Transmission 1

Clutch

A clutch is a device which enables the rotary motion of one shaft to be transmitted at will to second shaft, which has an axis that is coincident with that of first. Clutch lies in between engine and gearbox. When the clutch is engaged, the power is transmitted from the engine to the rear wheels through the transmission system and the vehicle moves when the clutch is disengaged, the power does not flow to the rear wheels and the vehicle stops, while the engine is still running.

• Clutch is disengaged in the following cases:

1. Starting the engine,
2. Shifting the gears,
3. Idling the engine

• Clutch is engaged only when the vehicle is to be moved and is kept engaged while the vehicle is moving.

Principle of Operation of a Clutch

The clutch operation is based on the principle that when two friction surface are brought into contact with each other and hard-pressed they are integrated due to friction between them. If one is in motion the other will also come into motion.

Fig 1. Components of a Clutch

One of the surfaces is considered as driving member and the other as driven member. The driving member of a clutch is the flywheel which is mounted on the crankshaft, and the driven member is the pressure plate which is mounted on the transmission shaft. Friction surfaces (clutch plates) are in between the two members i.e. driving and driven. On the engagement of the clutch, the engine gets connected to the transmission (gearbox) and the power starts to flow from the engine to the rear wheels through the transmission system. When the clutch is disengaged by applying force on a clutch pedal, the engine gets disconnected from the transmission and as a result, the power does not flow to the rear wheels while the engine is still running.

Mechanical Transmission

The mechanical drive train of the construction equipment is similar to that of the automatic transmission in a sense that a transmission is used in combination with a torque converter and shifting is carried out hydraulically when the operator moves the range selector lever.

1. Planetary Gearsets

Some power shift transmissions use planetary gear sets to perform the same functions as the transmission just described (Figure-2). A planetary gear set consists of three members i.e. sun gear, ring gear, and a planetary pinion carrier that holds the planetary gears in proper relation with the sun and ring gear. The planetary gears can move freely around the sun gear or inside the ring gear.

Fig 2. Planetary Gear sets

To increase or decrease the torque, there are six different possible methods of connecting this gear set to the power train. Direct drive can be achieved by locking any two members together, and neutral can be obtained by allowing all the gears to turn freely.

In actual application, planetary gear sets are used as single or multiple units, depending on the number of speed (gear) ranges desired. Power for turning the drive sprockets on tracked equipment may flow through a planetary gear arrangement that provides maximum reduction. The sun gear makes the planetary gears revolve in the stationary ring gear and move the carrier in the same direction in which the sun gear is rotating. The carrier is connected to the hub on which the sprocket is mounted, causing it to rotate with the carrier. This arrangement produces the maximum torque and speed reduction obtainable from a planetary gear set.

2. Planetary Steering

Some tracked equipment may be steered by a system that associates planetary steering and pivot brakes. The planetary steering system differs from the one previously described in the sense that the planetary pinion gears are two gears of different sizes, machined into one piece ( Figure-3). Two sun gears are also included. One sun gear is splined to the sprocket pinion shaft, and the other is machined on the steering brake hub. The sun gear, which is machined to the steering brake hub, performs the identical function as the ring gear in a conventional planetary system. Bushings are present to isolate the sprocket drive shafts and the steering brake hubs from the bevel gear carrier and the planetary carrier. Lubrication is provided from the oil sump that lies below the assembly.

Fig 3. Planetary Steering

When the tracked equipment moves in a straight path, its steering brakes are held in the applied position with the help of heavy coil springs. Braking prevents the steering brake hub and sun gear from rotating and forces the large planetary pinion gears to move around the sun gear. Then the power is transmitted to the sun gear on the sprocket drive shaft from the smaller planetary pinion gears.

When a gradual turn is being made, the operator moves one of the steering levers back far enough to release the steering brake on one end of the planetary system. When the operator releases the brake, the planetary pinion gears stop moving around the sun gear on the steering brake hub. This hub now rotates with the planetary carrier, and there is no power transmission to the sprocket drive shaft.

3. Steering Clutches

Steering clutches are used in the clutch-brake system, where both tracks directly are driven by the output of a single power source. Since they are physically connected to each other, the tracks must turn at the same speed and the vehicle will follow the straight path. To allow the turns, a clutch is provided to disconnect each track from the engine, allowing that track to slow and the vehicle to turn fairly softly. A brake allows the disengaged track to be slowed to take a more accurate turn, even to the point of stopping the track.

This system is very simple and easy to drive; however, it is not very much efficient. Braking one track slows down the vehicle and a large portion of the power produced by the engine is wasted to be converted into heat. While this is not a big deal in case of a small vehicle, but a large vehicle having a large engine can produce a tremendous amount of heat in a very short time. The braking one track also slows the vehicle down significantly, which is a big deal in the case of military vehicles where speed is paramount.

Hydraulic transmission system:-

Fluid coupling -: A fluid coupling is a hydrodynamic device that is used to transmit rotating mechanical power from one shaft to another. It has been used in automobile transmissions as an alternative to a mechanical clutch.

How fluid coupling can be act as a mechanical clutch?

In automotive applications, the pump typically is connected to the flywheel of the engine. The turbine is coupled to the input shaft of the transmission. While the transmission is in gear, as engine speed increases torque is transmitted from the engine to the input shaft by the motion of the fluid, providing propulsion to the vehicle. So, the performance of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.

Construction of a Fluid Coupling:-

It consists of a pump-normally known as impeller and a turbine generally known as rotor, both being enclosed suitably by a casing .They face each other with an air gap between them. The impeller is suitably coupled to the prime mover while the rotor has a shaft fixed to it by bolts. This shaft is further connected to the driven machine by the means of a suitable arrangement. Oil is filled in the fluid coupling through the filling plug provided on its body.

Torque Converter :-

Torque converter is a hydraulic transmission which escalates the torque of the vehicle decreasing its speed. It provides a continuous deviation of ratio from low to high. The important characteristic of a torque converter is its capability to multiply torque when there is a large difference between input and output rotational speed, thus functioning same as reduction gear. Cars with an automatic transmission have no clutch to disconnect the transmission from the engine. So, they use a torque converter that functions same as that of clutch.

Construction

There are four components inside the very robust housing of the torque converter. They are:

• Pump
• Turbine
• Stator
• Transmission fluid

The figure below shows the parts of the torque converter: turbine, stator, and pump (left to right).

Fig 4. Components of Torque Converter

The housing of the torque converter is bolted to the flywheel of the engine, so it turns at the same speed the engine is running at. The pump inside a torque converter is of the centrifugal type. As it spins, fluid is thrown to the outside. As fluid is thrown to the outside, a vacuum is formed that draws more fluid in at the center. The fluid then enters the blades of the turbine, which is coupled to the transmission. The turbine causes the transmission to spin, which provides motion to your car. The turbine contains the blades that are curved. This means that the fluid, that enters the turbine from the outside, has to change the direction before it exits the center of the turbine. This directional change causes the turbine to spin.

References

1. Technical book, “Construction Machinery Training”, Instate, Imlambad
2. Harris, F. and McCaffer, “Management of Construction Equipment”, Macmillan Education Ltd. London, UK.
3. Erich J. Schulz, “Diesel Equipment I and II”, Mcgraw-Hill book co.
4. Frank Harries, Ronald McCaffer, “Construction of Plant Excavating and Material Handling”, Granda Publishing.
5. SAE Handbook Volume 4
6. “Caterpillar performance Handbook”, Edition 33, Caterpillar Inc, Peoria, Illinois, USA.