Breather: what is it, how does it work and does it need to be serviced?
Cars have many tiny, hard-working parts that are completely invisible to the driver, but their importance is significant, and failure sometimes leads to serious problems. A trifle with a funny name “breather” – just one of them. Let's see where it is located, what it is for, how it works and whether it needs maintenance?
A breather is a valve for communication with the atmosphere, an indispensable element of all more or less large hollow equipment units in which lubrication and air are present.
As the shafts and gears rotate inside the sealed assembly, their friction heats the oil and the air above it. Accordingly, both liquid and gas increase in volume. After all, oil is never poured to the top – there is always an “air cushion” above it. Void, in other words. This empty space allows the oil to rise as it heats up, but not leak out. The assembly heated up from movement – the oil expanded, and part of the air escaped into the atmosphere. The car stood up, the unit cooled down – the oil decreased in volume, and the rarefaction pulled the missing air inside. Therefore, the pressure inside is always atmospheric, zero, if in a simple way.
To ensure equality of pressures inside and outside, in principle, a simple hole in the upper part of the assembly is sufficient – the gearbox housing or axle gearbox. However, you can't just make a hole: through it, abrasive sandy dust can get inside, to oil and gears – when driving on dusty roads, and moisture – when washing or driving in the rain. And when parking under a significant slope, the oil itself can also flow out. Although a breather in the form of a simple hole, covered with a breathable felt plug, can be found, for example, in units of small volume and working not with liquid, but with grease – for example, in gearboxes of large and powerful angle grinders or circular saws. But in cars where this valve is primarily found in gearboxes, transfer cases on all-wheel drive vehicles, as well as in axle gearboxes,
In a car, it is a small threaded fitting with a through hole, screwed into the body of one or another automotive assembly or mechanism. From above, this fitting is closed by a movable (capable of moving and rotating slightly vertically when pressed) cap-glass. And under the cap, the end of the fitting is closed with a rubber disk – a kind of “pill”, which is pressed against the fitting by a spring. Actually, this disc is the valve element of the breather.
The design is the most elementary, but there are several important nuances in its work that not everyone knows… Let's look at how it works, as they say, “in the basic version“!
In a good way, inside the body of the unit, where there is a breather, there should always be atmospheric pressure – the absence of both excess pressure and vacuum. But in fact, if you start to unscrew the breather, or at least just vigorously move its cap up and down and left and right, having achieved a “pill” shift, you can hear a quiet “zilch”. Moreover, the “zilch” can be the result of both light overpressure and light rarefaction. But both are the norm, not a malfunction!
The car drove off. Gears and shafts turn, friction heats them, and they are oil. It expands, warms and compresses the air layer above it. The air pressure inside the gearbox or axle rises. But the calibrated spring in the breather is extremely weak and presses down the rubber “tablet” with a negligible force of 10-12 grams. As soon as the pressure reaches some negligible value, barely above atmospheric pressure, it is released into the atmosphere, lifting the “pill”, overcoming the resistance of a frail spring.
The car stopped. The units started to cool down. The temperature and oil level decrease, the air pressure inside too. Gradually, a vacuum forms inside the bridge, since the spring presses a rubber tablet against the end of the breather fitting, sealing the bridge… However, in the end, the pressure inside, albeit with some delay, will become atmospheric. Why?
Remember that the spring is very, very weak. And the rubber “tablet” is not a precision valve with a perfectly lapped seat and disc. The breather is not designed to hold vacuum for a long time – this is inherent in its design, it is necessary! It passes atmospheric air into the unit at approximately the same rate as the vacuum grows there synchronously with the cooling of the oil. So the rarefaction inside, although it occurs, is insignificant and short-lived.
Well, the third situation. Actually, it is in it that the “pill” and the spring will play their role, because in the first two (systematic heating while driving and slow cooling in the parking lot) it was quite possible to do without them – the breather would work successfully, being just a “hole” communicating crankcases atmospheric units…
And this situation is a sharp cooling when the gearboxes of bridges, transfer cases and boxes are flooded with a sudden hit in a deep puddle or a conscious overcoming of fords. In this case, the vacuum inside the units occurs quickly and reaches noticeable values - for example, a rear axle heated to 90 degrees can fall into a deep gully of melt water with a temperature of about zero… Here, a vacuum jump is added to the force of the compression spring, which sucks the “pill” even more strongly to the end of the fitting, not allowing water, which can easily overflow through the level of the breather, be sucked into the gearbox or gearbox. The breather is self-sealing, and quite reliable.
With a short (albeit very sharp) cooling of the unit, the breather quite successfully protects its internal volume from the penetration of water from the outside, but what happens if you have to stay in the water up to the hub (or even deeper) for a long time? Say, when the assault on the ford dragged on or the SUV got stuck in a swamp? In this case, it is very likely that water will be sucked into the oil through rubber cuffs – stuffing box seals of axle shafts or CV joints. The gland seals have a profile that provides some self-sealing, but not from water from the outside, but from oil from the inside. The stuffing box as a whole is tight in both directions due to the constant pressure from the annular spring, but in the “outward” direction its elasticity is still noticeably higher than vice versa due to the shape of the profile. And if the new original seal can successfully resist being sucked in by the vacuum of water from the outside, then a battered and / or not very high-quality gland is unlikely. Although they can keep gear oil excellently in both cases!
For this reason, most owners of SUVs (especially not new ones) do not trust (and quite rightly) breathers in their regular version. They cut off the cap from them, throw out the spring and the “pill”, put thin hoses on the remaining fitting and bring them out in front into the engine compartment, and in the back into the cavity in the wing inside the trunk or into the cavity of the lamp. In general, to a clean, dry place and higher… In this case, the valve in the breather is no longer needed, and it continues to successfully fulfill its role of communication with the atmosphere. But owners of road cars do not need this procedure at all.
However, if you are not the owner of an SUV, then the breather can cause trouble for you. When it becomes clogged or simply turns out to be a product of a low-quality manufacturer. The fact is that for all its simplicity, this part requires very high precision and repeatability in the manufacture! The spring must be strictly calibrated in terms of compression force, the tablet must be made of very high quality rubber that does not allow deformation and sticking from oil and temperature. Distortions and jamming of the cap are strictly unacceptable, etc.
Defects and carelessness in the design of the breather cause it to quickly become dirty or sticky (which leads to squeezing oil through the seals when heated) and vice versa – easy passage of water inside (which leads to the formation of an emulsion from the oil). Even if the breather is initially of high quality and not “left”, then, despite its complete, at first glance, autonomy and self-sufficiency, it requires periodic “maintenance”, expressed in simple stirring and scrolling of the cap by hand. This prevents it from being overgrown with dirt inside and souring the “tablet”.
Although sometimes the low quality control of the manufacturer (this mainly concerns the domestic auto industry) forces car owners to intervene with modifications and in a conditionally serviceable unit… For example, there are frequent cases of oil emissions through the breather at the gearbox of front-wheel drive VAZs of old models. Since the garage practice (and even, it seems, some official letters from AvtoVAZ) recommended pouring oil into the box a little more than the nominal volume, the air cushion above the oil level turned out to be insufficient to compensate for thermal expansion – the lubricant began to ooze through the breather and flow through the box body… Drivers in such a situation, they carried it on a short hose higher, as is customary with jeepers.
Or take the rear axle breathers on the VAZ-classic, Volga and IZH of the vague 90s, when everything was done with your feet – the workmanship of the components was so terrible that at the factories they deliberately removed both the spring and the “pill” from the breathers, putting the cap back on and making from the breather, thus, a simple “hole” – if only it somehow worked and did not press the oil out… And if the owners of such cars did not drive along the channels of streams and did not park on the mountains, then they did not experience any special problems from such a simplification, and often they didn’t even know about it…