This article will take an in-depth look at timing belts.
The article will look at timing belt topics such as:
This chapter will discuss what timing belts are, their design, and function.
A timing belt is made of rubber with hard teeth capable of interlocking with camshafts and crankshafts cogwheels. It is an integral component of an internal combustion engine responsible for synchronizing the rotation of the camshaft and the crankshaft. It enables the proper opening and closing of the valves of the engine during both the intake and exhaust strokes of each cylinder.
The timing belt also plays an important role in preventing the piston from striking the valves, in an interference engine. A timing belt is usually a toothed belt with teeth on one or either side of the surface.
There are two main components of a timing belt: the molded cords inside the timing belt for carrying the torque load and the plastic compound used to shape the teeth and cover the cord itself. These components are available in different types of materials for different types of timing belts. To determine the type of materials to use, one has to consider the end use of the belt. Timing belts’ cords are usually made out of fiberglass, polyester, or Kevlar. They transmit power in the drive system through the belt.
The cord and the belt teeth are oriented at right angles with each other so that the cord can linearly transmit the power applied to the belt. An example of a belt that carries huge loads is the serpentine belt utilized in automobile engines. In smaller drive systems, elongation of the belt is minimal.
Belt stretch does not exist practically in small drive applications since the cord materials are so strong in relation to the loads that they transmit. Too high loads can cause breakage of cords as well as belt teeth jumping, or cogging over the pulley teeth. During the construction of timing belts, a mold in which the plastic is injected is present. The plastic is then injected into the mold that already contains the wound cord and accurate tooth profiles that are cut into the mold.
There is always a different mold available for each different belt length since there must be the exact number of teeth in the mold as there are on the finished belt. This is done to produce a finished, continuous belt that has no beginning or end. A mold can produce a sleeve that has a width of 18 to 36 inches having the number of teeth that is desired. The sleeve is accurately trimmed into the desired belt widths by special slitting tools. Food processing belts are made from urethane, when FDA requirements must be met. With urethane, any particles are less likely to be seen because urethane can be colored or left in a clear natural state. This is a different case with black neoprene belts.
The standard material for timing belts is neoprene because it exhibits good wear characteristics and from the mold, it accurately holds the tooth profile. To reduce wear, neoprene belts have a facing that is made from nylon fabric. To meet special requirements for low dust or particle applications like office copiers, clean rooms or medical, engineered polymers are used. To generate less dust than neoprene, an EPDM polymer is used as a core with all tooth wear surfaces over coated with nylon. This also ensures the accurate holding of tooth profile for many hours of use compared to urethane or neoprene belts.
Strong timing belt teeth are reliable for keeping the crank and cam shafts synchronized, and there is a wide range of metric pitches for the teeth. Pitch is the distance measured from the center of one tooth to the center of another adjacent tooth on a timing belt. Pitches impact other timing belt pulley factors that include the number of teeth and diameter. Older timing belts are designed using trapezoidal-shaped teeth, when it comes to teeth design.
However, new manufacturing techniques that allow for curved teeth to combat the challenges with noise and lifespan that are common in multiple belts designed using trapezoidal shaped teeth. An important point to note on the construction of timing belts is that a timing belt with shortened width offers improved performance due to the reduction of weight and friction.
There are many different arrangements of timing belt teeth depending on the desired application and environment.
As already mentioned before the first arrangement is the trapezoidal arrangement, while curvilinear tooth profiles are used by more modern timing belts.
For transmitting forces, trapezoidal teeth are extremely effective. However, when the levels of torque and speed are high, teeth of this blunt shape tend to wear down fast. Trapezoidal teeth remain common despite their faults, and for precision conveying and linear positioning belts, they are typically the primary choice.
Curvilinear toothed timing belts have a smoother and more rounded tooth shape. These types of belts reduce the risk of tension loss and they alleviate the high concentration of force that is experienced by trapezoidal teeth. Curvilinear timing belts have drawbacks of their own despite the great improvements they offer over other tooth designs. Curvilinear belt teeth are prone to a greater amount of play between the grooves of the pulleys and the teeth of the belt. This difficulty is referred to as backlash, and results in less accurate positioning of the timing belt, and the performance of the timing belt is potentially decreased.
These types of belts seek to combine the strengths possessed by both curvilinear and trapezoidal tooth profiles. Modified curvilinear belts have a shallower tooth depth and steeper sides. As a consequence, they are capable of transmitting forces of higher speed and torque effectively, without putting a sacrifice on durability. These types of belts are often primary choice industrial applications that are demanding.
The stages in designing timing belts are typically:
The first stage is the determination of the peak torque for the drive. This is mostly the starting torque of the motor. However there may also be momentary or shock loads that are unusual occurring during normal operation.
The second stage is the determination of the diameters of the largest pulley that can be utilized. This is done in consideration of the space limitations and the system’s drive ratio. This helps by increasing the drive’s torque capacity and prolonging the service life of the belt.
The third stage is the selection of the tooth profile of the belt. If for the selected profile, the torque for the peak drive is at the upper limits of torque transmission capability, consider making use of the next higher torque rated profile. For the selected profile, find the corresponding pitch. For the calculation of the required number of belt teeth, this value will be needed.
Calculate the T.I.M (teeth in mesh), bearing in mind the teeth in mesh factor. The peak torque from stage one must now be divided by the T.I.M. factor in order to determine the design torque. An important point to note is to check the belt pitch again to ensure that your application has not been moved outside the limits of the pitch that is desired for the chosen pulleys by this adjustment in torque.
This stage involves the calculation of the belt pitch length based on the distance of the design center of the drive.
The sixth stage involves the division of the length of the belt pitch by the selected tooth pitch and rounding of the result to the nearest whole number. This will be the number of teeth on the belt for the application. The nominal center distance of the drive design must be adjusted to match the belt.
This seventh stage calculates the effective tension (Te) on the drive by using the pitch radius and design torque of the loaded pulley that is the smallest in the system.
In this stage, the selection of the strength factor for the application is done. The effective tension from stage seven is then divided by the strength factor to determine the break strength required for the belt design. To represent a double span break, multiply by 2. For the determination of the reinforcement type that is required and the belt width, consult the table for the break strength. The value that is listed in the table must be larger than the break strength for the design.
This stage involves the selection of the belt width that is able to handle the torque of the design with the selected size of the pulley. An important point to note is that the required width of the belt for the system will be wider of the two.
The length of the belt can be calculated if the pulley diameter and center to center distance between the driven pulley and the driver are known.
The calculation formula is as follows:
length of belt=2L+1.57d1+d2+d1-d24L
Where: d1 and d2 are the respective diameters of the timing pulleys;
L is the distance between the centers of the timing pulleys
The various materials used in constructing timing belts include:
Rubber is the most famous timing belt construction material used across industries and applications. Many of the internal combustion engines found in a great diversity of cars use timing belts that are made out of some type of rubber or rubber compound. Even though rubber is the most commonly used material for timing belts, it does have some drawbacks.
Timing belts made from rubber are notorious for stretching and breaking relatively quickly if they are subjected to high temperatures and quantities of motor oil found in the engine blocks of vehicles. Nowadays, there are multiple types of rubber materials that are temperature resistant due to the advancement of technology. These rubber materials are now used to prolong the lifespan of your timing belts, and improve their resistance to distortion.
Greater strength and traction is also offered by improved rubber compounds and reinforcing fibers. These improved rubber compounds also guard against sheared teeth or any other potential damage to the belt.
Polyurethane has become a famous option for timing belts due to its properties which are resistance to high temperatures, natural greater elasticity and resistance to harmful effects of oil. For a broad range of applications, timing belts made from polyurethane are long lasting and offer extremely energy efficient solutions.
Polyurethane belts are capable of delivering high tensile strength and they also provide higher loading capacities. These higher loading capacities are crucial for the production of optimal torque. These types of belts are preferred by many in the industry because their cleaning and maintenance is relatively easy. Polyurethane is the best choice of material for making timing belts for multiple types of power transmissions and roller conveyor systems.
Timing belts made from fabric are the best choice when requiring high-performance and acceleration forces. In fabric timing belts, a wide array and combinations of materials are used. This results in splendid tensile strength, low coefficients of friction, and brilliant resistance to temperatures, both high and low. When the requirements to be met are high torque or acceleration, there is a great diversity of fabric timing belt options that will highly benefit.
Timing belts are used to allow for the operation of a vehicle’s engine. They are used to connect the engine’s camshaft to the crankshaft. They also control the pistons and valves in a vehicle. To explain in simple terms, a timing belt is a reinforced rubber band containing teeth or notches on the inner side.
A timing belt synchronizes the opening and closing of the engine’s valves precisely. As the crankshaft turns, the timing belt is set in motion. After that, the camshaft is then turned by the timing belt and opens or closes every valve and allows pistons motion in an upward and downward manner. For example, in four stroke engines there are four phases: the intake phase, combustion phase, compression phase and exhaust phase. During the intake phase, air and fuel are pulled inside the cylinders. During this phase, the intake valves are open and the exhaust valves are closed.
During the combustion phase and compression phase, the air is mixed with the fuel, compressed, and then the spark plugs ignite them. All of the valves are closed during these two phases. The exhaust phase is the final stage, the remaining air and fuel exists out of the exhaust valve. During this phase, the exhaust valves are open and the intake valves are closed. The timing belt is the one that does the work of controlling all of the opening and closing of the valves and the pistons' timing throughout every phase. The timing belt allows for the occurrence of each step in precise order.
This chapter will discuss the types of timing belts, including their possible causes for failure.
The types of timing belts include:
Open ended timing belts are available in different types of materials including extruded, thermoplastic polyurethane. These types of belts are dimensionally stable and they are made ideal for use in linear drive systems by their precise positioning.
They have a high spring rate and their tooth shear strength is excellent. For a linear drive with stiffness and repeatability that is high, the steel tension members are recommended over aramid fiber. The stiffest and strongest open ended timing belt is the move-series AT10 or AT15. Open ended timing belts are available in 50m rolls or specified cut length.
In many general purpose applications, spliced and welded are utilized. Their construction type is preferred in conveying as well as for profiles and backings that are welded. At the start of these types of belts, there is a length of roll stock that is open that contains finger splices cut into the belt and is then welded together using heat.
Spliced and welded timing belts have the same sealed edges and smooth back as open ended timing belts. These types of belts have 50 percent of truly endless belt strength because they are welded. They can be purchased in one tooth increments for a minimum length.
Truly endless polyurethane timing belts are extruded without a weld. This makes truly endless timing belts the strongest construction type, making them ideal for applications that involve power transmission. Truly endless timing belts have steel tension members as a standard and are found in different types of materials.
The AT is the most famous tooth configuration for more tooth shear strength. Double sided timing belts are also available for drive systems whose shaft direction changes. These types of belts are available with unsealed edges. If they are closely inspected, a pin-hole location of the tension member is found, where it exited the belt during the process of extrusion.
The distance or time periods for the replacement of the timing belts are recommended by the manufacturer. Failure to replace a timing belt in time will result in a complete breakdown or catastrophic failure of an engine, especially in interference engines. The owner must use the manual maintenance as a source of timing belt replacement intervals. The typical distances are 30000 to 50000 miles.
The timing belt tensioner can be replaced at the same time when the belt is being replaced, this is common. In some engines in which the timing belt runs the coolant pump, the coolant pump is also typically replaced. The common failure modes of timing belts are either delamination and unraveling of the fiber cores or stripped teeth. Stripped teeth leave a section of the belt where the cog of the drive will slip. Breakage of the timing belt, because of high tensile fibers’ nature is not common. The timing belt can be slowly wearied by debris and dirt that get mixed with oil and grease. This causes premature belt failure.
The correct belt tension is critical in the life expectancy of a timing belt. If the belt is too loose, it will whip, and if it is too tight, it will whine and apply excess strain on the cogs’ bearings. In either case, the life of the belt is drastically shortened. The belt tensioner also fails, apart from the belt itself. The other failures that can occur are from various gear and idler bearings and these cause the belt to derail.
When replacing an automotive timing belt, care must be taken. This ensures the correct synchronization of the valve and piston movements. If there is a failure in the correct synchronization of these parts, problems with valve timing can occur. This in turn will cause collisions to occur between valves and pistons in interference engines.
The various causes of timing belts failure include:
One of the main causes of timing belt drive failure is misalignment. Misalignment is caused by uneven or excessive tooth wear, tensile failure and belt tracking. To increase the lifespan of your timing belt, always check and align your shafts and timing pulleys. This saves lots of downtime.
Excessive load is the cause of the shearing of teeth in a timing belt. Excessive load or shock loads can also cause uneven excessive tooth wear and tensile failure. To eradicate this problem, the drive must be redesigned.
Ratcheting, which is the skipping of teeth, is due to a timing belt being under-tensioned. Under-tension results in the excessive or uneven wearing of the teeth, and excessive drive noise. To set the correct tension on a timing belt, use a tension gauge.
If there is an excessive vibration of a drive, or your timing belts are under belt stretch, there might be a weak drive structure. To stop this problem, try reinforcing the structure of the drive.
The lifespan of a timing belt is reduced by damaged or worn pulleys. If the teeth are worn out, they cause the belt to wear and or damage. The belt can be cut by nicks or gouges. It is important to inspect and replace pulleys that show signs of wear.
This is the most overlooked cause of failure in timing belts. A multitude of problems on a timing belt and pulley can arise from debris. Abrading of the belt can occur due to dirt on the teeth of the belt and the belt materials can be attacked by oil. To clean off the rust and dirt, use a stiff brush. Oil and grease must be wiped off the timing belt. To prevent all of the above reasons that debris can account for, the pulleys must be cleaned up and a shield fitted to the drive.
The various symptoms that indicate when the timing belt is about to fail include:
When the key is turned on to start a car and only the starter motor is heard engaging but the engine is not able to turn over, this is a sign that the timing belt is failing.
If the timing belt is broken, it might result in a continuous ticking or clicking sound that comes from the engine of the car.
In malfunctioning belts, misfiring is a common occurrence. If the belt slips on the drive of the camshaft, it causes the opening or closing of the engine’s cylinder earlier than it should, thereby impacting the rate of firing of your engine.
These noises can only happen when the vehicle accelerates, or it might only happen during the time one hits the brakes. This is a different case with a ticking sound that is constant.
This chapter will discuss the applications of timing belts, including their advantages and disadvantages when compared to flat or round belts. Considerations when selecting timing belts will also be discussed.
The various applications of timing belts include:
Timing belts are used in timing belt/cam belt systems that are present in most automobiles on roads across the whole world. It can be said that the mobility of the modern world is greatly made possible by the efficient operation of automobile timing belts. The toothed timing belts available in automobile applications are belts of high performance.
They consist of special materials for the coordination of the rotational motion of the crankshaft of the engine with the camshaft of the engine. To ensure the intake/exhaust valves in the engine’s combustion chambers open properly at the precise moment of expansion or compression, coordinated precision is necessary. It is also necessary for ensuring sustainable continuation of the engine’s combustion process. This coordination determines the pace of the engine, and if it doesn’t function properly, combustion will not be possible.
A high level of synchronicity must be maintained extremely by these belts, and they must also be able to perform under high rotational speeds. The timing belts must be strong enough to operate under high temperatures, wherever automobiles can be driven.
Another great example of a belt driven system is the treadmill. The timing belt in this drive mechanism must not be confused with the tread on which the runner directly runs. Rather, the timing belt drive mechanism is the toothed, synchronous belt that serves to transfer rotary motion from the central motor all the way to the drive pulley. The location of this drive is at the rear of the treadmill, but sometimes it is positioned in the front. In treadmills, flat friction timing belts can be used, as well as toothed belts.
A sewing machine is another everyday example of a belt driven system. The timing belt is utilized to transfer motion between the sewing pulleys and the motor. Although it is not a high performance, high load application, high speed system like the treadmill and the automobile, the sewing machine remains an excellent example of the application of synchronous timing belts. A sewing machine’s belt system is used to transfer the main drive motor’s rotary motion to the larger spoon head to drive the sewing mechanism.
Timing belts are also utilized in 3D printers, conveyor systems, CVT automatic transmission and industrial automation products.
When timing belts are compared to flat or round belt drives they may have disadvantages like:
However, these disadvantages are outweighed by the benefits which include:
The various considerations when selecting a timing belt include:
Clearly seen, there are many different materials used in timing belts due to their own different specifications to which they can handle certain circumstances. As already mentioned earlier, the different types of materials for timing belts include urethane, neoprene, rubber, fabric etc. Each material has its own unique properties that make it the most suitable for a particular application.
Apart from the type of material, there are also some characteristics that must be considered when opting for a timing belt. These characteristics will influence the smooth operation of the equipment. These characteristics are listed below.
Belt width deals with the plausible amount of tracking force of the belt. Wider timing belts result in greater tracking forces.
Shorter belts have greater tracking forces because of the connection that is between tensile cords and belt molds.
Small-sized pulleys create greater tracking forces. The diameter of the pulley must be greater than that of the belt.
Due to the impact of various loads of torque gathered by belt tracking, the timing belt’s magnitude of belt installation tension does matter.
Timing belts are as fragile as glass. If the operating environment affects the process of this synchronous device, it might be hampered from working. Dust particles can also cause malfunctions in the timing belt system.
The function of the timing belt depends on where it is going to be used. Not every material is capable of handling the requirements for all industries. Thus there is a need to conform to the application in selecting a timing belt.
This refers to choosing a standard design or custom design of a timing belt. The design of the timing belt involves questions about the amount of precision required.
Timing belts come in many different types with each type suitable for a certain application. Each timing belt is made out of a specific type of material that makes it perform best in a certain or particular environment. If you are to choose a timing belt for a particular application, always consider the type of material it is made from. This ensures optimal results in the performance of the timing belt for the particular application you choose it for. The replacement of a timing belt is also critical and time reliant.
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