1. Preventive Maintenance of Pneumatic System
The systematic maintenance of a pneumatic cylinder is necessary to ensure its long service life and reliability of its devices and components. Lack of regular maintenance may result in loss of air and associated pressure drops, premature wear of moving parts, production shortfalls and increased downtime of pneumatic components. The general maintenance procedure of a pneumatic system is explained the following subsections.
1.1. General Maintenance Procedure
A good maintenance procedure can be summed up in these words: ‘keep it tight, keep it clean, keep it lubricated and inspect it frequently and thoroughly’ to perform maintenance and not repair. More specifically the general procedure to maintain a pneumatic system comprises various activities like:
(i) removal of dust, moisture etc.
(ii) checking of possible loose bolts and nuts in the system components.
(iii) preventing air leakage.
(iv) reducing frictional losses
(v) ensuring safety of personnel
The first procedure in any organised maintenance programme should be the inspection of equipment and systems to prevent a future occurrence of any serious trouble. An important cause of failure of pneumatic systems is the presence of dust, moisture or any other foreign particle in the compressed air which must be removed to ensure the proper functioning of the system.
Another frequent cause of failure is loose bolts and connections in the system. Each connection should be periodically checked for tightness and the inspection should include checking of possible loose bolts and nuts on each components/sub-assembly of the system.
Leakage is another important concern of all compressed air systems, especially the older once. Leaks are caused by damaged/ corroded pipe-works, faulty seals, faulty pipe joints, poor quality of fittings and tubes, incorrect installation of airline couplers, dynamic wear, cuts in tubing and open drain taps.
Air leakage is a major reason for pressure drops. Leakage can occur but may remain undetected. The efficient operation of compressed air system demands the detection and stoppage of the compressed air leakage. The following measures can be taken to prevent air leakage:
» Maintain good quality of air in the system, contaminants in the air system will cause increased leakage and equipment malfunction.
» Take special care during assembly of all fittings and tubings. Use special tube cutters and leakage-resistant equipment.
» Stop air consumption during non-operational times.
» Estimate the correct air pressure for the system, over-pressurisation will result in more compressed air leakage and higher energy costs
2. System Malfunctions
A general account of malfunctions in pneumatic systems and their causes is given below:
2.1. Malfunctions due to contaminants
Pneumatic systems require the compressed air free of moisture and impurities, such as dust particles, pipe scales, etc., for reliable operation. Various conditioning devices-such as filters, dryers, lubricators etc are installed for the removal of unwanted and harmful contaminants. Lack of maintenance or servicing of these devices may result in the failure of the system operation.
Rust and scale particles are introduced within the system piping by moisture resulting from condensation. These free moving particles combined with oil and water sledge can scratch seals, abrade surfaces of precision parts of valves and cylinders, thus causing leaks. These particles can also block orifices, thus jamming valve spools. Further air passages may become restricted resulting in reduced airflow and increase pressure drop. Moisture can also wash away the lubricants from pneumatic valves, resulting in faulty operation, corrosive damage to surfaces and excessive wear. Moisture that is collected in pneumatic cylinders could cause cushions to become ineffective.
2.2. Malfunctions due to improper mounting
A cylinder, mounted haphazardly, will cause an undue strain on the mounting plate, thus increasing the stress level on the mounting bolts. A piston rod, if not properly supported or aligned with the centre line of a cylinder, will exert severe strain on seals and glands and will reduce their working life connections and supports that are subjected to vibration should be examined for tightness and strain.
2.3. Malfunctions due to inadequate air supply
Many a time, pneumatic systems/machines are added without enlarging the capacity of existing compressed air supply.
Owing to this, malfunctions may occur sporadically, and the fault-finding may become extremely difficult. For example,sudden pressure drops, caused by the actuation of additional nearby components, result in variations in piston force and piston speed for short durations.
2.4. Malfunctions due to Under-lubrication/over-lubrication
Lubrication of the compressed air may be necessary to provide seal lubrication. It prevents sticking of moving parts and help in controlling wear. Absence of lubrication or under-lubrication will cause the valves and cylinders to jam and the deterioration of their components due to increased wear, thus tending to make the whole system unsafe. Over-lubrication may produce sluggish operation of valves, cylinders and pneumatic tools.
3. Maintenance Tips
The following maintenance tips constitute general guidelines for maintaining the components of pneumatics systems. The exact maintenance procedure should be determined only after consultation with experts and taking into account the site conditions.
3.1. Maintenance of Compressors
Compressors should be located in clean accessible areas for inspection and maintenance. Avoid locations where the air may have high humidity and where winds whip up dust, grit and litter. It should also be possible to dissipate heat quickly from the compressor area.
Compressors have their own maintenance requirements. Their maintenance must be carried out in accordance with the manufacturer’s instructions. However, the following general maintenance aspects relating to compressors may be kept in mind. The essential routine maintenance activities are cleaning, visual inspection, running checks and servicing of filters,lubricators and cooling facilities.
3.2. Maintenance of Air Receiver
Condensate drain from air receiver should be automatic wherever possible, but they still need to be inspected regularly to make sure that the complete unit is working properly. Safety devices on air receivers, like pressure relief valve, must be maintained in satisfactory functional order.
3.3. Maintenance of Air Mains
Proper maintenance of air mains is very important. Regular inspection of air mains for leaks should be taken up, preferably after the close of the work when there is no interfering background noise. Build up the pressure to the operating level after closing all consumer lines. Determine the pressure drop at the air-receiver over a period of time. This gives an indication of the air leakage in the system, locate and repair leaks in the air-mains.
3.4. Maintenance of Air Service Unit (FRL)
The air service unit consists of Filter, Regulator, and Lubricator. If filters and lubricators are not taken care of and not cleaned when dirty, the money spent on their installation goes waste. A properly maintained filter system can eliminate about 75% of the potential cause of power system failure. In addition the life of the system components is increased considerably. The following regular maintenance of FRL is of utmost importance.
3.4.1. Filter: The condensate level must be checked regularly. The condensate must not exceed the maximum level marked, otherwise the condensate could be drawn into the system again. The drain screw must be opened to drain the accumulated condensate. Another maintenance activity is the cleaning of the filter cartridge is it is clogged.
3.4.2. Regulator: Usually this unit requires no regular maintenance, especially during the initial years of its service life.
3.4.3. Lubricator: The lubricating oil is used up in the process of lubricating the compressed air. Check the oil level and top-up if necessary.
3.4.4. Polycarbonate Bowls: Polycarbonate bowls, used for filters and lubricators must be treated with some care. The following measures need to be taken on polycarbonate bowls:
(i) Bowls which on visual inspection shows signs of mechanical damage, cracking, or hazing should be rejected.
(ii) Bowls which have been contaminated with paint should not be cleaned, but replaced, and
(iii) All bowls which have been in service for more than 10 years should be replaced even if they may appear acceptable by the visual inspection.
4. Maintenance of Pneumatic Cylinders
There are number of precautions that must be taken when installing and servicing pneumatic cylinders that will greatly increase their performance and operating efficiency. As a rule, it is not necessary to open the cylinder always for inspecting its internal condition.
4.1. Causes of Cylinder Failure
An important reason for cylinder failure is the presence of dirt which might lodge in between the piston and the barrel, and may score the barrel and seals. It may make the piston seal defective by occurrence of excessive leakage through the resulting gap between the piston and the barrel.
Among other causes of the failure of pneumatic cylinder are: worn-out or damaged seals, scratches, score marks, pitting etc on the cylinder barrel, piston rod and/or piston rod bearing. Incorrect installation of cylinders can result in irregular stresses being exerted on them. It can damage cylinder or distort them.
To keep the wear to the minimum, air service equipment should be fitted immediately upstream of cylinders, as close to them as possible, this will ensure that no grit particles, moisture, that are injurious to cylinder can reach it. When used with good quality air and not subjected to any abnormal operating stresses, cylinder will have a normal working life in excess of five million cycles. The following regular maintenance activities may be carried on pneumatic cylinders.
1.) Check the cylinder bearings and mountings at regular intervals. Improper mounting of the cylinder is the most common cause of damage to wiper ring, cup packing and bushing. Replace these parts if damaged.
2.) Piston rod should be examined for score marks
3.) Check the support of the piston rod.
4.) Check the rod bearings in the cylinder for wear and replace if necessary
5.) Use proper lubricants in sufficient quantity.
6.) Use clean piping to connect cylinder to the final control element (valve).
5. Maintenance of Pneumatic Valves
Pneumatic valves, in general, will have a working life of 10 million cycles if operated under normal working conditions. If a valve requires servicing before 5 million cycles, then check most carefully the installation, environment conditions and operating conditions as these might be the cause of its shortened working life.
The main cause of valve failure is the leaking of ‘O’ rings which results from the entry of abrasive particles into the valve and consequent wearing of the seals. For this reason, it is essential to have filtered and lubricated air supply to ensure that no dirt particles get into the valve. Use non corrosive piping, such as polyurethane piping.
Other probable causes of valves failure are blocked bleed holes and jammed springs. To detect this failure connect the valve to the air supply and operate it. The return movement of the valve must be snappy, and any sluggishness is indicative of the blocked bleed holes or jammed springs. This can be set right simply by cleaning the bleed holes, taking care that the very small holes are not enlarged at all.
Valves can also be damaged, in particular their operating mechanism, by incorrect installation and operation. Excess force on the operating mechanism, for example, could cause this mechanism to become disturbed. The coil in a solenoid valve, if damaged, it must be replaced with new one.
6. Faults in Pneumatic Systems
1.) Misalignment or mechanical jam
2.) Power supply failure
3.) Insufficient or low voltage
4.) Twisted tubing
5.) Burned solenoid coils
6.) Bend piston rod/barrel
7.) Flow restrictions
8.) Lack of lubrication
9.) Insufficient compressed air delivery
10.) No vertical operation permission for steam (US MODEL) 2 by 2 way valve
7. General Safety Measures
Adequate safety measures must be taken to avoid the risk of personal injury and equipment damage from your application or system. Minor problems can become major issues when they are not addressed. Following are some general safety measures that must be taken in pneumatic systems to avoid accidents.
1.) Keep your workplace clean before and after work.
2.) Use personal protective devices for all hazardous jobs.
3.) Follow the standard procedure while operating a machine.
4.) Know your job thoroughly
5.) Inspect regularly for damaged tubing, fittings and leaks
6.) Repair or replace components that show signs of wear or damage.
7.) Clean the spillage of grease, oil etc immediately
8.) Never direct compressed air towards yourself or anyone else.