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Linear actuator use in all electric mini-digger

  1. Feb 1, 2017 #1
    Hi all,

    I've got this idea in my head of building a mini-loader like a http://www.multione.com/mini-articulated-loaders/ . I'm interested in replacing the hydraulics with linear actuators and wondered if anyone here could shed some
    thoughts/advice.

    Given the driveline would also be fully electric, my reasons by going electric on the actuators are:
    a) Efficiency - running hydraulics is inefficient as it's continually running a pump.
    b) Servicing - hydraulics require servicing, linear actuators do not.
    c) Simplicity - Electrics are far easier to design and control than hydraulics.

    I'm mostly concerned with the main lifting actuator, I think (although not quite sure) that I could source an actuator with enough Torque and quick enough, but am concerned with potential side loads. I've attached a quick sketch of how I think the main lifting arm could be setup, but at position 2 is there a side load on the actuator? Or because the actuator would pivot at the bottom, is there no side load?

    Another question is, given a load (in kg) in the bucket, and the length of the arm, and the position of the actuator connected to the arm, how do I calculate the N force required to lift the bucket?
     

    Attached Files:

  2. jcsd
  3. Feb 1, 2017 #2

    Mech_Engineer

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    Several companies make what are sometimes called "electric cylinders"; they are typically a ball-screw linear actuator driven by either a stepper or DC brushless servo motor.

    Some examples:
    https://www.festo.com/cms/nl-be_be/21522.htm
    http://ph.parker.com/us/en/electric-cylinders

    While in some cases they might not match hydraulic for raw force, many electric cylinders are available with high force ratings. Parker's ETH125 series goes up to 114 kN for example. Speed will depend on the motor's max speed, and the total reduction between possible motor gearboxes and the ballscrew itself.

    Electric_Cylinder_Actuators_-_ETH125_zm.jpg
     
  4. Feb 2, 2017 #3
    Thanks so much mate, Googling Electric Cyliner vs Linear Actuator brings up MUCH different results. I was very suspicious that my search of linear actuators was peaking at a power level.
     
  5. Feb 3, 2017 #4

    Mech_Engineer

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    Regarding your other question, determining the forces required or involved will require Static analysis. Carnegie Mellon has a free-to-access module covering Engineering Statics, which might a good starting point if you're unfamiliar with Static Analysis.

    https://oli.cmu.edu/courses/free-open/engineering-statics-course-details/
     
  6. Feb 3, 2017 #5

    Baluncore

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    a) Efficiency - Hydraulics do not develop pressure unless work is being done. Without pressure there is high efficiency, good lubrication and cooling. As a generalisation, with electrics you always have pressure = voltage available, with hydraulics you always have current = flow. Study directional control valves, DCVs, to better understand the difference. There are also variable flow pumps and variable flow motors, they give hydraulics continuously variable transmissions with high efficiency.

    b) Servicing - Electric motors get hot and will need to be air cooled, so they will fill up with dust and mud, then need cleaning or they will fail. With hydraulics you change the filters and top up the fluid. Hydraulics are self lubricating and self cooling. If you applied the same forces with electrical actuators you would need to install an automatic lubrication system. The power to weight ratio of hydraulics is very high compared to electrical actuators. If slow movement with high forces is needed, select hydraulics. If very fast movement is needed, consider pneumatics. Both can be controlled by electrical systems and far exceed the performance of all-electric systems.

    c) Efficiency – You clearly lack understanding of the magnitude of the forces involved in earth moving equipment. The gearboxes necessary to slow down an electric motor to useful speeds, torques and forces will be just as inefficient as hydraulics and will probably weigh more. Electric controls applied to hydraulic systems combine the best of both worlds.

    Get serious about the breakout forces required. Then get hydraulics and get the job done.
     
  7. Feb 3, 2017 #6
    Mostly electric actuators are used only in indoor application ,such as hospitals ,clean room environment etc.,
     
  8. Feb 5, 2017 #7
    "You clearly lack understanding of the magnitude of the forces involved in earth moving equipment"
    Yes, you are probably right when I first posted this question. I mean, I knew that I was pushing the envelope, but thought given an unorthidox use case, electric might be possible (e.g. small machine, low use). Given the previous answers for calculating said breakout force, and doing some case studies, I now do understand the magnitude of forces, and have come to the same conclusion.
     
  9. Feb 7, 2017 #8
    I worked at a foundry where one of our forklifts was 100% electronic. It was much harder to learn to drive because the rates of speed that the actuators worked at were not as adjustable as they were with a hydraulic forklift. With hydraulics, the operator can finesse the levers and control the height of the tines down to the millimeter as well as do it slowly and very responsively (no lag to disorientate you). With the electric forklift, the actuators had lag and could only move as slowly as the digital controller was programmed to allow them to. The machine's responses to inputs were delayed and unpredictable.
     
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