I have been playing around with bearings designs lately and thought I would share this latest hybrid 8mm bearing. The video at the bottom demonstrates the finished product on a 8mm linear shaft.
It makes use of commonly available PTFE tubing for the area which is in contact with the linear shaft, combined with a printable bearing housing. This has quite a few advantages; as it's light weight, PTFE has an extremely low friction coefficient and if the tube wears over time it can simply be replaced, without the need to dismantling the linear shaft assembly.
The PTFE tube only protrudes out from the bearing wall by about 100~150 microns, which is enough to provide a glide surface with very little friction. The 3rd image illustrates the overlap of the tubing from the bearing inner wall.
Part number and variants
I try to design things with fairly high tolerances and I know from a previous bearing a few people had issues printing, this time I have made 10 variants of each bearing size. Additionally I have made 5mm test samples of each variant, allowing a smaller test prototype to be made a tested.
The part numbers should help to identify them and can be broken down as follows:
Part number: 24mm_Bearing_4xPTFE_8.2mmx2.2mm
Bearing length (2 variants): 24mm (LM8UU) or 45mm (LM8LUU)
Number of 2mm PTFE runners (2 variants): 4 or 6
Inner diameter (3 variants): 8.2mm, 8.3mm and 8.4mm
2mm PTFE tube tolerance (3 variants): 2.2mm, 2.3mm and 2.4mm
The 8.2mm ID will give the tightest fit, then the 8.3mm and finally the 8.4mm I/D will be the loosest. There will also be a subtle variation in fit between say an 8.4mm I/D with 2.2mm tube room and a 8.4mm I/D with 2.4mm diameter. The effect of this variation will be down to your printers accuracy and the PTFE's actual O/D, as the PTFE tube might be a good fit, loose fit or very tight fit causing it to squeeze and deform slightly out through the gap in the wall facing the linear shaft. There is nothing to be done here due to the large number of possible tolerance variations, hence why I have tried to offer enough variants to cover the majority of cases.
I have made a few of these myself and found the 8.2mm was a little too tight, especially as the 2mm PTFE tubing I am using is approximately 2.1~2.25mm. For best results I would first measure your PTFE tubing and then start with the 8.3mm ID version. The 5mm test samples should make the process less painful, just bear in mind sliding 2mm PTFE tube into a 5mm sample is very easy compared to a 45mm length.
I made all my test print in PLA on a layer height of 200 microns with a 50% infill, for final production bearings I will use 100 microns and slow everything down to get the best results.
Pending on the response to this I have some other ideas using this principle!
Print for quality not speed I would say! I slow down my settings for parts for the printer, to improve the part quality which ultimately goes on to improve the quality of future prints.
The aim of these bearings is to reduce the weight of any moving parts and then ultimately allow for greater print speeds.
If you can can print at 100 microns for even better results.