Caring for your car: cam belts
Most cars use a cam belts, often referred to as timing belt. These belts form an integral part of the engine and are vital for its function. Manufacturers recommend changing these at certain mileage/kilometres and this varies from make to make, sometimes they specify it to be changed when the car reaches a certain age, even though it may not have reached the recommended mileage/kilometres. A good mechanic will also, replace the timing belt tensioner at the same time as the belt is replaced. This is a more cost effective idea than changing it at a later date.
In modern engines cam belts and matching components are often subjected to a great deal of stress. High rotational speeds, vibrations and extreme temperatures (especially here in Tenerife) can easily have a negative effect on the functioning of parts. Cracks may appear below the metal surface, overheating, worn-out sealing rings of ball bearings can all lead to increased metal wear and even the jamming of pulleys, which in turn can easily cause belt damage. If a timing belt snaps when the engine is running, this can cause costly and sometimes irreparable damage to your engine. This would be due to the fact that some of the valves will have been held open when they should not be and thus will be struck by the pistons.
The usual failure of timing belts is either stripped teeth (which leave a smooth section of belt where the drive cog will slip) or delamination and unravelling of the fibre cores. Correct belt tension is critical – too loose and the belt will whip, too tight and it will whine and put excess strain on the bearings of the cogs. In either case belt life will be drastically shortened. Aside from the belt itself, also common is a failure of the tensioner, and/or the various gear and idler bearings, causing the belt to derail, hence why it’s advisable to change the tensioner at the same time as the belt.
In an internal combustion engine application, the timing belt connects the crankshaft to the 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 turns of the crankshaft. The timing belt does this very important job. It has custom teeth to turn the camshaft(s) synchronized with the crankshaft and is specifically designed for a particular engine. In some engine designs, the timing belt may also be used to drive other engine components such as the water pump.
A timing belt is typically rubber with high-tensile fibres running the length of the belt as tension members.
Rubber degrades with higher temperatures and with contact with motor oil. Thus the life expectancy of a timing belt is lowered in hot or leaky engines. Newer or more expensive belts are made of temperature resistant materials such as “highly-saturated nitrile”.
Older belts have trapezoid shaped teeth leading to high rates of tooth wear. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Timing belts are typically inaccessible and difficult to inspect. The manufacturer or your mechanic may also recommend the replacement of other parts, such as the water pump, when the timing belt is replaced because the additional cost to replace the water pump is negligible compared to the cost of accessing the timing belt. You may then find it much more cost effective to change the timing belt; tensioner and water pump all at the same time.