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View Full Version : Linear bearing spacing & Sizing linear rails?

fyffe555
12-07-2004, 12:35 PM
Hi all,

Does anyone have any suggestions or links to methods of calculating the spacing required between linear bearings on each rail on an axis?

That is, if you have an X axis of, say, 24" with a rail on either end, and using commercial linear bearings, what is the optimum distance between the two bearings on each X axis slide? If you have the distance too small then there's greater risk of binding, too far and the machine get's too big for a given cut area. Make sense? So how do you decide what that distance should be?

I've some information on Igus Drylin T and THK Linear slides. Working out the loads and dimensions for a prospective 36"x36"x6" cut area machine with a 3.25 router etc etc the info suggests a bearing spacing on an X rail of 14-18" for Drylin T 20mm and 10-12" for the equivalent load size THK.

From the machines I've seen here people are using less spacing. From some commercial machines, they use significantly less. The few high end vendors appear to use wider spacing more in line with my 'guesstimate'

Any thoughts?

Anyone comment on the use of Igus Drylin T or W?

cheers,

Andrew

ViperTX
12-07-2004, 01:37 PM
If it's a gantry set-up then the taller the gantry the wider the spacing between the bearings on each rail of the y-axis. It's a basic question of counteracting any torsional force created by the rapid start, acceleration and stopping of the gantry. If you have selected the correct size of rail / bearing to overcome this torsional force then the only binding that may occur will be due to improper alignment of the rails.

Also decide which axis is which...with you facing the front of the machine...x is generally the axis for the right/left movement, y-axis is the front to back and z is the up/down movement.

I would agree that for most gantry's that we as hobbyist use the outside measurement for y-axis bearing is 8 to 10 inches.

ger21
12-07-2004, 01:57 PM
I think you'd have a pretty hard time getting THK's to bind, even if you had them right together.

buscht
12-07-2004, 02:09 PM
I agree with Gerry on this. The reason to spread out the THK's seems to be more for table support than worrying about binding.

The IGUS people gave me these rules to go by. The spread on the bearings cannot be any less than 1/2 the distance between the rails. So if your rails are 36" apart, the bearings need to be 18" apart, more is better. This is to prevent binding.

I looked into using the Drylin bearings and decided against it. I bought THK and IKO rails on EBAY and have been very happy.