Maintenance and troubleshooting fundamentals for industrial automation
Maintenance and troubleshooting includes replacing, adjusting, and repairing elements of a machine or system so that it can properly perform its required functions
- Maintenance repair and overhaul (MRO) types include preventive maintenance, corrective maintenance and predictive maintenance.
- Many different types of mechanical assemblies include power transmissions, belts, gears and pulleys and more.
- Mechanical maintenance adjustments include ventilation, brushes, bearings, vibration and more.
Machine and system maintenance is sometimes referred to by the acronym MRO, an abbreviation for maintenance, repair and overhaul. Functions within this area include replacing, adjusting, and repairing elements of a machine or system so that it can properly perform its required functions. Some of the common types of maintenance that fall under MRO include preventive maintenance, corrective maintenance and predictive maintenance.
PM is a routine for periodically inspecting and servicing equipment. The goal is to prevent failure of components by replacing worn items, adjusting and lubricating parts of the equipment. Technicians also have the opportunity to examine equipment in detail to identify fatigued areas and notice problems before they cause breakdown. This helps to prevent downtime and allow equipment to make it from one planned service to the next.
Other terms related to this are Planned or Scheduled maintenance, where a scheduled service may take place based on time, or on the availability of a specialist. This can also be based on equipment’s running hours or the distance traveled.
Condition-based maintenance occurs when a technician or machine operator notices a problem, either during scheduled service or any other time before the equipment actually breaks down. This may require an unscheduled stoppage of machinery depending on the severity of the problem, but ideally this can be scheduled near-term during non-production time.
When equipment breaks down unexpectedly, it must be repaired or replaced before it can resume operation. This can be the most expensive type of maintenance, due to the production revenue lost and because equipment failure can cause damage to other parts of machinery. This can be differentiated from condition-based maintenance by the machine breaking before the problem is noticed.
If a repair part or redundant equipment is not available, additional downtime can result. For this reason, corrective maintenance as a planned event is usually undesirable. Having duplicate or spare equipment on standby can be a form of mitigation for this condition.
The form of corrective maintenance is usually a step-by-step procedure where the failure of the equipment triggers the event. Documentation for the equipment is used to either repair it or identify spare part numbers.
Data collection and analysis can help determine the condition of in-service equipment and estimate when maintenance should be performed. At a minimum, predictive maintenance can highlight spare parts that should be ready if a failure occurs before maintenance can be scheduled and performed. This can save money over other types of maintenance since tasks are only performed when warranted.
Equipment condition can be monitored by periodic (offline) or continuous (online) sensing of components. Vibration, temperature, power consumption or even visual condition via cameras can be included. While this can increase the reliability of equipment, it requires an investment in hardware and software.
Computerized maintenance management systems (CMMS)
CMMS software maintains a database of information that can be used to schedule maintenance for preventive and predictive maintenance and also locate spare parts. Some companies also require verification of regulatory compliance. This means that maintenance activities need to be recorded and parts tracked, automating these functions can save money and time.
CMMS can also help management make informed decisions, for instance calculating cost differences between corrective maintenance and preventive maintenance. They can produce status reports and keep records with details of maintenance activities and part usage. This software may be installed on a company’s computer servers or be cloud based with a subscription model.
There are many different types of mechanical assemblies and elements encountered in industrial automation. These are just a few to consider learning more about.
Bearings and bushings allow shafts to rotate or slide smoothly. Bearings often have some type of rollers or balls that require lubrication, there will be a fitting on the outside of the bearing housing if so. Grease or other lubricants are applied periodically using a grease gun, or it may be done automatically. Bushings may also contain balls or rollers, but typically do not have grease fittings. They are often just softer metal collars that a shaft can rotate or slide smoothly in. As bearings or bushings wear, they can often make noise or create heat. This is an example of using your senses to detect problems with these devices.
Gears and pulleys, like bearings, tend to be wear points. Gears can either mesh with other gears, or engage with chains. These also need to be lubricated periodically, either as part of a maintenance routine or as an automated system.
Pulleys are used with belts, the belt is usually the weakest link that must be replaced periodically and proper tension maintained. As with other moving parts, noise and heat are signs of possible problems, but belts may also create an odor of melting or burning rubber as they heat up. Motors also require periodic maintenance. In addition to bearings, motors may also have brushes and commutators that make electrical contact as the motor turns.
The windings of a motor are coated with an insulation that can break down, causing the motor to become less efficient and create more heat. This is often detected with a thermal overload that uses current to trip a bimetallic element in the motor starter.
Vibration is also sometimes monitored in order to predict motor failure before an event. Failure of a large or critical motor can cost a lot of money in down time or even damage other equipment when it fails. For this reason, motors are often monitored electronically in several different ways. For large critical systems this can involve expensive and complex monitoring solutions.
Couplings are used to connect the shafts of motors to other devices such as gearboxes and encoders. They are meant to compensate for vibration and slight misalignments, but because they move and can be less sturdy that the shafts themselves, they create another failure point, Spider couplings are made of elastomeric materials, and helical couplings can have very delicate structures. They are also usually located inside a housing, so they can be hard to view and take time to replace.
Loose couplings also allow for positional errors in servo systems. If a motor position starts to drift, the coupling should be checked for proper tightness. Keyed couplings are often used to eliminate shaft slippage.
Seals are built into many bearings or other places where shafts extend from equipment. Heat and wear can damage these, allowing dirt or moisture to enter delicate operating areas of components. Replacing seals on a periodic basis is part of a good maintenance program.
A clutch is used to engage and disengage power transmission from a driving shaft to a driven shaft, allowing the motor to run at a continuous speed. Brakes are used to stop rotating shafts. Both of these operate by using friction against pads or plates, which creates heat and wear. These are often combined in a single mechanism called a clutch-brake.
Figure 3 shows many of the elements listed in combination. This powertrain changes the output speed and torque from the driving motor and allows the load to be connected and disconnected, bringing it to a quick stop.
Six mechanical adjustment and maintenance tips
Motors need maintenance and inspection regularly to prevent problems. Following are some of the things to check on a regular basis:
Ventilation. Ensure fan area is free of dust, dirt or foreign objects.
Brushes/commutators. Most common in dc motors, these need to be inspected regularly. Springs are used to press the brush against the commutator rings, weaker springs can decrease brush life. Typical brushes last about 7500 hours, minimum life might be 2000-5000 hours, while 10,000 is about the maximum.
Bearings. some motors have lubrication ports for greasing the bearings. Follow manufacturer’s instructions for lubrication; too much and the grease will end up squeezing out of the seals, too little and the bearings will have reduced life. When bearings are completely sealed and don’t have grease fittings, they should still be inspected, and replaced if worn out. Excessive loading, hot motors and harsh environments can all contribute to bearing wear.
Vibration. Too much vibration can damage motor windings or create metal fatigue. This can be difficult to detect by hand, but there are vibration sensors available for preventive maintenance.
Windings. These may need to be checked periodically. This can be done visually or measuring the resistance with a meter.
Electrical connections. Over time, motor connections can loosen leading to higher current and eventual failure. Visible signs can include discoloration, but motor connections should be checked periodically for tightness.
Belts and chains should be monitored regularly for wear and proper tensioning. When they are too loose, they can slip, and when too tight they can contribute to reduced life in power train elements. Belts are usually tensioned to where they are taut but don’t exert excessive side loading to bearings, chains with gears may have a bit of slack.
Metal chains need to be lubricated regularly, this is often done automatically. Check sprockets for signs of wear, and replace links if needed. Belts should be replaced as needed or when recommended by the manufacturer. Cleaning of machine parts should be done to reduce corrosion and to prevent contamination of product. This can include painting and application of light oil such as WD-40 to fasteners.
A maintenance checklist or preventive maintenance software package can help track activity and keep records.
– This has been edited from the “Maintenance and Troubleshooting in Industrial Automation” book by Frank Lamb, the founder and owner of Automation Consulting LLC and a member of the Control Engineering editorial advisory board.