1.0 What is Open Girth Gear
Most of the published gear manufacturing knowledge is related to two main themes; material quality and tooth accuracy. In most cases, the mechanical accuracy of the gear blank is taken for granted. After all, preparing the gear blank usually involves applying the basic machine shop skills taught in high school vocational technical courses. However, when discussing gear manufacturing, the performance of large multi-segment large gears is not as relatively compact, rigid, and overall structure as we usually imagine. The large gear is a very large non-rigid structure, and special care needs to be taken when processing individual mating segments and the assembled gear blank itself.
As we all know, the mounting surface of the gear blank must have a certain geometric accuracy, and the pitch cylinder of the gear must have a certain geometric relationship with these mounting surfaces. In addition, when these basic geometric rules are not observed, the best structural materials and the most accurate tooth geometry have a limited impact on actual life. In fact, the American large gear rating standard-ANSI/AGMA 6014-B15-denies its validity when the required geometric relationship is not met.
However, for the manufacturer or purchaser of open girth gears, the conditions required to achieve those assumed geometric quality levels in the gearwheel may not be so intuitive. The following sections will describe common large gear design features, the normal sequence of steps in the manufacturing process, some typical challenges encountered along the way, and the downstream effects of failing to meet these challenges.
Examples of Open Girth Gears in Cement Industry:
Open Girth gears are used to drive large, rotating systems such as rotary kilns, ball mills, dryers, rotary or horizontal mills.
2.0 Open Girth Gear Applications
Girth gears are divided into two basic categories; tangential spring mounting and flange mounting.
Flange-mounted large gears are the more common of the two, and are most commonly used in “cold” processing equipment, such as semi-automatic grinding (SAG) mills and ball mills used in the mining and cement industries. According to current standards, large gears can have a diameter of up to 14 meters (46 feet), a face width of 1.1 meters (44 inches), a number of teeth of 50 modules (0.5 DP), and a weight of more than 120 tons. Flange-mounted gears rely heavily on the mill structure to provide the rigidity and geometric stability required for successful operation.
On the other hand, spring-mounted large gears are often used to drive (thermal) rotating high-temperature equipment, such as kilns and dryers. For these applications, when the temperature of the driven machine is usually hundreds of degrees higher than the surrounding gears that drive it, securely bolted flange mounting is not a viable option.
The following discussion will focus on flange-mounted gears, because so far, these applications represent a larger group of people.
3.0 Structural Designs of Open Girth Gears
The girth gear has two basic structural designs, including Y-shaped cross-section and T-shaped cross-section structure. The Y-shaped section design is almost exclusively used for cast steel, and to a lesser extent for ductile iron. The T-shaped section is widely used in steel and ductile iron castings due to its simplicity and light weight. The T-shaped section is almost exclusively used for assembled large gears with welded structures. The Y-shaped section design, when viewed as an independent structure, has greater torsional stiffness than the T-shaped section. However, both of these two designs have decades of proven reliability. This discussion does not indicate or imply any preference for any structural design.
Large gears up to 7.5 meters (24 feet) in diameter are usually manufactured in two sections. Beyond this size, most designs will use four to six segments-depending on the liquid metal pouring capacity of the foundry or the largest forging available for the weldment. Smaller individual segments can also make it easier to adjust the inherent geometric errors of original castings and weldments. The crack at the joint is located at the root of the tooth. When the number of teeth is not completely divisible by the number of teeth, the number of teeth of unequal length will appear on gear segments.
Tooth Alignment The most common girth gears are helical designs due to cost advantage. Spur gears are second in popularity, with double helical designs running a distant third due to their inherent complexity and higher cost of manufacturing. Double helical and spur gear designs may be necessary if the mill bearings cannot absorb the thrust force from single helical designs.
4.0 Applying Open Gear Lubricants
When selecting an open gear lubricant for use in a particular application, the appropriate method used for applicationof open gear lubricant, must be considered. The typical methods of application used in open gear lubrication systems are:
Generally, if the open gear lubricant is to be applied through a drip irrigation system, forced-feed lubricator, or spray system, it must have sufficient fluidity to easily flow through the application equipment. For brushing applications, open gear lubricants must have sufficient fluidity to be evenly spread on the teeth. In any case, the open gear lubricant must be sufficiently viscous to prevent extrusion from the gear teeth during operation. When lubricating an open gear by dripping into a splash plate or by using a splash and idler immersion system, the open gear lubricant must not be so heavy that it will flow when the gear teeth are immersed in it. Finally, when an open gear is lubricated, the consistency or grade and its ease of pumping must allow easy application under the prevailing environmental conditions.
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The most common spray/atomization system used to lubricate open gears is an intermittent mechanical spray system. Its use depends on the open gear lubricant remaining on the gear teeth through a few turns. The intermittent injection system uses a metering valve to direct the lubricant to the air/grease nozzle, which sprays the lubricant onto the open gear with the help of air pressure. The basic components of this type of system are pumps, controllers, metering valves, spray manifolds, and nozzles. The operation of this type of system is very simple; a signal from the controller turns on the pump and supplies open gear lubricant to the positive displacement metering valve.
The metering valve can be a progressive, two-line or ejector type. The metered lubricant is fed into a channel in the spray manifold where the open gear lubricant is directed to the nozzle. The second channel of compressed air (usually in the range of 80 to 120 psi) is directed to the same nozzle. This pressurized air blows open gear lubricant out of the nozzle and onto the open gear. After a predetermined amount of open gear lubricant is dispensed, the air system and pump will be shut down until the next lubrication cycle. There is usually a delay in shutting off the air to ensure that the open gear lubricant has cleared the nozzle. This removes the open gear lubricant from the nozzle tip, preventing it from drying out and clogging the nozzle.
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Gravity-feed or drip-feed systems.
Gravity feeding or drip feeding systems are used in mills, kilns, buckets, draglines and excavators. These systems consist of one or more oil injectors, cascade discs, pressurized feed lines or coating wheels. They allow open lubricant to drip into the gear mesh at a set speed. This application method is limited to open gears with a pitch speed of 1,500 ft/min (7.5 m/s) or lower. For these types of systems, asphalt, high-viscosity synthetic oil open gear lubricants are usually used. If pressurized feed lines or applicator wheels are used in these systems, semi-fluid grease or gel/polymer thickened type open gear lubricants can be used.
Oil bath (splash and idler immersion) system
The oil bath system is the easiest way to lubricate open gears. Allow the gear or idler gear meshing with the gear to be immersed in the open gear lubricant to bring it to the mesh. Idler immersion systems are usually limited to open gear systems with a pitch speed of less than 300 ft/min (1.5 m/s). Some systems will also include recirculation pumps and filtration systems. Splash and idler immersion systems can be found in mill and kiln applications.
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Hand, brush or pour apply it – Open Gear Grease Spray
Manual application is one of the oldest and most dangerous methods used to apply open gear lubricant by spray application or hand application. This method is generally used to apply asphalt-type and high-viscosity synthetic open gear lubricants.
For apply on open gear lubricants on mills, kilns, buckets, draglines and excavators, automatic Open Gear Grease Spray system is used as stated above.
However, using this method will not only lead to improper use of open gear lubricants, but also lead to the introduction of contaminants into the gears. Further use of this method during open gear operation may endanger safety and cause injury or death to persons who use open gear lubricants.
For more details for Open Gear Grease Spray(Open Gear Lube Aerosol) for small applications please visit: https://www.lubricants.center/lubricants/cement-industry-lubricants/aerosol-open-gear-lube/
5.0 Lubricating film thickness and selection criteria.
The main lubrication method required to lubricate open gears is elastohydrodynamic (EHD) lubrication. According to the EHD theory, the key factor is the oil film thickness of the open gear lubricant, which depends on the dynamic viscosity of the open gear lubricant at operating temperature, the average surface speed of the gear temperature, the load and geometry of the gear, etc.
It has been determined Figure 7 -Over-lubricated open gears. 40 Power Transmission Engineering February 2012 www.powertransmission.com The lubrication conditions present in most gears are mainly elastohydrodynamics. Gear teeth are exposed to huge contact pressures in a relatively small area (possibly up to 435,000 psi), but they can still be successfully lubricated with a very thin lubricant film. There are two reasons for this situation:
The high pressure causes the surface to deform elastically and spread the load over a wider area.
The viscosity of the lubricant increases significantly with pressure, thereby increasing the load-bearing capacity of the lubricant. Once the film thickness is determined, another important parameter that needs to be calculated is the Lambda ratio. This ratio is defined as the ratio of the lubricant’s EHD film thickness to the composite surface roughness of the contacting metal surface. When the Lambda ratio approaches 1—that is, the film thickness is in the same order of magnitude as the surface roughness—the contact between the two contact gears can be expected to increase.
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6.0 Procedure to follow on new equipment:
Clean all coating and debris from the gears.
Coat the gear and pinion with a light film of open gear lubricant employing some sort of spray method.
Run equipment slowly under no load to verify that there is lubricant throughout the entire load zone.
Gradually increase speed and load while turning on the automatic lube system.
Monitor continuously until a proper coating is maintained
For spray systems:
Prior to startup, purge the lube lines and check spray patterns for complete coverage.
Adjust the air pressure and volume as needed.
For drip systems:
Most open gear lubricants are designed to adhere where applied. Drip tubes should be spaced no farther than two inches apart.
7.0 Procedure for switching from one type of open gear lube to another:
Although it is best to completely clean the gear, pinion and gear guards, conversion of one type of open gear lubricant to another can be made by applying the open gear lubricant to be used directly over most existing applications.
Thoroughly clean the lubricating oil line.
Activate a timing setting that is 50% higher than the operating setting to ensure that all lines are purged and flushed and an adequate lubricant film is established before reducing the lubricant consumption rate to the operating setting.
Readjust the timer to maintain a sufficient lubricant film. The amount of lubricant should not be suddenly reduced, but should be reduced every 5 minutes, 150 to 200 hours for mills, and 100 to 150 hours for buckets, draglines and excavators. • Product performance should be monitored.
When reducing the consumption of the spraying system control unit, it should be set to ensure that the interval of the spraying cycle is as short as possible. Short and frequent spraying cycles ensure that the lubricant is evenly supplied to the components, thereby improving functional reliability.
Adjust the air pressure and air volume as needed.
The lubrication system, tooth surface condition and spray pattern need to be checked to ensure reliable operation. The spray system should be thoroughly maintained in accordance with the manufacturer’s instructions.
8.0 Learn About What Are the Prerequisite for Open Girth Gear Lubricant Selection
Excellent adhesion to open gears and resistance to extrusion under extreme pressure.
Tackiness (adhesive/cohesive properties) of lubricant on gears.
Provide gears an elastic retractable film which cushions the loads and lubricates load bearing surfaces and minimizes metal to metal contacts.
Resistance to fling-off from the gears.
No build up in the roots of the gear teeth.
Pumpability in all weather conditions.
Adequate film thickness – Lamda.
Resistant to water washout and spray-off.
Protection of the gears against rust and corrosion.
Spray ability and/or ease of dispensability.
Alleviate housekeeping and maintenance issues.
Considering the characteristics, applications and methods that are most suitable for open gear lubrication, there are many products on the market to choose from. Lubrication products are suitable for various lubrication principles, but the most effective is electrohydrodynamic (EHD) type lubrication for open gear oil spray. Some lubrication products used for open gear lubricating oil spray use boundary lubrication with the help of solid lubricants (such as graphite, desulfurization and molybdenum) as a component to separate metal-to-metal contact.
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9.0 The Open Gear Lubricants Types
The open gear lubricants types are:
High viscosity synthetics – Synthetic Gear Grease
Asphaltic type (also referred to as residual compounds)
Semi-fluid greases (also known as paste type)
Semi-fluid grease cutbacks
Useful Links for :
The Petron Gear Shield (https://www.lubricants.center/lubricants/cement-industry-lubricants/) meet the properties and performance criteria required to properly and successfully lubricate the open gears lube spray for open gears in Cement, Mining or other industries for spay type systems / applications.
For more details for Synthetic gear grease for kiln girth gear lubrication, please visit https://www.lubricants.center/lubricants/cement-industry-lubricants/synthetic-kiln-lube/
For more details on Petron Lubrication Products – Cement Industry lubricants: heavy duty open gear grease, best grease for metal gears, open gear oil, open gear grease, open gear lubricants, open gear oil and greases, please visit below:
Cement Industry Lubricants – Products Data Sheet
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