Minature Linear Slide Motion Problems

[sydneydog]

Long time "looker", first time "poster".
I am currently having trouble with the motion on a minature linear slide (less than 1" long) used to control the motion of cantilevered arm of small mass. The motion of the slide is affected when we torque the arm down to the threaded holes on the top of the bearing cage with M016 caps screws- the greater the torque, the greater the impact to the motion to the point it seizes the bearing cage on the rail. The application requires a minimum torque to prevent vibrational loosening. Thread locker is not an option. I've considered looking at alternate fastener material. Any other suggestions?

 

[CorBlimeyLimey]

What is causing the distortion? Mis-alignment, non-flat face contact, ???

Are you using an off-the-shelf linear slide? Which one?

A sketch or photo would help.

 

[desertfox]

Hi sydneydog

As Corblimey says a sketch or drawing would be helpful in understanding your problem.

desertfox

 

[sydneydog]

I've attached a model.

locating surfaces are tightly controlled for flatness. Bearing motion is great until torque is applied. We speculate we are distorting the bearing cage in some way - perhaps as the threads elongate?

http://files.engineering.com/getfil...4-af18-47f5-bbed-e9daba5a8512&file=slide1.JPG

 

[dvd]

Are your screws the correct length?
What is the rail fastened to?
Are you torquing the screws while holding the bracket or the linear slide block?
Are you matching/exceeding the bearing manufacturer's recommended tolerances?
Using the recommended number of fasteners on bearing and rail?
Have you called the bearing supplier?
Tried a different bearing?

 

[DesignerGuy16]

With that kind of size you're probably way out of the vendor specifications on side loading the rail.

It probably doesn't take much torque to sieze as you say because of the size.

Is there any way to redesign to center the load under the rail car? That may only be an improvement, but still may be out of the inertia range on the slide. The other options are to do the above and see if you can upsize, or add another rail to offset the loading. Depending on your space constraints you should hopefully be able to come up with something slick to remove the large offset loading.

Most of those rails are designed for a load to be directly above the car and not off to the side and underneath it.

James Spisich
Design Engineer, CSWP

 

[automationbabe]

Look up the manufacturer's specifications for this rail. A lot of manufacturer's have preferred methods for installing their rails to avoid this problem you are having. They vary from manf.-to-manf., and vary based on the type of rail you are using as well.

What you are experiencing is pretty typical for this application if installed wrong. I recommend talking to your rail manf. for additional info.

 

[vc66]

The slide is OK when the screws are threaded in there with a little bit of torque? And it only fails when you torque the screws down to prevent loosening?

If that's the case, I'm not sure it has to do with the moment on the cart. You'd have problems right from the start.

As dvd suggested, make sure your screws are the correct length.

Is this a stock rail and cart? If so, it would behoove you to call the manufacturer.

V

 

[BobM3]

The bolts "look" big for that slide. The top plate of the slide may be distoring. Also I see your fixture has a step in it near the side of the top plate. Could fastening it down be putting a load into the slide plate near that step? Again, that may distort the top plate (or the side) of the slide.

 

[CorBlimeyLimey]

I'm thinking the same as BobM3. As a test, place a shim or thin washers between the slide block and the cantilevered arm, and then see what happens when the screws are tightened.

 

[IRstuff]

There are two possibilities:

> Either the bolts are too long, in which case, you should see gouge marks from the bolt protruding onto the rail underneath. I would saw one of the arms to remove the load, and bolt to the slider and see how it feels.

> Your attachment may not be perfectly flat with the slider mounting surface, and torqueing the bolts may distort the slider to the point that it's off its bearing groove. The lip feature shown in your model is also suspect. If the slider lip does not mate perfectly with the corresponding feature on the arm, then torqueing the bolt will bend the the slider or the mount or both.


TTFN

FAQ731-376

 

[SincoTC]

Along the lines of what CBL said, put a couple of washers between the mounting plate and the bearing cart, they don't have to be thin, but should be the same thickness, a reasonable fit on the bolt and not too big on the OD, this will ensure that the pull from the fastener is concentrated around the holes and the cart will not be distorted if the surfaces are not flat enough. If you can now torque it up and it runs OK, then either the cart was being distorted by concave faces or ridges near the edges and pinching the rail, or the bolts were too long and thus binding on the top of the rail. If it runs but not as free as you expect, try supporting the offset weight with a finger and see if there's any difference.

Hope this helps.

 

[MintJulep]

What happens if only one screw is tightened?

 

[alansimpson]

Sounds like frame is distorting slide when screws are tightened. If interfaces between slide and body are not flat enough or there is some dirt or in sufficient clearance at corners the slide will be pulled out of shape when screws torqued up. Only a few microns could do it.

Check flatness of frame mounted to slide. Just a scratch will make a difference. A flat grinding stone will remove most of the defects.

A more forgiving slide like Del-Tron could also help.

Shake proof washers like Nord-Lock

http://www.nord-lock.com/default.asp?url=51.16.37

If they come small enough