Lego vs Metal

[Boxerlego]

Look at this first.

In this corner is the Lego differential and in the other there is a galvanized zinc steel washer. What do you think will happen between the two when the washer is use to shim the differential gear against the bevel gear (example picture)? Which one will WIN? Do you think the stone cold steel washer will destroy the Lego differential or will it be the other way around?

Regardless of what is consider Official building techniques. This is about real applied science and not some preconception.

Edited December 2, 2013 by Boxerlego

[Phoxtane]

I think you'll end up with the beams that are being shimmed against receiving most of the wear, in a large circle that matches the diameter of the shim. On the other hand, the shim will be just fine.

[DrJB]

I'm with the above post ... plastic will give in before the metal.

What did YOU find out?

[Boxerlego]

Before I say what I found out. I want to clarify why I made this thread. This thread was created to be informational and show the material factor that is involved when it comes to shimming up the differential. It is very possible for the idea Phoxtane suggested to happen when the shim material is more durable then LEGO.

Now everyone knows its fact that metal will cut in to LEGO and this is the standard realm of thought, but the fact is that for metal to be able cut into LEGO their must be a sufficiently sharp edge on the metal object to make the cut. Now understand the metal shim has no sharp edges and the shims flat metal surface is pressed against the flat Lego surfaces. When two flat surface are rubbing against each other both objects tend to wear each other down and basically to figure out which material is wearing down faster then the other is based around the hardness of the material.

Now for a quote from Wikipedia about ABS Plastic/LEGO "The most Important mechanical properties of ABS are impact resistance and toughness."

In the picture above we can see the shinning sign of metal wear that is on the differential and is evidence that the metal shim is losing material. Hear this, currently the metal shim is next to this part . When I first started using the shim against the differential it was a somewhat tight fit in that part making the differential not so easy to rotate, so I did not build with that frame style and had to rely upon the style of building seen in the example picture. Now the shim has developed the proper thickness and the differential is now able to rotate with less resistance in that frame then it did the first time. If the shim was made of ABS plastic then it would be consider a Lego on Lego surface and the rate of wear would be dependent on the quality/grade of plastic. Think about this. What if the differential did not need a shim and the gear border was thick enough to provide a solid support with the 20t bevel gear, what would happen?

[DrJB]

From what I recall from my tribology class, one of the wear mechanisms is when small particles detach from the two surfaces and get stuck in between. They typically provide some 'rolling film' and the original surfaces do not touch anymore. If the particles are rough, they act like grains of sand and eat-up the two surfaces. With time, some of those particles fuse with either surface ... So, there is a continuous process of particles detach (get created), rub, stick, ... and so on. The pic you showed suggests some small metal particles have encrusted the plastic differential. I bet some plastic particles would have filled small asperities in the metal shim as well.

To quote my former advisor: Tribology (unlike dynamics) is ... difficult to both conceive/theorize/explain, and thus, often full of surprises.

Edited December 6, 2013 by DrJB

[Boxerlego]

Here is a comparison picture of my control differential and one that has been shimmed. Notice how the metal wear skips around the differential and reflects the gear pattern. This a sign of the applied driving gear force on the differential and that portion is being pressed against the metal shim. Truly there is no visible sign of there being any plastic material on the metal shim as there is with the lego differential. Maybe if the shim was put under a microscope then we could see some material.

[DrJB]

very neat ... again, contact mechanics can be full of surprises.

[Boxerlego]

Yes it is full of surprises, I just notice that the shims inner hole's edge has indeed cut a very thin grove around the nub on the differential and there is a very little thread of plastic showing this. It is so small that taking a picture of it doesn't do it justice. Maybe you can see it in this picture here.

Edit: This picture is big 4000 x 2248 in size so you going to need to zoom out to see the whole picture. Here is the deep link to the picture. http://www.brickshelf.com/gallery/Boxerlego/Lego-parts/Differential-Spacer/metal_damage.jpg

Edited December 6, 2013 by Boxerlego

[Paperballpark]

Or maybe not, as the 'file is not yet public'.

[Junpei]

I can see the damage. But it is not that bad, I think it's fine.

[Boxerlego]

One of the biggest problem around using metal to shim against the differential, Is that metal can bend and any deformation in the shim can create some resistance on the differential. It has recently come to my knowledge that the differential in this picture was paired up with a deformed metal shim and the resistance between them allowed the shim to be able to cut in the differential like that instead of the shim freely rotating with the differential. The other metal shim I am using is a great fit with the differential. When there is shim resistance on the differential you can spin one tire by hand and the other adjacent tire will rotate and in the event when you have no resistance on the differential with the shim you will be able to spin one tire by hand and the differential will rotate. All that must be done without the motor connected to the drive axle.