Monday 10th Aug, 2020

BULKtalk: Choosing the right mechanical conveyor drive

Steve Davis, Senior bulk handling expert at Advisian, explains what to look for when selecting, installing and maintaining mechanical drives.

Steve Davis, Senior bulk handling expert at Advisian, explains what to look for when selecting, installing and maintaining mechanical drives.

Mechanical drives that have been installed and maintained well can last upwards of 50 years. Earlier failures or repetitive failures may be an indication of poor selection, operation or maintenance.

Most drives should be overdesigned for continuous operation to allow for starting, peak loads and temperatures, dirty conditions and upsets. Define the conveyor operating parameters and check conformity once the drive is selected. Most drive components are available in step capacity ranges, and when combined may show some risk if not matched correctly. All components in a drive system should be capable of carrying the load associated with the worst-case operating conditions.

Electric motors will deliver power and torque to capacity unless limited. They can run at 110 per cent of their rated power or close to breakdown torque for some time. Motors will reach locked rotor torque on every start where the system is overloaded beyond capacity.

In our mining industry, upset conditions are common. Ensure that all drive components are capable of the upset loads or are protected by mechanical or electrical means. If not, reduced life and/or failure will result.

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When a failure does occur, confirm the root cause. One situation involved failure of three fluid couplings at the same time in a triple conveyor drive. The conveyor ran back, and much damage resulted. The owner blamed the runback damage on the concept of having fluid couplings and was trying to find a fault with the couplings. The root cause was that holdbacks had been removed, and brake maintenance was incorrect, so the brakes eventually stopped holding.

When the inclined belt accelerated backwards with a full load, the resulting speed of the coupling caused casings to explode under the centrifugal forces. There was nothing wrong with the couplings. I have seen similar damage on a take-up winch maintenance drive, where the weight dropped and over sped the drive.

Drive components are rated for power, torque, and thermal capacity, often with a defined safety factor for specific conditions such as shock loading and vibration levels. Understand the relationships between the factors, as it may not be possible to run with all ratings at maximum. What are the potential upset conditions, and can these be limited by control or monitoring?

Temperature extremes and duration, exposure, cleanliness. Thermal ratings assume a clean radiating surface to radiate heat. Direct exposure to sunlight can raise the surface temperature well above ambient, reducing cooling. Cooling systems can extend thermal ratings, but is the added cost and complexity better than a larger drive component? If direct sunlight or spillage is unavoidable, add a shade/shedder system. Components may need air breathers, and these must be suitable for the environment and not allow dust or water transmission. Simple plug filters are rarely adequate in a mining environment, consider heavy-duty wash down resistant desiccant and filter systems.

Are all mechanical components capable of worst-case duty? For example, a perfect gearbox can be damaged through failure of an under sized coupling, hold-back or similar. Re-check all drive selections when the electric motor is selected as stall torques vary between makes by large percentages. Does variable speed operation change drive loading?

Standardisation can result in undersized or oversized drives. A horizontal fines conveyor with good feed control, and an inclined primary ore conveyor fed by apron feeder may have the same nominal power requirement and speed but will have significantly different operational loads.  On one project my client requested only two drive configurations for all conveyors, resulting in some on the limit and many significantly oversize. Was this a good selection?

Brakes and holdbacks are most effective when installed directly on pulley shafts, but also most expensive. High-speed brakes and holdbacks work through a gearbox and coupling, increasing modes of failure and stress on these components. High-speed brakes must be located on the gearbox input shaft, not the motor output. Remember that any conveyor in Australia where uncontrolled movement could result in injury or damage should have two independent holding devices, both rated for the worst case.

Lubricant selection impacts component life. Standardisation can result in use of inappropriate lubricant, leading to metal contact, high wear and temperature. Viscosity varies with temperature and may change starting torque demand. Don’t assume all lubricants of the same grade/description have the same lubricating properties. Inadequate lubrication generally shows after a period of operation. Failure modes for bearings and gears are well documented, so easy to determine if lubricant at fault.

Consider lubricant sampling, filtration and change facilities to maximise life and cleanliness. Install ground level fill and drain lines for large oil capacity units to avoid manual filling. What components need automated relubrication?

Installation and commissioning must be confirmed correct. Often a judgement call must be made on which upset conditions to design for. Poor installation and commissioning upsets cause many early failures and reduced life. Check and recheck alignment, preassembled and pre-aligned drives can change during transport and installation. Check all earth paths as stray currents through drive components are a source of failure and a potential safety issue.

Upset conditions often result in damage or failure of components. We can monitor many things and shutdown if necessary, but we monitor the effect, not the cause. Damage may already have occurred. Temperature levels over thresholds, high vibration levels, high motor currents, contaminated lubricants are all the result of an upset.

Early failure of faulty components is always a possibility, and these failures can be rapid and difficult to anticipate. If drives are critical, increase the level of non-destructive testing and quality assurance during manufacture and installation. Spare components and rotable drives reduce repair times.

Implement controls to limit impact from upsets. Examples include flow control (weight or volume), which limits the possibility of overfilling/overloading the conveyor. Weighers must be calibrated and checked regularly. Correct installation of skirts systems and good chute design reduce the pull-out drive torque when starting. Drive selection should allow for pull out when fully loaded with a bogged chute. Electric motor current and thermal capacity control can trip the drive before damage occurs. Electronic variable drive can be used to manage motor torque, limiting overload. Load capacity can be reduced when temperatures are extreme. Automated and remote operation of materials handling systems can leave control settings open to abuse.

Selecting an example

A typical shaft mounted conveyor drive consists of a four pole motor with high speed coupling, gearbox and low speed coupling mounted to a pulley. Demand power at design operating conditions is 195 kilowatts and motor starting torque is 3300 newton metres with a blocked chute.

We would normally select a 200-kilowatt or more likely a 250-kilowatt motor for power to give a margin, as calculations use many variable factors, and to give a margin for operating error. A good 250-kilowatt motor might achieve locked rotor torque of 3300 newton metres and could be accepted knowing the potential for a few aborted starts during the life of the system. Select a 315-kilowatt motor and Locked Rotor Torque is more than 4000 newton metres. Add an electronic variable speed (VFD) and move the motor torque curve to the left and we could approach 5000 newton metres breakdown torque at start up in an overload. Fluid couplings have a similar effect to VFD. Changing motor supplier or perhaps designing National Electrical Manufacturers Association (NEMA) B and buying NEMA C will change the torque values and possibly the power capacity of the motor.

Combining variability between different motors, control limits, and sizing couplings and gearbox (which also have stepped capacities and ratios) for the correct power and torque at all speeds to have an integrated drive needs some thought. Stability of the electrical supply is also a consideration, as torque is voltage dependent and high starting torques draw higher currents. The final drive system selection is a trade-off between cost, potential downtime and overall life reduction from exceeding component capacity during operation.

Poor and unsafe access for inspection and maintenance is inexcusable and can be designed out. If access is poor, inspection and maintenance are completed badly or not at all, leading to early failure.

For existing drives, many of which have many years operation and one or more upgrades, the design can be checked to confirm drive capacity. This could identify the root cause of any short service life and failure and lead to retrospective upgrades. Other than this, consider the measures included above to keep the drive clean and within capacity limits. Improve access if possible and add some monitoring.

System change

A common cause of early failure is change to a system without fully evaluating the consequences. The failure often occurs later and is not attributed to the change. Common root causes are:

Changing variable frequency drive (VFD) settings or oil fill on fluid couplings will change the torque that can be delivered and the duration of delivery. Changing to a larger motor generally increases power and torque.

Changed time, temperature and overload control settings may result in many aborted stops sending large forces through the conveyor.

A classic situation is to reposition a blocked chute sensor to reduce stoppages rather than mitigate the cause of the blocked chute. The load on the receiving belt increases in a blocked chute situation and it will not pull away. This doesn’t happen for several months. Analysis looks at drive components, motor and VFD settings, changes out the motor and so on, but forgets the sensor change.

Increasing conveyor throughput, because the motor draws less than full current at design load, and without considering the capacity of the mechanical components and additional upset loads.

Increasing speed and using drive components’ design margins can lead to early failures, and increased control and monitoring trips.

Use of a thicker belt cover to get better life. Almost any change to belt, idler rolls, chute and skirt design should merit a design check and a review of the likelihood of reduced mean time between failures.

Often outcomes from change are attributed to unrelated issues. For example, drive coupling failure increased from never to every eight months after a conveyor upgrade. The coupling was replaced several times by different supplier units before a new engineer asked questions. Site had blamed quality and ‘forgot’ the upgrade. There are many similar examples, where an assumption is made that the cause of a failure was at time of failure.

Lubricant specification change, extended oil change period without testing, changing filter cartridges to coarser filtration to extend filter life, bypassing filters, increasing oil temperature trip setting. These can all damage gearbox internals and bearings. Lubricants and filters are perceived high cost, and I have seen some failures through changing to save money.

I evaluated some bearings that were running hot after being in acceptable service for many years. Root cause was a change in lube supplier. The new grease complied with a poorly written specification and had a lower base oil viscosity than the old, making the grease inadequate. It took over a year for the problem to be reported after air and water cooling were used to lower temperature to avoid tripping, and at which time the bearings were unusable.

A final note when selecting drives: be open to options such as using motors with different synchronous speeds, gearless drives and other options. I selected a drive needing a 600-kilowatt motor to run continuously, but the staring torque for a few degrees of motion was much higher than this motor could deliver. We installed a low power pony drive with a one way coupling for break away with the main motor taking over after one revolution. Main motor and drive system were much smaller and cheaper, and we gained a creep drive for maintenance.