It’s important to understand the proper selection and implementation of pneumatic lubrication to facilitate operation of air-actuated equipment.
Learning Objectives
- Understand the pneumatic air flow through systems in plant solutions.
- Discover failure modes and pneumatic lubrication solutions.
Pneumatic lubrication insights
- Condensed water provides poor lubrication, imparts wear, facilitates rust formation and enables sticking.
- Low frictional force and coefficient of friction (CoF) between sliding surfaces are important to reduce power consumption, heat generation, cost, and inventory for air-actuated system components.
The impact of proper maintenance and lubrication of pneumatic equipment is vital for many plant operations across many industries and sectors. Ideally, subsystems remove impurities such as water and other debris from compressed air that derives from the work environment. Air leaks and use of an incorrect hydraulic lubricant can lead to varnish, corrosion, friction, wear, and slip-stick at mechanical and microscopic interfaces. Failure at lubricators, solenoids, cylinders and pistons and various air tools may arise. Increased capital expenditure, maintenance and electricity labor costs would be realized.
Fortunately, a well-designed pneumatic lubricant can mitigate several failure modes and allow continuity in operations. Suitable formulations integrate critical additives to enhance an appropriate base oil’s performance and lifetime. Positive results from lab tests and field trials provide verification and validation to end-users. Future advances include smart and safer technologies.
The pneumatic equipment market is stable, and analyses indicate solid growth to support broad industrial usage over the next decade in all regions. The market size is forecast to increase from over $18 billion in 2023 to above $34 billion in 2030 with double-digit compound annual growth rate (CAGR).
Pneumatics are critical to support vital infrastructure in most industrial settings, including chemical, water and wastewater, robotics, pulp and paper, petrochemical refining, textile, food and beverage and automotive.
Pneumatic equipment line and plant design
The purpose of pneumatic equipment is to provide smooth, controlled, accurate, simple, efficient and fast cycle intervals of pressurized air. Pneumatic equipment includes motors, pumps and compressors, lubricators, valves, cylinders, actuators, fittings and connectors, and various accessories. They are present in all most operations but highly concentrated in fluid flow control, material handling, packaging, robotics, production and other processes which require precision and controlled movement of materials. However, proper lubrication is important (see Figure 1).
A control panel coordinates timing between a compressor, dryer, oil/air separator, air receiver storage tank and user equipment (see Figure 2). Ambient air first passes through the inlet and the gas is pressurized during compression. This air contains water vapor and soluble impurities from the atmosphere. During compression, the air heats adiabatically. The warm air passes to a thermal dryer whose purpose is to remove water. The compressed air is directed towards receiver tanks for stowage and expands thereby lowering the temperature and dewpoint of air in the tank. Water condenses and typically accumulates at the bottom of the receiver tanks. This water should be drained frequently to keep relative humidity in the receiver tank low. Unfortunately, this action does not take place in practice. A distribution system delivers supposed dry, compressed plant air to pneumatic subsystems consisting of a filter, regulator, lubricator, solenoids, pistons, and various other air-actuated equipment.
Consider the pressure-reducing valve (PRV)/lubricator, solenoid and pneumatic cylinder/piston (see Figure 3). Pressure feed from the receiver tank is stepped down to a selected value at the PRV/lubricator inlet. Air-line lubricant present in the oil cup is drawn into the airflow. A needle valve controls amount of atomized oil that exits the PRV/lubricator assembly to the solenoid. This subsystem controls timing and delivery of air to a piston for linear motion, an actuator for rotary motion or other pneumatic devices.
Air delivery challenges in pneumatic systems
Unreliable air delivery in plants is common and can be extremely problematic. Insufficient pressure compromises overall plant reliability. Increased compressor cycling becomes excessively frequent. Energy consumption costs rise and can exceed both maintenance and capital expenditure costs. When not properly maintained, impurities attenuate functionality of the pneumatic system and user’s application both temporarily and permanently.
Improperly assembled fittings are a standard cause of air leaks. Under-tightened and over-tightened fittings from poor instructions and training are unfortunate failure modes. Cracked plastic or galled stainless-steel fittings are the resulting effects.
Bypass air is waste from poor sealing between a piston and cylinder. Interfacial metal or seal damage or deposits prohibit uniform contact. Leakage occurs past the seal when the cylinder is pressurized throughout and at completion of stroke.
Excess friction consumes energy. Two frictional bodies that generate resistance to motion give adhesive friction. Impediment of pliable interfaces introduces deformative friction. Finally, shearing of substrates with high surface roughness produces internal friction and heat. All three scenarios contribute to overall friction, can introduce hysteresis, chatter, and inconsistency.
Moisture and debris contaminate solenoids, cylinders, and other critical equipment. Condensed water provides poor lubrication, imparts wear, facilitates rust formation and enables sticking. Solid contaminates damage seals, plug orifices and yield surface finish wear.
Exposure of air to lubricant comprising paraffinic or mineral oil over extended time and temperature forms sticky varnish and gummy residue. The use of lower quality hydraulic lubricant that often comprises paraffinic oil compromises pneumatic systems. Inadequate or poor-quality lubrication leads to high operating pressure and degradation of hardware. However, the use of higher quality pneumatic lubricant which contains naphthenic, ester or Group V oils can limit these problems.
Instability of certain additives such as zinc dialkyldithiophosphate (ZDDP) to water depletes lubricant effectiveness. It is not recommended to use hydraulic lubricant that usually consists of primary or secondary ZDDP derivatives versus an exceptional pneumatic lubricant which relies upon antioxidants (AO) and anti-wear (AW) additives, which have better water compatibility.
Optimizing efficiency to achieve cost savings
Preservation of well-lubricated solenoids, seals and cylinders to optimize efficiency and reduce leakage can save money. Let us assume that a medium-sized company operates a 100 kW compressor station with a total runtime of all compressors at 6,000 hours per year. Given a rate from $0.08 per kWh to $0.20 per kWh, the annual energy cost is $48,200 to $120,500. If leakage and air bypass of compressed air can be reduced by 30%, a savings of $14,460 to $36,150 on energy costs can be realized.
System requirements for lubrication
All mechanical components in relative motion require lubrication. This is true for pneumatic systems where suitable movement of elements require thin film lubrication to promote efficiency and extend component life. Proper choice of a specialty lubricant involves complicated aspects that require thorough tribological knowledge.
Pneumatic systems function under total loss lubrication versus replenished lubrication circumstances. One must be concerned about additives that deposit when oil dissipates or decomposes. This phenomenon is impactful at large and small scales.
At the mechanical level, the tribological analysis must account for numerous items:
- Solenoid, cylinder, and rod materials
- Different seal materials and sealing edge geometries
- Contact surface pressure and surface micro-geometry
- Acting pressures and mounting position
At the microscopic level, the tribological system encompasses various gaps and surfaces that involve several factors:
- Intermolecular forces, thermal transmission, and conduction
- Friction and wear
- Chemical and electro-chemical corrosion
Characteristics of superior pneumatic lubricant
A proper pneumatic lubricant composition must consider sealing, base oil, additives and other factors. Verification and validation of a prototype is done through use of standardized test methodologies from recognized authorities, such as the American Society for Testing and Materials (ASTM) and field trial at an end-user. Finished product and safety data sheets, case studies and technical service support must be available to guide accurate decisions by the consumer. Here are some criteria formulators, applications engineers, plant managers and others should consider when selecting a pneumatic lubricant.
Sealing. Users should confirm compatibility between elastomer seals and lubricants before application and usage. Typical concerns include shrink/swell, hardness, and mass changes of the polymeric material. A guideline for failure may be set at +/- 5% change. In extreme instances, elastomers may degrade and break apart. A large dimensional change would result in poor seating and improper sealing. Leakage could occur, thereby conceding performance and introducing a safety hazard.
Base oil. One must use an appropriate base oil(s) for air-actuated systems. Naphthenic (see Figure 4), natural esters, synthetic esters and polyalkylene glycols are most compelling. Unfortunately, pure paraffinic oil (see Figure 4) is the common choice due to cost and availability but provides low performance. Paraffinic oil possesses low-oxidation stability, provides minimal water displacement, gives ineffective sediment dispersion and reacts with moisture to produce acidic sludge, varnish and surface corrosion. Hydraulic oils often contain pure paraffinic oil and are incorrectly used in pneumatic applications.
Additives. Some chemical and physical properties of the base oil can be enhanced to meet pneumatic system needs. Common ashless additives including AO, dispersants, corrosion inhibitors and friction modifiers are priorities. While limited amount of ashless anti-wear and extreme pressure additives could be used, certain technologies such as metal-based additives and tackifiers should also be excluded.
Thermo-oxidative stability of lubricants is a differentiating characteristic that ensures smooth cycling. Base oil stability is often enhanced by use of phenolic and aminic antioxidants to scavenge free radicals and prohibit oil polymerization. Base oil molecules can fragment, and then readily oxidize in the presence of dissolved air. Without antioxidants, subsequent polymerization, and deposition of films between sliding surfaces leads to gummy residue and slip-stick movement. The resulting varnish often does not readily resolubilize. Total acid number (TAN) will increase and promote corrosion.
A superior performing lubricant needs effective dispersion to remove deposits and rust in air-actuated systems. Dispersant base oil or additives associate with soot, prevent their agglomeration and settling, diffuse sludge, and prohibit polar molecules from sticking to a piston or solenoid surface. Â They are metal-free, do not leave ash, and possess limited acid-neutralizing capability. Dispersants have a large non-polar group, an amine/amide or alcohol/ester polar connecting group, and an oxygen or nitrogen polar head group.
Limited oxidation of metal interfaces is facilitated by proper functionality and extension of hardware lifetime. Rust inhibitors and yellow metal deactivators protect surfaces from residual amounts of water that are not removed from air intake. Examples of ashless options include alkylamines, amine borates and nitrogen heterocycles. Well-designed formulations can displace water molecules from a metal surface due to their miscibility.
Low frictional force and coefficient of friction (CoF) between sliding surfaces are important to reduce power consumption, heat generation, cost, and inventory for air-actuated system components. Solubilized friction modifiers may form weak interactions with surfaces, stack in alternating molecular arrangement, and shear. Common organic friction modifiers include carboxylic acids/amides, amines, imides, phosphoric/phosphonic acid, long-chain fatty amides, esters such as glycerol mono-oleate, and organic polymers.
Anti-wear (AW) and extreme pressure (EP) additives are used in moderation to reduce damage in some pneumatic lubricants. Common metal-free additives such as borate esters, phosphate esters, amine phosphates, disulfides, dithiocarbamates, and chlorinated paraffins containing boron, phosphorus, sulfur and chlorine elements that decompose and/or react with surfaces for protection. However, excessive concentration or improper choice of AW/EP additives can sometimes increase CoF. Exclusion of chlorinated species in formulations to support environmental impact concerns should be considered.
Viscosity. OEM’s may dictate a viscosity requirement for proper operation of air-actuated systems. The International Standards Organization viscosity grade is the consolidated industry approach to quantify fluid viscosity in industrial systems. The shear stable fluid should separate surfaces under repeated start/stop and reversible motion at high cycle rates. Such functionality requires low/nominal viscosity and minimal film thickness to support small load without drag.
Safety. Environmental, worker, and customer safety are desirable features associated with lubricant technologies in air-actuated systems. Biodegradable, EAL certified, and low VOC lubricants are available for applications where pollution in water, wastewater and air are regulated. Health and safety flags and controls include GHS symbols and statements on safety data sheets and labels. Choice of lubricants with no or low mineral oil content can be selected to reduce air mist concerns. Third-party H1 food-grade certification of lubricants for use in food-processing plants may be required.
Troubleshooting pneumatic lubrication challenges
Chesterton’s 650 Advanced Machinery Lubricant (AML) is compatible with several elastomers including polyurethane, fluorinated rubber, propylene-tetrafluoroethylene rubber, perfluoroelastomer, nitrile and polytetrafluoroethylene (see Figure 5). The blend of base oils and additives provides good thermo-oxidative stability, excellent dispersion of rust and dirt, superior water displacement and absorption, decent anti-corrosion capability, low friction and wear, virtually no odor and insignificant tack. 650 AML possesses an ISO 22 viscosity to separate sliding surfaces with low drag. The 650 AML is biodegradable and H1 NSF certified, while exhibiting low air mist hazards to address environmental, worker and customer safety in food processing and industrial settings.
The 650 AML restores, conditions, and extends life of pneumatic equipment. It provides fast cycle time, lowers downtime, lessons slip stick, stops clogging, improves productivity, and reduces maintenance and air costs. The product easily injects into in-line devices as a direct replacement for hydraulic and white oil.
Extensive field studies and repeat customers have benefited from Chesterton’s 650 AML in pneumatic systems (see Figure 5). Sticky residue was eliminated, hardware reverted to near pristine state and an odorless feature was emphasized at food processing plants. More accurate cycle counts, corrosion reduction, and non-staining characteristics were observed at facilities.
A pathway to improved pneumatic lubrication
Fundamental understanding of pneumatic components, airflow, leakage, applications and lubrication solutions is vital to address failure modes. Residual water and debris are primary culprits in processing. Short sighted decision to use low quality paraffinic base oil gives unforgiving failure. Advances in lubricant ingredients, internet of things (IoT) and artificial intelligence are improving equipment reliability, quality, and productivity. Regional emphasis may prioritize or require use of components with reduced environmental impact.
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