Properties of Lubricants | Purposes of Lubrication

Properties of lubricants

The essential properties of lubricants are given below.

  • Viscosity: Viscosity of lubricant is the resistance offered by it to the deforming stress i.e., it is the resistance offered to the flow of the lubricant. Viscosity of the oil decreases with the increase in temperature. Viscosity of the lubricating oil should not go down below a certain specified value at the highest operating temperature of the bearings. If too thick oil is used, it will lead to power loss, higher operating temperature and excessive wear and tear of the parts. If the oil is too thin, it cannot lubricate properly and lead to rapid wear of moving parts.

01 - PROPERTIES OF LUBRICANT - VISCOSITY

  • Oiliness: It is the property of an oil to spread and attach itself firmly with the bearing surfaces. Oilness of the lubricating oil should be high for better lubrication.

01 - PROPERTIES OF LUBRICANT - OILINESS

  • Flash and fire point: Flash point og an oil is the minimum temperature at which it gives off enough vapour so that a momentary flame is obtained when a naked flame is brought near the oil surface. Fire point is the minimum temperature at which an oil continuously burns. Fire point is always greater than the flash point. The flash point of the lubricating oil should be higher than the operating temperature of the bearing.
  • Volatility: When the lubricating oil is exposed to a high temperature for a long time, it may evaporate. This property is known as volatility. The loss of lubricating oil is known as loss of evaporation. The lubricating oil should have low volatility at the operating temperature. If it has high volatility, oil consumption will be more.
  • Detergency: The lubricating oil should carry away small particles to keep the interior of the engine clean. This property of lubricating oil is known as delergency.

01 - PROPERTIES OF LUBRICANT - DETERGENCY

  • Demulsibility (water separation): The lubricating oil should not form an emulsion when brought in contact with water. The property of resisting emulsification is known as demulsibility. The emulsion leads to collection of dust, dirt etc., and will increase friction, wear and oxidation. Hence it should be avoided.

01 - PROPERTIES OF LUBRICANT - DEMULSIBILITY

  • foaming: It is the condition in which minute air bubbles are held in the oil. This will reduce mass flow of oil and also increase oxidation. Hence the lubricating oil should be free from foaming trouble.

01 - PROPERTIES OF LUBRICANT - FOAMING

  • Corrosiveness: The lubricating oil should not attack the engine materials chemically. The oil should prevent corrosion and it should not contain sulphur.

01 - PROPERTIES OF LUBRICANT - CORROSIVENESS

In addition a lubricating oil should have a high film strength to withstand loads. It should be non-toxic and cheaply available.

Purposes of lubrication (or) Functions of lubrication

1. It reduces friction between moving parts.

2. It reduces wear and tear of the moving parts.

3. It minimizes power loss due to friction.

4. It provides cooling effect – During circulation, it carries heat from the hot moving parts and delivers it to the surroundings through crankcase.

5. It provides cushioning effect – It serves as a cushion against the shocks of the engine.

6. It provides cleaning action – Impurities such as carbon particles are dissolved during its circulation.

7. It provides a sealing action – It helps the piston rings to provide an effective seal against high pressure gases in the cylinder from leaking out.

8. It reduces noise.

Types of Live Rear Axles | Semi Floating Axle | Three Quarter Floating Axle | Full Floating Axle

Types of live rear axles

There are three types of live rear axles: Semifloating, three quarter floating and full floating. Let us see the details of these.

Semi floating axle:

01 - types of axles - semifloating axle

The semifloating rear axle is shown in the diagram The short rear axle shaft inner end is supported only be the differential side gear. The differential case carries the inner bearing between it and the axle shaft housing that supports it. The inner end of the axle shaft is thus relieved of the task of supporting the weight of the vehicle. The weight of the vehicle is supported by the axle housing. Now the outer end of the axle supports the weight of the vehicle and takes up end thrust. Hence, this construction is called Semi floating rear axle.

The inner en of the axle shaft is splined to the differential side gear. The outer end is flanged and the wheel is bolted directly to it. In some designs, the hub of the wheel is keyed to the outer end of the axle shaft. The axle housing supports the wheel bearing, which is placed inside the outer end of the axle housing. The bearing is held on the axle by a retainer. The bearing is held on the axle by a retainer. Most axle bearings are prelubricated. With this arrangement, the brake drum, the wheel, and the bearings retainer plate must be removed in order to withdraw the axle shaft. This arrangement results in the axle shaft helping to support the weight of the vehicle in addition to transmitting rotation to the wheels.

Three quarter floating axle:

The three quarter floating axle is shown in picture In this axle, the wheel hub is supported by the single bearing located in the center of the wheel hub. The wheel hub runs on the axle housing. The axle shaft is keyed rigidly to the wheel hub. This arrangement provides the driving connection and maintains the alignment of the wheel.

01 - types of axles - three quater floating axle.

The construction at the inner end of the axle shaft is the same as with the semi floating type. This axle is not supported by bearings at either end. The three quarter floating axle has only one bearing at the outer end. It is not as quite as the full floating type.

Full floating axle :

The full floating axle is shown in diagram In this axle, the wheel hub is supported by two bearings. The bearings are running directly upon the axle housing. The axle shaft is fastened to the wheel hub flange by means of a coupling. Through the coupling, the rotary motion of the axle shaft is transmitted to the hub and wheel. With this arrangement, the axle shaft can be removed from the housing without disturbing the wheel by removing the hub cap and the coupling.

01 - types of axles - fully floating axle

In the full floating axle, the axle shaft is not supported at either end by bearings. The position of the axle shaft is maintained by the way that it is supported at both ends. As such, the axle is relieved of all strain caused by the weight of the vehicle or end thrusts. Now, the only function of the axle shaft is to transmit the rotary motion or torque to the wheel. Because of this fact, the axle is called full floating.

Full floating axle is the only construction that holds the wheel in position even when the axle shaft breaks. In other types, the wheel comes off and causes the vehicle to drop. The full floating axle is used almost exclusively in trucks.

In all applications, in axle construction either tapered roller or ball bearings are used.