Jonathan

23-11-2010, 03:49 PM

I've decided on my router that I'm going to use supported linear bearing rails on the X-axis. Two parallel rails, with two bearings on each fixed to the gantry.

Let:

x = distace between two bearings

F = horizontal cutting force parallel to X axis

m = mass of gantry

Z = distance from cutter to centre point of Z axis linear bearings.

z = distance from X linear bearings to X axis leadscrew.

Using those I've arrived at the follwing formula (by taking moments and resolving the forces) for the force on a single bearing:

Max value = m*g/4 + F/(2x)*(Z+z)

Min value = m*g/4 - F/(2x)*(Z+z)

Which shows the force on the bearings could act upwards if the Z axis is long enough, or the bearing spacing is small.

My question is, have I mised anything here in the formulas and what would one normally chose for x in relation to Z:question:

If I put the numbers in for my router (x=0.3, z=0.5 ish), cutting steel, then I get -50N to 200N. Well within the 980N/-340N rating for 25mm bearings...and within rating even for 12mm bearings. I'm not sure which to go for. Two 25mm rails, or maybe get 4 rails and have 16mm...:question:

Let:

x = distace between two bearings

F = horizontal cutting force parallel to X axis

m = mass of gantry

Z = distance from cutter to centre point of Z axis linear bearings.

z = distance from X linear bearings to X axis leadscrew.

Using those I've arrived at the follwing formula (by taking moments and resolving the forces) for the force on a single bearing:

Max value = m*g/4 + F/(2x)*(Z+z)

Min value = m*g/4 - F/(2x)*(Z+z)

Which shows the force on the bearings could act upwards if the Z axis is long enough, or the bearing spacing is small.

My question is, have I mised anything here in the formulas and what would one normally chose for x in relation to Z:question:

If I put the numbers in for my router (x=0.3, z=0.5 ish), cutting steel, then I get -50N to 200N. Well within the 980N/-340N rating for 25mm bearings...and within rating even for 12mm bearings. I'm not sure which to go for. Two 25mm rails, or maybe get 4 rails and have 16mm...:question: