Maintaining the power transmission components of a high capacity, high-power bulk material handing conveyor belt requires treating it as system. This system includes a motor that creates the torque to drive the conveyor, a gearbox to reduce the speed and increases the torque, and a conveyor pulley assembly that transmits the torque to the conveyor belt. These components are connected with high-speed and low-speed couplings.
Replacing components is costly and time consuming, but by scheduling machine downtime to properly maintain each piece of the system, a user can increase the effective long-term life of machinery. Bearings, seals and gears are wear components. This means they will fail — it’s just a matter of when.
Inspecting and observing components should always be included in the preventative maintenance guidelines for a conveyor system. Monitoring lubrication, temperature, noise, vibration, wear and alignment will help uncover potential problems prior to failure.
Routine vibration measurements of the entire equipment train should be taken at regular intervals so that problems can be found well in advance of a component failure. The foundation and equipment base should be checked regularly for movement or looseness.
The majority of motor failures can be divided into two categories: bearings and windings.
More motors fail due to bearing problems than for any other reason. The leading cause of bearing failures relate to a variety of issues surrounding lubrication. Antifriction bearings should be re-lubricated on a regular basis. The lubrication schedule depends greatly on the motor’s operating environment and service conditions. While failures may occur due to lack of lubrication, bearings may also fail due to grease contaminated by water or other materials.
The second most common cause of motor failures is stator-winding failures. To insure long motor life, it is important the motor operate within the temperature class of its insulation system and be kept clean and free of particle build up on the frame surface, air inlet and fans.
There are several simple tests that can be performed to detect and prevent premature failure of a motor winding. First, motor current can be measured to determine if a motor is overloaded. Measurement of voltage imbalance is the second test. Voltage imbalance between phases may increase motor temperature and cause the motor to exceed rated temperature. The third test uses non-contact infrared pyrometers to help identify potential motor temperature problems by identifying abnormal hot spots, bearing problems, air flow and cooling problems. You can also perform an insulation resistance test. Motor insulation systems may deteriorate because of contamination, mechanical movement, cracking, attack by solvents, mechanical impact, or many other factors.
Technology today also offers the mine engineer sophisticated tools that evaluate the health of a running motor. There are tools available that evaluate the current signature and vibration of a motor concurrently to provide a comprehensive evaluation of motor health. This type of analysis can be performed as a routine survey throughout the mine, or these monitors can be installed on specific critical machines for continuous monitoring. These tools allow the easy transmission of motor data back to motor engineers who can perform a detailed analysis of the data collected from anywhere in the world.
The gearbox is a key component of the conveyor drive and one of the most expensive. Proper lubrication is critical to maintain long-term performance. The oil has two main purposes: it keeps the components from wearing and also keeps them cool.
An oil-sampling program is an effective way to monitor the health of a gearbox. Periodic oil analysis indicates if water is getting into the oil, if the oil is breaking down, or if there is gear or bearing wear. Sampling also can be used to establish oil-change intervals based on the actual condition of the lubricant. When inspecting the gearbox, check for leaks at the shaft seals.
Temperature monitoring is another useful tool. After establishing a baseline, subsequent readings can be used as comparisons, and the data can be trended. A rise in temperature or localized hot spots can indicate problems with the gears or bearings.
A person’s ears are also a useful preventative-maintenance tool. Abnormal sounds are often the first indicator that something is wrong with the gearbox. Vibration readings can be a good indicator of gearbox health, and this analysis can help detect coupling misalignment, improper foundation support (soft foot), and gear or bearing damage. Take a baseline reading with the gearbox installed and connected to the conveyor. Like temperature measurement, trends can help tell what is happening inside the gearbox.
Conveyor Pulley Bearings
Routine maintenance and proper lubrication will ensure a bearing’s maximum life span. The bearings on a conveyor pulley normally run at low speeds and should be filled 100-per-cent full of grease before they leave the factory. This helps prevent water from getting into the bearings during shipping and storage. This fill also helps keep contaminates out during operation.
Effective lubrication is critical to prevent premature bearing failure. If the bearing is not re-lubricated properly then its life is essentially only as good as the service life of the grease. Therefore, re-lubricating the bearing at predetermined intervals is recommended. Most instruction manuals list re-lubrication intervals based upon speed and hours of operation. However, these are typically general recommendations and don’t reflect how temperature and environment may impact the bearing. The manufacturer will be able to supply more detailed recommendations.
When it is time to re-lubricate the bearings, it is important to use with the proper type of lubricant. Not all lubricants, whether grease or oil, are compatible. Contamination, including dirt, dust, moisture, etc., will wreak havoc on a bearing once it has penetrated the seal cavity. It is best to lubricate just before shutdown, especially in moist or humid environments.
Noise and audible vibrations are other easy characteristics to identify. When bearings begin to show audible signs of noise and vibration there is something wrong and a scheduled inspection is due. Vibration is a key characteristic growing in popularity to identify bearing trends and predict failure. Accelerometers can be used to measure vibration on the equipment. Bearing frequencies that correlate with vibration measurements might identify inconsistencies on the raceways or rollers; signs of on setting fatigue failure. Routine measurements should be recorded for future comparisons.
The pulley needs to be monitored for rim or lagging wear. Typically a pulley is not designed with additional material added for wear. So if the pulley in use on a conveyor is not lagged, and the user is experiencing rim wear, lagging should be applied. However, lagging wear also needs to be monitored because uneven wear can cause problems with belt tracking.
On the drive pulley you can monitor the wear by looking at the depth of the grooves. The grooves are there to allow water and other material that gets between the pulley and the belt to move to the edge of the pulley and out from under the belt. When the grooves are almost gone, or you start to have problems with the belt slipping, it is time to replace the lagging. On non-drive pulleys, the lagging should be replaced before it has worn down to the rim.
The pulley may also be monitored for noise and vibration, which can detect cracks in the rim or end disc.
Metallic, grid or gear couplings are most commonly used on large conveyor drives. They require grease lubrication, which should be monitored and changed normally every six to 12 months. Particles found in the grease would indicate the coupling is wearing, typically caused by misalignment. Vibration monitoring of the gearbox can also help determine if there is a problem with misalignment.
Instead of focusing on one or two components of a conveyor system, creating a maintenance program that encompasses the entire system will lead to longer effective life of machinery. Inspecting, observing and caring for the system as a whole will help prevent unplanned downtime and increase productivity.
David Keech is a mining industry engineer with Baldor Electric, a member of the ABB Group. For more information, visit www.blador.com.