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Laurel Tree Charter School human powered blender

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Team Pink Elephants designed a human powered blender during the Fall 2012 for the Humboldt State University Engr215 Introduction to Design class. The human powered blender is a device intended to be used by students in grades K-12 at Laurel Tree Charter School in Arcata, California. The device will blend food in the school's cafeteria, while teaching students about alternate methods of energy production.


Humboldt State University’s engineering department partnered up with Laurel Tree Charter School. Laurel Tree Charter School is a K‐ 12 school with a mission to "create a sustainable model of education which provides all of its students with accessible curriculum, based on college preparatory standards, while developing life and social skills in a mixed age setting" [1].

Problem Statement and Criteria[edit]

Problem Definition[edit]

Laurel Tree Charter School, located in Arcata, California, recruited Humboldt State University’s Engineering 215 students to design an electric, commercial food blender that is powered by human energy to use in the school’s cafeteria.


The criteria for the project, Frankenstein's Axle were identified and developed to ensure that any generated solutions would fit the minimum qualities desired. The team deliberated on its generated list of criteria with the client in order to clarify and focus on the needs that must be satisfied by the project. The criteria below are constrained to fit the definition of our problem as discussed with the client.

  • Safety-All moving parts should be housed so as to not come into contact with the user in any possible way.Material should be sturdy, so they are not broken during use, and not extraneously flammable.
  • Level of Educational Value-Students should know more about mechanical, kinetic, or electrical energy and human power production than they did before use.
  • Usability -Any time the blender is used there will be at least ten students present, and will be used for at least thirty minutes at a time. The blender will also be used at school events, involving up to 50 people and a much longer duration for the use of the blender. The blender should produce at least one, delicious blended beverage between the works of at least two people, within at least ten minutes.
  • Durability-This device should be able to withstand multiple uses per week by different age groups.
  • Cost-The blender must cost less than $410.00 to build. The breakable parts must cost less than one hundred dollars per year to completely replace. The working blender, with every part present and working, must cost no money at all to function.
  • Repairability/Maintenance-Repairing this device should be as easy as opening up the user guide and diagnosing the problem.

Description of Final Project[edit]


The human powered blender takes advantage of the convenience of basic electric kitchen blenders meaning that using the blender itself is as simple as plugging it in. Electricity for the blender is drawn from the battery, and the battery charging system is the definitive process that makes the blender human powered. The design uses a 350W 36V permanent magnet DC motor with a two inch v-belt pulley mounted onto the shaft of the motor. A 45 amp, 600W blocking diode prevents the battery from powering the motor and draining the battery unnecessarily. The motor is wired to the battery and the battery to an inverter which provides AC power to a variety of appliances rated at 400W with a peak output of 600W.


  • Wooden Base and Parts - The platform that each individual component is mounted to is a piece of plywood that is 3/4” thick X 32” wide X 78” long. Each crank is mounted to a 10” long two by four. The 32”X12” plywood bench is supported with three 17” four by fours. The rear axle is mounted with two by fours that are equal in height to each crank that they are linked to. At the rear there is mounted a stack that houses the generator, battery, blender, and monitoring equipment.
  • Chain Drive - There are two pedal cranks on the device that are fitted to perform an equal amount of work to the rear axle. The driver side pedals include a gear with 48 teeth while the passenger side pedals have a gear that has 46 teeth. The driver side pedals are connected by bicycle chain to the bicycle wheel attached to the rear axle, which has a gear with 24 teeth. The passenger side pedals are connected by chain to a gear attached at the end of the axle opposite of the bicycle wheel. This gear has 23 teeth. The two gear ratios equal each other at 2:1, which provides tension enough to turn the rear axle at an evenly distributed work ethic.
  • Pulley System - The shaft of the generator is turned by pedaling the cranks at the front which turns a fixed axle with a mounted bicycle wheel. The 26” bicycle rim is connected by a drive belt to the 2” diameter on the shaft of the motor. The gear ratio between the diameters of each pulley is 13:1.
  • Electrical Production - The motor included is rated to produce just over 12V at 1056rpm which is the minimum speed required to charge the battery. The ratio from the bicycle wheel to the pulley on the shaft of the generator is 13:1, (multiplied by the gear ratio created by the pedal cranks, 13:1 X 2:1= 26:1)which means that one person pedaling alone would have to pedal 42 times a minute instantaneously in order to charge the battery.
Fixed to the face of the permanent magnet DC motor is a weld on V belt pulley.

Wired to Blend- Included in the system are the electrical components required to charge the battery and power the blender. The parts included are the 12V battery, the 400W blender, a 30 amp fuse, a 40 amp blocking diode, and 10 gauge copper wire leads. When setting up the system for use,the leads should connect the components as follows: The fuse is connected directly to the negative battery terminal. The fuse is then connected to the negative terminal on the motor. On the positive battery terminal, the blocking diode runs between the battery and the motor so that the battery does not discharge to the motor. The inverter is then connected to the battery when wanting to power the blender.


The cost of the design and production of this model is measured in dollars spent as well as hours worked by each team member. Below is a table of every material purchased and otherwise acquired to build PINK ELEPHANTS.

Materials and Expenses[edit]

The project budget was set at $410.00 and every component purchased is logged. There are estimated retail dollar values assigned to each material or component that was donated in order to represent a more realistic production cost.

Quantity Material Cost ($) Total ($)!
1 36V DC Motor 50.00 100.00
4 Bicycle Chains Donated 39.88
1 12V Battery 35.47 35.47
1 Bicycle Wheel Donated 35.00
1 81" Drive Belt 30.00 30.00
2 Steel Bearings (3/8 in) 23.59 23.59
1 Plywood (5ft by 5ft) Donated 22.51
1 Plywood (8ft by 6ft) 22.51 22.51
1 Green Paint (1 quart) 11.50 11.50
1 Yellow Paint (1 quart) 11.50 11.50
1 Multimeter 9.97 9.97
2 Waterseal (12 Oz) 8.63 8.63
1 Blocking Diode 7.49 7.49
1 Jumper Cables 6.50 6.50
2 4 by 4 (6ft) 6.00 6.00
2 Project Housing Box 5.39 5.39
1 Rim Tape (Roll) 5.10 5.10
30 2" Stainless Steel Screws 5.00 5.00
1 2 by 4 (8 ft) 5.00 5.00
7 2 by 4 (Various Lengths) Donated 5.00
1 Battery/Alternator Testor 4.99 4.99
1 10 gauge Copper Wire (5 ft) 4.50 4.50
1 Corner Brace (1 1/2 in) 3.99 3.99
20 3" Stainless Steel Screws 3.80 3.80
1 Fuse 30 A 2.99 2.99
1 Steel Rod (1/2 in, 3ft) 2.97 2.97
1 Sandpaper (Package) 1.50 1.50
1 Another thing - 3' x 2', Yellow 240.00 240.00
Total Cost $270.88 $423.27

Design Time Input[edit]

Time Spent on Design and Construction (Hours)

Below is a pie chart representing the archived number of hours spent working through each stage of the design processes. The team spent on the entire design process is broken down into percentages, recognizing the proportion of time spent on each phase.

Testing Results[edit]

Human Powered Blender in use!

The results of building the design model concluded that the side-by-side tandem recumbent bicycle powered blender was an effective design. With moderate cadence of pedaling, 14 volts is easily achieved, and with two people adding the power, it is a light exercise for both. The multimeter used capped out at 10 amps, thus it produces at least 10 amps, which yields 140 watts. The blender is 420 watts, thus for one minute of blending; only three minutes of pedaling are required. Since the battery used was nearly full, only six watts were effectively charging it, and as it drains, the maximum charge is more likely to be obtained.


Here is a link to our project video


Pedal cranks
Rear axle

Each component that makes up this project was designed and built individually. The individual parts are the cranks, the bench, and the rear axle, which are all mounted to one piece of plywood to form one whole piece.

1. The Cranks Two scrap bicycles were donated to be used for parts for PINK ELEPHANTS. To build the cranks in this project, the pedals from the bicycles were hacksawed off and mounted to 2 by fours with metal brackets. Each standing crank was then mounted to the plywood and reinforced with additional 2 by fours and two-inch screws.

2. The Bench The bench sets on 3 equal length four by fours. The seat is made up of a piece of plywood 32"X12" and the backing for the bench is a 32"X18" piece of plywood.

3. The Rear Axle The rear axle for this system is designed so that each set of pedal cranks will engage the bicycle wheel. aq 21 inch steel bar was salvaged from the axle of an old go kart and fixed with a bicycle wheel on one end and a flywheel scrapped from another bicycle wheel is fixed to the opposite end. This allows bicycle chains to be attached to either end of the axle.

4. The Mounting Board The mounting board is a sheet of plywood that is 3/4" thick by 32" wide by 6.5' in length. The plywood was cut to this size in order to define our spatial constraints as far as storage space and transportability.

Steps to Build the Pink Elephants Bleder
Mounted bench and dual crank system Step 1 : After cutting the plywood mounting board to desired size (6.5' 3'), fix the bench to the face of the plywood. Once the bench is in place, you are then able to measure a comfortable distance to mount each set of pedal cranks.
1chain.JPG Step 2 : Once the pedal cranks have been mounted in a relative distance to the bench, the next step is to mount the rear axle which is done by screwing in the 17 inch tall 2x4 wooden posts that have mounted pillow blocks with 3/8 inch bearings to allow the axle to spin. Next, you can fix a chain from the cranks to the axle and tighten it accordingly.
Step 3 : The next step is to cut 3 two by four pieces of wood to the length of the entire mounting board. After everything was mounted to the project board and the chains were tightened on both crank sets, the mounting board bent to an arc that does not rest flat on the ground. For this reason, we mounted the project to 3 two by fours that are less likely to bend and provide lasting support. Finally, you can paint the project.

Thank You[edit]

We as a team would like to thank the people that made this project possible. Thank you Rebecca Schuler of Laurel Tree, for providing a workspace, tools, and guidance. Thank you Marty Reed, for helping us build the metal parts of our machine. And finally, thank you bicycle Revolution Bicycle Repair in Arcata, Ca for donation of parts and time.


  1. Laurel Tree Charter School Mission Statement