Linear slides or wheels for Cantilevered Load

[TimSchrader2]

Hello

Consider a 20 ft long box beam with 4 Cars (or linear slides) on one end spaced at about 62". So the beam can travel a certain distance. Assume the two most loaded cars take 25,000LB reaction load for each car. The other two cars would be somewhat less.

You could use something similiar to Hevi-Rail wheels and rails by PBC (Pacific bearing) Or thier standard linear slides that come in the pillow blocks. But since the load is way out it breaks this 2:1 rule they have about the drive force (OR LOAD) needing to be within 124" of the cl of the cars. I say that if the drive force is centered along the cars and overcomes the friction it will move the load OK. I have a video of a similiar product that does so on plastic slides, which is, or should follow the same rules of Physics. Of course, if the drive force is too high the slides will bind and friction increase as load increases. The 2:1 rule is based on friction of about .2, so wheels with a very low friction can exceed the 2:1 rule by quite a bit. I have done 8 to 1 with 10,000lbs capacity wheels but I would prefer to just buy the slides and not design a 25,000lb wheel , but I may have too. The hevi rails would work if I doubled them up but that is not the best way unless the wheels are pivoted to share the load equally.

So the question is: has anybody broken the 2:1 rule as far as the load being farther from the 4 sliding cars then the MFG would like(not the drive force).

 

[tmschrader]

HELLO wheels was chosen. In case anyone is curious. Less friction less risk of shiming or motions. Hevi rails can be considered a type of Cam wheel roller and they come with higher toleranced rails that should be more straight then steel off the shelf.

 

[Jboggs]

Good choice. It's generally not good policy to override manufacturers' recommendations. I always picture myself trying to explain to a frustrated customer (or worse yet an opposing attorney in a civil liability trial) that I did that because some guys I never met said it should be ok. Not good.

 

[TimSchrader2]

Hello
I am not sure the point of the post was fully understood.

The below video shows one application where the distance between the sliding plastic guides is roughly 6FT yet it telescopes out to 105FT. Clearly breaking the 2 to 1 rule by a large margin. The ratio of 6Ft to the reach is much more then 2. Maybe 10-14 or more. I have done ratios of about 4 -5 on telescopic sections with plastic sliders. I do not see why this physics should be any different then those sold by PBC or Igus. And neither could they tell me why or what is different.

Note the rule of thumb of 2:1 applies more to the drive force being higher then the plane of the sliders, not the load itself. IMHO and some experience. Although two MFG's policy is to treat both the same.
I believe acceleration plays a major role when applying the 2;1 rule to the load and not the drive force.

In the video the drive force is the wire ropes and it is centered between the plastice guides. At around 3;16 time you can see the plastic guide on the bottom at the end of the truss.

in any case I went with wheels to eliminate some uncertainty.

https://www.youtube.com/watch?v=i3gXL963-ik&list=PLvRZp_kckarc5hRfXe-3qvn2vzps0gmdw