even if they were solid steel bars, there would be enough movement over several
feet that you would be able to pull the sled out an inch or so.
get a good heavy rope or chain, attach it between two cars that are on a hill
(the uphill car in park, the downhill car out of gear)
then pull on the center of the rope/chain and see how far you can move it before
you move the downhill car noticably (and note that you will be able to move it
noticably if you have a good long rope)
this is a similar problem to chain sag, you cannot pull a chain tight enough to
eliminate sag. some back-of-the-envelope numbers
if you have a 20 pound sled, with 5’ of line on each side (10’ total), a 1
degree sag would move the sled about 1", would put 280 pounds of tension on
the lines, and would strech the lines (or move the mounts) only about 0.02" Even
if we had a welded steel frame and brackets, you aren’t going to prevent the
mounts a few inches out from the main support behind the workpiece from flexing
that much.
you just cannot pull tight enough to eliminate sag, the multiplier as you get
closer and closer to zero sag just keeps going up. even solid steel bars will
strech
this isn’t exactly right, but do a chain sag calculation with tension of 4500N
(~1000 pounds), over 3.3m, with a total chain weight of 3Kg/M (~10Kg) and the
chain will sag ~9mm
https://www.spaceagecontrol.com/calccabm.htm?F=4500&a=3.3&q=3&g=9.81&Submit+Button=Calculate
in reality the sag will be a bit more as it would be a point load, not a heavy
chain as this calculates, but I couldn’t quickly find a proper calculator.
realistically, we are not going to get much above 100 pounds of tension, because
the wood will start flexing at that point (we already see it a little bit on the
current machines)