Chain tensioner
A chain tensioner is a device for tensioning a chain ; it is used in a wide variety of machines and drives. Chain tensioners are known for the derailleur gears of a bicycle, the timing chain of an internal combustion engine and tracked vehicles .
Chain drive on a bicycle
When using a derailleur system, a bicycle chain must run on sprockets of different sizes with the same tension in order to ensure correct shifting. For this purpose, the derailleur of a derailleur has a spring (usually attached in the housing of the mounting screw on the frame / derailleur hanger) and a cage with two pulleys through which the chain runs. The deflection via the cage is variable in position and angle. The starting position can be adjusted using a special adjusting screw.
Bicycles without gears ( single speed dial, Fixie ) may have a chain tensioner. This allows the changing use of sprockets of different sizes (front or rear) for different areas of application without the chain having to be shortened or lengthened each time.
Chain tensioners ensure that the chain runs smoothly and without vibrations and guarantee quiet, long-lasting operation with loss of lubrication. In the worst case, an insufficiently tensioned chain can skip the teeth when accelerating hard or when driving uphill. The driver then steps into space and falls. A chain that is tensioned too tightly has high internal friction, runs with difficulty, wears out quickly and makes cracking noises.
Even historical bicycles often have horizontal rear dropouts that are open to the rear, where the chain must be tensioned with chain tensioners that can be adjusted using nuts.
Timing chain in the combustion engine
The chain tensioner is used to tension the timing chain of a camshaft drive . The chain must always be slightly tensioned because a loose chain has play and the valve timing is not exactly correct. It would also cause noise (rattle) and excessive wear of the chain and sprockets would be the result.
The chain tensioner does not press directly on the chain, but rather on a tensioning rail along which the chain slides. This tensioning rail is pivotably attached to the engine block on one side and on the other side it is pressed against the chain by the chain tensioner.
Because the chain expands when it is heated and contracts again when it cools down, a chain tensioner is required that can continuously compensate for this change in length. A fixed jig, e.g. B. with an eccentric, can therefore not be used. Likewise, purely spring-operated tensioners are not suitable because they would have to press the chain with too much tension in order to reliably prevent chain vibrations.
That is why hydraulic chain tensioners are often used, which work in a similar way to hydraulic valve lifters in valve trains . In these, a relatively weak spring generates enough contact pressure to keep the chain pretensioned with a minimum of backlash. This spring is built into a cylinder and presses on a piston. In addition, when the engine is running, the cylinder is filled with lubricating oil via a check valve using the oil pressure circulating lubrication that is common in combustion engines . If pressure acts on the piston (e.g. when the chain cools down), the oil is forced out of the cylinder through a small bore or annular gap . If the force is only effective for a very short time, as occurs periodically due to the chain oscillations, the tensioner remains almost rigid because the oil cannot be pushed out through the opening provided so quickly. If the chain becomes longer, the spring pushes the piston against the chain. This happens in a phase in which the chain is just a little loose due to the chain oscillation. Oil immediately flows into the cylinder through a ball valve. The chain is thus immediately retensioned.
