Monday, February 13, 2017

[2.70] Seek and Geek Week 1

Seek and Geek: Ballscrew assembly from Reuse


Since the desk I'll be working on as my main project was most likely going to use a leadscrew/ballscrew, I thought that poking a leadscrew/ballscrew would be the perfect first Seek and Geek assignment. 

I knew a friend of mine that found a ballscrew assembly off of reuse@ (an email list that's the source of many mechanical parts for future MIT projects) and I borrowed it from them. 

Ballscrew assembly!

The ballscrew assembly is made up of a threaded rod driven by a motor to move a ballscrew (a fancy nut with ball bearings to reduce friction between the threads) linearly up and down the rod. In order for the rod to transfer the rotary motion of the motor into the linear motion of the ballscrew, the base of the motor must be constrained in the axial and radial direction and the ballscrew itself must be constrained in the radial direction. This particular ballscrew assembly did not have a radial constraint for the ballscrew, but the metal block attached to the ballscrew had holes where rails (most likely metal rods) could be used to prevent the ballscrew from rotating with the threaded rod. 

If used to linearly actuate an object (a desk for instance...), the object could be attached to the ballscrew itself, and be moved up and down the threaded rod with the activation of the motor. Engineers would have to carefully spec out the various component of the ballscrew depending on whether the object was being moved vertically or horizontally. If the object is actuated vertically, the threaded rod must be constrained from swaying side to side and if the object is actuated horizontally, the threaded rod must be constrained from bending - both of which would result in the ballscrew jamming along the threaded rod. The motor torque output and ball bearing strength must be also carefully selected to support the weight of the object. 

Once the power and strength of the leadscrew/ballscrew has been analyzed to ensure that the object is supported well, the physically placement of the rails comes into effect. The rails must be as long as the distance that the ballscrew is to be moved, and if an extension is added to the ballscrew, an object can be moved past the rails. If I were to use a leadscrew to move my desktop up, I might want to use a ballscrew extension so that the rails don't extend above my desk. But the stability of this ballscrew extension is in question and something I would like to investigate. 

2 comments:

  1. its amazing the great stuff here on reuse!

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  2. I think that model of ballscrew is this guy from THK: https://tech.thk.com/en/products/thkdlinks.php?id=359

    It's kinda hard to tell because the model type only says "IK" and it should have 3 letters.

    The nut is a deflector type - the rolling balls get picked up by a deflector at the end of travel (one revolution of the shaft), pass through a deflector tunnel then return to their initial position but on the adjacent groove.

    It also gets preloaded by changing the groove pitch in the middle of the nut, which forces the balls to different sides of the shaft thread. http://www.cbmind.com/linear/thk/pdf/Ballscrew%20Product%20Specifications.pdf

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