Guide Shaft Assembly Problem

[utson]

Hi everyone,

We want to mount these yellow guide shafts which have 2 meter length to the pink base.( Both of them are steel ) They are carrying linear bearing which can be traveled all through the shaft, so that, it's not possible to use a fixing instrument except the edges of the shaft ( by the way, it's no seen on the picture but the other edge of the shaft is also same. ) Both shafts should be perfectly vertical, hence parallel to each other. Lastly we can reach to bottom side of the base plate ( in case of using an instrument from there.)

So to achieve this precise assembly, which instruments you can suggest to me? And what techniques i can use?

Thanks in advance..

 

[MikeHalloran]

You cannot make two shafts perfectly parallel, so if you are planning to have some sort of rigid carriage ride on the linear bearings, the carriage will bind, and you will fail.

The trick is to make the carriage relatively insensitive to center distance between the shafts, e.g. by providing flexibility or articulation in that direction.

Additionally, you cannot make the shafts perfectly coplanar either; there will always be some twist in the carriage's trajectory. The usual solution for that problem is to make one of the linear bearings substantially shorter than the other, so the carriage is effectively riding on three bearings.

Note also that you can buy linear bearings that don't completely surround the shafts, so you can use relatively thin shafts and have them continuously supported by an adjacent column of arbitrary size and stiffness.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

You only need to align the shaft axes and bearing bores to within the net radial clearance between the shaft ODs and bearing IDs. For example, if there is a net radial clearance of .002" between the bearing IDs and shaft ODs over the plate length/width under the worst case conditions of shaft bending and bearing installation misalignment, then that's what you have to work with when rigging the shafts. They don't need perfect alignment, just good enough alignment. As for an adjustment mechanism, I would suggest an eccentric bushing at each shaft end that can be rotated to precisely adjust radial position and then be clamped in place.

As MikeHalloran points out, in practice your bearing system is over-constrained. One shaft and bearing set should provide 2 rotational and 2 translational constraints, and the other bearing set should have one less translational constraint (float) in the plane defined by the two shaft axes.

 

[utson]

Thanks Mike,

As you said, we are trying to built a system to guide the load( actually it's DropTower to make an impact test.) First times, we were also thinking about using that linear bearings which are not completely surrounded but the bearings will be in a falling weight unit like in the document. Do you think it will bring about adverse side affects if we use them in a system like that. If yes, it can be logical to use 2 different sized linear bearings. Last ditch, it can be employed 2 linear bearings more one under the other on the guide shaft with some spacings to prevent binding.

 

[utson]

Hi Tbuelna

I saw some applications of eccentric bushings to correct misalignments on lathes. But really, i couldn't imagine that they can be useful in that application. I think, they can be a good way to have best alignment while doing the assembly. Thank you very much.

 

[Tmoose]

Each Motorcycle "triple clamp" has 3 holes.

http://cdn.motocross.transworld.net...iles/2012/03/KTM_Triple_Clamps_Hi_Res_1_.jpeg

The large pair at the ends support the fork tubes, which must be pretty parallel if the forks ( using plain bearings with a few 0.001s " clearance) are not to bind .

As shown starting 4;00 here -

https://www.youtube.com/watch?v=FKZQkf56MPE

Boring both at once would help keep the holes equidistant and parallel.

 

[utson]

Yes Tmoose,
We were thinking exactly the same thing to drill the mounting holes for upper and lower bases to which we need to assemble guide shaft!