hardened shaft vs unsupported length

[project5k]

ok, so i have resigned myself to building a smaller machine that i had originally wanted... its just a $$ thing.. so, now rather than using the angle iron and rollers idea, im considering using the hardened round rods and linear bearings... but what size rod do i use for what lentgh of travel when the shaft is only supported at the 2 ends? obviously the bigger shaft in a short span is stronger, but what do you consider to be the minimum diameter for say a 2 ft span, 3ft? 4ft?5ft? 16mm? 20mm? 25mm?

I'm sitting here looking at the vxb website, trying to decide what i should compare for prices against any other sources...

[project5k]

36 sets of eyes and no one has an opinion?

[dertsap]

i was in the thrift shop the othe day and saw a weight set , and it got me thinking about how good that type of bar would work if a guy cleaned it up , flex wouldn t be a problem over a long distance

[fkaCarel]

Supported is when you calculated the assembly 1/6*B*H^2, where unsupported rods are only 1/10*D^3. And for the rods you should also take in consideration the momentum on the mounting points. But this is of course just an opinion.

[Guldberg]

I would'n do my 1.2m 25mm stainless unsupported, but thats just me:-)

[dertsap]

vibration would be a concern over a long distance 3/4 drill rod over 3' should be fine , my first table used drill rod over the 2' width of the gantry ,and it is solid and doesn' t sag

[Tom Brown]

Originally Posted by Guldberg I would'n do my 1.2m 25mm stainless unsupported, but thats just me:-)

That's why the skate bearing solution is so compelling. You can use tubing and support it across the whole span.

[jeffs555]

The deflection of unsupported rod is much higher than most people would think. A 3 foot piece of 3/4" steel rod with simple supports on each end will deflect .02 inches for each 10 lbs of load. It will sag .006 inches just from its own weight. You can get the freeware program beamboy to calculate deflections for various loads with different size shafts.

[Mcgyver]

i think the initial lack of response isn't cuz people don't want to help, its because you're asking the wrong thing, no one can tell you the size unless you specify how much deflection you are willing to live with. determine how much weight it is supporting and how much deflection is acceptable to you, then use a progy like beam boy to figure out the dia you need.

[dertsap]

Originally Posted by jeffs555 The deflection of unsupported rod is much higher than most people would think. A 3 foot piece of 3/4" steel rod with simple supports on each end will deflect .02 inches for each 10 lbs of load. It will sag .006 inches just from its own weight. You can get the freeware program beamboy to calculate deflections for various loads with different size shafts.

are you referring to low or high carbon steel , high chromium ,tungsten type , moly ,d2 , t1 ? what about heat treat ?

[Geof]

Originally Posted by dertsap are you referring to low or high carbon steel , high chromium ,tungsten type , moly ,d2 , t1 ? what about heat treat ?

Doesn't matter much; the Elastic Modulus, E, which is a measure of the deflection under load is nearly the same for all steels. It also does not matter whether it is heat treated or annealed; heat treated only means it will deform to a greater amount elastically but it does not alter the amount of elastic deformation for a given load.

[dertsap]

i'm genuinly surprised , when i crossed my gantry rails from 10-18 to drill rod , it had made a difference , when it sat for days with the 10-18 the y axis would bind up ,so i would roll the rods to a different position , i haven't needed to do that with the drill rod