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High mechanical output on demand, completely independent of wind or water power – the spread of the steam engine unleashed the industrial revolution in the factories. The individual production machines were driven via steel shafts mounted on the ceiling of the building, pulleys and flat drive belts made of leather. Introduction 3 Timing belts 4 Function 5 D esign/materials 6 Profiles/handling 9 M aintenance and replacement 10 C hanging the timing belt 12 Timing chains 13 The first cars and motorcycles also used this power transmission principle. However, the flat belts in this application...

Timing belts Timing belts guarantee absolutely synchronous power transmission since a positive-fit connection is created between the drive pulley and the belt by means of the teeth. In internal combustion engines they are used to drive camshafts, fuel injection pumps, balancer shafts and water pumps. The timing belt transmits the rotary motion of the crankshaft to the camshafts. Their cams operate transmission elements such as tappets, rocker arms or cam followers, which ultimately transfer the motion to the valves. Starting from the camshaft, the valves are therefore opened and then closed...

Timing belts Timing belt design Image Fabric backing Highly stressed timing belts are reinforced on the back of the belt with a temperatureresistant polyamide fabric which also increases the wear resistance of the edges. A timing belt is made up of four main components: > Polyamide fabric > Elastomer body > Tension members > Fabric backing (depending on finish) In addition, there are a few special cases, for instance: > iming belts which run in oil and enable a slimmer engine T design. Their components are specially equipped for this application environment and are resistant to oil and...

Timing belts Operation of a 4-stroke engine: The engine only runs properly if the rotary movements of the crankshaft and the camshafts are synchronized. The first timing belts used a trapezoidal shape which was already in use in industrial applications (L profile). As requirements relating to noise properties and load transmissions increased, curved tooth shapes (HTD and STD profiles) became established. The circular shape ena- Profile identification bles uniform distribution of the forces acting on the tooth and avoids tension spikes. The pitch (t) is the distance between two teeth and is...

Timing belts Maintenance and replacement Problem Timing belts are maintenance-free, i.e. they do not require retensioning. They endure high levels of stress as a result of the high temperatures in the engine compartment and the constant flexing and are subject to aging and constant wear. Their condition should be inspected as a precautionary measure during servicing in accordance with the vehicle manufacturer’s specifications. Irregularities are then identified in good time. If the timing belt snaps while the engine is running, the engine valves and pistons can suffer high-impact...

Timing belts Changing timing belt When changing the belt, all the steps specified in the vehicle manufacturer’s instructions must be carried out. It is essential that any special tools specified as necessary be used. This ensures that the relative positions of the crankshaft, camshaft and, if appropriate, fuel injection pump to each other are not changed. Under no circumstances may force or levering tools be used when mounting a timing belt on the pulleys. The running direction is unimportant unless it is indicated by a direction arrow. Timing belt with markings Some timing belts have...

Timing belt drive components The timing belt precisely controls the combustion process in the engine. For the timing belt to operate safely and reliably, various components are required to guide it and ensure the correct tension. All the belt drive components are subjected to extreme stresses in modern engines, such as vibrations or large fluctuations in speed and temperature. They affect the entire timing system and call for exacting quality standards. Idlers and guide pulleys The position of the driven belt pulleys normally requires the timing belt to be guided using idlers and/or guide...

Timing belt drive components Tensioners Manual tensioning pulley The entire pulley is turned via the eccentric fastening bore until the required belt pretension is achieved and the pulley is then fastened in that position. This simple system cannot compensate for changing factors (heat, wear) and performs no damping function. Other tensioning systems have therefore gained in popularity since the 1990s. Various tensioning systems are used to generate the belt tension in the timing belt drive and keep it as constant as possible. They are fitted on the slack side. – Short-term changes in...

Timing belt drive components Water pumps The high temperatures generated in an i.c. engine have to be dissipated in order to prevent damage as a result of overheating (defective cylinder head gasket, cracks in the cylinder head). Liquid-based cooling is the method of choice in automotive engineering. The thermally stressed areas of the engine block and cylinder head contain channels (cooling jacket) through which the coolant flows. This transports the generated heat to the radiator which discharges it into the atmosphere. The water pump conveys the coolant in a circuit which ensures that...

Timing belt drive components Typical fault pattern Coolant A blend of water (distilled or demineralized) and ethylene glycol forms the basis of the coolant. Ethylene glycol lowers the freezing point while at the same time raising the boiling point of the blend, which enables more heat to be dissipated. With a ratio of 1:1 in the blend and at atmospheric pressure the freezing point is approx. –35°C and the boiling point approx. 108°C. Many different materials are used within the cooling circuit and can cause corrosion when they are in contact with each other. In addition to its function as a...