Airless Spray Technology - For Lubricating Open Gears

Chris Holland, Lincoln
Tags: automatic lubrication, gear lubrication

An innovative grease spray technology that requires no air now is available for lubricating open gears in such heavy industries as mining, concrete and gravel operations. An open gear is not enclosed, usually because of its size. Proper lubrication of the open gear drive is critical for long and trouble-free service life and proper operation. The cost-effective airless spray lubrication system provides dependable, high-pressure lubricant spraying of open gears with low maintenance.


Figure 1. Open Gear Airless Spray System

When compared to an air-assisted system, the airless spray system requires less plumbing. Because no air is used to atomize the lubricant, more grease is applied to the gear and less is wasted in fogging and coating the gear housing with lubricant. Consisting of four main components - an airless spray valve, controller, lube filter panel and pump - the airless spray lubrication system provides a wide range of spray patterns, so lubricant can get where it is needed. The system handles NLGI #2. It can spray some lubricants previously considered unsprayable.

When spur gears are operating, the line of contact between the driven gear and the pinion is constantly changing. In a typical industrial bearing, the same area of bearing surface is continually carrying the load and absorbing the heat generated during operation. In an open gear, the load is changing as the teeth pass in and out of mesh.

As the teeth first begin to mesh, a sliding motion predominates. As the teeth approach the pitch line contact, the sliding motion changes to a rolling motion, and at the pitch line the motion is virtually all rolling. As the teeth pass out of mesh, sliding motion progressively increases with a decrease in the rolling motion. Although the teeth contact is commonly referred to as the “contact line,” it is of an appreciable width. It could be as large as 3/16-inch or more at contact pressures of 3,000 psi or more.

An airless spray system can spray the lubricant with enough force to reach the lowest points of the teeth contact line, even below the pitch line to ensure all bull gear contact areas are lubricated.

How the Airless Spray System Works
The airless spray valve in Figure 2 is shown in stage 1, the initial stage ready for lubricant.


Figure 2. Stage 1, Airless Spray Valve Ready for
Charging with Lubricant

Nitrogen gas in the top section of the accumulator, on the left, is precharged to 1,500 psi. Hydraulic fluid is on the bottom side of the accumulator diaphragm. The hydraulic fluid is pressurized to 2,000 psi. The lubricant to be applied by the airless spray system is pumped into the airless spray valve on the right side of the measuring piston. The pressure switch indicates low pressure to the controller. The control signals for the pump to start and disable the spray solenoid valve.

Stage 2 is the operating sequence. The lubricant enters the system through the inlet check and moves the measuring piston against the hydraulic fluid pressure, which in turn acts against the nitrogen gas pressure on the other side of the diaphragm in the accumulator. When the pressure reaches 3,500 psi, the pressure switch changes state, turning off the pump and enabling the spray solenoid valve. The airless spray valve is now ready to spray.

Stage 3 is the spraying process. When a spray cycle is initiated, the spray solenoid valve opens and the pressure in the accumulator causes the hydraulic fluid to act against the measuring piston. This action forces the lubricant out through the spray valve and spray nozzle onto the surface to be lubricated. The pressure switch changes states, the spray cycle times outs and the spray solenoid valve closes.

Lubricant Considerations
The proper grease for each machine is recommended in the specifications provided by the machine manufacturer. The type of lubricant specified, asphaltic, synthetic or semifluid greases, will dictate where the lubricant is applied to the gears.

Asphaltic
Asphaltic lubricants are lubricants with an asphaltic base diluted with a nonchlorinated solvent. The purpose of the solvent is to make the grease thin enough to spray. After it is sprayed, the solvent evaporates, leaving a film of grease behind to lubricate the gear mesh. Asphaltic lubricants are widely used in the Western Hemisphere.

Asphaltic-type lubricants use post-mesh application where it is applied to the driven gear just after the gear meshes with the pinion or driving gear.


Figure 3. Post-mesh Application

The grease should be applied to the loaded tooth of the driven gear. The goal is for the solvent to evaporate before the lubricated area of the gear reaches the driving pinion gear. Lubrication frequency is 10 to 30 minutes. It should be noted that asphaltic-type lubricants have a tendency to dry over time due to the evaporation of solvents in the drum. The use of a follower is recommended in the lubricant reservoir.


Figure 4. Pre-mesh Lubricant Application

Synthetic
Synthetic lubricants are made from a synthetic base material and do not require solvents to make the grease sprayable. Some of these lubricants may replace asphaltic-type lubricants and may be applied with the post-mesh method, or applied directly to the pinion.

Many hydrocarbon bases and synthetic lubricating oils contain tackifiers to increase the tenacity of the oil, or make it stick to the gear surfaces. This tacky property will not allow the lubricant to be sprayed at the system’s standard operating temperature and may require a higher-temperature thermostat. Testing proves that little, if any, heat is added to the gear surfaces due to the convective cooling action of the atomized lubricant, and the lubricant retains its desired viscosity.

Semifluid
Semifluid greases are commonly specified on equipment manufactured in Europe. The European manufacturers often recommend applying the lubricant to the loaded tooth before the gear mesh. Semifluid greases have no solvents to evaporate; therefore, they may be applied directly to the mesh of the pinion or drive gear. The lubricant should be applied intermittently in small quantities.

Determining the Application and Number of Spray Heads
The tip angle and spray distance determine the shape and size of the spray pattern. The tip angle is the included angle of the spray pattern as it comes out of the spray tip. The spray distance is the distance from the spray tip to the gear face or target surface. The distance between the spray tip and the target surface will determine the spray width and the force of impact. As the distance is increased, the impact force will be reduced.


Figure 5. Spray Pattern Components

The distance from the gear to the spray tip will vary depending on the room available in the application. The distance from the tip to the gear face should not exceed 18 inches. Air turbulence from the moving gears may reduce the impact force of the lubricant on the gear face and cause unpredictable results. Reducing the tip-to-gear distance will reduce the pattern width. Smaller spray angle tips are available for applications requiring a narrower spray pattern.

Gear width, diameter and space constraints may require more than one airless spray valve to lubricate a gear. If the gear lubricant requirements exceed the capacity of one airless spray valve, a second airless spray valve can be added. In some situations, a single airless spray valve cannot provide a wide enough pattern to cover the entire gear face. Another airless spray valve must be added, thus doubling the spray pattern width. A controller is available that will control two airless spray valves installed on a single machine.

Spray Pattern Factors
The lubricant’s viscosity, specific gravity, type and temperature greatly affect the spray pattern. Each lubricant should be tested to determine the proper spray temperature and tip required for the application.

To achieve a uniform spray pattern, the airless spray system is equipped with a heater to maintain the lubricant temperature that, in turn, will control the lubricant viscosity in a wide range of atmospheric conditions. The lubricant is heated just prior to application so that solvent evaporation does not occur. Heating of the lubricant in the drum is usually not required except in extremely cold conditions.


Figure 6. Airless Spray Pattern

The system’s thermostat is set to maintain the lubricant temperature at 120°F to 150°F (49°C to 66°C). The standard thermostat will be suitable for application of most asphaltic- and petroleum-based greases. Synthetic lubricants and some greases may require higher temperatures to obtain a suitable spray pattern. A high-temperature thermostat that will maintain a lubricant temperature from 160°F to 175°F (71°C to 79°C) is available.

Keeping the Lubricant Clean
For trouble-free operation, it is recommended that the lubricant be filtered before entering the airless spray valves. Solid particles in the lubricant can clog the spray valve and spray tips, requiring frequent attention to clear and service. Keeping lubricants clean in the reservoir will help, but not prevent, some contaminants from entering the system. A dual filter panel is available to keep lubricants clean and reduce tip and valve clogging.

Spray Nozzle Tips
Standard spray tips attach directly to the outlet adapter on the side of the airless spray valve assembly. Standard tips are not recommended for any lubricant that may cause frequent clogging of the tip. Other available tips can be cleaned easily and replaced without tools. These tips may feature a rotating tip handle that allows it to be flushed out with a lubrication cycle.

A swivel assembly that directs the spray toward the target area without aiming the entire airless spray valve assembly is available for use with some lubricants. After the spray has been directed to the desired target point, the swivel assembly can be locked in position by tightening the swivel lock nut.

System Requirements
To properly lubricate open gear and pinion in bull gear applications using the typical airless spray system, the following items must be determined:

  • The number of airless spray valves required to spray the gear face.
  • Required film thickness.
  • The area at the outside diameter of the gear to which the lubricant is applied.
  • The gear lubricant capacity.
  • The rotation speed of the gear and the spray time.
  • The gear coverage area with one spray cycle.
  • The pause time between spray cycles.

After making the initial calculations, the system should be set up and observed in operation. It should be determined if the application rate must be increased or decreased, and if necessary, the pause time recalculated and adjusted accordingly.

Pump and Size of Supply Lines
The typical pump should develop pressure of 3,500 psig at the connection to charge the system. Determine the position of the pump and make the sketch of the system. Always position the pump as close to the system as possible. Check with the manufacturer for specific pump assembly requirements, as well as hose and controller selection.

Maintenance
Once installed and fine-tuned to the application, the filters and spray tips will require regular maintenance. Clean, well-filtered lubricant will reduce tip and system maintenance to an occasional tip cleaning. Lubricant contaminated with dust and dirt will cause frequent tip clogging and possible spray valve clogging.

 

Many factors can affect the spray pattern. Temperature, viscosity, pressure and tip wear can all cause the pattern to change. If the spray pattern fails to fan out and sprays out in a solid stream, any of these factors may be at fault. Care should be taken to ensure that the tip is clean and free of any clogging or buildup of dried lubricant.

The system filters must be kept clean. A clogged filter may result in filter bypass, thus allowing dirt and debris to enter the system. In extreme cases the filter element can burst, causing filter media and dirt to enter. Keeping the lubricant clean in the reservoir will reduce filter maintenance. A follower and tight-fitting drum cover with no openings should be used.

Although air-assisted systems may work well for many lubrication situations, the airless spray system has a definite role in large, open gear lubrication applications. Its unique technology provides many benefits that are worthy of further investigation.


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