Can the camshaft knock and what to do
Content
The problem can arise in engines with high mileage or in those whose maintenance was practically not monitored, they filled in counterfeit and cheap oil, rarely changed it, saved on the quality and timing of filter replacement.
Previously, there were motors in which rapid wear of the camshaft was the result of design and technological errors, now this does not occur, all engines are almost the same.
The principle of operation of the camshaft in the engine
It is possible to ensure the most efficient conversion of the chemical energy of the fuel into the mechanical energy expended on moving the car only if the optimal conditions for combustion in the cylinders are strictly observed.
A four-stroke engine must timely load the working volume with the required amount (and quality) of the air-fuel mixture, compress it, set it on fire in a timely manner and allow thermal energy to be spent on expanding the volume at maximum pressure on the piston.
A significant role in this is played by the valve timing. In fact, these are the angles of rotation of the crankshaft at which the valves open and close. There are two of them - inlet and outlet. If there are more valves, then this only means an increase in the number of intake and exhaust valves in order to interfere with the flow of gases as little as possible.
With the exception of unique and racing engines, the valves are closed by powerful return springs. But they open under the influence of eccentric cams of complex shape (profile) located on shafts rotating synchronously with the crankshaft. Here "synchronously" means a clear and unambiguous connection of rotational frequencies, and not their identity.
This shaft, and there may be one or more, is called a distribution or camshaft. The meaning of the name is to distribute the flow of the mixture and exhaust gases through the cylinders by opening and closing valves.
The angles at which the protruding cams are oriented relative to the drive gear or sprocket determine the valve timing. The shafts are driven by gears, a chain or a toothed belt from the crankshaft.
Any slippage or other change in the frequency ratio is excluded. Typically, the camshaft makes one revolution every two revolutions of the crankshaft. This is due to the fact that the cycle is determined by the gas distribution, and within the cycle there are four cycles, two cycles per revolution.
The main tasks of camshafts:
- ensure the accuracy and timeliness of opening and releasing (closing by a spring) of each valve;
- set all parameters of valve movement, speed, acceleration and change in acceleration of each stem during the opening-closing cycle, which is important at high speeds;
- provide the desired valve lift height, that is, resistance to the flow of filling the cylinders;
- to coordinate the intake and exhaust with each other over the entire speed range, often phase change systems are used for this - phase regulators (phase shifters).
Between the camshaft cam and the valve stem there may be intermediate parts: pushers, rocker arms, adjustment devices.
They always have the ability to set the thermal gap, manually during maintenance or automatically, using hydraulic compensators.
Reasons for knocking
Most often, in the form of a knock from the side of the gas distribution mechanism (timing), a change in valve clearances is manifested, as well as the appearance of backlash in the pushers and rocker arms. For example, the knock of a pusher in its cylindrical seat of the head when worn.
But over time, the knock begins to publish and the camshaft. This is due to the wear of its sliding fit in the beds (plain bearings) or a strong change in the profile of the cams, when silent operation is no longer possible with any setting of thermal gaps.
Due to wear of the bearings, the shaft can gain undesirable freedom both in the radial and in the axial direction. The knock will appear anyway. By ear, the knock of the camshaft should be distinguished from the knocks of valves, pushers and parts of the crank mechanism.
The knock of the valves is more sonorous, like that of the pushers, it varies in frequency, and at the crankshaft and pistons, the knocks are localized below the head. You can also distinguish by the frequency of rotation, which is half the camshaft, but it is more difficult.
What to do if there is a knock from the camshaft
They wear out, and unevenly, both the camshafts and their beds. Previously, there were repair technologies that included the replacement of liners or housings with bearing assemblies and grinding of shaft journals. Unfortunately, now the developers of motors are no longer thinking about repairs.
However, it is far from always necessary to purchase a block head with beds. There are repair technologies for spraying, followed by a groove for the exact size of the new camshaft. The shafts themselves, with strong wear, have to be changed.
But if we are talking about unique parts that cannot be purchased due to price or rarity, then spraying and camshafts on the necks and cams is possible, followed by processing to size and grinding.
For minor damage to the necks, polishing is applied, but this case does not apply to the topic, such shafts do not knock. Knocking will be a sign of extreme wear, when it is no longer possible to do without replacing large parts.
