View source for Jamming of linear slides

the opening/flexing of the table slides could be caused by a fairly well-known mechanical engineering problem. It's called jamming 
of linear slides.  

That's alot of words - here's a picture:
[http://www.maelabs.ucsd.edu/mae3/Assignments/slider/Slider_FBD_Problem.htm Imagine the sketch rotated 90degrees clockwise]

'''Some good news - there is a way to fix the problem:'''<br/>
Make the ratio of H to W as small as possible.  So, make the bearing surface longer, or move the centre of mass closer to the bearing surface.  The bad news is that this will use up space for vertical travel, or reduce the table size (or maybe both) - but engineering is all about compromises....

'''An experiment to demonstrate:'''<br/>
Find a ruler, and a table that sits against a wall.  Put the ruler flat on the table so that the long edge touches the wall.  Now push the ruler in a direction parallel to the wall, from a point halfway along the short edge - it will move along the wall fairly nicely.  <br/>
Next turn the ruler 90degrees, so that the short edge is touching the wall.  Again, push the ruler along the wall from a point halfway along the long edge.  This time the ruler will rotate away from the wall.<br/>
This is what causes jamming.  <br/>
If the ruler is 10" long and 1" wide, the ratio is 1:10 in the first case, and 10:1 in the second.  

In general, a good rule of thumb (used by manufacuting machine designers who think they are too busy to work out the maths - yes 
i've been there) is that H:W should be 1:2 maximum.  With very smooth surfaces, and very rigid systems you may get away with less bearing support (1:1.5).  With rough surfaces, and loose machines more support might be required (1:3?).  

Finally, it is true that reclining the machine back fixes the problem (but not the root cause of the problem).  Reclining artificially reduces the distance h, because F is taken from the centre of mass of the table, and always points vertically downwards.  However, reclining the machine back may also introduce new problems:
* Potential additional friction
* Difficult/dangerous work setup on 45degree table
* Reduced effective table mass/rigidity

[[Category:MultiMachine]]