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Industrial gears like hypoid, bevel (spiral, straight), spur, and helical often operate under heavy loads and require extreme-pressure protection for gear components. Gearboxes play very important role for many industrial power transmission systems. Many components in gear drives have tight tolerances and optimized gear geometry, which are required for transferring working loads as smoothly and efficiently as possible.

For successful operation and long life of a gearbox, it is directly related to proper maintenance. Many gearbox failures take place due to a few problems and basic preventive maintenance practices will minimize these failures. We will focus on four areas for proper functioning of gear: lubrication, temperature, vibration and noise.

Lubrication

Lubrication is one of the most important components of a gearbox. Effective lubrication is extremely critical to all gearboxes and will help prevent gear and bearing failures. Many gear and bearing failures result from insufficient or interrupted lubrication

Maintaining proper lubrication includes using the appropriate lubricant, keeping oil clean and free of foreign materials, and maintaining a sufficient supply of lubricant. Since selecting a lubricant is based on many independent factors including gear type, load type, speed, operating temperatures, input power and reduction ratio. Foreign materials present in the lubricant can cause abrasive wear. When lubrication problems occur they can cause several gear problems. Failures, like scoring and galling, are generally caused by oil film breakdown resulting in metal-to-metal contact.

 

Temperature

A rise in temperature or localized hot spots can indicate that the gearbox is not operating as efficiently as it once was due to a problem with either the gears or bearings. Temperature control is also important for oil life. For sump temperatures above 200° F, R & O mineral oils start to degrade rapidly and gear and bearing wear may occur along with shortened seal life. Synthetic oils have been used successfully in operations up to 225° F, but are more expensive than mineral oils.

High temperatures resulting in tooth surface damage and oil degradation. Most manufacturers offer cooling packages such as shaft-driven fans, electric-motor-driven fans or heat exchangers to keep gearboxes running at lower temperatures.

Vibration

Each machine fault generates a specific vibration profile, and a single vibration measurement provides information concerning multiple components. The frequency of the vibration is determined by the machine geometry and operating speed. By analyzing shaft vibration,  machine fault is detected as due to the imbalance, misalignment, general looseness or wear, bearing defects, gear defects, or some other unforeseen problem.

 

Noise

Abnormal sounds alarm that something is wrong with the gearbox. An increased sound level may indicate worn or damaged gears and bearings. Knocks can be the result of broken teeth or bearings. Rattles may be caused by loose fasteners or high vibration.

 

 

 

 

   

Hydraulic oils, also called hydraulic fluids, are the medium by which power is transmitted in hydraulic system. Hydraulic fluids are also responsible for lubrication, heat transfer and contamination control. Common hydraulic fluids are petroleum-based or mineral-based fluids ,water-based fluids , synthetic fluids are used. Examples of equipment that might use hydraulic fluids include excavators, brakes, power steering systems, transmissions, backhoes, garbage trucks, aircraft flight control systems and industrial machinery.

Pascal's Law: It states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same.

Pascal constructed the first known hydraulic device, which consisted of two sealed containers connected by a tube. The pistons inside the cylinders seal against the walls of each cylinder and prevent the liquid from leaking out of the cylinder and prevent air from entering into the cylinder. When the piston in the first cylinder has a force applied to it, the pressure moves everywhere within the system. The force is transmitted through the connecting tube to the second cylinder. The pressurized fluid in the second cylinder exerts force on the bottom of the second piston, moving it upward and lifting the load on the top of it. By using this device, Pascal found he could increase the force available to do work, just as could be done with levers or gears.

 

One of the most important examples is the hydraulic brake system to stop moving vehicles.

 

  The hydraulic brake system consists of a master cylinder joined by tubes to four smaller cylinders, one for each wheel of the car. They are called brake cylinder .All cylinders are provided with oil tight pistons.

A forward push on the brake pedal causes a force on the piston in the master cylinder and a consequent pressure on the brake oil. These undiminished transmitted pressure forces the piston in each brake cylinder to act on the brake shoe attached to a caliper or against a rotor in the case of disk brake. The resulting friction stops the car.

A simple hydraulic system consists of hydraulic fluid, pistons or rams, cylinders, accumulator or oil reservoir, a complete working mechanism, and safety devices. These systems are capable of remotely controlling a wide variety of equipment by transmitting force, carried by the hydraulic fluid, in a confined medium.

Fluid Properties
Viscosity, viscosity index, oxidation stability and wear resistance are very important characteristics of a hydraulic fluid. These characteristics will determine how your fluid operates within your system. Fluid property testing is done in accordance with either American Society of Testing and Materials (ASTM) or other recognized standards organizations.

 

1. Viscosity (ASTM D445-97) is the measure of a fluid’s resistance to flow and shear. A fluid of higher viscosity will flow with higher resistance compared to a fluid with a low viscosity. Excessively high viscosity can contribute to high fluid temperature and greater energy consumption. Viscosity that is too high or too low can damage a system, and consequently, is the key factor when considering a hydraulic fluid.

2. Viscosity Index (ASTM D2270) is how the viscosity of a fluid changes with a change in temperature. A high VI fluid will maintain its viscosity over a broader temperature range than a low VI fluid of the same weight. High VI fluids are used where temperature extremes are expected. This is particularly important for hydraulic systems that operate outdoors.

3. Oxidation Stability (ASTM D2272 and others) is the fluid’s resistance to heat-induced degradation caused by a chemical reaction with oxygen. Oxidation greatly reduces the life of a fluid, leaving by-products such as sludge and varnish. Varnish interferes with valve functioning and can restrict flow passageways.

4. Wear Resistance (ASTM D2266 and others) is the lubricant’s ability to reduce the wear rate in frictional boundary contacts. This is achieved when the fluid forms a protective film on metal surfaces to prevent abrasion, scuffing and contact fatigue on component surfaces.

 

 

 

 

 

 

Grease -As per ASTM D 288(American Society for Testing and Materials) lubricating grease can be defined as: "A solid to semi fluid product of dispersion of a thickening agent in liquid lubricant. Other ingredients imparting special properties may be included" .

Greases are typically applied in areas where a continuous supply of oil cannot be retained, such as open bearings or gears.  When selecting suitable grease, factors considered are operating temperatures, water resistance, oxidation stability etc. Grease's characteristics, including viscosity and consistency also consider.

There are numerous applications for grease, most are for lubricating bearings of various types. There are two main categories of bearing  i.e., plain or anti friction. In addition, these types of lubricants are often used for the lubrication of ways and guides.

As a general rule, grease used for lubricating ways and slides are sodium-base greases. Plain bearings on the other hand use grease for limited speeds, typically below 300 RPM with a practical maximum of about 400 RPM. On the other hand, greases for anti-friction (high-speed) bearings include those used for plain bearings with the exception of barium greases. Barium should never be used for high-speed applications.

Extreme pressure greases are commonly used in heavy-duty ball and roller bearings, as well as plain bearing applications that are subjected to high-loading conditions. A gear set is a perfect example where EP grease is used to overcome high-load conditions. However, operating temperatures are typically limited to a range of 150o to 200ºF for this type of lubricate.

  

NLGI Grease Classification (National Lubricating Grease Institute)

The consistency of industrial greases are classified by the distance in tenths of a millimeter, that a standard cone penetrates a sample of the grease under standard conditions at 25ºC

 

Lubrication of Gears

The purpose of lubrication is is to minimize friction and wear to extend equipment service life..  To provide extreme temperature and pressure protection in order to prevent wear, pitting, spalling, scoring, scuffing and other types of damage that result in equipment failure. Other important purpose for lubrication is to protect against thermal degradation, rust and corrosion to equipment.

Methods of Lubrication

There are generally  three type of gear lubrication.

           

  • Grease lubrication
  • Splash lubrication
  • Spray lubrication

 

Grease Lubrication (0 to 6 m/s tangential gear speed )
Grease lubrication is suitable for any gear system that is open or enclosed, so long as it runs at low speed. The grease should have a suitable viscosity with good fluidity especially in a enclosed gear unit.   Grease is not suitable for high loads and continuous operation and there is virtually not cooling effect.  The must be sufficient grease to ensure the gear teeth are lubricated but an excess can result in viscous drag and power losses.

Splash Lubrication(4 to 15 m/s tangential gear speed )
Splash lubrication is the normal method for lubricating spur, helical, bevel and worm gears.  The gears simply dip into a bath of oil as the rotate.   Splash lubrication needs at least 3 m/s tangential speed gear speed to be effective.  It is important that provisions are made to ensure the teeth are not immersed in the bath such that excessive losses result from the oil being churned up.  The oil level should be monitored under static and dynamic conditions to ensure it is correct for the application 

Spray Lubrication (above 12 m/s tangential gear speed )
For the higher speed units (10 to 20 m/s peripheral speed) engineered spray lubrication is genally provided using shaped nozzles with oil at a circulated pressure of about 0.7 barg. At higher speeds the system for directing the oil at the teeth needs to be carefully engineering to ensure the oil actually reaches the contacting surfaces as centrifugal forces and escaping air flow will tend to deflect the oil jet.

 Types of  Gears

 Gear Design Dictates Lube Design

Gear designs vary depending on the requirements for rotation speed, degree of gear reduction and torque loading. Transmissions commonly use spur gears, while hypoid gear designs are usually employed as the main gearing in differentials. Common gear types include:

Spur


Spur (straight cut) gears are widely used in parallel shaft applications, such as transmissions, due to their low cost and high efficiency. The design allows for the entire gear tooth to make contact with the tooth face at the same instant. As a result, this type of gearing tends to be subjected to high shock loading and uneven motion. Design limitations include excessive noise and a significant amount of backlash during high-speed operation.

Bevel


Bevel gears (straight and spiral cut) transmit motion between shafts that are at an angle to each other. Primarily found in various types of industrial equipment as well as some automotive applications (differentials), they offer efficient operation and are easy to manufacture. As with spur gears, they are limited due to their noisy operation at high speeds, and are not the top choice where load carrying capacity is a requirement.

Worm


Worm gear sets employ a specially-machined “worm” that conforms to the arc of the driven gear. This type of design increases torque throughput, improves accuracy and extends operating life. Primarily used to transmit power through non-intersecting shafts, this style of gear is frequently found in gear reduction boxes as they offer quiet operation and high ratios (as high as 100:1). Downfalls with this type of gear set are its efficiency, high price per HP and low ratios (5:1 minimum).

Hypoid


Hypoid gear sets are a form of bevel gears, but offer improved efficiency and higher ratios over traditional straight bevel gears. Commonly found in axle differentials, hypoid gears are used to transmit power from the driveline to the axle shafts.

  Planetary


Planetary gear sets, such as those found in automatic transmissions, provide the different gear ratios needed to propel a vehicle in the desired direction at the correct speed. Gear teeth remain in constant mesh, which allows for gear changes to be made without engaging or disengaging the gears, as is required in a manual transmission. Instead, clutches and bands are used to either hold or release different members of the gear set to get the proper direction of rotation and/or gear ratio.

  

Helical


Helical gears differ from spur gears in that their teeth are not parallel to the shaft axis; they are cut in a helix or angle around the gear axis. During rotation, parts of several teeth may be in mesh at the same time, which reduces some of the loading characteristics of the standard spur gear. However, this style of gearing can produce thrust forces parallel to the axis of the gear shaft. To minimize the effects, two helical gears with teeth opposite each other are utilized, which helps to cancel the thrust out during operation.

Herringbone


Herringbone gears are an improvement over the double helical gear design. Both right and left hand cuts are used on the same gear blank, which cancels out any thrust forces. Herringbone gears are capable of transmitting large amounts of horsepower and are frequently used in power transmission systems.

  The differences in gear design create the need for significantly different lubrication designs. For instance, gears normally seen in automotive differentials are hypoid gears and require GL-5 concentration and performance of extreme pressure additives.

“This is because of the spiral sliding action that hypoid gears have,” said Dinwiddie.

Most manual transmissions have helical gears and do not require GL-5 performance.

“The helical gear is almost a straight cut gear, but on an angle,” said Dinwiddie. “There is spiral action and very little sliding action, hence there is less need for extreme pressure additives.”

Gl-4 gear lubes have half of the extreme pressure additives of GL-5 lubes.

 

 Gear  oil

Gear oil ,a fluid lubricant made specifically for transmissions, transfer cases, and differentials in automobiles. It is formulated  with specially processed base oil and sulphur phosphorus additives to ensure  exceptional chemical and thermal stability. It is used in gears (gearboxes) for reduction of friction and wear of the gear tooth surfaces, removal of the heat generated by the operating gear and corrosion protection of the gear parts. The high viscosity ensures transfer of lubricant throughout the gear train.

Most lubricants for manual gearboxes and differentials are hypoid gear oils. These contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears.

American Petroleum Institute (API) established a performance grading system for gear oils (mostly automotive gear oils). According to the system gear oils are designated by the letters GL (Gear Lubricant) followed by a number 1,2,3,4 or 5:

  • GL-1-GL-1 gear oil has rust and oxidation protection effect but it does not contain extra pressure (EP) additives. the oil is used in low load applications only.
  • GL-2-GL-2 gear oil contain more additives than GL-1, but without EP effect. It is used in medium loaded worm gears.
  • GL-3-GL-3 gear oil possesses light EP effect. It is used in non-hypoid gears.
  • GL-4-GL-4 gear oil possesses moderate EP effect. It is most widely used oil.
  • GL-5-GL-5 gear oil possesses high EP effect. It is used in hypoid and other highly loaded gears.
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