Structural question? Re-purposing Bicycle Crank

[dolorm]

Hi.

Background: I am fabricating a debarker for my band sawmill. A debarker is like a circular saw with 1/4" dado blade riding about a foot in front of the bandsaw blade. It cleans the pathway of debris, mud, sand, etc.. Typically, it more than doubles the life of each bandsaw blade.

Structural evaluation: There is a pivot point on which a 15 pound motor hangs. Since I do not have a lathe yet, and to save money, I am thinking about using the sprocket, crank, and housing off an old bicycle I have on hand.

Would any of you know if this is a functional solution? I like to make stuff, but I do not know how to evaluate repurposed components for structural capabilities. My only evaluation criteria is, when a boy, we used to jump our own little grand canyon and put hundreds of pounds of force on those cranks and they never skipped a beat, ever. Of course, we were not hanging them sideways and hanging 25lbs while we drive down bumpy roads at 50mph either.

https://postimg.cc/PpXkHVWR

 

[matt_i]

Will it work, of course, not known how long.

Keep this in mind, bicycle components are optimized for weight savings, meaning they are not necessarily the most robust and long-lived components.

Something like pillow block bearings would make a robust design. You can use regular old 1018 in good clean condition to make the shaft without a lathe (its usually +/- .002" from what I've found which is well within the range of emery cloth hand-finishing), or W-1 drill rod is ground to a .0005" size, and is still weldable and has slightly more carbon and tensile strength (typically use 48-50ksi)

 

[dolorm]

Thanks for that tip Matt. Pillow blocks and regular shaft sounds very good. I will check on price shortly. Thank you for that idea. -Mike

 

[brianh]

It should work fine the bearings even in a cheap bottom bracket can take a lot of load.

 

[ckcorder]

also check out some of the shafts at mcmaster.com.

They have a variety of different shafts, materials - some cut to size lengths. They will also have almost any of the pillow block bearings you can imagine.

 

[dolorm]

I am going to take a stab at using the crank. If it starts to wear early, I will replace that section of the assembly. It gets about a third of a rotation every bark section of a log. I guess that comes out to maybe 50 rpm per day of operation on a very productive day.

Thank you!

 

[bullnerd]

So its not spinning, just a pivot point?

Keep it sealed if its caged bearing and not the "modern" sealed type. They used to make little rubber boots for BMX bikes to keep crud out, made a big difference.

What does the sprocket do?

 

[dolorm]

The small sprocket has a 1/30HP (25W) 60RPM 12V motor. It is wired through an on/off switch and another switch: reversable momentary on switch. You move the swing arm toward or away from the log by toggling the momentary switch. The springs in the chain keep tension on chain and more importantly allow the saw to move in/out with the imperfections of the log. The saw blade has a second 1/4" disc that is mounted right next to cutting blade. This disc is smaller in diameter by about a 1/2" or so. It keeps the debarker blade from digging in beyond the bark layer. The debarker is left out (away) from the log after the bark is out of the picture. So it only gets used on the first 4-8 cuts typically, or every cut for natural edge slabs. I attached a few pics to show my design and a couple of factory images. The springs may not work, so I am looking at another option. They can't go through pulley.

 

[bullnerd]

In my opinion the crank set is strong enough.

Capture the chain to the larger sprocket, so it can rotate, but not enough to feed the springs into the teeth.

For some reason your springs remind me of the charlie brown cartoons.

 

[brianh]

Instead of a motor could you put the debarker on a spring loaded pivot with a guide wheel in front and behind the debarker?

 

[dolorm]

I am actually toying with the idea of a locking lever to control the swing rather than the geared motor. My friend has one on his sawmill and it works in this irder: eyebolt, spring, cable around pulley, second spring, cable, through to operator side of saw head, and locking lever. You pull and lever clicks on locking teeth and the debarker swings toward the log. When you want to move it out, you pull locking handle and it wants to swing back away from log do to springs. I could go eye bolt, spring, chain around sprocket, second spring, cable through to locking handle.