Selection of lubricant for the gearboxes.

The selection of lubricants are very essential requirement for the mechanical engineers for the mechanical reducers / gear boxes. The lubricant which is selected can provide lubrication for both the gears and bearings as well.

Here are the essential parameters were given for choosing the proper lubricant.

1. Extended Drain Intervals:

The lubricant that is filled in the gearbox must have a tendency to retain its property over a long duration of time. This will enable the lubricant to be used for a long period of time without changing the oil. The intervals between the oil changes in the gearbox should be relatively long. This also reduces the cost involved in changing the lubricant.

2. Seal and Paint:

Compatibility: All gearboxes have an oil seal to prevent oil from escaping outside and to prevent the external dirt from entering the gearbox. The Oil Seal is normally made up of rubber. Similarly, all gearboxes are painted on the inner surface where the lubricant is contained. The lubricant should be chemically neutral which will prevent it from reacting with the paint and the oil seal.

3. Superior Demulsibilty:

A majority chunk of the gearboxes work in a atmosphere containing high humidity levels. There is every chance of water entering into the gearbox via its breather and mix with the lubricant. The lubricant must have a high demulsibilty, which prevents it from mixing with water.

4. Exceptional Temperature Stability:

The lubricant must have good temperature stability i.e. it must have a flatter viscosity temperature slope. This will enable the lubricant to retain the viscosity even at higher temperatures.

5. Good Oxidation and Non- Foaming Stability:

The lubricant must have good oxidation stability, which will prevent it from oxidizing with contaminants in the lubricant. Non Foaming stability is required to keep the oil from over flowing out of the gearbox.

6. Cooler Running of the Gearbox:

The lubricant must possess good thermal conductivity. The heat generated in the gears must be effectively carried from them and transferred to the casing of the gearbox for conduction of the heat to the atmosphere. The real properties of the lubricant that need to be looked into before selecting them for use in a gearbox.

GEAR BOX CONTAMINATION - LUBRICATION SOLUTION

GEAR BOX CONTAMINATION:

•“Wear and tear” is a common expression used to describe the ageing of the mechanical systems.

•Wear commonly occurs as a result of the contamination and degradation of the lubricant.

•The contaminants can… not only cause a breakdown in the health of the lubricant, but will also interact with the moving surfaces to cause wear, leading to component failure.

•Adhesion, abrasion and corrosion of component surfaces will typically result from oil contamination as will poorly specified or unhealthy lubricants.

•The wear rate can be reduced as a result of some basic measures to protect the gear box from wear.

LUBRICATION:

•Proper Lubrication with timely addition / replacement plays vital role in maintaining the gear boxes with efficiency and increasing its working life.

•Oil level should be checked in all the gear boxes oil indicators / dip sticks.

•Oil level should be in between the minimum and maximum limits of the dip stick (or indicator).

•Testing of the lubricating oils used to be carried out periodically. The following tests are to be done…

1. Dirt contamination
2. Moisture
3. Volatile materials (Benzene, kerosene, spirit etc,)
4. Viscosity
5. Acidity
6. Alkalinity due to soda (Na2 Co3) etc.,

LUBRICATION MANAGEMENT:

1.Right Selection of the Lubricants.
2.Quality of the oil is to be maintained free from contaminants, should be stored properly before use.

•During recent developments, gear boxes designs changed… new surface hardening techniques & metallurgy has enabled the manufacture of smaller gear boxes for a given hp rating.

•These changes have resulted in much more aggressive surface loading, and the thickness of super-hard material is often razor-thin.

•These harder surfaces are more resistant to particle-induced sliding wear, but they are highly susceptible to particle-induced rolling wear, which occurs at the pitch line of the gear tooth where load is transferred.

•When a particle is squeezed in the load zone, it can fatigue the hardened material, resulting in small formation of dent, producing a proud area around the dent’s crater.

•The plastic deformation that occurs where the surface is dented can dramatically alter the material’s physical properties, making it more susceptible to wear and fatigue.

•Resulting that the gear boxes tent to run hot, increasing the risk to the lubricant.

IDENTIFYING THE SOURCES OF CONTAMINATION:

•Contaminants are hard particles, moisture, high temperature and aeration. The other examples are radiation, process chemical or physical matter from the environment.

•Ingression points of the gear box contamination are seals and the breathers. This will be during maintenance activities such as top –ups, drain and refill etc.,

•Wet conditions may ingress moisture into the gear box.

•Dust particles are ingresses in the gear boxes from the environment.

CONTROLLING THE SOURCES OF CONTAMINATION:

•SEALS: Standard lip seals are low-cost item, but require frequent replacement. Their ability to seal against oil leakage and dirt/water ingress is poor by comparison to labyrinth seals. These seals cannot be directly loaded or pressurised with external forces.

•BREATHERS: snorkel type, or vent plug type breathers can prevent large bodies to ingress into the gear box, but cannot stop a destructive 10 µm particle from entering into the gear box. (like a ball rolling into a open door).

•Upgrading the vent into a proper contaminant-exclusion breather should minimise the ingression of hard particles and moisture. (1 µm filters).

• For applications where volume changes are minimum and the dusty environment, the bladder type (also known as expansion chamber) of breather is recommended to use.

•The bladder allows for expansion and contraction of the air within the casing as a result of temperature changes.

FILTRATION OF GEAR BOX OILS:

PORTABLE OFF-LINE FILTRATION: Some gear boxes may incorporate a small pump with a filter.
Many gear boxes don’t have filtration facility. In such cases filter carts can usually adopted by replacing the fill and drain plugs with quick –connect fittings.

At least 5 to 7 times the volume of the oil in the system should be passed through the filter cart to ensure adequate clean up.

PERMANENT OFF-LINE FILTRATION: Larger units where large volumes of oil and high levels of cleanliness must be maintained, a permanent off-line circuit should be employed.

GEAR NUMENCLATURE & DEFECTS OF GEARS

Spur Gears are the very common gears for many gear boxes. As the helical gears are the development of the Spur gears, learning spur gears can help almost any type of gears. The design parameters for the gears depending upon the torque which is to be transmitted at a certain required speed. The area of contact will dictate the load capacity of the gears. For designing and application of these gears one should able to know the numnclature of the gears. I have given the details in the below picture.

For measuring the tooth thichness there always some mistakes people used to do. Here in the below picture I have given the method and the various tooth thicknesses acording to the gear modules in the below picture.

DEFECTS OF THE GEARS:

1. Wear is the major defects on the gears. The wear may be for one tooth or few or all the teeth.
2. One or few teeth broken or twisted.
3. Burrs on the bore or the key way.
4. Burrs on the internal splines of the gear bore or the tooth surface.
5. Crack or damage on the rim or the bore of the gear wheel.

LIMITS OF WEAR FOR DISCARDING:

Limits of wear for discarding is one of the important thing which many engineers will try to search here and there for the details. Hence I have given here for easy referrence.

OPEN GEAR TRANSMISSION and the surface speed up to 2 meter / second – The tooth thickness (Sx) reduces due to wear by (0.5m) mm, where m is the module, or about 33% of the original tooth thickness.

CLOSED GEAR TRANSMISSION (GEAR BOXES) for surface speeds over 2 meter / sec. – the tooth thickness (Sx) reduces due to wear by (0.3m) mm or about 20% of the original tooth thickness.

MESHING OF GEARS:

Perfect Meshing gears is one of the prerequisites for an healthy transmission of the power.
The spur and helical gears must be assembled in such a way that the axes of the meshing gears are parallel and the distance between their axes is correctly established.

The correct meshing of the spur and helical gears is decided by applying blue paste on the teeth of the driving gear, giving it several rotations and seeing to the blue marks on the teeth faces of the meshing driven gear.

The defects due to mis alignment which found during blue matching is shown in the below picture.

..... Suggestions are invited in the posts.

GEAR TRANSMISSION & TYPES OF GEARS

Gearing is a very vital form of transmission of mechanical power. Gears are universally adopted for almost all machines. We can say that without Gear boxes there hardly exists Mechanical Equipment. Hence the knowledge on these gear boxes are very essential for the Mechanical Engineers.

Type of Gear Transmissions:

1. Spur Gear,
2.Helical Gear,
3.Herring bone Gear,
4.Bevel Gear,
5.Rack and pinion,
6.Worm and worm wheel.

1. SPUR GEARS:

Spur Gears in any transmission system are employed for changing the speed, the torque and are fixed on shafts either rigidly or with sliding traverse.

2. HELICAL GEARS:

Helical Gears can be used for transmitting motion between parallel shafts as between perpendicular shafts. Helical gears used for transmission at an angle are called SPIRAL GEARS.

Advantages of Helical Gears:
•Noiseless motion even at higher speeds.
•Smooth transmission of small gears with fewer teeth, at large transmission ratio ( up to 15:1)
These gears must be rigidly installed on the shaft. These gears result in an axial force in one direction depending upon the direction of rotation and are used for transmitting small power.

3. HERRING-BONE GEAR:

HERRING-BONE Gears are pairs of Helical Gears. These gears are used for large power transmission.

Advantages of Herring-bone Gears in addition to Helical Gears:
•No Axial Force. It is balanced axially in both direction of rotation.
•Large power transmission can be done at impact loads having frequent starts (inching).
These gears also must be rigidly installed on the shaft.

4. BEVEL GEAR:

Bevel Gear transmission is employed for transmitting motion from one direction to the other at any angle. In general 90° transmission is used mostly.

5. WORM & WORM WHEEL:

Worm and worm wheel gives a large single stage speed reduction in perpendicular direction. Such transmission gives lower efficiency of transmission due to increased frictional losses. For a noiseless motion, worm 1 is usually made of medium carbon / alloy steel and the worm wheel 2 of bronze (or in some cast iron).

6. RACK & PINION:

The rotary motion of pinion 1 (small gear) is transformed into linear motion of the rack 2. For transmitting large power worm & worm rack transmission is employed.

These gears are used separately or with combinations as per the requirement of the transmission.