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  1. #1
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    calcs required for linear actuator

    Hi all, maybe someone can point me in the right direction for what I need.

    I found a source in china for reasonably priced ballscrews and nuts. I am using them on a CNC router.

    That got me thinking about the possibility of using them for a diy linear actuator. Building it is simple enough, I though about using a servo plus 25mm ballscrew/nut and putting it in a housing.

    What I dont know is how to calculate what size servo I would need to hold any specific weight. I.e. a weight will be pushing down on the screw trying to turn the servo, how much torque is required to counteract that and give me enough torque to move the weight.

    Any ponters would be appreciated!

    Thanks,
    Jason

  2. #2
    500+ This must be a daytime job autocadplease's Avatar
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    Ask the question at cnczone.com and you should get a response from some experts ASAP.
    Grant D.
    Nelson,B.C. Canada
    Win7 32bit, FSX, PM Boeing, TH2GO, GEX, VoxATC

  3. #3
    Jason,
    If your trying to do that why dont you just use car windshield wiper motors thats what a guy did in NZ and it worked fantastic. http://www.jimspage.co.nz/intro.htm

    Tim

  4. #4
    75+ Posting Member Leo Bodnar's Avatar
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    Hi Jason,
    Have a look here: http://www.nookindustries.com/ball/BallGlossary.cfm
    Also a lot of servo motor user manuals have load calculation worksheets at the back.

    In practice you have to take into effect frictional torque of the servo motor and the ballscrew assembly.

    After you counteract weight and friction whatever is left will be used to accelerate the load. After acceleration the torque requirement will subside.

    Servo motors have continuous rated torque and peak torque (3..4 times continuous). I guess in your case you can safely assume continuous torque requirement based purely on weight+friction and then use higher torque to get load moving.

    I hope I did not confuse you...

    Quote Originally Posted by metalnwood View Post
    What I dont know is how to calculate what size servo I would need to hold any specific weight. I.e. a weight will be pushing down on the screw trying to turn the servo, how much torque is required to counteract that and give me enough torque to move the weight.

  5. #5
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    Hi Autocadplease and Tim, thanks for your replies.

    After a fair amount of searching I found a website that had the calculators I was after. It was at http://www.racointernational.com/screw_formula.htm

    Tim, I emailed Jim some time ago, probably a couple of years ago. I wish he was closer to me, I would have liked to pop in and have a look.

    One of the reasons I dont want to go the way Jim has gone is because I am a bit of a machinist and I like the building as much as the end result. For me, to build the actuators and machine most of the components is something I want to do. Building the CNC machine is showing me that the control of the servos is quite easy and achievable.

    At first I liked the idea of of the geared motors with an arm but what I didn't like was the price if you were to get everything new. I know I can scrounge aroudn for second hand stuff but it's a bit up in the air about what you can get. It's not like I will ever build more then a couple, maybe three of these but I like to be able to do it consistently with the same parts. For me, that doesnt mean consistently spending $1k per actuator but doing it myself

    So, it turns out that I will need about 1200 oz/in to turn these is they were directly coupled to the ball screw. Probably a bit steep so I think I will gear a servo 2:1 to achieve it. I should still get the speed I need and servo's at an ok price to do it.

    This was based on a 3dof platform with no spring, the actuators taking all the weight. I want heave but still may revise this to have a central point with a spring. Lots to think about. When I finish my CNC router I should have the electronics behind me to give it a go

    Thanks again for your reply.

    Jason

  6. #6
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    Thanks Leo, we must have posted at the same time.

    I used a effieciency coefficient of about 80% I think, this may be higher then my ballscrews get but at least I overcompensate then the other way around.. I think this is essentially the 'friction' of the actuator.

    I just ordered about 250 lbs of ballscrews and linear rails for another couple projects. It's the first time I have got these from china. I will evaluate them and see if they are suitable for the project. Hopefully so.

    Thanks,
    Jason

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