Machined Parts: No Clearance

[Rod Sheridan]

David, did you buy a precision axle or merely a piece of cold rolled round steel?

Cold Rolled Round often isn't accurate enough to be used in a bearing...............Rod.

[David Winer]

David, did you buy a precision axle or merely a piece of cold rolled round steel?

Cold Rolled Round often isn't accurate enough to be used in a bearing...............Rod.

FIRST, I want to thank everyone who responded with advice about a matter that I got myself into from ignorance. I didn't know enough even to state that the axle and pillow block type bearings were not designed to be mated. I purchased them from different sources on the internet. The shaft is stainless steel. My plan for the cart has been to make the axle go from wheel to wheel instead of using short pieces for an axle on each wheel. And this scheme means that the bearings would need to be put on after the cart's wooden frame is finished with full length axle in place. (I see that it all might not work out the intended way.)

You all have come up with several approaches that I may use, and may indeed use some of the ideas in combination. One thing I have gleaned is that abrading the shaft to fit for this application does not involve precise fitting in either dimension or roundness. So "sanding" could end up a dirty way to get 'er done if more sophisticated ways are not practical. Taking the problem to a machinist (one of the suggestions) sounds like the most straight forward.

Further ideas are welcome. This forum is a gold mine.

[Ronald Blue]

Ian and Ellery are correct. The issue is with your axle. As Ian said look closely for a nick or ding or even possibly a swell on the end. If you can measure it accurately check it's diameter. You can easily remove excess stock with emery cloth in the 150-200 range. If you work your way around evenly you won't get it out of round enough to bother anything. After all it's just an axle on a hand cart. Another thing to watch is if the clearance is tight if you get even slightly cocked it will lock on the shaft. Also if it starts on when you remove it if there is a ding or swell at fault it will be shiny at that contact point. You should only need to go in a few inches on each end unless this is different than I envision. I am not so sure the forstner bit is a good way to gauge the bearing size.

[Greg Urwiller]

Another thing to consider: I assume your wheels will spin freely on the axle? Otherwise your cart won't turn well if the wheels are locked onto the shaft. When turning your cart each wheel is going to need to rotate at a different speed just like your vehicle axle/differential. If so then you don't need the axle bearings. The axle can be locked solid to the cart by other means rather than a bearing. Or am I looking at this wrong?

[David Winer]

"You can easily remove excess stock with emery cloth in the 150-200 range. If you work your way around evenly you won't get it out of round enough to bother anything. After all it's just an axle on a hand cart. Another thing to watch is if the clearance is tight if you get even slightly cocked it will lock on the shaft. Also if it starts on when you remove it if there is a ding or swell at fault it will be shiny at that contact point. You should only need to go in a few inches on each end unless this is different than I envision."
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Now that I have a better understanding of the precision needed in this case I am leaning toward a trial with your specific grit (150-200). This should bring down the axle diameter enough to slide readily. Mating of axle to the inside diameter of the bearing does not need to be exact (you all assure me). Once the pillow blocks are attached to the wood frame the inner-bearings will be tightened to the axle and the axle will then turn within the pillow blocks on their races.

[David Winer]

Another thing to consider: I assume your wheels will spin freely on the axle? Otherwise your cart won't turn well if the wheels are locked onto the shaft. When turning your cart each wheel is going to need to rotate at a different speed just like your vehicle axle/differential. If so then you don't need the axle bearings. The axle can be locked solid to the cart by other means rather than a bearing. Or am I looking at this wrong?

You are correct in pointing out that the axle bearings are not actually needed. But as you say, locking the axle to the cart "by other means" raises other design problems. Use of pillow block bearings seemed straight-forward until I ran into the problem of machined surfaces tolerances.

[David Winer]

David, did you buy a precision axle or merely a piece of cold rolled round steel?

Cold Rolled Round often isn't accurate enough to be used in a bearing...............Rod.

Rod's comment seems to address the problem on the nose.

My attempts to reduce axle diameter by abrasives were hopeless--minutes of work with coarse grits have not allowed an end of the axle to fit into the bearing, both "nominal" 3/4 inch.

The wheels arrived by UPS. Lo and behold, they have built-in bearings and fit my stainless steel rod very nicely. So, in good ol' American boy tradition I will drop back three paces and punt. I am changing the cart design to eliminate pillow block bearings, and am now going to mount the wheels on the axle simply with stainless washers and cotter pins. This scheme will also allow the wheels to turn independently to avoid skidding during tight turns, a possible problem mentioned in one comment.

When the project is finished I will send a picture of the cart to Sawmill Creek. Everyone's comments have been most helpful.