How Timing Belt Works

8/9/2019 How Timing Belt Works

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How timing belt works?

To understand the working of timing belt, lets first understand a belt drive.

A belt is a looped strip of flexible material, used to mechanically link two or more rotating

shafts. They may be used as a source of motion, to efficiently transmit power, or to track relative movement. Belts are looped over pulleys. In a two-pulley system, the belt can either drive the pulleys in the same direction, or the belt may be crossed, so that the direction of the shafts is opposite.

Fig. 1 Open belt drive: Both shafts rotate in the same direction

Fig. 2 Crossed belt drive: Both shafts rotate in opposite directions



Fig. 3 Open belt drive

Fig. 4 A pair of V-Belt Credit: wikipedia



T iming belt

A timing belt, or cam belt (informal usage) is a part of an internal combustion engine thatcontrols the timing of the engine's valves.

In the internal combustion engine application, the timing belt connects the crankshaft tothe camshaft(s), which in turn controls the opening and closing of the engine's valves. A

four-stroke engine requires that the valves open and close once every other revolution of thecrankshaft. The timing belt does this. It has teeth to turn the camshaft(s) synchronized withthe crankshaft, and is specifically designed for a particular engine. The job of the timing beltis to turn the camshaft(s) at exactly 1/2 the speed of the crankshaft while maintaining a

precise alignment. This means that the crankshaft will make two revolutions for every

revolution of the camshaft. Engines will have at least one camshaft, or as many as four camshafts in some of the V-type engines. The camshaft causes the intake and exhaustvalves to open and close in time with the pistons, which move up and down in the

cylinders. The valves must open and close at exactly the right time in relationship to the piston movement in order for the engine to run properly.If the valve timing is not correct,the pistons will come up and hit the valve, causing major damage to the engine by bending a

valve, destroying a piston or bending a piston rod.

Why use belt drive and not chain drive?



Gear and chain systems can also be used to connect crankshaft to the camshaft at the correct

timing but they constrain the relative location of the crankshaft and camshafts.As the

distance between centers of shafts increases, the driver and follower for such methods of transmission become large, unwieldy, and costly, and rigid links may be used to connect therotating pieces, as in the case of parallel rods of a locomotive. For a further increase of

distance, the transmission is attained by means of belts and pulleys, and, if the distance is

very great, by wire ropes and sheaves. Even where the crankshaft and camshaft(s) are veryclose together, as in pushrod engines, most engine designers use a short chain drive rather

than a direct gear drive. This is because gear drives suffer from frequent torque reversal asthe cam profiles "kick back" against the drive from the crank, leading to excessive noise andwear.

W orking

The timing belt rides on a crankshaft pulley and either one or two camshaft pulleys,depending on whether the car is a single overhead cam, double overhead cam, or, in the case

of rear wheel drive vehicles, a mid-engine cam. Each of the pulleys has a timing mark onthem. The marks must all line up according to the timing specifications on the car. The

pulleys are toothed, as is the belt. This keeps the belt from sliding around on the pulleys.



As the crank turns, it turns the crankshaft pulley, sending the timing belt in motion. The

timing belt, which runs up to the camshaft pulleys, turns the camshafts of the engine. In

turn, the lobes on the camshaft push the valves into an open or closed position. In a four-stroke engine, there are four phases:

1. I ntake: Air and fuel get sucked by a vacuum through the intake manifold and into the

cylinders on the first down stroke of the piston. At this time, the intake valves are open andthe exhaust valves are closed.

2. C ompression: The air and fuel mixture is compressed on the first upstroke of the piston. At this time, all valves are closed.

3. C ombustion: The compressed air and fuel mixture is ignited by a spark from the spark plug on the second down stroke of the piston. At this time, all valves are closed.

4. Ex haust : What comes in must go out---any excess air and fuel left in the cylinder gets pushed out through the exhaust valves on the last upstroke of the piston. At this time, theintake valves are closed and the exhaust valves are open.

If a timing belt stretches or breaks, it does not keep the valves in time with the rest of the

engine. The valves must open and close at the proper time, hence "valve timing." If the

valves are not open or closed at the appropriate time, the car will either run badly or not atall. There is one major thing to keep in mind when dealing with timing: most engines withdouble overhead cams and a good percentage of engines with single overhead cams are

interference motors. If the timing is off in an interference engine, the valves will open or close at the wrong time, causing them to hit the piston on the upstroke.



Automobile engines can be classified as either Free-Running or Interference,

depending on whatoccurs if piston/valve synchronization is lost. In the free-running

engine with the crankshaft still moving, there is enoughclearance between the valve

and piston, even if the cam stops with a valve fully open.

However, interference engines usually sustain damage if synchronizat ion is lost. As

illustrated,disrupted synchronization allows the piston and valve to collide causing

damage and veryexpensive repairs for the vehicle owner. Damage can occur tovalves, pistons, or heads, and insome cases, completely ruin the engine. Most i mport

(with higher compression engines) andmost car and light truck diesel engines are

interference.



Links:

Importance of timing belt replacement

http://www.youtube.com/watch?v=DGh-3fvK61I

Documents

How Timing Belt Works