ani_pump_medium.gif (17013 bytes) title-reports.jpg (7248 bytes) ani_pump_medium.gif (17013 bytes)

 

HOW A STICHING MECHANISM IN A SEWING MACHINE WORKS
QUESTIONS OR COMMENTS
elgart_jonah.jpg (1914 bytes) AUTHOR: Jonah Elgart
E-MAIL: bologna@mit.edu
COURSE: 13
CLASS/YEAR: 2

MAIN FUNCTIONAL REQUIREMENT:  To stitch thread on fabric

DESIGN PARAMETER:  Sewing Machine Stitching Mechanism


GEOMETRY/STRUCTURE:

sewing_machine-1.gif (33314 bytes)

Detail of Stiching Assembly Components
Stiching Mechanism
bobbin case bobbin hook race needle

 

sewing_machine-2.gif (23496 bytes)

Cut-Away View of Geometry of Sewing Machine Power Transmission
Sewing Machine (the components that goes from the motor to the needle and bobbin)
housing motor belt crankshaft

EXPLANATION OF HOW IT WORKS/ IS USED:

a-sewing_machine.gif (74548 bytes)

Animated Operation of Stiching Mechanism
(this is sooooo cooool!!!!!!!)

 

The steps of a stitch: (put the picture for each step before the corresponding explaination)

Step 1

19.GIF (3020 bytes) The fabric has two threads running through it. One thread runs through the needle from a spool and the other comes from the bobbin. The needle lowers and penetrates the fabric, carrying its thread with it.

Step 2

2.gif (3042 bytes) The needle begins to lift and forms a loop of thread on the fabric’s under side. As the hook case turns the hook approaches, catching the looped thread.

Step 3

5.gif (3078 bytes) The needle rises above the fabric. As the hook case continues to turn, it carries the loop of thread.

Step 4

14.gif (3161 bytes) The needle is still above the fabric. The hook case finishes carrying the loop over the bobbin.

Step 5

17.gif (3029 bytes) The needle’s thread is pulled taught by a lever upstream (not visible in the diagram). This action pulls the loop from the hook and thus completes the stitch. The stitch is now ready to be repeated.

The needle:

  1. The main shaft is powered by a belt (5) that runs from the motor.
  2. At the needle end of the crank shaft is a protruding lever (6) that rotates with the shaft.
  3. This bar connects to the lever at one end (6) and the needle rod (7) at the other. Since the needle rod can only travel up and down, the bar transfers only the vertical movement from the lever.
  4. 4. The needle oscillates vertically.

DOMINANT PHYSICS:

sewing_machine-2.gif (23496 bytes)

Cut-Away View of Geometry of Sewing Machine Power Transmission

 

Variable Description Metric Units English Units
Pin Motor power Watt Horsepower
Pfrict Power lost to friction Watt Horsepower
Pother Power used to drive other parts of machine Watt Horsepower
Pn Needle power Watt Horsepower
rm Radius at motor meter feet, inches
r2 Radius at 5 meter feet, inches
r3 Radius at 6 meter feet, inches
ws Crank shaft rotational speed rad/s rpm
wm Motor rotational speed rad/s rpm
x(t) Vertical needle position meter feet, inches
v(t) Vertical needle velocity meter/s feet/s, inches/s

The power provided by the motor is equal to the power consumed. From this we can find the power provided to the needle.

Pin = Pfrict+Pother+Pn   =>   Pn = Pin-Pfrict-Pother

To find the velocity of the needle we first must find the crank shaft’s rotational speed, ws. As the power at the motor equals the power at the crank shaft (assuming 100% efficiency): Pin = F *  rm * wm = F *  rs * ws. Since the force applied to the belt is the same force applied to the crank shaft: rm * wm = rs * ws which means ws = rm * wm / rs.

The vertical position of the needle is a function of time, x(t), that depends on the radius of the lever, r3, which protrudes from the crank shaft and its rotational speed, ws. This function is approximately a sine wave.

x(t) = r3 * sin(ws * t)    (because ws = rm * wm / rs)   
      = r3 *  sin[rm*wm *t/ rs]

To find the approximate velocity with respect to time, differentiate:

v(t) = r3 * (rm * wm / rs) * cos[rm * wm * t/ rs]


LIMITING PHYSICS:

None Submitted


PLOTS/GRAPHS/TABLES:

None Submitted


REFERENCES/MORE INFORMATION:

Macaulay, David. The Way Things Work; Houghton Mifflin Company, Boston: 1988

Brother vx-807 sewing machine


ADMINISTRATIVE:          Home | General Info | Syllabus

STUDENT RESOURCES: Gear Pump Resources | Workshops | Labs | Field Trips | Final Reports | Design Tools

MISCELLANEOUS:         Students | PrizesMachine Gallery | Class Gallery | Comments | Links | Sponsors

MARTIN L. CULPEPPER 1998 & 1999