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  1. #1
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    Default Does Cone Pulley Configuration change Mechanical advantage?

    Ill probably get shot for asking but I am at a loss as to the answer to this.

    Does the cone pulley configuration change the mechanical advantage??

    Everything in my simple mind tells me that configuration E-D even though the slowest spindle speed does give the most mechanical advantage?? Or is there no difference regardless of the belt configuration?

    Preet Lathe Machine.jpg

    Thanks for any help.

  2. #2
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    Default

    Yep. Less speed, more torque.

  3. #3
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    Whenever the smaller pulley is on the motor side of things and the larger on the output side it will always give more torque but less speed.
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  4. #4
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    Default

    To say the same thing a slightly different way?

    The power stays the same. One goes down the other has to go up.


    But more torque doesn't let you do more work. If that's what you are thinking.

  5. #5
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    Default

    Thanks gents, that was what I was thinking.

    It seemed logical to me to run my lathe in the slowest pulley configuration because my 3ph motor with a vfd runs up to 70hz which is way faster than the old lathe should ever run.

    So now I can be running the motor at a higher hz but still get a slow enough spindle speed. I still have the back gears if I want to really slow it down.

  6. #6
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    Default

    Quote Originally Posted by Stustoys View Post
    But more torque doesn't let you do more work. If that's what you are thinking.
    In theory. In practice, if you don't have enough torque you'll stall your lathe and then you won't be doing any work.
    Chris

  7. #7
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    Default

    Depending on the age and design of the motor, it may not operating at reasonable efficiency at 70Hz supply, so the power output may be somewhat lower than if operating at 50Hz. A lot of older motors drop off fairly quickly above 50/60 Hz due to magnetic losses in the cores as the materials where specifically optimised for the 50-60Hz range. Some newer motors have a wider tolerance, and specialty motors are available designed to run from VFD's over a broad range of frequencies.

    Ultimately, you may have more available power by using an upper limit of 50Hz and moving up a step on the pulleys, it would be a function of pulley ratios and motor characteristics.

    Assuming that the drive mechanism is native to the lathe, the lathe if in good condition should be reasonably happy running 50Hz with the belt driving from the largest of the steps to the smallest step on the spindle. The manufacturer would not ship the lathe so equipped and with a 50Hz motor unless it was capable of handling the speed safely. Of course the spindle speed needs to be optimised to the diameter of the turning and the task being undertaken.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  8. #8
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    Default

    Malb, I agree with everything you said, but I would like to share my logic for comment or clarification. If the PO is using his lathe up near top speed he is likely to be machining aluminum or a small diameter item. In both instances he is unlikely to be taking heavy cuts and can tolerate a loss of efficiency.

    In my uneducated opinion I would aim to set up the motor so that for the majority of intended use the motor would be between 35 and 65 hertz.
    I don't think it has been mentioned here but caution should be used when running motors at low speed for long periods. Most motors have a fan on the back that blow cooling air across the motor. When the motor is running at slow speed it is not as efficient so will run hotter and have lower airflow, so less cooling. In contrast to my opening paragraph, if you are running slow you are likely to be taking a heavy cut on a large hard item, therefore running the motor hard and producing heat. It may not be an issue in a Tasmanian winter but would quickly push the friendship in a Birdsville summer.

    Steve

  9. #9
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    Default

    True, also for lathe headstocks that rely on splash lubrication for the spindle, running very slow motor rpm may lead to insufficient splash lubrication on the spindle bearings.

    Probably not an issue if only running down to 30hz or so but getting lower and it may be an issue worth considering, investigating.

    Simon

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  10. #10
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    The general practise in industry for use of VFD is to gear the motor at or close to your slowest speed required as it creates far fewer problems running a motor faster than it does slower. Having said that, there are plenty of examples of VFD controlled motors that have been specifically engineered otherwise.

  11. #11
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    Default

    Thanks very much for the detailed replies.

    In my instance I think I have it pretty close to being where it should be. The good thing about the south bend lathe is the information online from others that have done exactly what i have done before me.

    I actually have the VFD programmed to run the motor up to 75hz, this is from the advice of a person who specialises in VFD programming and I also asked the sparky at work what he thinks of running it up to this speed and he said itll be fine.

    However, in reality, I definitely do not need to spin the motor that fast for this old lathe, so Ill most likely back it back off to around 60hz.

    With belt in the 'middle' of the cone pulley I get a good operating range (95% of what I do) from approx 20-50hz. If I need to run slower I have the back gears, and if I need to polish something I have the extra speed in those very rare cases.

    The reason I started the thread is because I always felt like I should probably be running the belt in the configuration I showed in the original post to give me the most mechanical advantage and it would allow me to run the motor more in the approx 30-60hz range. I feel this is the optimum set up in my instance.

    A lot of guys put higher hp motors on the south bends than the original but I can spin the belt off the pulley if I push it too far, so this motor has enough grunt, it is only a little hobby lathe in reality. Being a hobbyist I like the built in 'safety' feature of having a belt that will spin off the pulley when I try my luck with big cuts or usually when i am parting off a little too aggressively.

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