Welcome to Our Community

Some features disabled for guests. Register Today.

Z-Axis support question

Discussion in 'General Talk' started by BobC, Apr 7, 2017.

  1. BobC

    BobC New
    Builder

    Joined:
    Apr 5, 2017
    Messages:
    6
    Likes Received:
    0
    Hi all - I'm new to this forum, so apologise if this has been addressed before somewhere.
    I am looking to build a vertical elevator for a model railroad application. It will lift a 1.5m section of track up and down between levels. I intend to have a screw either side, with the two stepper motors driven in sync. I expect the "payload" attached to the carriages to be about 4kg.
    My question is how the screw is supported. The C-Beam and V-Slot options appear to just have the screw attached by a coupling to the motor at one and and running in a bearing block at the other, but with the motor at the top, it seems to me that the entire weight of the screw, carriage and its payload is hanging off of the motor. If the motor is at the bottom, the motor is carrying the weight, but that's not how most z-axis builds are configured.
    I would have expected some sort of thrust bearing at the lower end if the motor is at the top.
    Maybe I have missed something important, but can somebody comment on how these things are built, please?
    Many thanks,
    Bob
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
    Staff Member Moderator Builder

    Joined:
    Dec 20, 2013
    Messages:
    1,441
    Likes Received:
    583
    Motors are generally placed at the top of systems built here mainly to keep them out of the way. In reality it doesn't really matter. I do often question however whether putting the vertical load into the flex coupling and into the motor shaft is a good idea on the heavier systems. The coupling acts as a spring affecting how well the bit stays in contact with the material and the stepper is not designed to be operated with significant axial load. Truly most would be better served by having a thrust bearing lodged between the top plate and the flex coupling.

    In your case however, the loads are really not all that significant. The only issue here would potentially be the unbraced length of the screw. For a longer screw you would definitely want it to be supported from the top. Using an 8mm thrust bearing with a lock collar above the top bearing plate is usually a good approach. If having the motor in line with the shaft is inconvenient you can also easily offset it with the NEMA 23 Reduction / Stand Off Plate Set.

    If the screw is more than about 250mm, the axial bearing should be at the top. Loads should always hang from screws, keeping them in tension. Compression in long slender elements like screws caused them to bow out sideways.
     
  3. BobC

    BobC New
    Builder

    Joined:
    Apr 5, 2017
    Messages:
    6
    Likes Received:
    0
    Hi Rick.
    Thanks for taking the time to reply, I appreciate it. In the case I described, the screw will be 1000mm, as I need about 850-900 travel.
    Those thrust bearings look what I would have expected, but do the lock collars (which I've seen in a V-Slot build video) hold well enough with just one grub screw to carry all the load?
    No problem with having the motor in line. Just might need a bit longer spacer to accommodate the bearing and collar between the top plate and the coupler.
    Thanks,
    Bob
     
  4. Rick 2.0

    Rick 2.0 OpenBuilds Team
    Staff Member Moderator Builder

    Joined:
    Dec 20, 2013
    Messages:
    1,441
    Likes Received:
    583
    If you are concerned with the capacity of the collars, add another screw. There are a number of examples here on the forum where this was done. I don't think it should be an issue with the light loads you are using but it is always best to take the safe route.
     
  5. BobC

    BobC New
    Builder

    Joined:
    Apr 5, 2017
    Messages:
    6
    Likes Received:
    0
    OK, I'll look into that. I didn't think a 4kg load would be deemed "light" but then I haven't actually built one of these things yet!
    Thanks for your help.

    Bob
     

Share This Page