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  • Problem in controlled lifting of heavy load

    Discussion in 'The main mechanical design forum' started by resonanceshah, Aug 9, 2011.

    1. resonanceshah

      resonanceshah Member

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      Hi,

      I have an application where I need to lift something and then bring it down in a controlled manner. The weight would probably be around 10,000 Kg. While lifting up I can understand that the electric motor + gearbox would exert the torque that will allow the load to be lifted. Further a brake would keep the load in position when the motor is inactive. However, when I try to bring the load down, the load because of gravity and inertia will cause the motor to run faster than it should and hence the load would come down in an uncontrolled manner.

      I am trying to figure out how to bring the load down in a controlled manner. The only way I can figure out is to use a worm gearbox with self locking properties. However, due to some design constraints I am unable to use the same (apart from the fact that the efficiency would be hardly 40% if I were to achieve self-locking property). Is there a better way to get around this problem.

      Thanks
      Reso
       
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    3. tjohnston

      tjohnston New Member

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      Hello,

      SEW-Eurodrive has a website called PT Pilot that can help you solve this problem:

      http://v4.ptpilot.com/PTPilotNET40/Home.aspx

      It seems to me you need to design your system with a motor and closed loop feedback. You motor drive will follow a speed and acceleration profile, checking itself with encoder feedback from the motor. During the descent, the motor will become a generator, so dynamic braking will be necessary.
       
    4. resonanceshah

      resonanceshah Member

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      Hi,

      thanks for your reply. However, firstly the link needs a registration of some sort. Secondly it is definitely possible to do this using a closed loop system (which is why servos would work). However, it is possible to do so in open loop also i feel. In fact it made me wonder, why are chain hoists non - reversible....or cranes also for that matter. Cranes use planetary gears yet do not face the problem I foresee. There is a point I am missing somewhere i feel. The brake is employed only when the motion is stopped. But during motion if the energy in the load is high, why wouldnt it cause the motor to overrun.

      Reso
       
    5. resonanceshah

      resonanceshah Member

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      Also, I presume that if I use a VFD maybe it will allow electronic dissipation of energy in terms of I2R losses and hence allow the load to come down in a controlled fashion. However, cranes work even without the drives :)
       
    6. SCIYER

      SCIYER Well-Known Member

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      Your problem definition is akin to a 10T crane. The speed at descent would be around 10 to 15 % higher. Secondly use a brake that would have atleast 150% calculated capacity. This is to prevent the load from creeping down. While a worm gear drive would be self locking, when you have a failsafe (If the motor were to stop, the brakes should apply) brake, one may even use a planetary gear box or an epicyclic gear train.
       
    7. PWASS

      PWASS Well-Known Member

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      This application seems ideal for a Hydraulic Solution.
       
    8. Engr_Stan

      Engr_Stan New Member

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      I would agree with Pwass, A hydraulic system will be an ideal system for the application as you have indicated. You could use a Hydraulic whinch and tackle. A hydraulic system also offers you multiple variations and control of speeds.
       
    9. Your Engineering Pro

      Your Engineering Pro New Member

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      It is even better to do it electric. Use frequency controllers and use brake resistors. I just delivered cranes to a US client. They were used to hydraulic cranes but now they have seen the electric version they ask me to quote again for the next project. I had made the design including a proportional wireless remote control. The hoist and lowering spead was proportional operated by a joystick. At full working load you even could change the joystick from full lifting to full lowering at once and the load was changing from lifting to lowering very smoothly without any strange hickups and without using the brake.

      Best regards,
      Bas
       
    10. Tom1959

      Tom1959 Member

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      We use a hydraulic pump in similar application in our factory. The weight is lifted to a point slightly above where it is needed. It is then descended under gravity. The descent valve can then be closed at exactly the correct spot.
       

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