The Mechanism behind the Sewing Machine

The Mechanisms that Operate the Sewing Machine

By: Natalie Bukowski

Patented in 1790 by Thomas Saint, the sewing machine revolutionized the garment industry. This revolutionary mechanical device also pioneered the way for development of smaller machines that could be used in homes and small places of businesses. The many components of a sewing machine operate in unison to form stitches that connect individual pieces of fabric together. The original home sewing machines were powered by a treadle which was pushed up and down by the operators foot. Although most sewing machines today are powered by an electric motor, the basic structure and function of the machine has not changed since its invention.

Upper Spool of Thread

The Home Sewing Machine

The sleek, modern appearance of today's home sewing machine belies the mechanical function of the machine. The light, easy to clean plastic casings hide a true engineering marvel of gears, cams, and belts, all of which work in precise synchronization to operate the machine.

View of Sewing Machine

Imagine From: http://www.sewingmachines.us/shop.htm

A Look inside the Casing

An electric motor drives the mechanical elements of the sewing machine. The speed of the motor is controlled by the operator through the use of a foot pedal. When the operator presses on the foot pedal, the motor engages a drive belt which is connected to a drive wheel. A diagram of the mechanical parts hidden by the plastic housing of a sewing machine is presented below.

View of the Inside of a Sewing Machine

Image from:

http://home.howstuffworks.com/sewing-machine2.htm

Upper Drive Shaft Operations

View of Upper Drive Shaft Operations

Image from:

http://home.howstuffworks.com/sewing-machine2.htm

When electrically engaged, the drive wheel rotates the upper drive shaft, which is connected to several mechanisms which work in conjunction to operate the machine. Located at the end of the upper drive shaft is the crank. The crank operates the needle assembly and the thread-tightening arm. The thread-tightening arm creates slack for a loop in the top thread to be formed. After the thread is released from the shuttle hook, the thread-tightening arm tightens the loop of top thread around the bobbin thread to form a stitch. The thread tightening arm works in conjunction with the shuttle hook and bobbin assembly, which is described in detail in the following sub-section.

Crank

The Needle Assembly and Needle

The needle assembly is composed of a needle bar and the needle. The needle bar moves up and down, pulling and pushing the needle through the fabric to be sewn. The needle, which is manually attached to the lower end of the needle bar, can vary in thickness depending on the type of fabric to be stitched. The needle is formed to facilitate the correct forming of the stitches. Unlike a needle used in hand sewing, the eye of a sewing machine needle is found close to the point. One side of the top of the needle is flat. This side is attached to the needle bar which holds it in place. The short groove is located at the point of the needle. This portion of the needle holds the thread from the upper spool, which is pierced through the fabric. As the needle is pulled back up, the thread is held between the cloth and the long groove part of the needle. The loop of the thread is formed by friction between the cloth and the long groove.

Needle Assembly and Needle

Image from:

http://home.howstuffworks.com/sewing-machine2.htm

Needle Bar

Needle

Diagram of a sewing machine needle

Image from: http://www.tfsr.org/pub/technical_info/sewing_machine_manual/Getting_started_how_sewing_machines_work.pdf

Lower Drive Shaft Operations

The lower drive shaft controls a set of gears that rotate the shuttle hook and bobbin assembly. The lower drive shaft also moves linkages that control the feed dog mechanism.

Lower Drive Shaft Operations

Image from:

http://home.howstuffworks.com/sewing-machine2.htm

The Shuttle Hook and Bobbin Assembly

The shuttle hook and bobbin assembly is integral to the stitching process. Located in the lower casing of the machine, this assembly is accessible to the operator. The shuttle hook holds the bobbin, which is a small spool of thread. Bobbins can be made of metal or plastic but must be precisely shaped to feed the second thread properly. The inside of the shuttle hook is stationary and the part around the outside rotates and grabs the top thread from the needle. The shuttle hook then rotates around the bobbin causing the thread from the needle to loop around a strand of thread from the bobbin. The two strands of thread are pulled together creating a tight knot called a stitch.

Needle Thread

Shuttle Hook

Bobbin Thread

Bobbin

View of the shuttle hook forming a loop around the bobbin thread

Image from: http://sewingschool.org/2010/11/04/how-a-bobbin-works/

The Feed Dog

The feed dog, controlled by linkages along the lower drive shaft, is the mechanism that moves the fabric as it is being stitched. Able to move forward and backward, as well as up and down, the feed dog presses tightly against the fabric being sewn. Serrated teeth found on the feed dog also help guide the fabric as it is being stitched.

Loop Stitch

According to an internet article by Tom Harris, the heart of a sewing machine is the loop stitching system.[endnoteRef:1] Although all of the mechanisms previously explained are necessary to operate the sewing machine, the loop stitching system is what connects fabric together. Although the operator can adjust the type and length of a stitch, the loop stitching system is the basis for sewing applications using a sewing machine. This system allows the needle to pass part-way through the fabric and then pull a small loop of thread from one side of the fabric to the other. This loop of thread is then wrapped around either another loop of the same piece of thread or another piece of thread coming from the bobbin. [1: Harris, Tom. "How Sewing Machines Work - HowStuffWorks."HowStuffWorks. InfoSpaceLLC, 2015. Web. 17 Mar. 2015. .]

Conclusion

In conclusion, the sewing machine is a unique piece of machinery which utilizes many mechanisms working in precise synchronization, over and over again, to form a series of stitches.

From the moment the motor engages the upper and lower shaft mechanisms, all components are working in unison to operate a needle and two strands of thread, one from an upper spool and the other from a lower bobbin. The shuttle hook and bobbin assembly, working in conjunction with the thread-tightening arm, facilitates the forming of a loop stitch. This process repeats over and over allowing a series of stitches to be formed which then allows fabric to be sewn into a variety of finished products.