Need help with load calculation

[SatanKlawz]

Ok I am getting my Z axis together on my home built machine. I sat down today to design the Y axis and how it will move the Z axis. I had planned on using the same bearing blocks that I bought for my X axis for my Y axis. While looking at everything that I already have today I found out that the mounting pattern on the back of my Z axis lines up perfectly with the bearing blocks that I bought for my X axis. Here is the issue, I had planned on using 4 blocks to do the moving on my Y axis but the mounting pattern will only support 2 blocks. If I can get away with just using the 2 blocks this will save me form buying 2 extra blocks and will save me the trouble of designing another plate to mount the z axis to 4 blocks. I have attached a picture of a document that lays out the load capabilities of the block I want to use (The data that refers to my blocks is highlighted in blue) and a picture of how they would be mounted to the Z axis. I need to know if this will be sufficient or do I need more bearing block support. I plan on using this router for wood and plastic if that matters.

[dgwatson]

SatanKlawz,
If you want to find all the loads which apply, you will need to do a force vector diagram. They are not that difficult once you read about them. The force to begin with is the load which will be applied by your motor and screw, taken from the torque curve diagram. Start off by assuming the router is off, the screw has full load and the system has stalled, this is a static state. Then apply the vector diagram to a moment diagram where all the moments are equal to zero. although this is unlikely it is a start, it would be the maximum in the event of an error. In the catalog, there are formulas to determine the load you are going to have (max load) which will be known frm your moment and force diagrams. That will tell you if it is ok or not. the other thing is that there are two brg blocks which would counter act each other for moment. Also, I believe the bearings you are using have a reduced maximum load if mounted on their side.
OR... seeing you are near finished just do it. If the system bends and stalls, you will need to add the other bearings. If things work your off to completion. Its all in the fun of DIY.
Regards
DGW

[SatanKlawz]

Thats the deal, I am not finished yet and at this point have the option of buying whatever I need. If what I have will work I will order more as the bearings in the pic are for another axis on my machine. I have to buy parts either way it goes so I want to get what I need the first time. Where can I find the torque curve diagram? Is it for the stepper motor? I need help with this and you seem to know what you are doing. The steppers I have are these and I will run them with a G540.

http://www.kelinginc.net/KL23H2100-35-4B.pdf

They have torque curve charts for other steppers on their site but not this one.

[dgwatson]

SatanKlawz,
the max torque for this motor is 381 oz-in. I finished one last year and used smaller motors (125 oz-on). These proved to be ok however, would have been nice if larger like your choice. I used Hiwin linear bearings HGH20C for the bridge which were a good choice, same size as yours. I needed 4 brgs as the twisting was too large (determined by calculations). I also placed one set (2) on the top plane of the bridge and one set (2) on their side. This helped stabilize the bridge and carriage and keeping the max load ratings at the largest. If you are not finished with this concept of only 2 brgs, I would suggest you change your intent and use 4. Having both on their side reduces the load max dramatically. Hope this helps.
DGW

[SatanKlawz]

I can go ahead and buy 4 bearings for this and redesign it a bit. I was just hoping that 2 would be good enough. Should of known better! On another note I looked up the specs of the bearings that you listed for yoru project and it appears that they have a highly reduced load ability when compared to the ones I listed. Did I read that right?

[dgwatson]

SatanKlawz,
The brgs that I selected are rated in kN (kilo Newtons). The chart that you provided for your bearings are in Kilograms. The conversion is approx 1 kg = 9.81 N. Regardless of the units if your catalog uses Kg then you should use kg in your calculations. Also the brgs that I used are easier and less forgiving when It comes to mounting and alignment. I did not look up your brgs, I do not know what the proper method for mounting are. All linear brgs have documentation on how to use them. This is important information to ensure long life of the brgs. When I selected these brgs, they were less expensive (aprox 2 years ago). I also added the plate that you mentioned. It was to allow the Z slide to be module making it easier to assemble and play with when setting up the drives.
DGW