Why the asymmetrical bed carriage?  

Re: Why the asymmetrical bed carriage?

Is there a particular for the bed carriage to be asymmetrical, in the way that there are 2 bearings on one and just 1 on the other side? That construction is likely to cause vibrations (and is, in my case), since the side with 1 bearing wobbles more than the side with 2 bearings when going back and forth. Of course, it is also less stable when applying vertical pressure on the side with 1 bearing.

Has anyone made a modification with 2 bearings on both sides?

I asked myself the same question and searched a lot at the forum. but because the answers didn't really conviced me (self-aligning ? avoid blocking ?...), I build my clone with symmetrical bed carriage... and I even used two lm8luu at each side.
the carriage and the heatbed are rock-stable. no vibrations, no wobbeling, no skewing (at heavy y-direction changes).

I've tried to get my symmetrical design working with drylin bearings (8 at all) and drivelin rods... but the friction was way to high, so I went back to the good old steel rods and the cheap lm8luu's.

by the way: I tried the drylins (& drivelin rods) at my genuine i3 mk2 kit with asymmetrical design, but it didn't work for me because every heavy movement change in y-direction was accompanied by a slight skew of the heatbed (due to the assymmetrical design and the clearance of the drylins).
ok, the printer was much more silent then... but at the price of print quality/accuracy.

Re: Why the asymmetrical bed carriage?

Jeff, your response convinced me even more to find an alternative solution to the original Y-carriage.

I build my clone with symmetrical bed carriage... and I even used two lm8luu at each side.

In that thread you stated that you were not too happy about that particular part because of the misaligned bed mounts.

What do you think about printing that part (Y-carriage) in ABS? It is pretty far from the heat bed, so it should't melt or soften it.

There are quite a few designs here https://www.thingiverse.com/search/page:1?q=y+carriage . 😉 Has anyone tried some of it yet and is there one to be recommended?

Re: Why the asymmetrical bed carriage?

...
In that thread you stated that you were not too happy about that particular part because of the misaligned bed mounts.

yes, exactly.

What do you think about printing that part (Y-carriage) in ABS? It is pretty far from the heat bed, so it should't melt or soften it.

There are quite a few designs here https://www.thingiverse.com/search/page:1?q=y+carriage . 😉 Has anyone tried some of it yet and is there one to be recommended?

I tried to print a two part* version from the genuine prusa carriage in pla, just to test if it'll work in principle. but it was not stiff enough... maybe with abs or pc ?
*) because in one piece it didn't fit inside the 250x210 print area.

furthermore the y-carriage needs to be absolutely plane and mechanical very stiff, otherwise your heatbed will most likely warp when heating up, so I'm not really convinced that a printed part will do the job at the prusa design approach.

Re: Why the asymmetrical bed carriage?

This is indeed a good question! In my opinion, the Mk2S should have come with a symmetrical carriage.

Re: Why the asymmetrical bed carriage?

This is indeed a good question! In my opinion, the Mk2S should have come with a symmetrical carriage.

I am not sure this is true. With four - imagine them at the outer edges of the carriage - each needs to be perfectly parallel or they will bind. With three, the bed will stettle into a minimum friction between the alignment of the side with two (the average parallel-ness) and the third side can move slightly to accommodate.

Think of the three vs four legged stool comparison, instead of length it is how parallel are they. The three linear bearing design will be more tolerant to slight misalignments of the bearings.

Re: Why the asymmetrical bed carriage?

This is the same as the table with 3 or 4 legs.

Which stand better ? Yes, the table with 3 legs. The table with 4 legs always wobbles if the floor is not leveled.

P.S.: I'm using the RJZM and it's very stiff and smooth 😉 Yes, correct, you have to align very well.

Thomas

Re: Why the asymmetrical bed carriage?

so no reason that really convinces the world not to use tables with 4 legs

Re: Why the asymmetrical bed carriage?

or with 5 legs 😮

Meine Hund der hat 4 Beiner

http://www.songtexte.com/songtext/fredl-fesl/ein-pferd-hat-vier-beiner-53c0ef25.html

Thomas

Re: Why the asymmetrical bed carriage?

It is a good practice for mechanical engineers to follow this principle: https://en.wikipedia.org/wiki/Degrees_of_freedom_(mechanics)
Avoid to overconstrain your static or cinematic assembly. Even with 3 linear bearings you are overconstrained, you would need a planar joint on one side.
For linear motion systems on 2 rails now, you have to take care of a good alignment of the rails. It is a matter of backlash management, cost effectiveness and mounting simplicity for a kit. It can even works well without being aware of (theory vs practice).
4 legs tables have many advantages and it is not an issue if they are wooden made as they are quite flexible and the soil is quite flat. If you have an iron made one or put it on a very rough soil, you have to compensate the level.