap, even if large forces are applied to the shaft.

This is how plain bearings work in all engines. When it rotates, the shaft does not touch the sleeve at all, these parts practically do not wear out. Usually they think that rolling bearings create less resistance than sliding. But this is absolutely wrong. They are better than ball ones, withstand shock, dampen vibration, and make less noise. The only thing such bearings do not like is low rotational speeds, when the oil does not form a wedge and the trunnion metal rubs against the sleeve metal.

The same oil wedges are formed between the teeth of gears and gears working in oil. The gear teeth practically do not touch each other and do not wear out, between them the same oil, driven into a trap, is an oil wedge.

The second miracle is the "glazing" of oil at high pressures. Locked in a trap, sandwiched between two rotating and pressed against each other forces, the oil thickens, as it were, "glass", and begins to transmit the shear load. But thin films of oil at rapidly occurring and discontinuing pressures of the order of 1 GPa (1000 MPa) begin to behave like jelly, jelly or even glass - to transmit the shear load. The force transmitted by the "glazed" oil is not large, it is several times less than the usual friction forces under the same loads, but there is no intensive wear so typical for dry friction. Therefore, such a "glazed" oil is used in continuously variable transmissions - variators, which are replacing today's gears.

How are stepless gears arranged?

How are stepless gears arranged? Let us explain their principle by the example of the simplest of them (Fig. 2). It is called frontal, because two ice rinks - disks - are pressed here against each other as if by foreheads. If the large disk (left) rotates at a constant speed from the motor, then the speed of the small one depends on its position on the axis. In the lower extreme position, this speed is maximum; as it approaches the center, it falls, in the very center of the large one the small disk stops altogether, and on the upper side of the large one the small disk starts to rotate in the other direction. The large disk is connected to the engine, and the small disk to the wheels, through a reduction gear, of course. For variators even developed a special, high-traction oil - a tractor, glassing stronger than others.

This is how a liquid behaves in a trap: it reduces friction in bearings and increases it in CVTs, helping to transmit rotation without steps, increasing, at the same time, the service life of machines and mechanisms.