Steel Rod

[georgebarr]

I am considering using Precision Polished Steel Rods as a linear guide since it is cheaper than THK or Hiwin slides. I plan to just support the ends of the rods and not provide any support in the middle like some bearings do. My question is, how can I determine the amount of weight the rod will support before bending say 0.01" in the middle? What is this type of mechanical measurement or calculation that is used here? Is it called Deflection? Are there software or references that can tell me how much weight a 1" dia. Steel Rod will hold before bending a certain amount?

Also, the Rod supports are expensive. Is there a cheaper alternative to support the rods?

Are there cheaper sources than Hiwin.com for sliding rails?

Thanks,

[ger21]

Try this. http://www.geocities.com/richgetze/

[jeffs555]

I use BeamBoy, that Gerry posted the link, quite a bit. A 1" dia steel rod weighs about .9lb per linear inch, so be sure to add a distributed load for this amount. A lot depends on how rigidly the ends of the rod are supported. A 1" dia steel rod 2 ft long with simple end supports will deflect 0.01" with a 35lb load in the middle. Using cantilever end supports, it will take nearly 200lbs to deflect it 0.01".

Another free program that comes in handy is Engineering Power Tools. http://www.pwr-tools.com/
It has a lot of tables of material properties and many engineering calculators built in. It will do deflection calculations, but not as easily as BeamBoy.

Jeff

[andy_ck87028]

Hi

I too was concerned about deflection caused by weight however the feedback I received was that sideways forces can be greater than downward pressure (dependent upon material being cut, router mass and rate of router deceleration, etc, etc).

Consequently, I have since worked to the assumption that I have to incorporate additional support apart from at the two ends. My approach has been to use trucks that are supported by abec bearings. Those bearings leave the underside of the rail free thus allowing support points under the rail.

I am going to use araldite to secure the supports to the underside of the rail and if that doesn't work, I will tap a shallow hole. The support will be nothing more than bolts secured to the frame.

Hope that helps but remember its still theory for me as well at this point

Andy

[ger21]

Depending on the size of the machine, I'd try to design for no deflection with at least a 30lb load for a smaller machine (2x3), maybe even 50lb. This is in addition to the weight of the gantry.

[fyffe555]

George, What bearings will you be running on this rod?

If it's ball linear bearings like thomson then be careful of both the hardness and sizing tolerance, have a look at thomson shafting as its not much more than most other precision rod stock.

Note that Precision rod spec is +/- the nominal where proper linear shafting is -. For hardness thomson shafting (mcmaster.com page 958) is rc 60-65. Commonly available Drill rod is c56-60 hardened. Unhardened it's around 45-50. Too soft. Unhardened precsion rod is usually around rc25.

Anything too soft or too tight a fit due to tolerance can gall, score or otherwise damage the rod and the bearing and generally annoy you later. Don't ask me how I know that.

Note also theres some general purpose rod with higher rockwells in the 70's. Problem is the tolerance both on diameter and roundness doesn't fit linear ball bearings and so they're usually listed not suitable.

best bit is that Thomson shafting isn't much more than most precision rods so I'd suggest a look first.