Tech Information
Timing Belts
- 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.
Serpentine Belts
- 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.
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