In a piston engine, either a timing belt (also called a cambelt) or timing chain or set of timing gears is a perishable component used to synchronize the rotation of the crankshaft and the camshaft. This synchronisation ensures that the engine's valves open and close at the correct times in relation to the position of the pistons.
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In most piston engines, the camshaft(s) are mechanically connected to the crankshaft. The crankshaft drives the camshaft (via a timing belt, timing chain or gears), which in turn actuates the intake and exhaust valves.[1] These valves allow the engine to inhale air (or an air/fuel mixture) and exhale the exhaust gasses.[2]
The most common devices to transfer the drive are toothed rubber belts, metal timing chains or a set of gears. The teeth of the belt/chain/gears mesh with both the crankshaft and camshaft(s), thereby synchronising their motion.
In many older overhead valve engines, the camshaft is located in the block near the crankshaft, therefore a simple gear system is often used to drive the camshaft. Overhead camshaft engines mostly use timing belts or timing chains, since these are better suited to transferring drive over larger distances. Timing belts were common on mass-production cars until the s and s,[citation needed] however since the s timing chains have become more common due to the replacement intervals required when using timing belts.
The phrase "timing belt" usually refers to a rubber toothed belt. The advantages of timing belts are typically a lower cost, reduced friction losses,[citation needed] less noise and that belts traditionally do not require lubrication.[4] The main disadvantage is that belts wear over time, therefore belt replacement is recommended at specific intervals.[5][6] Replacement of the engine's water pump at the same time is often recommended, since the water pump is also subject to wear and easily accessed during the replacement of the timing belt.
Timing belts are typically located in front of the engine and are often behind a cover for protection against dust and debris. However a few engines since have used "wet timing belts", whereby the belt is lubricated by engine oil to reduce friction losses by 30% and thus reduce fuel consumption by 1%.[7] In some engine designs the timing belt may also be used to drive other components, such as the water pump and oil pump.
A timing belt is typically made from rubber, although some belts are instead made from polyurethane or neoprene.[8][9] The structure of the belt is reinforced with corded fibres (acting as tension members)[10] and the toothed surface is reinforced with a fabric covering.[11]
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. Also, the life of the reinforcing cords is also affected by water and antifreeze, so it is important that belt that can be exposed to water is able to drain the water away quickly.
Older belts have trapezoid shaped teeth leading to high rates of tooth wear.[citation needed] Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Manufacturer-specification timing belts may stretch at high rpm,[citation needed] retarding the cam and therefore the ignition.[12] Stronger aftermarket belts will not stretch and the timing is preserved.[13] When designing the timing belt, a wider belt increases its strength however a narrower belt reduces weight and friction.[14]
The usual failure modes of timing belts are either stripped teeth (which leaves a smooth section of belt where the drive cog will slip) or delamination and unraveling of the fiber cores. Breakage of the belt, because of the nature of the high tensile fibers, is uncommon.[15] Often overlooked, debris and dirt that mix with oil and grease can slowly wear at the belt and materials advancing the wear process, causing premature belt failure.[16]
Toothed belts were invented in the early s, for use in textile mills.[17] The first known automobile engine to use a timing belt was the American Devin-Panhard racing car, used an engine converted from pushrods to overhead camshafts through the use of a toothed belt made by the Gilmer Company.[18][19] This car won the Sports Car Club of America (SCCA) National Championship in .[20]
The Glas was the first mass-produced vehicle to use a timing belt. The Pontiac OHC Six engine was the first US mass-produced vehicle to use a timing belt,[21][22] while the Fiat Twin Cam engine was the first mass-produced engine to use a timing belt with twin camshafts. Carmakers began to adopt timing belts in the s and compared to timing chains are less expensive, smaller, lighter, quieter, isolate harmonics of the crankshaft from the valve train, require less power than chains and can potentially function without lubrication. Timing belts are usually made of Neoprene or HNBR.[23][7]
Metal timing chains have become more widespread in car engines produced since the s, due to the lack of the regular maintenance task of replacing a rubber timing belt. While the chains themselves are subject to minimal wear, lubrication of the chain or failure of the tensioner and chain guides can cause maintenance issues. By contrast to rubber-based timing belts which give no indication of snapping while one is driving, early symptoms of a timing chain system becoming worn exist, and these include a rattling noise from the front of the engine.[24]
Most pushrod engines, where the crankshaft and camshaft are very close together, use a short chain drive rather than a direct gear drive. This is because gear drives suffer from frequent torque reversal as the cam profiles "kick back" against the drive from the crank, leading to excessive noise and wear.[citation needed]
Timing gears are used in various overhead valve engines, due to the close proximity of the camshaft to the crankshaft.
Fiber or nylon covered gears, with more resilience, are often used instead of steel gears where direct drive is used. However, commercial engines and aircraft engines use steel gears only, as the other materials can fail suddenly and without warning.[25]
Failure of the timing system will prevent an engine from running. Many modern cars use interference engines, which can suffer catastrophic damage in the event of timing system failure,[26] since the loss of synchronization between the crankshaft and the camshaft will cause the valves to collide with the pistons.
Warning signs that a timing chain needs to be replaced include:[27]
Aside from the belt/chain itself, also common is a failure of the tensioner, and/or the various gear and idler bearings, causing the belt/chain to derail. Furthermore, in engines where the timing belt drives the water pump, failure of the water pump can cause the pump to seize, which can break the timing belt or chain. For this reason, timing belts and chains are often sold as part of a kit with the water pump, tensioner, and idler pulleys so that these parts can be replaced to prevent timing belt or chain failure due to failure of these parts.
The buzzer sounds off; it's halftime. Your band takes to the field. Before you know it, though, the trumpets play off-beat, the cymbals slam into each other and the saxophone player simply walks off the field. This, although exaggerated, is what could happen if the drum major, the keeper of tempo, went missing. And in the world of automotive maintenance, visualizing this scenario can help illustrate the timing belt's role and what can go wrong without routine replacement.
Think of the synthetic rubber timing belt, which is reinforced with fiber cords, as the drum major of car maintenance -- meaning it keeps everything in the engine in sync. When the engine is on, it's in constant, timed motion, thanks to the belt, the connection between the crankshaft and camshaft.
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So why is this important? The crankshaft converts linear energy from the pistons, which move up and down, into rotational energy that eventually turns the wheels. The camshaft opens and closes the engine's valves to allow air and gas in and out of the engine. The timing belt links the two in harmony. Without it, the pistons and valves would collide.
Obviously, this is bad news for vehicle maintenance as this internal collision can cause destruction fair amount of engine damage. Therefore, it's important to stay ahead of your belt's lifespan -- traditionally replacing it every four years or 60,000 miles (96,561 kilometers) or, in newer vehicles, every 100,000 miles (160,934 kilometers). Be sure to check your vehicle's maintenance manual to see what your car or truck's manufacturer suggests.
So we've all accepted the timing belt's importance, but that doesn't mean you have to spend your days worrying that your belt will force you to pay a large maintenance bill. Instead, you can act as the band director and keep that timing belt in check by watching its wear and arming yourself with the knowledge to replace it. Read on to learn about the wear of your belt, typical tools needed in a repair and how to replace it.
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You may not even think twice about your timing belt until it nears replacement time, but don't be lulled into a false sense of security because you haven't hit 60,000 or 100,000 miles (96,561 or 160,934 kilometers) yet on your timing belt. If you happen to be working on your engine and can see the belt, take a look for cracks, shredding or excessive slack.
Furthermore, keep in mind that your timing belt will attain natural wear and tear from the engine environment -- a toasty world that can get up to more than 500 degrees Fahrenheit (260 degrees Celsius). If there are any additional outside factors to the belt's wear, you may need to pick up the pace on vehicle maintenance of your belt. Examples of other factors affecting your auto maintenance schedule include:
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Considering these factors and routine car maintenance, chances are you'll need to replace your belt during your ownership of your vehicle. And when that time comes, there are certain tools needed for a timing belt replacement. If you're accustomed to doing your own repair work, you'll likely have most of the tools you need. These include:
In addition to these tools, you will also need some materials specific to timing belt replacement, such as a new timing belt, timing light, timing belt cover gasket set, belt tension gauge, and bolts or pins to hold the camshaft position during your work. Depending on your vehicle's make and model, you may also need a harmonic balancer puller or three-jaw gear puller to remove the crankshaft pulley if it doesn't just slide off.
And, as with most auto maintenance procedures, one size doesn't fit all. Procedures, tools and belt type needed for timing belt replacement vary depending on vehicle make and model. Therefore, perhaps the most valuable tool of all during a belt replacement is a technical manual from a reputable automotive maintenance publisher, such as Chilton or Bentley Publishers [source: Trottier].
Now that you have your tools lined up and service manual handy, let's take a look at how to remove your old timing belt.
Special ThanksSpecial thanks go to Dan Trottier from Bob's Auto Service Inc. in Saco, Maine, for his help with this article.
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Now, right when you're ready to jump into your car maintenance project, you might become wary of your ability to accomplish this task. However, if you have a moderate knowledge of engines, you will most likely be able to tackle a timing belt replacement [source: Trottier].
In general, a car enthusiast who is not a trained professional should allocate about eight hours to complete this vehicle maintenance procedure [source: Trottier]. The first thing you'll need to do is to remove the old timing belt. Although the procedure will vary based upon the technical manual you're using, in general you should:
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Once you reach this point, you're halfway there. Next up is installing that new timing belt. Continue to the next section to learn what steps to take, as well as the significance of setting tension and lining up your timing marks.
When Your Ride Has a Timing ChainDon't think you have a timing belt to begin with? You might be correct. Your car may have a timing chain instead.
Timing chains work just like timing belts in keeping time between the crankshaft and camshaft. Belts and chains dual it out for popularity, though -- changing places leading the pack for auto manufacturers. Currently, timing chains seem to be winning, most likely because they usually don't require maintenance and last the life of an engine [source: Trottier].
However, timing chains can stretch with age and are noisier, heavier and more expensive to repair -- if automotive maintenance ends up being necessary [source: Carley].
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If you're replacing your belt as part of routine vehicle maintenance, then chances are your old belt -- the one you've successfully removed -- kept your engine in time for several years. To do that belt justice and keep your vehicle going strong, people embarking on their own auto maintenance need to take care when attaching new timing belts.
In fact, if Dan Trottier, an automotive technician with Bob's Auto Service Inc. in Saco, Maine, has any advice for timing belt replacement it's to take your time. He says, "Just take your time and you can do it. Don't feel you have to move quickly. Move at your own pace."
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Although your technical manual will provide instructions for your car maintenance needs, the basic steps of attaching a belt during this automotive maintenance procedure are to:
From these steps, the most important ones and those that deserve further discussion are the adjusting of tension and lining up of the timing marks. Just like a bandage that can cut off circulation if it's too tight or not provide enough protection if it's too loose, there's a middle road to take with tension. A belt that is on too tight will not last as long; a belt that is on too loose may skip or jump and fail to keep proper time. This makes your car less efficient and creates the possibility that your pistons and valves will collide. Always refer to your technical manual for exact tension specifications.
Once you have the tension right, you need to confirm all your timing marks are lined up. Lining up your timing marks is similar to setting your watch. If you don't set your watch right, you may be late or early. If you don't set your timing belt correctly, being late or early means your engine won't run efficiently and you run the risk of damaging your engine. Your automobile will have specific timing marks to set. Pay attention to your technical manual when confirming your marks are lined up. Then, proceed with reassembly.
Once you have everything under your hood back together, your new belt is set for its role as drum major of your car. You can take to the field for your halftime show -- or the open highway -- knowing you've enhanced your chances that all the players in your engine will remain on tempo.
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