Final Drive Mechanism: Types of Rear Axles in a Tractor

Table of Contents

• Introduction
• What is Final Drive in a Tractor?
• How Does a Final Drive Work?
• Different Types of Reduction Technology
  • Single Reduction
  • Inboard Reduction
  • Hub Reduction
  • Epicyclic Gear Reduction

Introduction

A tractor requires a combination of high torque and low speeds to perform any farm task efficiently and there is an inverse relation between the two. Now how does this combination changes as per the operational requirements depends on the final drive mechanism of a tractor.

In addition to this, a final drive, depending upon the reduction technology used, also increases the shock absorbing and load distribution capacity so that the gearbox remains safe.

So, now the question is what exactly is a final drive in a tractor? Let us understand from the sections given below.

What is a Final Drive in a Tractor?

A final drive is a mechanism that uses different reduction technologies to finally transmit power to the rear axle and the drive wheels. It is a gear reduction unit placed between the Differential and the Drive wheels. This unit reduces the final speed and provide more torque to the rear wheels to perform different types of agricultural or commercial tasks.

The final drive is the last part in a tractor’s transmission system and the final component for increasing torque through gear reduction or changing the direction of the power flow.

Final drives consistently handle heavy torque loads; thus, they are designed to perform under challenging farm conditions and absorb shocks & jerks while they operate.

A final drive on a tractor can be standalone or housed in the axle housing. The position of final drive depends on the needs and functions of a tractor.

How Does a Final Drive Work?

The efficient transmission of power from the engine to the rear wheels is necessary for a tractor to perform its desired tasks. The power transmission is to be such that rear wheels get more torque and less speed. This reduction in speed happens through the final drive.

A tractor’s final drive reduces the speed and provide more power to the drive wheels with the help of different reduction technologies. These speed reduction technologies use either a crown pinion gears or a pair of bull pinion gears or planetary gears to convert power into torque.

In doing so, final drive of a tractor allows the two axle shafts, ball bearings, and gears to transmit power smoothly and efficiently in a high-torque system.

What are the Different Types of Reduction Technology

There are mainly four types of gear reduction technology that helps convert tractor’s power into torque. These are:

• Single Reduction or Straight Axle
• Inboard Reduction or Double Reduction
• Hub Reduction
• Epicyclic or Planetary Gear Reduction

Single Reduction

In this system, the final gear reduction takes place once at crown pinion wheel. Upon transmission of engine power to the crown pinion wheel, the pinion (smaller gear) rotates faster than the crown (bigger gear). The ratio is typically in the range of 3:1 to 4.5:1, that is, if pinion rotates for 3 times, then the crown rotates only once. As a result, the speed gets reduced and torque increases. This reduction in speed and increase in torque is called reduction.

After gear reduction, the final transmission of power to the drive wheels in the single reduction technology happen through two straight half shafts (axle shafts), therefore, this system is also known as straight axle gear reduction.

Benefits: In this system, the friction is less, thus the power loss is minimum. Further, the fuel consumption is low and is fuel-efficient but the output is higher. Since, the friction of wheel to roads is higher, the pulling power of tractor is greater.

Inboard Reduction

In this system, the gear reduction happens twice, one at the crown pinion wheel, and second at the bull pinion gears attached to the axle shafts of both sides. That is why, it is also known as double reduction technology. Since, the bull-pinion gears are housed in the axle housing, thus it is called inboard reduction.

After gear reduction, the final transmission of power to the drive wheels happen through bull gears. The impact of sudden load or shock on gearbox in this system gets reduced by almost 50% as the shock gets absorbed at bull gears.

Benefits: This reduction technology is highly useful in heavy-duty applications (pulling tractor trailer) and wet applications (puddling activities). Furthermore, due to this reduction technology, the tractor can easily operate cultivator, rotavator and MB ploughs.

Hub Reduction

This system is similar to the Inboard Reduction technology in terms of working except that the bull-pinion arrangement is housed in the hub of the wheels. That is why, it is called hub reduction technology.

In this system, the shaft is not at the centre of wheel as in Inboard Reduction. Rather the wheel centre shifts backward and gets near to hitching point. As a result, the stability of the tractor increases and the front-lifting of tractor decreases by 40% in this technology.

Benefits: This technology makes a tractor ideal for haulage activities such as operating trolley. However, this is not ideal for wet applications as the chances of mud sticking on wheel increases.

Epicyclic Gear Reduction

EPI gear reduction is heavy-duty, robust and most advanced technology. In this system, the gear reduction happens twice as in Inboard or Hub Reduction. However, in place of bull-pinion gears, there are three Planetary Gears (smaller gear), which are attached to the Sun Gear (bigger gear) through ring gears. The arrangement of these gears is near the axle shaft of both sides.

In this system, the smaller planetary gears rotates faster than the sun gear to provide enhanced rotational torque to the rear wheels.

Benefits: First, the arrangement of gears decreases the chances of slippages. Second, the load gets distributed equally among the three planetary gears such that the shock on gearbox is reduced to zero. Third, this technology is ideal for heavy-duty applications such as operating double trolleys, loaders and dozers. Fourth, the EPI technology on higher HP tractors ensures that fuel saving and output is optimum.

However, this technology is not that useful in mini tractors and for operating small implements because after reduction not much power will be left to drive the rear wheels.