Explanation:

The Davis steering gear is not used because it has sliding pairs. Let's understand the reasons behind this choice:

1. Introduction to Davis steering gear:
The Davis steering gear is a type of mechanical mechanism used to transfer motion from the steering wheel to the front wheels of a vehicle. It consists of a series of linkages and gears that enable the driver to control the direction of the vehicle.

2. Definition of turning pairs:
In mechanical engineering, a turning pair is a combination of two elements, such as a shaft and a bearing, that allow for relative rotational motion. Turning pairs are used in many mechanical systems to transmit motion and torque.

3. Definition of sliding pairs:
In contrast to turning pairs, sliding pairs allow for relative linear motion between two elements. This can be achieved through the use of sliding surfaces or sliding contacts.

4. Importance of correct gearing:
Correct gearing is essential for the efficient and smooth operation of any mechanical system. It ensures that the motion and torque are transmitted accurately and without any loss. In the case of a steering gear, correct gearing ensures that the driver's input is translated into the desired steering motion of the vehicle.

5. The problem with sliding pairs in steering gears:
Sliding pairs, such as sliding contacts or surfaces, are not suitable for steering gears due to several reasons:

- Higher friction: Sliding pairs tend to have higher friction compared to turning pairs. This increased friction can result in a higher force requirement to operate the steering gear, making it harder for the driver to steer the vehicle.

- Wear and tear: Sliding pairs experience increased wear and tear due to the sliding motion. This can lead to the degradation of the steering gear over time, reducing its effectiveness and lifespan.

- Reduced efficiency: Sliding pairs introduce additional losses due to friction, resulting in reduced efficiency in power transmission. This can lead to energy wastage and decreased fuel efficiency in the vehicle.

6. Cost consideration:
While cost can be a factor in the choice of steering gear, it is not the primary reason for the avoidance of sliding pairs. The main concern is the performance and reliability of the steering gear, which is compromised by the use of sliding pairs.

Conclusion:
In conclusion, the Davis steering gear is not used because it has sliding pairs, which are unsuitable for steering gears due to their higher friction, increased wear and tear, and reduced efficiency. The choice of steering gear is crucial for the proper functioning and control of a vehicle, and turning pairs are preferred over sliding pairs in this application.