Simple Machines (Part-3): Wheel-Axle, Screw & Gears Explained

With a round wheel fastened to the axle, the axle and wheel constitute a particular kind of simple machine. Combining these two gives you a mechanical advantage when lifting loads when the force needed is less than the weight of the thing.

When force is supplied to the wheel, the attached axle rotates, producing the torque, also known as the rotational force. Given the disparity in wheel and axle sizes, it can be assumed that the two separate components are moving in a distinct length of time. This is because, as the picture below illustrates, the wheel and axle's different circumferences provide a mechanical advantage.

Mechanical Advantage (MA) = [ Radius of Wheel / Radius of Axle ]

*This implies that, compared to a smaller wheel, a larger wheel will take less force to move the same load.

6b. Application

The wheel and axle mechanism are utilised in everyday life, as discussed in the example below :

• Automobiles: Wheels on bicycles, cars, and other vehicles reduce the drag and enable the rolling motion for higher loads.

• Doorknobs: The spindle (axle) in the door knob takes the mechanical advantage of the difference in circumference of the knob and the spindle to facilitate the locking and unlocking in a more accessible manner.

• Wrenches: Utilises the Wheel and Axle mechanism to tighten or loosen a bolt or nut, converting the handle into a type of wheel.

• Windlasses: As shown above in the example, a device utilises the system to draw/pull the water out of the well.

6c. Advantages

• Significant mechanical advantage caters for the easy lifting of heavy objects.

• Versatility: Adaptable to several applications and the foundation of modern machines.

• Friction Reduction: By reducing the amount of friction between surfaces, wheels facilitate easy movement

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7. SCREW

7a. Introduction

The screw is an example of a Simple machine that converts rotational force into linear movement. It consists of a helical thread, unlike an inclined plane revolving around a cylindrical axis. The force is applied using screws in a controlled way to lift loads as well as to hold objects together. The diameter of the shaft and the distance between threads, also called pitch, are used to determine a screw’s mechanical advantage.

A screw advances or retreats when it is turned because the inclined plane (threads) changes the rotational force to a linearly applied force. A screw's mechanical advantage can be computed as follows.

Mechanical Advantage = Circumference of Shaft of Screw/ Pitch of the Screw

7b. Application

The screw has multiple applications as discussed below :

• Fastening: In construction and fixing of furniture, for instance, screws are often used to hold materials together.

• Lifting: Screw jacks can be used to lift or lower heavy loads through the turning of the screw.

• Machines: Screws are used in machines to control motion, exert force, and alter a part's shape.

• Screw-style bottle caps twist to expose threaded surfaces, which securely shut containers.

7c. Advantages

• Mechanical Advantage: The ability to apply force and lift weights makes screws favorable in terms of mechanical advantage.

• Precision: The ability to apply force and regulate movement is made possible by screws.

• Screws are incredibly versatile tools that work well with a wide range of materials.

• Screws are widely available and can be applied to almost any material or situation.

8. GEARS

8a. Introduction

Gears are one of the most significant factors for torque transfer and Speed Exchange. This is because when the driver gear is turned, the teeth of the driver gear push against the teeth of the driven gear and thus cause the driven gear to turn in the opposite direction. The system’s speed and mechanical advantage are defined by the number of teeth on the gears and the number of gears in the system.

Gear Ratio can be computed as follows :

Gear Ratio = [ Teeth in Driven Gear/ Teeth in Driving Gear]

Gear Ratio = 25/75

= 1:3 (proportionately, the speed varies)

Important to know: Gears Type

• Spur Gears transfer motion between parallel shafts and have straight teeth.

• Helical gears: Operate more smoothly and silently thanks to their inclined teeth that engage more gradually.

• Bevel gears: Conical in shape, they transfer motion at right angles between intersecting shafts.

• Worm Gears: Having a screw (worm) that meshes with a gear (worm wheel), worm gears can self-lock and offer high reduction ratios.

# Trick to find Direction of Gear Rotation

1. Even combinations of gears always rotate in the opposite direction. For Example, if the driver gear is rotating in an anticlockwise direction in a combination of gears, the last driven gear would be rotating clockwise(opposite direction).

2. Odd combinations of gears always rotate in the same direction. For Example, if the driver gear is rotating in a clockwise direction in an odd combination of gears, the last driven gear would also be rotating in the clockwise(opposite direction).

8b. Application

Since gears transfer force and motion, they are used in many processes. Below are some function of gears:

• Automobiles: The wheels are driven by the engine’s speed and torque, which are controlled by gears in the transmission system.

• Watches and clocks: The hand movement is regulated through gears to measure the time well.

• Machinery: Industrial gears are used in the machinery to provide and control the movement of various parts.

• Bicycles: Bicycle gear systems enable riders to change the effort required to pedal depending on the terrain.

8c. Advantages

• Efficiency: Extremely effective at transferring force and motion.

• Precision: Precise management of torque and speed.

• Versatility: Employed in a range of settings and applications.

CEED 2021

(Question Mechanical Aptitude using the concept of Gears & pulleys)

Q2: If the Gear X rotates at the constant speed of 20 RPM, the Gear Y will turn…?

A. Clockwise 10 RPM

B. Anticloclwise 20 RPM

C. Cloclwise 20 RPM

D. Anticlockwise 5 PRM