Bevel gearboxes: A versatile solution for efficient power transmission

Bevel Gearing 101: What is a bevel gear, and how does it work?

Simply put, a bevel gear translates rotation and torque at a 90-degree angle. Bevel gears consist of two cone-shaped gears with intersecting axes. When the input gear rotates, the gear teeth engage and transfer the rotational motion to the output gear and shaft.

Bevel gears are typically made from steel, cast iron, aluminum, or bronze. (Choice of material depends on a variety of factors such as cost, load/speed conditions, noise requirements, and lubrication provisions.)

Bevel gearbox assemblies are quite versatile, can handle large overhung loads, and can be highly modified. A light-duty load may use a spiral bevel gearbox made from aluminum, whereas a heavy-duty load with significant torque and speed may use a cast-iron gearbox with a forged bevel gearset. An example of an ideal application for a spiral gearset could be a small military robot, while a heavier duty forged gearset may make more sense for an outdoor agricultural vehicle.

Key Considerations when Choosing a Bevel Gearbox

Application engineers will ask themselves several questions when selecting a bevel gearbox. Is the application inside or outside? Does the application need stainless shafting, venting, sealing? Answers to these questions often come down to RPM, duty cycle, and expected performance.

For example, imagine a kitchen mixer, which uses bevel gearing. If the user is only mixing eggs, that’s not too tough on the internal gearing system, and may require low torque for the majority of uses. But once the user throws in flour, butter, other ingredients … they can really hear the drive start working. That mixer will experience various loads throughout the mixing process. Mixers used once for 10 minutes will require a lower service factor than one used continuously 8 hours per day. Regardless, the operator will need to use the mixer within the design capacity or damage may occur. This is a hypothetical example that illustrates the various considerations for designers using bevel gearing.

Key considerations

Speed and Torque Requirements

Determine the speed at which the gearbox will operate and the amount of torque it needs to transmit. Higher torque requirements may necessitate a larger gear set to handle the expected load.

Application and Service Factor

Consider the specific application and its duty cycle. What is the application? Is it one that runs for five minutes, once a week? That’s pretty light duty, from a service factor standpoint. If the application is running 24/7, and/or heavy shock loads are involved, designers should consider a heavier duty gearbox with a higher torque capacity. (If questions around application and service factors persist, designers can consult AGMA or a Boston Gear catalog for more information.)

Noise Sensitivity

Evaluate whether the application requires quieter operation. A “spiral bevel” engages more than one tooth at a time. At higher speeds, spiral bevel gears are known for their quieter operation. “Forged bevels,” louder yet more rugged, are commonly used in agricultural applications.

Overhung Load Capacity

Assess the overhung load (or radial load) on the extended shafts and select a bevel gearbox with the appropriate capacity to handle the load.

Gearbox Ratio

Bevel gearboxes are typically 1:1; however, other ratios exist such as 1.5:1, 2:1, etc. There may be an application advantage to halve the speed and double the torque as a 2:1 ratio. In some applications, they could also be used as a speed increaser.

Rotation of Output

Consider the rotational direction of the output shaft based on the input direction. Bevel gearboxes can often be configured for opposite rotation, but one should always understand the rotational direction of both shafts.

Lubrication

Some gearboxes are grease-filled or oil-filled, while others arrive without any lubrication. Based on the speed, loading, and ambient operating temperature, different lubrications may be recommended. The right lubrication will cool the gears and bearings at running temperature while maximizing performance.

Shaft orientation

If an oil-filled bevel gearbox has a vertical shaft, it may require a grease fitting to ensure the upper bearings are properly lubricated.

Naked or enclosed

While some engineers may choose to manually design “naked gearing” into their drives, others may prefer to buy pre-assembled enclosed gearboxes. Often referred to as “ready to use,” enclosed bevel gearboxes will contain bearings, gearing, and lubrication. This reduces the design complexity, freeing the engineer to focus on critical machine details.

Bevel gears are all around us, and engineers must consider all these factors when working a bevel gearbox into their designs.

Figure 3: An enclosed gearset would include shafts, shims, bearings, oil seals, pins, and other elements pre-assembled, whereas “naked gearing” forces the designer to engineer these elements independently.

Advantages and Disadvantages of Bevel Gears

Bevel gears offer several advantages that make them a popular choice for power-transmission systems. Firstly, their ability to transmit rotation and torque at a 90-degree angle makes them ideal for applications where space is limited or where a change in direction is required. Additionally, bevel gears are highly versatile and can handle a wide range of loads, from light-duty to heavy-duty applications.

However, it is important to consider the disadvantages of bevel gears as well. One of the main challenges is the potential for noise generation, especially in applications where noise sensitivity is a concern. In such cases, a spiral bevel gear may be a better option due to its quieter operation. Another consideration is the overhung load on the shafts, which can put strain on the bearings. Proper sizing and selection of the bevel gearbox are crucial to ensure optimal performance and longevity.

Once bearings and gearing begin to fail, it creates a snowball effect for the application. A failing component will generate a lot of hardened wear debris. Once that failure begins, propagation of damage to other components will accelerate.

It is crucial for designers to avoid taking shortcuts when selecting a bevel gearbox with a torque capacity lower than an application’s loading. Consider a scenario with a design for an application demanding 500 inch-pounds of torque. What happens if something becomes stuck in the machine, causing a sudden torque spike beyond 500 inch-pounds? Occasional momentary overloads “within reason” won’t harm the gearbox. However, if an application continuously subjects its gearbox rated for 1,000 inch-pounds to a 10,000 inch-pound loads, the gearbox will quickly fail. Repeated instances of excessive torque spikes will shorten the life of the gearing and bearings.