A question on linear blocks

[Drools]

I was looking over the specs for my THK HSR25 blocks and there are several numbers that I don't understand. Maybe someone here can set me straight.
1. what is this "Static permissible moment kN-m"
2. "Basic load rating" is much higher than the above rating. What do both ratings mean?
kN-m does not mean much to me, I should convert it to ______.

Thanks for the help.

Similar Threads:

• Want to buy linear rails for IKO LRX15 blocks
• IKO linear blocks, no rails
• Abba linear Blocks
• Need Help!- Where to get Linear rails and blocks?
• Linear Pillar Blocks

[jsheerin]

Static means stationary - not moving.
A moment is the same thing as torque, or a force applied at a distance from the center of rotation, so it causes the part to try to rotate.
So static permissible moment would be the maximum allowed torque applied to the block when it's not moving.
kN-m should really be written kN*m. It's kilonewton*meters, or 1000 Newton*meters. A newton is a metric unit of force equal to 1kg * 1m/s^2, and obviously a meter is a unit of length. Torque = force * radius that the force is applied at. You could convert that to English units of oz*in or ft*lbs if you wanted.

Basic load rating is probably defined in the THK catalog, but it's likely a load applied to the center of the block (ie, it doesn't create any torque). Usually there would be a horizontal and vertical load rating, although they could be the same value depending on the block design.

Hope that helps.

[Drools]

Thanks for the help jsheerin.
I found this thread kN/m = lbf/ft - OnlineConversion Forums It discusses the conversion in detail.
What I was trying to is, figure out how much force my design could safely tolerate.
I’m using 4 THK HSR25B blocks on 2 rails for my Y axis.
I’m using 4 THK HSR25A blocks on 2 rails for my X axis.
I’m using 4 THK SHS25C blocks on 2 rails for my Z axis.
Each axis will have its own rating. Details of each block can be found here LM Guide [ THK ]
I included the basic details in the pictures below. Even more detail can be found it the datasheets on each block. Static permissible loads are much lower are much lower than the Basic Load Rating.
A question since I’m using 2 rails per axis could I effectively double the rating?
Ex: As per the last picture, Ma with double blocks is 1.71kN*m since I have rails it will be 3.42kN*m which converts to 2522.5lbf*ft.

Thanks

[jsheerin]

If I look at the HSR25A here: https://tech.thk.com/en/products/thk...th.php?id=1137
The basic load ratings in all the directions are 19.9kN dynamic (moving) and 34.4kN static. Then if I look at the catalog page here: https://tech.thk.com/en/products/pdfs/en_a01_212.pdf the basic load rating given there is the same, so I'm not sure exactly what you're asking about... If you are comparing the load ratings to the moment rating, those are two different measurements and it would be expected that they'd be different.

The moment with two blocks assumes they're right next to each other, I believe. If you'll be spacing them apart as most people do, the moment your carriage can tolerate will be higher. Look at the example calculations in the THK catalog. I did not specifically use the moment values - I reduced all loads applied to the carriage to loads on the bearings. If you had 1 carriage by itself, the moment rating might be important. If you have 4 blocks at the corners of a plate, then a moment applied to the plate is probably better represented as loads applied to the blocks, as they'd see the load as uniform along their length depending on how far from the center of the carriage they were.

The process I used when I was working on a big design, as outlined in the THK catalog, was to calculate the loads on all the bearings on a carriage (4, for a typical example) including all moment arms and inertia forces due to acceleration, and calculate a safety factor for each bearing compared to the dynamic load rating. THK says the safety factor for a bearing should be between 2.5 and 7 for a machine tool with vibration. This is probably because there could be momentary loads which are much higher than the typical load and this safety factor will prevent you from brinelling the rails (putting dents in them from the balls in the blocks). Then I took the minimum safety factor to calculate the expected life. The life is measured in distance traveled. If you go with higher loads, the life will be shorter.

The other half of the picture is what the deflection of the bearings will be under load and what stiffness that gives for the machine. There's stiffness data from THK in my mill thread if you search for it.