About Lubrication

Lubrication function

Regardless of which gear mechanism is used, the main functions of lubricants are:

-Reduce gear wear

-Prevent corrosion of precision parts

-It is estimated that only 2-3% of the lubricant is used to lubricate the gears, while the remaining 97-98% is used as coolant.

-Improve Gear Efficiency

Lubricants are generally available in liquid, fat/polymer and solid forms. In the gearboxes we are dealing with, planetary reducers are usually lubricated with fat/polymer lubricants.

Liquid lubricants are generally mineral oils with additives used to improve mechanical and chemical properties. Similarly, there are two main components of the grease, namely the base oil and the thickener which can maintain the oil phase by capillary action. The thickener can precipitate out the lubricating oil at a certain rate to meet the lubrication needs of the gear and absorb the excess oil at the same time. Its working mechanism is not very clear to date. Polymer lubricants are similar to greases in that they consist of a base oil and a filling matrix, but have a higher oil content than grease. Solid lubrication is generally used in vacuum environments and space applications.

Liquid lubricants are generally mineral oils with additives used to improve mechanical and chemical properties. There are two main types of base oils: mineral and synthetic oils. Thickeners also fall into two broad categories: soap-based and non-soap-based. Soap-based refers to fatty acids and metal compounds. Most commercial lubricants are soap-based, with lithium-based lipids being the most widely used. The non-soap-based groups are organic compounds, including urea and acyl

Amines and dyes have better high temperature performance and oxidation stability than metallic soap-based ones. Polymer lubricants are similar to greases in that they consist of a base oil and a filling matrix, but have a higher oil content than grease. Solid lubrication is generally used in vacuum environments and space applications.

The upper limit of the temperature of the general thickener is 110℃ to 140℃ and the lower limit is -20℃ to -30℃. This is why suppliers are wary of operating temperatures below -30 ° C.

Another special note is that different lubricant mixtures may lead to incompatibilities, making lubrication fail. If the soap base and non-soap base or different types of soap base are mixed together, the consistency will vary greatly, either too thick to be suitable for lubricant, or too thin and lost. Similarly, mixing grease with different base oils (such as petroleum and silicone oil) will cause two-phase fluid and affect continuous lubrication. We sometimes need to replace the grease in the gearbox to meet the special needs of customers. Be sure to clean it thoroughly before adding new grease.

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For motion-control technicians, there are too many new terms to go on without explaining:

1.Lubricating oil film：

This is a very important concept in lubrication theory, which is really about the creation, maintenance, and failure of oil films. Under certain speed and load conditions, a layer of oil film exists between a pair of friction bodies, which effectively separates the contact surfaces.

2.Viscosity：

The viscosity of lubricant is a measure of liquid flow resistance under the action of shear stress, which plays a decisive role in the formation of lubricating oil film. It should be noted that the viscosity of the same brand of lubricant increases exponentially with the increase of pressure, and decreases exponentially with the decrease of temperature.

Good lubrication viscosity helps to buffer forces between gears and limit operating noise. Lubrication with high viscosity results in additional friction damping and degradation of the high temperature properties of the lubrication. The choice of viscosity depends on the speed of operation
Viscosity is the most important characteristic of lubrication. It must be high enough to meet load requirements while maintaining a certain degree of fluidity even at low temperatures.

3.Pour point：

The pour point is related to the lowest temperature at which the lubricant will begin to flow. This value, together with the low temperature viscosity, represents the low temperature resistance of the oil. At low temperatures, the lower pour point limit of the lubricant must be lower than the ambient temperature. It's usually five degrees lower

4.Flash point：

The flash point is the lowest temperature at which the oil in the lubricant evaporates into a flammable gas.

5.Drop point：

Dropping point refers to the temperature at which grease becomes liquid. Also called melting point.

6.The oil film strength:

Good oil film strength helps to prevent scratches caused by contact between metal gears

7.Adhesion agent:

Helps prevent loss of lubrication due to gravity or centrifugal force, especially at high speeds

8.EP:

EPLubrication provides additional wear protection for heavily loaded gears, especially when oil film failure occurs. EP lubrication can also be used in harsh (hyperboloid gear) applications

9.Extreme pressure lubrication：Extreme Pressure Gear Oils (EP)

EPGear lubrication is usually recommended for high load conditions, high sliding friction, high power output conditions, helical bevel gears, and hyperboloid gear mechanisms.

It is important to note that EP lubricants usually contain some chemically active additives, so special attention should be paid to corrosion of bronze or brass when used in heavy load turbine worm systems.

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Because EP lubrication may also contain solids suspended, such as graphite or molybdenum disulfide, EP lubrication is not suitable for applications requiring braking. When the clutch (brake) action is, EP lubrication of the chemical substances will cause the sliding of the gear bite and lead to the sliding of the mechanical mechanism, easy to cause safety accidents.

The vast majority of losses in a gearbox come from three components: lubricating oil (grease) agitation losses, sealing resistance, and friction between gears, all of which depend to a certain extent on lubrication.

Sealing ring lubrication usually uses light oil. When considering gear box lubricants, attention should be paid to their compatibility.

Let's focus on the relationship between friction and lubrication between gears.

We know that the involute profile is not completely rolling in the meshing process, but has a little sliding, and the wear is mainly related to this sliding.

The oil film acts as a divider, which may be complete or partial. Full separation is that the friction is mainly dependent on the performance of the lubricant under contact temperature and pressure conditions. In partial separation, the wave peaks of the rolling/sliding surfaces are in contact with each other under boundary lubrication conditions. In conclusion, it is important to establish a lubricating oil film on the contact surface.

In the 1960s, the lubricating oil film parameter (relative film thickness) Λ was introduced to indicate the extent to which the lubricating film separated the contact surface. Λ≥3 indicates complete oil film isolation.

H is the thickness of oil film, and is the root mean square value of roughness of two contact surfaces.

Let's first discuss the relationship between different contact states and oil film

Stribeck curve

The Stribak curve clearly shows the relationship between the friction between bearing surfaces and the relative oil film thickness (Λ) and the lubrication system.

The parameter Λ indicates the possibility of rough surfaces contacting each other at a given oil film thickness.

Concrete is divided into：

1）  Boundary friction

The boundary friction refers to the situation where the bearing surfaces engage with each other, which is easy to occur when the relative velocity of the meshing surfaces is low, and there is a large load and a change in the direction of motion.

Lubrication with anti-wear (AW) or extreme pressure (EP) additives should be selected to reduce friction.

2）  Friction under full oil film isolation（Λ≥3）

When the lubricating oil film is completely separated from the two bearing surfaces, there is no machine-to-machine contact. The life of the machine depends on the purity of the lubrication, and the friction force increases with the increase of the oil film thickness

3）  Friction under partial oil film condition（Λ＜3）

In this case, when the lubricating oil film is not enough to completely separate the contact surface, some convex summits on the surface penetrate the oil film and contact each other, then the thickness of the oil film is higher than the boundary lubrication, and is between the boundary lubrication and the full oil film lubrication.

In the process of gear movement, it is a combination of sliding and rolling. Which of the two movements is dominant? It depends on the type of mechanism of the gear

Gear type	features	Dominant movement pattern
Straight wheel (including rack and pinion)	Very common. Sliding friction occurs when the gears meet and are not locked. High noise may be produced at high speed	rolling
Hypoid gear	Hyperboloid teeth are close to helical bevel gears except that the center lines of the shafts do not intersect. The transmission gears of this all-metal mechanism require special lubrication conditions, namely lubrication with oil additives and Antiweld additives, to withstand high tooth pressure and high friction speeds	sliding
The turbine worm	A turbine worm works in a similar way to a screw. The pressure on the tooth surface is evenly distributed, so bronze with a low relative coefficient of friction can be selected as the turbine. Lubrication depends on the formation of a fluid film and requires a large amount of lubrication to improve lubrication performance and prevent metal-to-metal contact. So Dynabox uses liquid lubricants	sliding
Spiral teeth	Lower noise and vibration compared to straight gears. In any condition, the load on helical gears is distributed across multiple teeth to reduce gear wear; But the axial force produced when the gear bites requires thrust bearings to digest it.	Roll + Slide

Types of gears, characteristics and lubrication considerations

factors	demand
Gear type
1，Spur gears, umbrella teeth；	Low speed sliding
2，Spiral teeth	Medium speed sliding, medium and high load capacity
3，Hyperboloid gear	High speed sliding, high load capacity
4，The turbine worm	Very high sliding speed, medium to high load capacity
Load capacity	High-load industrial gears require EP lubrication, known as extreme pressure lubrication
Surface roughness	Rough surfaces require high viscosity lubrication while smooth gear surfaces require low viscosity lubrication
Power transmission	As the load increases, the viscosity increases
The gear speed	The higher the speed requirement, the lower the viscosity
Compatibility of additives	Some extreme pressure lubricants contain additives that corrode brass and bronze components
The temperature	The choice of industrial lubrication viscosity must be based on the high and low temperature conditions in the actual work.

General principles of lubrication selection:

For gearbox users, there is not much opportunity to choose the internal lubricant. This is only possible in special environments (usually special temperatures) and when the gearbox is inefficient due to high speed and low torque, so there is no need for a detailed discussion here, just a general understanding.

1）  For gearbox users, there is not much opportunity to choose the internal lubricant. According to the type of gearbox, transfer power to choose the type of lubrication

2）  Choose according to linear velocity lubricant viscosity, linear velocity of the gear is that the choice of lubricant viscosity is very important factor, gear linear velocity determines the length of the gear and gear contact, namely high linear velocity corresponding to the gear lighter load and short contact time, for this type of gear, low viscosity lubricating is preferred; Low linear velosities usually correspond to high gear loads and long gear contact times. For these gears, high viscosity lubrication is preferred, and EP (extreme pressure) additives should be considered when loads are very high.

3）The temperature

The environment and the operating temperature of the gear also determine the choice of gear lubrication. Typically, industrial gear lubrication operates at temperatures ranging from 50 to 550, generally higher than ambient temperatures. When applied at high temperatures, lubrication is required to have good viscosity, strong oxidation resistance and anti-foaming properties. It is necessary to consider the abnormal high temperature environment, lubrication viscosity is too large, too much lubrication, abnormal overload may cause the temperature rise. Low temperature applications require lubricants with high fluidity to provide sufficient viscosity. Therefore, these lubricants must have a high viscosity index and a low pour point.

4）Chemical stability and antioxidant capacity

Prevent the thickening of lubrication and the formation of grease. This requirement is particularly important when the gear is running at high speed.

An overview of lubrication selection：

Lubrication type	Spur gear	Spiral teeth	The turbine worm	Umbrella tooth
Antirust antioxidant lubrication	The standard	The standard	Light load, low speed	The standard
Extreme pressure (EP) lubrication	Heavy load & impact load	Heavy load & impact load	The most commonly used The most commonly used
Mixed oil	Not commonly used	Not commonly used	nc	Light load
Synthetic lubricating	Heavy load & impact load	Operating temperature over 82 degrees	Heavy load & impact load	nc

Summary of Lubrication Selection：

1）When choosing a gear lubricant, consider load, linear velocity, temperature, type of gear, gear surface finish, and type of application.
In general, high viscosity lubrication is usually directed at:

-High load

-High temperature

-Low speed

-Rough surface of gear

-Turbine Worm Applications

EP Extreme pressure lubrication is usually used in heavy loads, medium temperature environments, but not for yellow metals (bronze, brass)

2）In the choice of viscosity grade for industrial use, prefer to choose a high viscosity grade of lubrication rather than a low viscosity grade of lubrication, because the high viscosity of lubrication can provide a relatively high load capacity and maintain a better oil film strength; Low viscosity lubrication may result in adhesive wear, low load carrying capacity and accelerated wear of gears. Ensure that the lubrication is not sufficiently viscous to be distributed across all gear meshing sources.

3）Pay attention to the sealing of the gear box, control the dust in the air into the housing, can reduce the gear wear by at least 50%

4）Adding too much lubrication is also undesirable. Excessive lubrication can lead to lubrication bubbles, gear overheating can lead to lubrication leakage, and over time, may produce lubrication oxidation.

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