Spider Gang Cultivator Caster Design 2

[Lonjevity Farms]

Hello forum, I posted this in the Agricultural forum but it seems nothing ever gets posted there. I am working to improve our 3 point spider cultivator and want to weld up a pair of casters to mount on it. The implement weighs approximately 3000 lbs. and mounts to the 3 point hitch of our tractor. I have the arm design worked out but wanted some input on the pivot section. Looking at zero turn mower pivot tubes as an example, they typically use bushings and/or bearing combinations. Do you think that this product would work with a 1 1/4" shaft pressed fit into schedule 80 tube?

https://www.mcmaster.com/#2938t63/=16hi5ll.

These bushings have quite a large dynamic load capacity.

I have attached a few pics of the design from Solidworks. The square tubing is 3 x 3 x 1/4 wall A36 and the shaft is 1 1/4" CR 1020 steel. The pivot tube is X Strong Sch 80 1 1/2" x 1 3/4" long pipe to press the bushings and shaft into. The shaft has a 1/2-20 tapped end for the bolt to apply the necessary preload. The shaft is welded to the square tube via a through hole. The hub and spindle are rated for 1750lbs, two casters total 3500 lbs capacity. Not being familiar with typical tapered roller bearing applications do manufacturers machine out the pivot tube for the bearings or use stock material and press fit?

I could also use these as well but there is significant price difference versus the bushings. Also from a maintenance point the bushings seem a better fit.

https://www.mcmaster.com/#5709k86/=16hjkkm

Overview image of caster design

Cross section of end bolt, shaft, pivot tube, and bushings; one on top and one on bottom.

FEM of caster shaft showing displacement. von Mises stress is well within FOS.

Thank you for any suggestions or information.

 

[grunt58]

I would like to see the SolidWorks files. I've been following this thread, it's been pretty interesting. Can people message on here? I don't want to post my email address.

 

[Lnewqban]

Lnewqban,

As far as the heat on the spindle from welding, don't trailer companies weld the spindles into round and square tube for the axles with no problems with bearing fitment?

As jgKRI has well explained above, I was referring to the hub.
Predicting the deformation of the surfaces to take the bearings is as difficult as to achieve all hub-frame joints to be welded exactly alike.

I am not criticizing your idea of the bearings, just transferring my practical experience with similar assemblies:

1) The outer ring of the bearing should not rotate or slide within the hub. For the interference fit between bearing and hub, the allowance per inch of diameter usually ranges from 0.001 inch to 0.0025 inch. Anything bigger than that makes installation very difficult. Heat, steel and tight dimensions don't mix well.

2) For high loads and little rotation, flat bearings work longer because they have a big surface. Ball bearings transfer loads through several points and roller bearings through lines. Because dynamic lubrication is marginal and pressures huge, the tracks tend to develop pitting or/and valleys after some time.

3) Those pressures can spike and quickly destroy the bearings when the two cavities for the bearings are not perfectly aligned and concentric. Machining must be precise regarding that; making a mandrel is recommended.


"God will not look you over for medals, degrees or diplomas, but for scars." - Elbert Hubbard

 

[Lonjevity Farms]

Thanks jgKRI. Does the caster pivot spindle look satisfactory with the changes I made?

Grunt58, I am not sure how to send you a message. My email is colddainhell@gmail.com if you want to send an email to me first. What version of SW do you have? I am version 2016 SP2.

 

[Lonjevity Farms]

I am not criticizing your idea of the bearings, just transferring my practical experience with similar assemblies:

I know you are not criticizing and I hope you keep the recommendations coming. Are the flat bearings you are referring to the ones they use in casters?

 

[Lnewqban]

Sorry, I call flat the plain type bearing, consisting of just a bearing surface and no rolling elements.
It could be cylindrical a thrust bearing combination.

"God will not look you over for medals, degrees or diplomas, but for scars." - Elbert Hubbard

 

[SwinnyGG]

Yes, I think the designs you have come up with look pretty reasonable.

Just make sure:

1) You're accounting for dynamic loading
2) You're sizing bearings based on their STATIC load ratngs. This is not a dynamic application (no constant rotation).

Just make sure your forces all check out and I think you're ok.

 

[gruntguru]

You could use stub axles and hubs from a wrecked car or truck. Don't you have a couple of those rusting away somewhere on the farm?

je suis charlie