Gearbox: main types and possible failures

The gearbox is a device used to reduce the rotation between the driver and what will be driven. In addition to performing this rotation adjustment, the reducer also generates an increase in torque force. This is because it multiplies the input torque by the transmission or reduction ratio.

There are four main types of reducers: with gear train; planetary; worm and wheel; and bevel gear. But, before addressing each one, it is important to differentiate between the reducer and a gearmotor:

Gearbox vs. Gearmotors

In the case of motors, a common question is the choice between using a reducer or a gearmotor. The basic function of both is quite similar: they are mechanical devices that reduce the speed of a driver. However, the way this reduction is performed differs.

In the case of gearbox, there is a solid shaft keyed at the input, allowing for different types of reduction. Gearmotors, on the other hand, operate coupled to the motor, thus being limited in the reductions of rotational speed.

Another difference is that gearmotors have direct rotational transmission. Thus, any overload will occur in both the reducer and the motor.

In practical terms, the gearmotor tends to be an economical solution. Now, if there is a need for a large variation in reduction, the reducer is generally the most suitable choice. Therefore, the choice between one or the other may consider the cost-benefit ratio, but should be guided by the intended objectives.

Main types of Gearbox

Gear train reducer

Gearbox with gear trains are usually present in conveyor belts and other transport systems. Therefore, they are large reducers, capable of supporting heavy loads, have high torque, and have the input and output shafts arranged in parallel. They may, therefore, contain one or two outputs for one input.

Planetary gearbox

These reducers have a set of epicyclic, planetary, and ring gears. One of the main characteristics is the low inertia of the mechanical components, contributing to high transmission ratios. Thus, the main application is when high speed, lower weight, and minimal space are required. The planetary has applications such as a timing belt system, rack and pinion, or turbine motor.

Worm and wheel gearbox

Worm and wheel reducers are the most commonly used in the market. They use the gear and worm on top of bearings and covers, also standing out for low noise. The input and output shafts are arranged perpendicular to each other. In them, therefore, the path of torque transfer and reduction is unique: the rotation input will be on the worm shaft and the output on the shaft or hollow hub.

Bevel gear reducer

They are a common form of angular reducers, in which two bevel gears interact to create an angle (usually 90°) between the input and output. These types have a transmission by angular contact, providing high torque powers. In this type of gearbox, it is possible to change the direction of the input and output shaft, being very common in agricultural machinery.

Main failures in gearbox

Reducers are basically composed of shafts (input, output, and intermediate – in some), gears, bearings, sealing and fastening elements, as well as covers and housing.

Thus, based on the composition and the type of activity performed by this equipment, the main failures in gearbox are:

• Lubrication deficiency: The most common failures in reducers are related to lubrication, whether due to lack or excess of lubricant. Thus, the main consequences are: reduction of component life, loss of reducer efficiency, excessive heating, and even contamination of the reducer.
• Misalignment: Misalignment is a very common failure in shafts and can cause abnormal wear of the components. It can even cause meshing problems due to misalignment between the gears.
• Bearing wear: Bearing wear is a very common failure in various assets. It can be caused by improper installation, accumulation of residues increasing friction, or even resulting from misalignment.
• Gear defect: Gear can present various failure modes. Among the most common are wear due to interference or abrasion, breakage due to overload, cracks, and backlash. The problem then increases vibration levels, as well as can cause overheating and the presence of iron shavings in the oil.
• Overheating: Overheating is among the main failures in bearings. Among the causes of the problem are the presence of impurities, reduction of internal clearance, overload on the bearings, and pressure on the seals, external heat sources, etc. The consequence is bearing failure due to seizure or fatigue.

Thus, these problems can lead to unplanned downtime, increased maintenance costs, productivity loss, wear of other system components, delays, etc.

Each of these failures may require a specific type of monitoring. For example, identifying leaks through sensory inspection. Now, misalignment, wear, and overheating require other approaches, such as condition-based maintenance.

How to perform condition-based maintenance on the gearbox?

An effective way to continuously monitor an asset is through the collection of vibration signals. These data allow the identification of possible failure modes through frequency spectrum analysis. The signal spectrum provides a visualization of the various vibrational components occurring simultaneously in an asset. Next, let’s look at the fault diagnosis in a gearbox.

How to diagnose faults?

To understand the fault diagnosis, we can analyze the case of an ore pulp agitator containing a crown and worm gear gearbox. In this equipment, the rotation reduction is from 600 rpm to 32 rpm (0.5 Hz). The low output rotation and the existence of a bearing below the gear can make diagnosis difficult. Therefore, technicians installed sensors on the gear and at the bottom of the shaft. Locations that allowed monitoring of the bearing as well.

With monitoring, it was possible to identify faults in the outer race of the bearing. Typically, this problem manifests as a succession of evenly spaced peaks in the time domain, representing the passage of rolling elements over cracks formed throughout the bearing’s lifespan.

In the case of the mentioned gear and worm gear reducer, the linear and circular waveform indicates an error in the transmission of force and motion in the component, highlighted mainly by the different amplitudes in the waveform. Thus, in the circular waveform, one can exactly see the number of teeth of the bronze gear (53).

The application of envelope analysis shows rotation harmonics, indicative of backlash in the gear. The origin is the output shaft. This causes a misalignment of tooth contact, with one being shorter and the other longer. Additionally, the envelope waveform indicated symptoms of backlash and rubbing in the output bearing.

From the analyses and identification of faults, it was then possible to plan the intervention and corrective maintenance of the equipment.

Do you want to know more about this diagnosis? Read here: Internal sensor installation identifies gearbox gear defects (dynamox.net)