Power Scissors

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Bucknell Mechanical Design Home


Power Scissors Dissection

Figure 1: HP Deskjet 600c Before Disassembly
Figure 2: HP Deskjet 600c Without The Case


The primary function of the Power Scissors is to cut through different materials. There are two blades, the bottom one stays stationary, and the top blade moves rapidly, opening and closing. This action allows the tool to cut through things.

2) Another stepper motor, undocks the printer carriage. The dock is used to secure the carriage, and to keep the printer head clean.

Figure 5: Docking Station For The Carriage
Figure 6: Stepper Motor That Drives The Dock Using A Worm Gear

3) The printer carriage is operated by a drive belt, which is powered by a third stepper motor.

Figure 7: Belt Drive System
Figure 8: Stepper Motor That Drives The Belt Using A Toothed Pulley

4) The printer cartridge applies ink onto the paper according to the instructions supplied by the CPU through the copper contacts.

Figure 9: Cartridge To CPU Interface


The table belows lists the components for the HP Deskjet 600c printer:

Table 1: Power Scissors Component List
Part # Part Name Category Function Material Picture
1 Motor Input Serves as Power Supply for blades
2 External Casing Structural Component Protects internal components and holds them together Plastic
3 Jaws Ouput The bottom blade remains stationary, while the top blade moves continually up and down Steel
4 Battery Input Provide power to motor Wrapped in cardboard
5 Cam Motion Conversion This forces the bracket to move by translating rotation motion of the motor, to translational movement in the bracket. The bracket then causes the upper blade to open and close Plastic
6 Screws Structural Holds Casing Together Metal
7 Bracket Support element Ball and socket joint limits movement of jaws. The socket is ovular so it allows some movement of the jaws. Plastic
8 Button (spring) Motion Conversion Closes switch to start motor by closing circuit. Button – plastic

Spring – metal Switch - metal

Side View

Cool Animated Videos

<embed src="http://www.youtube.com/v/8_iTSznToHU" type="application/x-shockwave-flash" width="425" height="350"></embed>

Animated Printer Carriage

Right-click here and select "Save Link As" to download the video (.avi)

<embed src="http://www.youtube.com/v/iayt04lR08E" type="application/x-shockwave-flash" width="425" height="350"></embed>

Animated Paper Feed Mechanism

Right-click here and select "Save Link As" to download the video (.avi)


Analysis Of The Belt System

Scope of Analysis

The two engineering specifications that are quantified for the printer belt tensioning system, are the force on the belt required to accelerate the printer head to its maximum speed, and the force to stop the printer carriage and change direction. Both of these specifications pertain to the Dots Per Inch design parameter. The best design is to obtain the maximum DPI rating in the quickest printing time. To achieve this goal, the belt must be able to cope with the forces to accelerate the printer carriage.

Key Properties

  • Printer Prints 4 pages a minute at 300 DPI
  • Mass of Carriage With Ink Cartridge = 0.14 kg
  • Moment of Inertia of Pulley = 0.5 (.002kg)(0.005^2m) = 2.5 x 10^8
  • The belt is a trapezoidal design, meaning each tooth has the shape of the trapezoid
  • The belt is made out of polyurethane which has good wear resistance and low friction


Friction force exerted by the slider on the carriage is always constant when moving, and should have been reduced to a minimum by the manufacturer. Lower friction would reduce the force the stepper motor has to supply to move the carriage. Since it is difficult to measure the exact amount of friction force, and it is relatively small compared to the acceleration forces, friction can be neglected.

Finding The Speed

Since, the actual printer head speed I could not be found, a few assumptions had to be made with the available information. The printer has a 300 DPI rating, which means that it can print 90,000 dots per square inch. It can print 4 pages of text per minute. Assuming that it prints 8.5” x 11” pages with a 1” top and bottom margin, and 1.25” side margins, it leaves a total of 54 square inches of text. This equates to 4.86 x 10^6 dots per page. Multiply the dots per page by the pages per minute, and that results in 2.43 x 10^7 dots per minute. 2.43 x 10^7 dots per minute is the average printing rate for the printer. However, if we assume the format is 12 point, Times New Roman font, the total area per line of text is 1.95 in^2 (6.5” x .3”) 0.54 square inches per page multiplied by 4 pages per minute, divided by 1.95 square inches per pass, and taking the reciprocal, yields .009 minutes per line, which is 0.54 seconds per line. Every line is 6.5 inches long, which means that the printer head moves at 12 inches per second. Since information on the acceleration of the printer carriage could not be obtained, it was instead estimated. After careful observation of other inkjet printers, the carriages reach their top speed almost instantaneously, so it was estimated that it takes 0.2 seconds to reach maximum velocity, and 0.25 seconds to stop and change direction.


  • Acceleration of Carriage From Rest

Acceleration of Carriage = max velocity/ time = (0.3048 m/s) / 0.2 s = 0.15 m/s2

Mass of Carriage With Ink Cartridge = 0.14 kg

F = ma = (0.14)(0.15) = 0.021 N

Therefore, it takes .021 N to accelerate the carriage from rest to the max velocity

  • Stopping and Changing Direction

Change in velocity = (0.3048+.03048) = 0.6096 m/s

Change in Time = 0.25 s

F = (Mass*Change in Velocity)/ Change in time = 0.34 N

Therefore, the maximum force exerted on the belt due to the change in direction is 0.34 N.

As seen in Figure 10 below, the maximum force to pull the carriage is related to the torque supplied by the motor pulley. Furthermore, the carriage is fixed to one side of the belt, so it translates directly with the belt. During the analysis, the entire carriage is treated as a point mass and represented as a block. Figure 11 shows the free body diagram of the motor pulley. It is evident that the torque from the pulley is directly related to the force on the belt during acceleration.

Figure 10: Free Body Diagram Of System
Figure 11: Free Body Diagram Of Motor Pulley

The analysis through Adams produced results that were similar to the calculated values. The maximum force, as seen in Figure 12, reached 1 N which was very close to the calculated value of 0.34 N.

Figure 12: Force of the Drive Belt During Acceleration


To increase the maximum force that the belt could handle, a few options are available. The cross-sectional area could be increased to decrease the stress on the belt. According the stress equation, stress = load / area , the stress can be reduced by increasing the cross sectional area to compensate for load force. The cross sectional area can be increased by making the belt wider or thicker. Making it wider would be more beneficial because it would provide a greater surface area for the toothed pulley to grip onto. The drawbacks for increasing the surface are the increase in stiffness and cost of manufacturing. Making the belt thicker would reduce its flexibility, making it harder for it to move around the pulley. It would require more torque form the input motor to compensate. Making the belt wider would force the pulleys to be wider, which drives up the cost of the system.

Another method to increase the stress capacity of the belt would be to use a curvilinear design instead of the current trapezoidal design. The curvilinear design looks very similar to the gear sprocket of a bicycle. Instead of a trapezoid shape, the curvilinear design uses a half circle shape. Therefore, the teeth are deeper in the gear which makes it less probable for tooth jumping during high accelerations. Furthermore, there is less material at the edges of the gear, which lowers the moment of inertia, allowing the gear to accelerate faster. [1]

Figure 13: Photoelastic Stress Pattern [1]

Figure 13 shows the photoelastic stress pattern of both designs, and it is evident that the curvilinear shape distributes stress more evenly. This is due to the larger tooth cross section. Since the curvilinear shape handles stress better than the trapezoidal shape, a narrower curvilinear belt could be used in place of a wider trapezoidal belt, thus, saving material and cost. [1] The trade off may be in increased cost to manufacture, since curvilinear belts are not as common as trapezoidal belts.

A final option to consider is to change the material of the belt. Using a more durable and stress resilient material such as steel or composites, may work better. However, it is most likely that these materials are more difficult to manufacture, which increases the cost

User Requirements

a. Blades are sharp enough to cut different materials

  i.	   Have a sharpening function

b. Easy to use

  i.	   Has a low weight
  ii.	   Has a small size
  iii.	   Comfortable to hold
  iv.	   Wireless
  v.	   Easily accessible buttom to start

c. Aestheically Pleasing

  i.	   More likely to be bought

d. Long battery life, chargeable e. Changeable blades for different materials to cut f. Doesn’t break if dropped

Engineering Specifications 1. Material for jaws is sharper than cutting materials strength. This corresponds to E. 2. Battery Life is ______ long. This correspsonds to D. 3. Force of scissors is greater than _____. This corresponds to A. 4. Weight is less than _______ (5 lbs?). This corresponds to B. 5. Scissors retain sharpness for _______ (amt of time). This corresponds to A. 6. torque on motor is ______. This corresponds to A. 7. External casing can withstand ______ force, to avoid being dropped. This corresponds to F. 8. A certain percentage of people when surveyed said that the scissors are pleasing. This corresponds to C.