Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Linear force transfered from an electric motor to a screw.

  1. Jan 1, 2009 #1
    I have a 1/4 hp electric motor. I plan on attaching a piece of threaded rod to the shaft of the motor. If I put a nut on the threaded rod, and welded 2 bars to either side of the nut (running parallel to the threaded rod) how would I calculate the amount of linear force I am exerting? The threaded rod would be stabilized with a piece of teflon or something so as to minimize the strain on the end of the rod, as well as reduce resistance through friction.

    I am trying to think of how to go from cylindrical force to linear....

    I am going to go back into the old text books because I know I did something like this before... I'll post what I find, but if anyone remembers off of the top of their head, please feel free to post.

  2. jcsd
  3. Jan 1, 2009 #2
    You want to know the torque of the motor. We know it's power is 1/4 HP, but to get the torque from the HP, you need to know it's RPM.

    And how many thread per inch is your threaded rod?

    BTW, I would not use a nut, but a thread coupler, if I could. It looks like a long nut.
  4. Jan 2, 2009 #3


    User Avatar
    Science Advisor

    Maybe I'm misunderstanding the post, but if you want rotational to linear motion, how about a rack and pinion?

    If you're trying to calculate the tangential force of this setup, I believe it's just the torque divided by the radius. Dynamics wasn't my strong suit, but I think the tangential force at the end of a stick attached to a spinning wheel is still just torque divided by the distance of the tip of the rod to the center of the wheel. If it's not the tangential force, I think you also have to divide by the sine of the angle (torque is the cross product of radius vector and force).
  5. Jan 2, 2009 #4
    Yeah, a threaded coupler would work. I am trying to make a real basic electric wood splitter, only for splitting kindling wood which should not need much force. It's an old electric motor, so I am going to have to look the model number up on the internet, if I can find one, or measure it...

  6. Jan 2, 2009 #5
    I could use the same thing. Let me know how it goes, if you continue.

    Most electric motors we end up with are 3600 RPM AC syncrounous motors. They're the most common. If it's about 8 inches long it will be about 1/2 HP.

    It turns out, that you don't need to know the torque once you know the threads per inch. At 20 threads per inch and a 1/2 HP syncronous motor, the numbers may not add up. It depends on the splitting force you will need.

    At 3600 RPM = 60 RPS, the splitter moves at 6" per second. It's a bit fast. The linear force is 1100 pounds. That would be like you and 5 friends standing--not stomping--on the wedge.

    Power = Force times Velocity. So the same motor will give you move force at less velocity if it were geared-down. 1/2 HP at 1800 RMP would give you a ton of force minus losses.

    Edit: just to make sure, I checked out a washing machine motor I have. It's 1800 RPM. Maybe I've misforgotten common values.
    Last edited: Jan 2, 2009
  7. Jan 2, 2009 #6
    Yeah, I am going to give this thing a shot, just to see what happens. I saw somewhere on the internet a guy that was using some sort of multiple flywheel attachement as a wood splitter, and it looked really cool. I would love to build a hydraulic one, but unless you can scrounge every single part, you might as well buy new...

    I miss the story's my old man would tell me about going to a junk yard and finding "gold"... But then again, my generation does have the internet.
  8. Jan 2, 2009 #7
    I know someone that tried to power ready rod and had very little success. Even with tons of lubrication the friction so high it would seize up. He then tried acme threads with very little more success. Commercial drives use either a ball nut or something like Al Bronze nut with special lubrication.
    Regardless here are the formulas for an acme thread:
    http://www.utm.edu/departments/engin/lemaster/Machine%20Design/Notes%2027.pdf [Broken]
    Last edited by a moderator: May 3, 2017
  9. Jan 3, 2009 #8
    its a simple mechanics question.

    Get the the tangential force at the thread using F = T/radius. Now try to visualize the situation, it is similar to lifting a block up the incline, make a free body diagram of a mass on an inclined plane with the force just calculated being applied horizontally towards the incline. Since you need to know the maximum force that could be exerted, assume acceleration to be zero & hence calculate the vertical component of normal reaction, this is the axial force you need.

    Alternatively, you can use the energy method(though it would be approximate, coz frictional loss is neglected). Energy input equals force times velocity, axial velocity can be calculated using the rpm of motor & the pitch of the screw.
  10. Jan 3, 2009 #9
    Last edited by a moderator: May 3, 2017
  11. Jan 6, 2009 #10
    Phrak says, "Most electric motors we end up with are 3600 RPM AC syncrounous motors." Actually, this is not quite so. Such motors would be pretty rare and quite expensive.

    Most common AC motors are nominally 1800 rpm induction motors which means that their rated speed is around 1750 rpm. This speed is usually on the name plate, and is the speed at which the motor makes rated power. With a very high load, the speed with fall below that. Induction machines "slip" in order to develop torque which is to say that their speed drops significantly below the nominal 1800 rpm.

    Synchronous machines do not slip at all, although they may lag by a fraction of a revolution, referred to as the "torque angle." They may be readily found in both 1800 rpm and 3600 rpm models, but they are much more expensive to build and consequently to install. For this reason they are far less common.
  12. Jan 6, 2009 #11
    You say that you are building a wood splitter. Let me mention a couple of matters for your consideration.

    1) If you are planning on having the splitter push directly against the motor bearings, you will destroy the motor in fairly short order. The motor bearings are not made for this sort of duty, so they will fail rather quickly when loaded axially like this. Be sure that you do not expect the motor to do anything more than provide power for your machine; that is all it is designed to do.

    2) You should make some motion calculations regarding the speed at which the splitter will move with the shaft turning at motor speed. This is apt to be moving far to quickly for good control, and it also is like to mean that you do not have adequate mechanical advantage. You will probably want to look for a system that provides a greater mechanical advantage and a resulting slower splitter motion.
  13. Jan 7, 2009 #12
    How many phases are on the field windings of a 1800 rmp syncronous motor DrD? Or are they called poles?
  14. Jan 7, 2009 #13
    In the typical construction for 60 Hz AC currrent, a 3600 rpm synchronous motor will have a two pole rotor and an 1800 rpm synchronous motor will have a four pole rotor.

    Those poles can be established either by a DC winding or by a permanent magnet structure in either case.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook