What is a Timing Belt Pulley?

Timing belt pulleys are used to connect and synchronize the rotation between two shaft systems. Automobiles, for example, use a timing belt pulley component to link the cam and crankshafts. The gear body of the pulley is lined with teeth that ensure the shafts do not slip. By directly coordinating the rotary motion of each shaft, the engine valves are able to open and close in proper timing and sequence.

What is a Timing Belt Pulley Used For?

Pulleys are designed in a number of ways to meet various applications. Generally, a pulley belt is smooth and tasked with power transmission between components. The belt generates frictional force along the pulley’s surface (often ribbed), causing rotation of the gear body. Traditional drive belts allow for asynchronous, variable speed operations. However, the ribbed, level surface introduces slippage and timing risks.

Timing belt pulleys – also referred to as synchronous timing belt systems – are primarily used to align the motion of multiple gear shafts. Synchronous systems generally maintain a higher degree of longevity and emit less noise than gear or chain drive units.

Teethed Gear Body

The pocketed structure of a timing belt pulley secures aligned movement between shafts. The raised surfaces provide additional grip and reduce slippage significantly.

The slip-resistant properties of synchronous pulleys lowers the amount of belt tension necessary to maintain consistent rotation. Less belt tension reduces lubrication requirements and minimizes bearing load on each shaft element. Relative to asynchronous chain or gear drives, timing belt pulleys are far easier to implement and maintain.

Homogenous Rotation

Smooth belts are susceptible to slipping off track. If belt continuity is interrupted, a shaft component may cease rotating altogether and fail to transmit power to critical engine elements.

The teeth built into a timing belt pulley guarantee the rotation of each shaft is linked to one another. The mode of contact between the belt and pulley gear ensures the rotary alignment and timing. As such, engine valves open and close in sync across multiple shafts – ensuring that cylinders fire at the appropriate times.

Different Types of Timing Belt Pulleys

Timing belt pulleys can be arranged in multiple ways depending on the power transfer or conveyance requirements of the system. The specific orientation of the gears – termed belt drives – dictates the frictional distribution and rotational speed of the pulley unit.

Open Belt Drives. Drives wherein two parallel shafts rotate unidirectionally. Generally, sides of the pulley will be split into low tension (slack) on one end, and high tension (tight) on the other. The larger gear will bear the slack in order to generate more frictional force on the smaller gear.
Crossbelt Drives. Two parallel shafts rotate in opposing directions. The belt crosses over itself, generating friction at the center point of contact. Due to the inherent wear on the belt, these drives tend to be larger and low-speed.
Jockey Pulley Drive. Similar to an open belt drive, but with the implementation of “idler” pulleys. Open belt drives often have a low arc of contact, and struggle to sustain belt tension. The idlers interface with the belt between the driver and driven pulleys, increasing tension in the system and resulting power transmission.
Fast & Loose Pulleys. Used to start or stop the driven shaft on command without stopping the driver shaft. A sliding bar pushes on the slack end of the pulley and ceases rotation of the driven shaft.

How to Choose a Timing Belt Pulley?

Synchronous pulleys may be used for:

Conveyance. Applications such as conveyor belts, processing, packaging.
Power Transmission. Delivering power to ancillary or auxiliary components in an engine.

Packaging, automobile, and large scale industrial applications will require different timing belt specifications.

Selecting an appropriate timing belt depends on a number of factors including material, environment, and sizing. Evaluating the belt unit’s operating conditions will affect its application (large vs small equipment), durability, and general safety.

Sizing Options & Applications

There are countless applications and subapplications for timing pulleys. For example, a car’s power steering complex may use 6 or more gear components in a single pulley system. Alternatively, simpler systems may only require a 2-gear, 1-belt unit. Operators should consider teeth volume, time rating, pitch, belt and gear widths, belt lengths, and component weights.

Environmental Factors

Environmental variables can significantly affect timing belt pulley function. Pulley systems should be regularly inspected for component integrity, debris build-up, and general wear and tear.

Temperature and Pressure. Depending on the material construction, operators should closely monitor ambient temperature and pressure levels. For instance, plastic pulleys will fare worse in high temperature environments.
Humidity and Corrosion. Certain metals will be more susceptible to corrosion and grease accumulation. Polycarbonate structures degrade in the presence of various gases and oils.
Oil and Gas Exposure. Urethane and polyurethane units may be preferable in environments where oil, grease, or caustic gas is a consistent exposure risk.
Debris and Particulates. Material build-up in the pulley can cause blockages or frictional imbalances. System maintenance should include de-greasing and debris removal.

Material Considerations

Steel

Steel is the hardiest of materials available for timing belt pulley design. The metal is largely immune to harsh environments and maintains the highest tensile strength among commonly used pulley materials. Steel components are resistant to heat, cold, and chemical exposure – even over long periods of time.

Plastics

Plastic timing belt pulley options typically include nylon, polycarbonate, and acetal materials. Plastics easily mitigate oxidative conditions and generally require less maintenance than alternative materials. In addition, plastic pulleys provide the lightest and most affordable options for operators. The effectiveness of plastic drops off in more extreme operating conditions.

Nylon. Naturally lubricative surface, low noise, and non-abrasive. Low maintenance requirements. Limited tensile strength relative to metal or plastic options.
Acetal. Lightweight and high level of chemical resilience. Low melting point.
Polycarbonate. Physically durable and malleable material. Easily degraded by petromaterials (oils, gases).

Aluminum

Aluminum represents a balance between steel’s metallic durability, and plastic’s lightweight malleability. The versatility of aluminum makes it one of the more common material variants implemented by timing belt pulley systems. Almost entirely resistant to heat conditions, aluminum is perfect for intensely high temperature environments.

How to Tell Your Timing Belt Pulley is Failing?

A failing timing belt pulley will interfere with an engine’s turn-over and ignition. If the pulley is damaged beyond a certain point, the system may be unable to start up at all. Common signs of pulley failure or dysfunction include:

Oil Leaks. When the timing belt cover is worn through, cracked, or unsecured, your engine can begin leaking oil. If the issue is not caught in reasonable time, your engine will be at risk of overheating and incurring much more significant damage.

Excess Exhaust. Timing belt failures can place a greater burden on your engine (e.g. oil leaks). When your engine works harder, or begins to overheat, your vehicle will emit more smoke at the exhaust.

RPM Irregularity. Non-typical RPM readings can indicate a worn or slipping timing belt. Issues can range from dulled or missing gear teeth, a frayed belt, or environmental conditions impacting pulley function.

Time Since Last Inspection. Timing belt maintenance inspections are recommended every 60,000 to 100,000 miles. Verify the status of your vehicle with your owner’s manual and a mechanic to ensure there is no excess wear and tear.

Timing Belt Pulleys Brought to you by Motion Systems

Determining the optimal timing belt pulley specifications for your project is a multivariable process. Motion Systems specializes in the design and implementation of pulley systems. Looking to reduce project lead times? Contact us for a consultation on pulley solutions.