long shaft design 1

[gida2]

I’m trying to design a 20-foot shaft to roll up 100 feet of tarp. The shaft would be simply supported on two pillow block bearings. For a bare bones budget the shaft would be hand cranked (very low use ~ 60-80 rpm’s and used maybe once a day).

For a preliminary design I came up with a (SCH 40) 2.5-inch steel pipe with a 10-inch PVC pipe on the outside. The PVC pipe would be bolted to (QTY-3) 1-inch steel flanges (cut out to reduce weight).

My concern is (based on a fixed-simply supported beam calculation) the pillow block bearings would be taking moment loads of around 730 in-#’s along with a 150 # normal force.

Should I be looking at a different type of support like a loose fit pipe hanger wrapped with Teflon or Delrin? Timken makes a spherical (4 bolted) pillow block bearing which could probably handle these loads but would be too expensive for this application.

Any advice on long shaft design would be appreciated.

 

[GregLocock]

Use some sort of sleeve bearing, make sure it can rotate around the normal axis to acommodate the bending in the shaft. I think your speed/load calcs may be a bit optimistic - that's a couple of horsepower.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dvd]

You might look at a bearing arrangement using crowned yoke rollers and forego the 2.5" pipe. Two yoke rollers on the lower third of the pipe and a hinged cap with two (one would work) above would be good. I use McGill yoke rollers quite often in somewhat similar applications. A fairly small yoke roller (I'd start with 2" so that you weren't fighting rolling resistance too much) would handle your loads, and the crowned face would accommodate the tube deflection.

 

[gida2]

Thanks for the advice - Can you explain the hinged cap part of it – I’m having a hard time visualizing this.

 

[dvd]

In order to get the tube out of the bearing block you have to open it up, thus the hinged cap with the two upper yoke rollers mounted on it. Use something like a swing bolt from Reid Tool opposite the hinge to hold the cap down during use. You can also use crowned cam followers instead of the yoke rollers. Osborn makes a very nice shaft for their yoke rollers that will work with the McGill yoke rollers.

 

[gida2]

I like this idea - Thanks

 

[ccw]

Hi gida2,
I would suggest that you simply support the ~300#? load of tarp and 20 ft. x 10" dia. PVC by the ends for a few weeks to assure that you are not going to get some sag and permanent deformation in the PVC. I am always suspicious of using PVC for structure. Otherwise, you may end up really needing the 2HP that Greg calculated.

 

[Barry1961]

Most pillow blocks have self aligning bearings with a spherical outer race that allows a great deal of movement. I would guess over 20 degrees, which would be much more than your shaft deflection.

Barry1961

 

[gida2]

Thank you all for the advice!

Any ideas on the shaft design?
I figured it has to be roughly 21-feet long (20 feet for the tarp (standard length) + around a foot for bearing supports, etc...).

Most pipe comes in 20-foot lengths max. stiffness/weight should be maximized. The shaft may be hand cranked so the amount of revolutions should be kept to a minimum. An option in the future may be to add a motor or gearing.

Any ideas are appreciated.

 

[MeDuh]

What kind of space/weight/environmental considerations do you have?
My reccomendation, should it be plausible would be to make your spindle at least 4 inches in diameter, and consider your material options more carefully, since your proposed aspect ratio seems to be deflection prone no matter the application. were it me i think i'd be happy with the standard 20ft length with pieces that can be turned, attached at the ends. This would allow you to explore bearing options.