[greenene]

Can someone please explain the meaning of the double numbers in some rows in the attached data sheet.
As an example, I've highlighted one cell in the second row to point to two numbers under "Basic Load Rating", an upper one and a lower one.
I simply cannot figure out why there are two numbers. When does one apply the upper number, and, similarly, when does one apply the lower number in the calculations.
Thanks.
tony

[jfong]

You need to look at the first part of the data sheet just above this one. It’s different block size/type. They split the chart in two sections.

[mactec54]

Can someone please explain the meaning of the double numbers in some rows in the attached data sheet.
As an example, I've highlighted one cell in the second row to point to two numbers under "Basic Load Rating", an upper one and a lower one.
I simply cannot figure out why there are two numbers. When does one apply the upper number, and, similarly, when does one apply the lower number in the calculations.
Thanks.
tony

As jfong said they have different Block lengths, chart below is what you are asking, you see the different size rail like 15mm or 20mm Etc and each one has 2 Bearings Blocks that are a different length with different load rating

[greenene]

Thank you for the info. One more question. In the "moment' boxes, Mc refers to only ony block, and not two. I am confused as to why there is moment data for one and two blocks for Ma and Mb, but not for Mc.

Essentially, if I am building an axis with two rails and a total of 4 blocks, I understand that the two block number refers to two individual blocks literally touching each other on one axis. I don't understand the Mc as it applies to two block located together.
tony

[jfong]

https://tech.thk.com/en/products/pdf/en_b01_056.pdf

You need to look up the tech notes for your particular lm guide and blocks but this is a starting point for the calculations.

[greenene]

This was very helpful reading, but I am still confused about how to interpret one of the three moments numbers for selecting linear rails.
On the x axis, the one on which the z axis is attached, there will be two linear rails and two blocks on each rail. According to the data sheet, the moment data makes sense for two blocks on each rail( or a total of 4 blocks on two rails) holding the z axis. It's the third moment, Mc, which is referred as the radial moment, that confuses me.
First, under what circumstances is this moment relevant?
Second, since I have a total of 4 blocks, what is the math to determine if this moment limit is acceptable.

I guess I am looking for some common sense explanations

[genixia]

Mc is a rolling moment, as if you're trying to roll the block around the rail axis. One side of the rail sees a radial load, the other a reverse radial load.

I assume you're talking about a gantry configuration if Z is riding on X. For the X blocks, the entire mass of the Z carriage assembly causes an Mc moment on the lower two blocks as the carriage tries to pivot on the lower rail. The upper blocks see a reverse radial load. Both blocks also see lateral loads. When cutting with axial force on the spindle those forces are reduced, and theoretically even be reversed if the upwards moment caused by the axial force exceeded the downward moment caused by the weight of the carriage.

In most situations on a dual rail system that Mc moment isn't large. Imagine a vertical mill cutting a slot in a workpiece mounted in the middle of the table, the slot being aligned with the X rails. One side of the cutter is climbing out of the workpiece, the other diving in. The Z axis as a whole will see a pitching moment (Ma). Most Z configurations will effectively shift that moment across two blocks, translating it into radial forces on each block. The workpiece, and the table, will see a moment that is Mc in the X direction. If you only had one rail mounted under the middle of the table, it would be simple Mc on that rail's block. But most (all?) mills have two parallel rails under the table, and we're cutting between them. The table translates that Mc moment into opposed radial loads on each rail.

You could imagine that if cut that slot right above one rail that it would be possible to get that rail to see a rolling Mc moment, much as if you only had one rail. In a two rail system though, that moment is resisted by a radial load at second rail. The stiff table acts as a lever and the distance between the rails means that a relatively small radial force at that second rail can resist a larger rolling moment at the first. The wider the gap between the rails, the more effective that leverage is.

As to what constitutes an appropriate limit, I don't know. Practically speaking, I would say that keeping forces across two rails means that (because of the above reasoning) you only really to worry about radial and lateral loads. Keep to common and safe design configurations, and ensure that you can handle the expect radial and lateral loads, and the rolling moment should take care of itself.

[greenene]

Finally, a clear and thorough explanation of the "rolling' moment, which I had difficulty understanding. Thank you very much.
tony