Bushings or bearings?

Hello, Just wondering what most of you rep-rappers use on your X carriage?
I like that bushings can be printed and not wear out the rods.


Atm i use bearings and they are fine,

I think i have just answered my own question,
Wait for the bearings to fail and then change to bushings if the rods are still good.


What do people typically use here tho ?

I am a big sucker for bushings. You don't need hardened shafts to run them on while bearings will wear out unhardened shafts (tracks where the balls of a LM8UU run on the shaft become noticeable rather fast).
Since I am also interested in the original goal of RepRap, self-manufacture is rather important, so I made bushings out of UHMWPE, which run extremely well (no wear noticeable after some months running). After I put together my printer, taulman 618 became widely available and I did some tests with it. As a bushing material, it is equally formidable with the advantage that you can actually print your own bushings that last a while (PLA bushings are supposed to wear out after a short time, so I didn't even consider to use them).

how much last a bush printed with taulman 618 ?
how many hours ?

thanks,
mihai

uGen Wrote:
-------------------------------------------------------
> I am a big sucker for bushings. You don't need
> hardened shafts to run them on while bearings will
> wear out unhardened shafts (tracks where the balls
> of a LM8UU run on the shaft become noticeable
> rather fast).
> Since I am also interested in the original goal of
> RepRap, self-manufacture is rather important, so I
> made bushings out of UHMWPE, which run extremely
> well (no wear noticeable after some months
> running). After I put together my printer, taulman
> 618 became widely available and I did some tests
> with it. As a bushing material, it is equally
> formidable with the advantage that you can
> actually print your own bushings that last a while
> (PLA bushings are supposed to wear out after a
> short time, so I didn't even consider to use
> them).

I am not sure how long they will last exactly when installed on a printer, but I have done some tests with un-anodized aluminium. Here is the original thread: LINK.
Unfortunately, the photos in the first post are already gone since I uploaded them to imgur, but the follow-ups with the results are still there...

I think that the bushings might last a very long while (several months of continuous use), especially when you make them adjustable to account for wear.

Ive experimented alot with printed bushings and bronze bushings. After using ball bearings there really is no comparison IMO. The main problem i had with printed or bronze bushings was the small amount of static friction. When using bushings my circles always came out a tad bit oval shaped, no matter how hard i tried to aleviate it i couldnt. When using bearings the circles imediately came out perfect due to the reduced friction. A tiny amount of friction makes a huge difference in print quality.

I'm in favor of Igus 8mm plastic bushings. I just gave them a try on my tool steel that just arrived and man are they smooth and very little friction.

igus self aligning bushings and hardened linear rails work great for me, some 3 in 1 oil every once in a while and they're like NO friction

Tool steel rods and other unpolished rods that have been ground to a reasonably accurate dimension can be polished very quickly if chucked in a drill press or hand drill.

I used fine sand paper(400, 800, 1200 grit) on drill rod and it made a big difference even though I kept it quick and simple.

Bushings will last longer and run smoother. It might not help LMUU type bearings last longer, but they will roll smoother. Probably reduces the wear on the dust shield though.

I will try printing nylon bushings though, just to see. Maybe on the Z axis?

---

Yvan

Singularity Machine

GITRDUN Wrote:
-------------------------------------------------------
> Ive experimented alot with printed bushings and
> bronze bushings. After using ball bearings there
> really is no comparison IMO. The main problem i
> had with printed or bronze bushings was the small
> amount of static friction. When using bushings my
> circles always came out a tad bit oval shaiped, no
> matter how hard i tried to aleviate it i couldnt.
> When using bearings the circles imediately came
> out perfect due to the reduced friction. A tiny
> amount of friction makes a huge difference in
> print quality.

Is this due to lack of torque/motor current?

The coefficient of friction for ball or roller bearings is at least 10 times lower than for a plastic bearing. This isn't the kind of thing you can feel by sliding or turning a bearing, but it makes a huge difference to your motors. Remember too that stepper motors are essentially springs. A higher load on a stepper reduces it's ability to reach the position of a step. A good reason to have considerably oversize steppers.

I have Igus self aligning bushings ground and polished rods etc etc, and had the same results as any other bushing. You can feel the static friction with bushings. With no belts attached put a finger on the Y axis or X and push just a tiny bit, if it initially has any resistance at all there is to much friction. Im not saying bushings cant be used im just saying the difference between any bushing and the ball bearings is drastic.

If you want to use bushings, or linear bearings either is ok. Look at the tolerances of each so as to reduce slop and play. Make sure the rods used in the printer are precise as possible. Many people still use pla printed bushings as well.

It takes less time to setup pre loaded linear bearings because they compensate for the slop. so if you are new to building a 3d printer, this may be the best way to go. for example 8mm linear bearings likely will still work with 5/16 drill rod.

If you want very little play and know what you are doing, bushings are probably the most accurate, and hold precision better at high speeds.

reprap vendor info and bushing types common are here:
[reprap.org]

For linear bearings read here:

[reprap.org]

here is a blog about pla printed bushings:

[blog.reprap.org]




As for friction of each in practice it is negligible. It is easier to install and use linear bearings, but more precise to use bushings. Bushings can be easily machined, even found at local hardware stores. Linear bearings need to be shipped typically.


whatever choice you make you will likely be fine. Belt tention and precision of rods are more of a concern.

Edited 1 time(s). Last edit at 09/05/2013 09:03PM by jamesdanielv.

Besides, the Ultimaker uses brass bushings on the X/Y rods. And we all know how the output of those things look like.
I guess partly, it is alignment that plays a big role in how well the bushings run and partly stick-slip (or, the difference between static and dynamic coefficient of friction). The bigger the difference, the more the bushing tends to "skip" or "stutter". PTFE for example has the same static and dynamic coefficient of friction on steel (Cf53 IIRC), so it slides extremely well, without skipping. Unfortunately, PTFE is neither printable nor hard-wearing. So the next best things are PA (there are several kinds, some of which are better known as nylon), PET and UHMWPE. With the right lubrication (I use silicone spray), the UHMWPE bushings don't exhibit stick-slip.

There are PTFE filled plastics that have a very low static friction and very low wear.

Here in South Africa the local brand is Vesconite Hilube:

[www.vesconite.com]

Not sure if it could be extruded but one would have to make fillament first. Having (split) bushes machined should be pretty practical. The material is more expensive than Nylon and similar but the bushes will require very little and the rod stock does not need to be very large.

When I am up and running I will give it a try.

They have reps all over the planet but not sure how much variety they keep in stock.

---

Kalle
--
Lahti, Finland
The only stable form of government is Open Source Government. - Kalle Pihlajasaari 2013

It seams like the issue with bushings could be with how long they are. The longer they are the higher the resistance is going to be. I just had a set of prints done for my 3d printer on a machine using igus bushings and o1 tool steel and from my perspective the results are great as it produces very accurate prints.

With that in mind igus makes some very "short" length bushings but I suspect that as the length decreases you get the increased chance of slop.

@KalleP: Unfortunately, the thermal decomposition point of PTFE seems to be lower than the melting temperature. So, no chance to get this working. There are other fluoropolymers like FEP that might work.
Personally, I would try PET first, as there is already some filament available. Strangely, the melting point is far below the one found on Wikipedia, so probably, there are some variations (taulman has a PETT filament and Vivak sheets are made out of PET-G for example)...

@hcker2000: The formula for friction doesn't include surface area of the interacting partners, so length of the bushing shouldn't play a role in resistance (of course, the added weight will indeed add an extremely minimal amount of friction). The only things that should matter are coefficient of friction and normal force (= the force applied perpendicular to the surface). Which makes perfect sense: if you take a 1Kg weight and an interface area of 1mm², the pressure per mm² is higher than if you take the same weight and an interface area of 10mm².
So, a single, extra long bearing might be useful for better alignment vs two shorter ones if you don't have self-alignment features built-in.

You guys are incredibly intelligent, thanks for the reply's

Tonight and tomorrow, I am going to give PLA and NYLON bushings a go, I really do not mind one bit if they needed to be changed every couple of months, printing them is great, and i like to print as many parts as i can

What about PTFE beads or power in a lower temp plastic binder? You don't really need to melt the PTFE...?

I wonder if this is something that could be experimented with fairly easily with a filament extruder...

---

Yvan

Singularity Machine

@uGen: @Yvan: The Vesconite Hilube is PTFE loaded plastic. One would only have to melt the matrix and the PTFE filler should move with it as it is already well mixed in.

I have heard of other lubricant loaded plastics, some have graphite, others use molybdenum disulfide, (the original Vesconite is black in colour and uses one of these) there may be more options I have not heard of. The point is that the matrix plastic is usually choseen to be a high quality engineering plastic to start with as it has to handle the mechanical loading while the filler is the friction reducer. I expect some easy to extrude combination with ABS or PLA could be formulated if the matrix specifications were not all that high.

As far as choosing bushes with more or less surface area, in my experience looking at well engineered products, the trend has always been towards minimising the bush contact area within the limits of handling the static load carrying capacity and wear reserve. In theory the amount of surface area does not affect the force of friction but in practice I believe at least three effects will still make it better to have less surface in contact. Firstly dust and grit will get trapped in the surface and this will increase the friction and the more surface available the more dust can be trapped. I have a gut feel that sticktion may be influenced by the amount of surface area in contact but may be wrong here. And lastly any misalignment over longer distances on a rigid bearing will cause increased forces, over constraining is something to be avoided. This is why slightly compliant bushes (split) with groves (to clear dirt) that are self aligning (ball shaped bush in spherical socket) should offer the lowest friction for any choice of material. Having a PTFE or similar filler or liner in a bush may bring the performance close to that of a low cost linear bearing. The Igus people have much more money to spend on development than we do so we can learn from them, having a clip in slippery liner (a split length PTFE tube) in a prepared pocket on the carriage may be a possible workable compromise, it would have low friction, be a bit self aligning and cheap and easy to replace.

There was a report some time back of substantial increases in sliding force on a linear bearing if an undersized rod was used. Other reports of cheap bearings sticking when they have insufficient clearance, so too large or too small causes an increase in the force as the balls do not run freely.

---

Kalle
--
Lahti, Finland
The only stable form of government is Open Source Government. - Kalle Pihlajasaari 2013

Also for those who bought cheap chinese LM8UU, the rubber seal can be a major source of binding. I removed them on mine just leaving the clip to hold it together and they slide much better.