machine design

Fall 2020 Page 1 of 3

Machine Design – MEIE4102 Farooq Al Jahwari

Sultan Qaboos University College of Engineering

Department of Mechanical & Industrial Engineering

The Spring-powered Air Rifle Project

Warning: Please do NOT fabricate your design or even attempt to make a prototype. Submission of the report

is enough.

Preamble

Air rifles rely on a high-pressure gas source to propel a projectile. The gas is almost always air (hence the name),

and the pressure source may be a tank of pre-compressed gas, or a compression chamber where gas is

compressed by a piston propelled by a compressed steel spring. The two biggest advantages of a spring powered

air rifle are power and ease of use. With a spring-powered air rifle, you only have to load the rifle once to achieve

maximum power. Most springs are rated at 1000 FPS (feet per second) and can be used for small game hunting

and also for target shooting. Higher velocity means greater accuracy over longer distances.

Air guns - or more generally gas guns - come in many configurations and power levels, from BB (bullet ball)

smoothbore guns suitable for children all the way to hyper-velocity helium cannons used in crater formation

research, capable of propelling projectiles at hypersonic speeds.

Working Principle of Conventional Spring-powered Air Rifles

Conventional means the spring is a standard steel round spring, as opposed to springs that rely on compressed

gases, much like shock absorbers in automobiles. Within conventional spring-powered air rifles there are several

possible configurations. The most common configuration consists of a single spring which is compressed when the

rifle is loaded. The compressed spring acts on a piston, and when the rifle is fired, the trigger releases the spring

which in its turn propels the piston forward, compressing air in a compression chamber. The compression chamber

communicates with the breech, where the projectile is seated, through a so-called transfer port, which delivers

high pressure air to the breech and this in turn propels the projectile down the barrel.

Figure 1 illustrates the concept of a conventional spring-powered air rifle. The figure is highly stylized and not to

scale. The top illustration shows the rifle action in the loaded position, where the piston is held in place by the

trigger sear (the sear and its connection to the trigger blade are greatly simplified, triggers are far more complex

than shown in the figure). The bottom illustration in figure 1 shows the rifle action after firing and before the

projectile has travelled all its way to the muzzle of the barrel.

Fall 2020 Page 2 of 3

Machine Design – MEIE4102 Farooq Al Jahwari

Figure 1. Stylized schematic

of a conventional spring

piston air rifle action - the

trigger mechanism is highly

simplified.

Tavella, Domingo. (2015).

Internal Ballistics of Spring

Piston Airguns.

Scope of Innovation

 First problem with spring-powered air rifles is the balance between the pressure generated by the spring

and the force required to load it. Some springs can be tough to load for a younger or elderly person. The

more powerful springs use bigger (or multiple) springs and therefore are harder to load.

 The other drawback is that all spring air rifles (with a few exceptions) are single shot which means you will

have to load a bullet after each shot.

The Design Challenges

 To provide compressed air capable of accelerating the bullet at higher speeds which requires a spring that

demands more loading force and hence more difficult to load. So, how efficient is it to develop a battery-

powered electric motor gearbox to do the job of loading the spring ?.

https://upload.wikimedia.org/wikipedia/commons/4/44/AEG.gif

Figure 2.

An illustration of the working

principle of an electric motor

gearbox.

Fall 2020 Page 3 of 3

Machine Design – MEIE4102 Farooq Al Jahwari

 Is there another mechanism which can utilize the idea of gearbox without using motors and batteries ?.

 Can the rifle be made with multiple shots and automatic loading (maybe via a spring !!) ?.

 Can the spring-powered air rifles accelerate a bullet with comparable speeds to the ammunition-based

rifles ?.

The Design Task

You will work on keystones of design (i.e., selecting materials, establishing geometries, mechanisms, studying

force and stress analyses, design stages and integrating parts and assemblies into a whole rifle configuration that

will safely and reliably meet both engineering and marketing requirements). You may focus the design task to the

spring piston air rifle action only or extend it to the whole rifle design (with extra marks).

Deliverables

Final report in PDF format by Thursday, 24-December-2020 on Moodle. Content of the report and template are

provided.