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Making Energy Generating Gym Equipment

  1. Jun 19, 2017 #1
    Hello Everyone,

    I am an engineer and I wish to implement a gym system that generates energy from the user. I am originally an electrical engineer by trade, and have limited knowledge in mechanical design. This is why I need your help.

    My current design follows the outline of a basic cable machine commonly found in gyms for exercising the triceps, biceps etc.

    I have the brief design sorted:

    • A rectangular frame is used to support all the components and weights used
    • A pulley system is used to provide a means for the user to pull a weight up and down
    • The rotation of the pulley is transferred to a shaft
    • The shaft is connected to a motor
    • The motor is therefore run by the users energy
    • The output voltage of the motor is fed into electronics where the resulting energy is stored
    I require advice upon the mechanical aspect of this project. I wish to make a system that is:
    • Efficient
    • Feasible to implement within 3 months
    • Can handle a weight of 10kg
    Current component parameters are:
    • Motor with 15V rated output and 6mm diameter D-Shaped shaft
    • Frame material is 45mm-by-45mm Rexroth beams
    I have provided some basic design sketches to give you a visual idea of what I am thinking:
    The first image is the frame. It is 1 meter in height, and 0.5 meters in width and length. There is also a middle beam connected across the top as shown in the image.

    This middle beam is where the pulley and motor are connected, and is illustrated in the second image.
    The second image is a basic representation of how I wish to connect the components together.

    As shown in the second image, I wish to screw the motor under the middle beam, connect it to a coupling shaft, have the coupling shaft connect to the pulley, have the whole shaft supported by hook-like supports, and have the pulley rotate by having the cable pulled up and down by the user on one end and connected to a weight on the other.

    Please give feedback on my design, and I need better ideas so I can improve the project.
  2. jcsd
  3. Jun 19, 2017 #2


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    Staff: Mentor

    Can we back up a sec? What is the purpose of this project? Is it for school? An invention for commercialization?

    Have you considered;
    How much energy is available to be harnessed?
    How harnessing it will affect the workout you get?
  4. Jun 19, 2017 #3
    Of course.

    This project is something I proposed myself to do in the electrical department at my university. It has been accepted that I can do this for my main project. The purpose of this is to provide an in-depth investigation and implementation of energy generating gym equipment. I understand this has already been done with the use of bike machines, but the specific area I wish to focus on is the use of weights. This is partly due to my interest in weightlifting.

    I have done rough design sketches for generating energy from other types of weight equipment, like the leg press machine. But I have chosen the pulley-cable machine since it is the most simplest, and a good place to start.

    Since I already have good knowledge on electrical engineering, I have no problems doing that side of the project. However, I would like a wide range of opinions from people with knowledge in mechanical engineering, since I am not entirely confident in my design.

    I have calculated the best case scenario for the amount of energy generated for a few cycles of pulling the handle up and down. This has involved basic equations such as Energy= m*g*height. Per set of exercise, with a weight of 10kg, with a height of 0.5 meters= 50 Joules

    Given that entire system is perfectly efficient, the amount of energy transferred to the motor shaft per set will be 1000 Joules. This is for a set of 10 repetitions. One repetition being the user pulling the 10kg weight down by 0.5 meters, and then allowing the weight to fall down by 0.5 meters. Thus, each repetition will have 50*2= 100 Joules. And each set will have 10*100 = 1000 Joules.

    Given that in a commercial gym the average number of sets performed on such equipment will be 40 per day, the amount of energy transferred per day will be 40*1000 = 40,000 Joules.

    Therefore each year will be 365*40,000 = 14.6 Mega Joules.

    Therefore, for a voltage of 1V, the amount of electrical energy stored per year is 14.6 Mega Joules/3600 = 4055 Amp/hours

    In the best case scenario, the energy can be accumulated through multiple energy generating machines through the year, and save the gym owner a small amount off of their bills. The customers will be using the equipment anyway, so it will be a good way to generate energy out of activities people will do anyway.

    Of course, the commercialization of such a product may not be possible due to the small amount of energy generated, and the significant costs of the installation and maintenance fees.

    But the main purpose is to make the system and investigate, and try new ways to make the system more efficient.

    The equipment will be made in such a way that it will not affect the functionality and movement patterns of the original equipment. This is done to allow for the gym users to not notice any difference in usability, and keep the effect on the muscles similar.
  5. Jun 19, 2017 #4


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    Staff: Mentor

    As in a senior design project? Or is this a lower level? Is it individual or are you part of a team? In either case, just as a demonstration that it can be done, the project idea is fine. For such a project, the idea doesn't need to be comericially feasible...though you will want to examine that when you present it and write a report about it.

    There are a lot of details to be worked out and I'm not sure if you know what they are or not, but there is a fundamental problem: your design has a 10kg weight in it. Your weightlifter can either lift a 10kg weight *or* he can spin a motor to generate energy, simulating a 10kg weight. But he can't do both. This means your weightlifter only gets the benefit of lifting the "weight", but doesn't get the benfit of lowering it (the bar will lower itself via gravity). Is he going to be ok with that? It also means your energy extraction is half what you calculated. Continuing the thought:
    Conservation of energy tells us that you only get to use the energy once. You can either use it to lift a weight or you can use it to spin a generator, but you can't do both. It almost sounds like you think the electricity is generated for free, with no torque on the motor/generator. This means that the person doing the weightlifting is going to notice the difference.

    This also beggs the question: what did you think the motor/generator is acutally doing? Volts, amps, rpm, torque? I think if you look into selecting the motor as your starting point and looking at how exactly you extract energy from it, that will tell you a lot of what you need to know in terms of what the mechanical design will need to look like (pulley/gear ratios and such). That part really isn't difficult (you probably took an introductory physics course already...?).

    I feel like I should be disappointed that an engineering student is missing something so fundamental, but I think this is why design projects are so critical; this is wha happens when an electrical engineer looks at amps and volts and doesn't think about what that means in terms of RPM and torque....and vice versa for the mechanical engineer.
    What is the purpose of this calculation? Electric companies charge for kWh, not amp-hours at 1V...there isn't anything I'm aware of that you can do with 4055 amp-hours at 1V (you can't even charge a cell phone with it).
    Last edited: Jun 19, 2017
  6. Jun 19, 2017 #5
    Your starting point. Find out what the arm muscles are doing when raising and lowering a weight

    Designated design parameters before are a no no - you are restricting yourself already.
    In other words, what you have classified as being the "current component parameters" may not be a good fit after the analysis of the system nor fulfill the expectations of the new design ( those starting with "efficient, ... )
    Last edited: Jun 19, 2017
  7. Jun 22, 2017 #6
    I agree but since it is a school project those restrictions may be valid - i.e. it may be all that he has access to without pouring money into the project.
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