sagging shafts

[smarbaga]

i have 4 thompson SPB-16-ADJ pillow blocks, 1 inch shafts ( 2 per shaft ).
these blocks are for unsupported shafts.
what would be the maximum length of shaft i should get before the shaft starts to sag. ( say with a 100 pound load ).
will i be able to use longer shafts if i use 2 shaft supports at each end, say 3 to 4 inches apart, before sagging.
thank you

[Scott_bob]

Yeh, Shafts are best when they don't sag...

[ViperTX]

Go to the thompson industries website for a deflection or sag calculator.

No, you would be better off using a shaft that was supported along it's length and open bearings. You'll quickly learn why most of us are leaning towards linear rails and even they need to be supported.

[smarbaga]

i have been to the dannaher sight
no calcuator and no reply to my e_mails
these blocks are what i have to work with
i have been looking for an answer to this question for over 2 months now.

[Halfnutz]

Well, at least its the 16 stuff. That 1" is some heavy duty material. Its all a matter of span and load, of course, I know Ive seen specs somewhere, maybe McCarrs has something equivelant or, yes I know, THK and IKO both have comparable assemblies, try them.

[vladdy]

If you're using , say, 1040 cr [30,000] an eight foot rod with supports two inches from each end, and distributed load of 50lbs [one bar] across the center eight inches will bend about 0.38 inches down..

and yes, two supports at each end will help, primarily in reducing a 'tilt' effect on the support, which will reduce rod deflection to some extent..

ideally, a weight of 100 lbs on two bars, even 4 feet long, 'should' be done with a supported rod..

hard to give a better answer 'off the top' without

not sure if this the answer you were looking for, but??

[smarbaga]

i am thinking of a 36" travel on 1" thompson ( or equal) shafts with 2 supports at each end about 3-4" apart.
100 pounds might be high... i really don't know what my z axis will weigh
mostly for cutting wood, plastic and maybe aluminum
probably more like 25 lbs, i always like a little elbow room,
i guess 50 lbs is a big enough elbow

[OCNC]

Originally Posted by smarbaga i am thinking of a 36" travel on 1" thompson ( or equal) shafts with 2 supports at each end about 3-4" apart. 100 pounds might be high... i really don't know what my z axis will weigh mostly for cutting wood, plastic and maybe aluminum probably more like 25 lbs, i always like a little elbow room, i guess 50 lbs is a big enough elbow

I ran the following numbers through a program that computes the deflection for a simply supported beam. I assummed a 1" solid steel shaft with an E of 30 x 10^6 psi. The support points were pinned at one end and rolling at the other. The spacing between supports was 36". A load of 50 lbs. was placed midway between the supports. Under these conditions the deflection at the load was .033 in. Fixed supports and double supports would reduce this further but the analysis is of the indeterminate type which the program can't handle. Let me know if you want to run any other scenarios.

[OCNC]

Originally Posted by OCNC Fixed supports and double supports would reduce this further but the analysis is of the indeterminate type which the program can't handle. Let me know if you want to run any other scenarios.

After my previous post I remembered a more sophisticated beam analysis program called Beam 2D. I downloaded it and entered your shaft using the previous criteria except I added an additional support at each end set 4" inside of the outboard supports. These were all pinned supports so your actual supports are still adding additional stiffness. Under these conditions the deflection was reduced to .00489 in. (a third of a 64th). Beam 2D seems pretty simple to use and gives you 30 full use sessions before it is crippled. Let me know if you want me to run any other scenarios.

[smarbaga]

thank you ocnc for that valuable (to me) info.
i supposr with 2 parallel shafts the sag will be half .0025 or so.
i think this number will be more that adiquate for what i do.
i downloaded beam 2d and am about to plug some # .
http://www.download.com/3001-2054_4-10329541.html?idl=n
i had know idea that would sag so much in that short of a distance.
i will buy two 48" shafts and double support them at each end 4" apart.
now it not a shot in the dark,
thanks again

[OCNC]

Originally Posted by smarbaga thank you ocnc for that valuable (to me) info. i supposr with 2 parallel shafts the sag will be half .0025 or so. i think this number will be more that adiquate for what i do. i downloaded beam 2d and am about to plug some # . http://www.download.com/3001-2054_4-10329541.html?idl=n i had know idea that would sag so much in that short of a distance. i will buy two 48" shafts and double support them at each end 4" apart. now it not a shot in the dark, thanks again

If you lengthen the shaft to 48" the deflection will increase to .0129, almost a 64th.