Alternative to a square linear bearing?

[CWatters]

Anyone know of an alternative to the 7mm Drylin square linear bearing? They seem on the expensive side, 30GBP for just the plastic insert/bearing surface.

https://www.igus.co.uk/product/983

Ideally I'm looking for something smaller, say for a 3mm shaft/rail?

 

[JBA]

Without any information on your application requirements it is difficult to make any rational suggestions.

 

[CWatters]

Its for a 3D printer. Most cheap 3D printers are one of two types : "Direct" (where the extruder and it's stepper motor are mounted on the carriage) and "Bowden cable" (where the extruder and stepper are remote and connected to the carriage by a flexible pipe that carries the filament). Both have advantage and disadvantages.

I thought it might be possible to make a third type that has the extruder on the carriage but a remote stepper motor.

I initially thought about using a flexible drive shaft but then it occurred to me I could mount the stepper at one end of the linear rails that the carriage rides on and add a "third rail" that was rotated by it. So I would need something like the Drylin bearing on the carriage that could slide on the third rail but would be rotate by it and transmit motion to the extruder in the carriage.

The extruder stepper is typically a size 17 stepper motor which turns a gear in the extruder at low rpm (say 100rpm?) with a torque of less than 40Ncm (0.4Nm).

I might be able to use the 7.5mm version of the Square Drylin bearing as it's well within its torque specs. However the bearing itself is about 15mm in diameter and would need a ring gear around it to drive the extruder gear. This would mean an unavoidable step up gearing because the ring gear would be bigger than the extruder gear. This step up increase the torque on the linear bearing.

Sorry if that's hard to follow, I'll try and post a sketch tomorrow.

 

[Nik_2213]

Will dropping the diameter thus allow excessive dynamic flexing / 'whipping', or is that handled elsewhere ?

 

[CWatters]

That's a possible problem but other types of system such as the Bowden cable type also have a lot of backlash between extruder and print head so some flex in the drive might still be acceptable.