A timing belt is a part of a combustion engine that controls the timing of the engine's valves. Some engines use timing gears. The term "timing belt" is also used for the more general case of any flat belt with integral teeth. Such belts are used for power transmission or to interchange rotary motion and linear motion, where either high loads or maintaining a specific drive ratio are important. A common non-automotive application is in linear positioning systems. Such belts have also been used in efforts to make a cleaner, lower- maintenance bicycle transmission but have never become popular in this application.
In the internal combustion engine application, the timing belt connects the crankshaft to the camshaft(s) that 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 turn of the crankshaft. The timing belt does this. 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 and oil pump.
Gear or chain systems can also be used to connect the crankshaft to the camshaft at the correct timing. However gears and shafts constrain the relative location of the crankshaft and camshafts. A belt or chain allows much more flexibility in the relative locations of the crankshaft and camshafts. Camshaft drives, whether gears, belts or chains are also able to even out wear, since the chain or belt can be made such that the number of teeth on the belt is co prime to the number of teeth on the crankshaft and camshaft sprockets, thus ensuring that each tooth on sprocket doesn't end up on the same tooth on the belt repeatedly.
While chains and gears may be more durable, rubber belts are quieter in their operation (in most modern engines the noise difference is negligible), are less expensive and are mechanically more efficient, by dint of being considerably lighter, when compared with a gear or chain system. A timing belt is a specific application of a synchronous belt used to transmit rotational power synchronously.
Timing belts are typically inaccessible and difficult to inspect. The manufacturer recommends replacement at specific intervals. The manufacturer 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. Failure of the timing belt will seize the engine and often leads to damage that is uneconomic to repair.
Interference. Depending on the design of the engine, the piston and valve paths may "interfere" with one another and incorrect timing in their movements may result in the piston and valves colliding. (Such designs are also called "interference head" or "interference engines", and include virtually all diesel engines. Conversely, non-interfering engines, such as the Mazda B engine, are called "free-wheeling" or "non- interference" engines.)
In interference designs, regular service is especially important as incorrect timing may result in the pistons and valves colliding and causing extensive engine damage and therefore costly repairs. The piston will likely bend the valves or if a piece of valve or piston is broken off within the cylinder, the broken piece may cause severe damage within the cylinder, often also affecting the crankshaft. Some manufacturers, such as Nissan, have switched back to timing chains for the majority of their engines because of the breakage problems associated with belts. However, in some newer engines, timing belts are designed to last the effective life of the engine.
When a timing belt is replaced, care must be taken to ensure that the valve and piston movements are correctly synchronized. The usual failure mode of a timing belt is stripped teeth (which leaves a smooth section of belt where the drive cog will slip) rather than an outright snapping of the belt, which is very uncommon. 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 dramatically shortened. All engines feature an adjustable tensioning roller to allow correct adjustment of belt tension.
A timing belt is typically rubber with high-tensile fibers (e.g. fiberglass or Twaron / Kevlar) running the length of the belt.
Rubber degrades with higher temperatures and with contact with motor oil and antifreeze. 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" (HSN). Older belts have trapezoid shaped teeth. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Aftermarket timing belts may be used to alter engine performance. OEM timing belts "will stretch at high rpm, retarding the cam and therefore the ignition." Stronger, aftermarket belts, will not stretch and the timing is preserved. In terms of engine design, "shortening the width of the timing belt reduce[s] weight and friction."
The first known timing belt was used in 1945. The German Goggomobil micro car was the first mass produced vehicle to use a timing belt in 1950. The first American vehicle to use a timing belt was the 1966 Pontiac Tempest. The Vauxhall Slant Four was the first production overhead cam four-cylinder design to use a timing belt, a configuration that is now used in the vast majority of cars built today.
A serpentine belt is a single, continuous belt used to drive multiple peripheral devices in an automotive engine, such as an alternator, power steering pump, water pump, A/C compressor, air pump, etc. Idle pulleys, and/or belt tensioners may also guide the belt. It was invented by Jim Vance while working for the Gates Rubber Company and first used by Ford Motor Company for the 1979 Mustang.
It is more efficient than the older multiple belt system. By using a single, wider belt instead of multiple, thinner belts, the belt may be put under increased tension without stretching. Higher tension reduces slip, which increases belt life and mechanical efficiency. Reduced slip can allow the use of lower-ratio pulleys; this reduces the load on the engine, increasing gas mileage and available power. Additionally, it is easier for the driver to know when the belt has broken, since this will cause the steering resistance to suddenly increase. With multiple belt systems, if a single belt breaks, such as the alternator belt, the driver may not realize that there is a problem until the engine fails completely.
Serpentine belts are also much easier to maintain and change, since there is no need to remove multiple belts to replace a single broken or worn belt. Removal of belts can itself cause stress and premature wear.
The drawback of this single belt is that if the belt breaks, the vehicle loses all peripheral devices. There are some cars that use two serpentine belts for their system, such as the 95–99 DOHC Nissan Maxima.
It is estimated that one in every three cars in U.S. roads, or approximately 70 million vehicles, is equipped with a timing belt.
On a four-cycle engine, the timing belt turns the camshaft(s) at half the crankshaft speed. For every two turns of the crankshaft, the camshaft turns once.
OHC (overhead cam) engines will have at least one, and, in case of large V-8s, as much as four camshafts.
On some engines, checking the timing belt is as hard as it is to change it.
The best protection against expensive engine damage, caused by a timing belt failure, is to check it at regular intervals, and to replace as recommended by the manufacturer.
The intervals recommended are usually between 60,000 to 100,000 miles.
Technicians also recommend that the water pump be replaced at the time of a timing belt change.
A professional qualified mechanic, using the proper procedures and tools for the job, should perform replacing a timing belt. We discourage the replacement of OHC components by the general public.
Severe and costly engine damage can result from simple mistakes when installing a timing belt.
TUF Auto Parts 5690 Bandini Blvd., Bell, CA 90201 Ph: 323-266-0099 Fax: 323-266-0666 www.tufautoparts.com email@example.com