Drive belt tensioner for your car

Securing a high engine speed with the tensioner for the poly V-belt

A belt drive is used for the transmission of torque and engine speed, where no transmission of control times is required. In the past, the fan belt was the standard used component. Nowadays, the old fan belt drives no longer meets modern requirements. For this reason the poly V-belt was developed. It offers a more reliable transmission at varying rotational speed. For maintaining the necessary tension, the tensioner (clamping arm) is used.

More performance from the dynamo

A dynamo or alternator supplies the power. For early cars, 3 Volt was enough. When headlights were developed, 6 Volt became necessary. With the increasing number of electricity users such as automatic window openers, etc. the operational tension increased to 12 Volt. Modern cars need at least 14,4 Volt. Hybrid cars are not operated below 24 Volt. Electric cars have an even higher voltage. A dynamo or alternator can supply this tension in two ways:

• Larger diameter of stator and rotor
• higher engine speed

Running an alternator with higher engine speed is the easy way. This saves space, weight and costs. Nevertheless the higher rotational speeds have an unfortunate side effect: the bearings wear quicker. However, this can be avoided with a trick.

Longer life span with freewheeling

Not the higher rotational speeds affect the alternator bearings, but rather the speed variation. At every load change and gear shifting of the engine the alternator is accelerated and decelerated. Particularly the deceleration affects the belt drive and the bearings of the alternator. To avoid this, modern dynamos have a so-called OAP (overrunning alternator pulley) installed. It is installed on the receiving belt pulley and ensures that the alternator shaft can rotate in only one direction. When the engine speed decelerates, the alternator runs on without straining the bearings and the belt drive. The deceleration and acceleration run smoother and have a positive effect on the life span. The traditional V-belts are not suitable for this sophisticated technology. Therefore a poly V-belt with a tensioner is used.

More friction with a poly V-belt (ribbed V-belt)

The poly V-belt is a flat belt, used for the transmission of engine speed and torque. Practically it is the successor of the V-belt (fan belt) which was standard for decades. The poly V-belt has small, wedge-shaped ribs on its inside to fortify and increase the friction surface. These ribs give him its name of ribbed V-belt. The broad layout of the poly-V belt ensures an optimal support between the giving and the receiving belt pulley. It is considerably thinner than the traditional fan belt and therefore more prone to rupture. In order to keep it tensioned within its technical parameter, the tensioner is used. The tensioner is a lever arm with an installed spiral spring and a tension disc. With its pressure roller or its tensioner disc it presses against the belt, ensuring an equal work load at variable engine speed.

Combined component tensioner

The tensioner is a combination of components, consisting of

• lever
• tensioning spring
• pressure roller or tensioner disc
• tension damper

The tensioner arm for poly V-belts is fixed on the engine block. Its replacement is very easy, but should always be performed within the prescribed maintenance intervals.

Defects of the poly V-belt tensioner

The tensioner of a poly V-belt is technically rather complicated, with different possible causes for malfunction. Therefore it is important to replace the component within its prescribed maintenance interval. This is not automatically the moment of belt renewal. Nevertheless the instructions of the manufacturer should always be observed.

A defect can occur:

• failure of the tensioning function
• jamming of the pressure roller
• loss of the damping function

When the poly V-belt tensioner can no longer maintain the necessary tension, the belt slips. This is heard as a loud squeak. Modern cars provide an error alert. The rotational speed of the alternator no longer corresponds with the engine speed. When the pressure roller jams, the poly V-belt ruptures after a short squeak. The alternator ceases functioning and the corresponding check light on the dashboard lights up. When the tensioner of the poly V-belt no longer dampens adequately, the belt can slip or rupture.

Life span and costs for a tensioner for a poly V-belt

The poly V-belt tensioner should be replaced according to the manufacturer's instructions. The spectrum is very wide. Many manufacturers prescribe a replacement every 40.000 kms, others indicate an operation time up to 240.000 km. Prices for a spare part start at ca. 23 EUR (± £20), and can, depending on type, run up to 240 EUR (± £ 215).