Supported linear rail question.

[IN-WondeR]

In my design that I'm still working on, I'm thinking of mounting supported linear rails. But, and here comes the question.
If I mount them on the side, so the Pillowblocks is mounted sideways instead of upright, would they have any problem of working in this way without making any slack over time? The reason is, that this kind of linear rail is somewhat cheaper than THK linear guides are.

But if anyone knows if supported linear rails can be mounted on the side instead of the top, please let me know.

[Dan Brown]

i've seen it done a few times and it is what i'm planning for my first build as i'm only building it to make the parts for my second build.

on my second build i want the rail supported in the upright position not just because it makes more sense in respect to were the load is coming from but also because i imagine the area were the rail joins the support getting completly full of sawdust, chippings, coolant spills and all sorts of other rubbish.

[Gary]

While it can be done, dont expect to put the rated load though the bearing.
The bearing is open and the case is ally so will bend if too much load is applied, and when this happens it will go all sloppy.

[Ross77]

Bit weird but ive just been looking at that today, From what Ive found (not finnished yet tho) Basicly for the open bearings used on supported rail then depending on the manufacturer the rated load is only if applied vertically from above, side loads reduce it to 70% and upward forces at 50%

So if the pillow block is mounted on its side then its vertical capacity is reduced to 70-75% but the bigger problem is the open face which is reduced to 30-50% capacity. As the open face is now in a direction of travel you effectivly have full rated strength in one direction but only half in the other.

If the rails are mounted as a pair, back to back ( ie top and bottom) then the weak spots are evened out, and at first glance you end up with an approx 75% reduction from the rated load capacity in all directions.

As I said not finished yet but will post some pic's latter to see if I'm on the right track.

[Ross77]

heres what I have so far. Any comments....:smile:

[Dan Brown]

if only i understood :(

[Ross77]

Hi Dan
Sorry, The red area is the effective load capabilities based on a 70% reduction for the horizontal and 30% for an upward force (reverse radial). the dotted line is the combined valule for two rails and blocks.

Hence the total vertical load is 100 + 30 = 130N ect....

[irving2008]

dont forget that the rail is acting as a cantilever as well so there will be increased flex in the rail

[Ross77]

dont forget that the rail is acting as a cantilever as well so there will be increased flex in the rail

Not quite sure what you mean? are you talking about the actual rail support? if so I wouldnt have thought it would deflect much?

[IN-WondeR]

Ross

I'm sure it's a nice calculation you have made. But the look of it, is not the setup that I will use. I will for my X axis, only have one rail pr. side and not two.

[Ross77]

But the look of it, is not the setup that I will use. I will for my X axis, only have one rail pr. side and not two.

Yes it is, just rotate the diagram 90deg. in your case the distance between the rails will be greater but the load distribution will be the same. for the x axis you need to allow for all the weight of the y and z axis so a greater capacity in the vertical is better, ie the first diagram. As you want to mount them on their side then diagram 2 will apply so you will have reduced load capacity and therfore require more pillow blocks to get the same effect.

So if the initial design required 2 blocks per rail then to get the same strength when mounted on the side you will need 3 blocks per rail.

[IN-WondeR]

Ok, I see then. However, I think I have overcome the problem, by mounting an heavy angle iron on the side, and mounting the rails on top of these.

[Ross77]

Ok, I see then. However, I think I have overcome the problem, by mounting an heavy angle iron on the side, and mounting the rails on top of these.


That may still cause problems if the angle can flex......How big is your machine and what are you cutting?

[IN-WondeR]

my machine is 4'x8' so it's going to be quite big.
the angle iron will be run at the machines full length if used that way.
mainly cutting wood materials, Nylon sheets, plexi and so on. Not cutting metals.

[Lee Roberts]

While it can be done, dont expect to put the rated load though the bearing.
The bearing is open and the case is ally so will bend if too much load is applied, and when this happens it will go all sloppy.

But isn't that the same wether you mount on the X, Y or Z. Somthing to do with load planes being the same ?

[Gary]

The supported round rail is designed so the load is directly on top.
I have seen it used in different orientations, but it is not designed for that.
As an example the TBR16 has a max dynamic load of just under 40 KG, but if you mount it on its side this will go right down and would definitely fail with sustained load of 40KG.

[Ross77]

Am I missing the point here, but that is per bearing? As most x and y axis will have 4, and even with a 70% reduction, thats still 112kg. Add another bearing per side and that goes up to 168kg. thats quite alot no? :smile:

[Lee Roberts]

The load planes are in the same direction wether you mount the rail laying down or tip it on its end are they not ?

[Ross77]

The load planes are in the same direction wether you mount the rail laying down or tip it on its end are they not ?

What do you mean by load "planes"? are you refering to the loads/forces placed on the bearing or resistive load capabilities of the bearing?

If the rail is mounted vertical then it is not technicaly subject to gravitational forces(because the bearing moves in this direction) so it only has to deal with cutting forces. X and y have to take the weight of the machine as well so loads are not the same in all planes.

As the bearings are open on one side then this area has reduced capacity by about 50% (so depending on the mounting orientation this will be different for each axis/plane) which is why I believe that they should be mounted back to back to even it out.

[Gary]

The rail is designed as a supported type, but you put it on its side and it is not supported as well anymore and you could bend the rail if the weight is too high.

[ptjw7uk]

Surely any rail for a given length will suffer some sag. The more massive the supports then the more the sag due to its own weight it will all be down to some form of compromise between size and allowable sag. This is the reason why 8 x 4 tables are massive with many legs all supported on a concrete floor. As in most things as you go up in size the problems all get that bit harder to do on the cheap, best thing is to build them on the floor!

Peter