Here is the assembly from the Railcore ZLT. What's missing in this view is the bottom plate.
This is what the assembly is accomplishing
- ZLT Bearing Block
- Hold the 608 bearing with clearance for the shaft coupler
- Mounting screws for the block attach to the NEMA17 motor
- Allow access to the shaft coupler
- Bearing
- Provide rotary support/alignment on lead screw
- Shaft Coupler
- Adapt 5mm motor shaft to 8mm lead screw
- Provide compliance for shaft misalignment
- Stepper Motor
- Provide rotary motion
- Support the weight of the lead screw and partial weight of the build plate and build
At first glance, I see three issues with this part
- Thin wall around the socket head counterbore
- Lack of clearance between the rotating part of the bearing and the bearing block
- Reliance on screws to provide concentricity of motor shaft and bearing
1 & 2 are easy to fix, but #3 is a challenge. Ideally the bearing block would mate or reference itself to the round feature on the stepper motor face. This is perfectly concentric with the motor shaft. But this design is relying on the shaft coupler (which is flexible) and the screws/holes to determine the position. This is not ideal.
On top of that, I'd make 2 more changes.
- Make the top of the bearing slightly proud of the bearing block so that a shaft clamp could be used so the vertical force on the lead screw is supported by this shaft. This could be done with a shaft spacer or an extended inner ring bearing, but why eliminate the option?
- Beef up the standoffs a bit
This part addresses most of the issues.
Looks a bit beefier than the part it's replacing. The overall height of the part is the same to allow for using the same hardware. Here, the bearing is sitting a couple of mm higher in the assembly.
Section view shows a structurally stronger part, which should allow us to support the weight of the lead screw into the bearing block. The bottom of the linear block has a 32mm bore on center, you could use that to align the bearing block to be concentric with the motor.
The choice of shaft coupler, a zero backlash jaw coupling, is fine for this application.
Here you can see the bearing outer race only contacts the bearing block and the inner race contacts the shaft clamp with a shim washer to prevent any parts rubbing. This will eliminate any axial load on the motor shaft and it should maintain a the lead screw at a constant height, without worrying about the shaft coupler introducing any vertical movement while rotating.
It goes without saying, the design relies on gravity, If one was inclined to fully constrain the lead screw, then you would simply capture the bearing and add a shaft spacer between the shaft collar and the bearing.
Now being a printed part, I would recommend testing this configuration and replace the bearing block with an aluminum version if it's not stiff enough. You can determine that by putting a dial indicator or height gauge on top of the shaft clamp and press down on top of the lead screw and see how much deflection you'd see with about 1 or 2 kgf.
Now checking it in the assembly, it's looking pretty good. I'm fairly happy with it. I added part numbers and updated the color. Moving up to the build plate brackets next.
MCR-003-0 - MECHACORE Z BEARING BLOCK
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