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Did I Just Make a Perpetual Motion Lathe?

12 comments
11 years ago
General Articles     Woodturning

Time for a treadle lathe update and something to make me go “hmmmmm”.

Treadle lathe drive center

Look Ma! My drive centers line up too!

The lathe is almost done. I haven’t strung up the belt yet, but that is my next step. I’m afraid once I get it running, I’ll have a hard time finishing up all the little details and instead just start playing. I have already gotten a taste of this by installing the spindle and crank arm. Really once I finished “hanging” the flywheel and started it spinning there was very little work done because I just played around with adjustments the drive center. The flywheel is astonishing however and runs like I just created a perpetual motion machine.

I cranked it up and let it spin and I kid you not it was still ticking over on it’s own 3 minutes later at least. I have not technically trued up the wheel, but rather just cleaned up the bow sawn edge with a spokeshave roughly to my pencil line. It runs quite true with only a small hump in one quarter. However that small hump throws a bit more weight around and acts to give the wheel a little kick each rotation. That I think explains the seeming perpetual motion.

Treadle lathe in progressSo here is the head scratcher. Should I really focus on getting this wheel perfectly balanced? The way it runs now the momentum gets a little more energy with each rotation and requires an incredibly small amount of energy to keep it up to speed. If this were my electric midi lathe it would create a lot of vibration at higher RPMs and shake the body all over the place. With this lathe, I have a very massive frame and I doubt I will ever exceed 1000 rpm at the spindle and a much lower rpm at the wheel. When I had the speed cooking along at high speed with me kicking the treadle lightly once ever 1-2 seconds I felt no vibration in the headstock or the rails. I actually set a glass of water on the rails and got no ripples. So first of all, hooray for sturdy lathe body construction! So now I’m wondering if I can even introduce vibration into the frame due to this imbalance at my relatively low RPMs.

Not being an engineer, I can’t help but wonder what I’m missing. Am I trying to impose modern machinery ideals onto a DaVinci design? I know that if I balance the wheel by removing that extra bit of stock I won’t be able to put it back on. Realistically it may not even make a difference in the perceived effort on my part while turning and the wheel will still be at least 60 lbs.

If you hadn’t guessed by now I’m really leaning towards leaving it out of balance to see just how that translates to performance once I chuck up wood and apply load via the gouge. For now it is too dang fun to crank it up and watch it spin effortlessly without slowing. I’m anxious to experiment a bit and see if the dreaded vibration creeps up after all.

Here is a short video I took showing the wheel in motion:

Sep 28, 2013 | Hark the wheel! She spins!! by RenaissanceWW on Keek.com

Its still going…

Sep 28, 2013 | It’s unstoppable! by RenaissanceWW on Keek.com

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Aaron Marshall

Looks great! It’s been way too long since my college engineering courses, which is why I’d say – leave it alone for now. Give it a test with some actual turning. If you don’t feel the pulse of the wheel, forget about it.

I’m sure that in theory that the “hump” is slowing things down a bit by causing more stress and friction on the hub. You mentioned a little “kick” when the high spot comes around, but there must be a corresponding slow-down on the other side of the rotation. Otherwise the wheel would speed up infinitely and create a time vortex!

Adam Maxwell

I think Aaron Marshall is spot-on. As an example, my bench grinder (tailed) used to stop within a few seconds. After I balanced the wheels (using a stupidly dangerous procedure that I won’t mention), it keeps spinning an order of magnitude longer. Although I knew this was the theory, I was still shocked at just how much difference it made.

Since your bearings are good and the frame is solid, you won’t have problems right away, but imbalance will cause increased wear. Over time, I’d guess you might see holes elongate, and parts might loosen up. You’re not going to get it perfect unless you start drilling to lighten it or screwing lead weights to the sides, but a “perfect” geometric symmetry is probably good enough.

Also great job!

Jamie Bacon

That’s a sweet looking lathe Shannon. And smooth as can be. I have no clue if the bump in the wheel will cause you problems down the road or not, but as smooth as it’s running, I don’t think it’d require much effort on the treadle if you made it perfectly round.
Seeing the treadle lathe almost done makes me want to build one. Or convert my spring pole.

gary

I wonder, if after you put on the belt, what kind of vibes you get. Maybe it won’t make a difference. But……….

Roger Davis

You DON’T HAVE an imbalance, even if you do have a bump; the glass of water proved that. If you remove that hump, you will introduce an imbalance. It is apparently offsetting a less obvious defect on the other side or a difference in density in the wood. And no, you are not getting a kick from that hump or any other imblance. Gravity is a conservative field, and whatever it gives taking a mass downhill, it takes back exactly as much going back up.

Jim Paulson

Beautiful work Shannon. For me the task you face is getting the wheel round. If you are going to use a drive belt like I have on mine, then you’ll want the rim slightly crowned and round. That is important so the belt will track, i.e., stay on and not wander off. It will matter less if you opted for a rope or cord. In my case I built the wheel out of pine and it wasn’t as much work to make round. However, I did have to shim the hub (pipe flanges based on Roy’s design) to get the spin balanced. I also put a large chunk of scrap lead on the side of the wheel to be a counterweight and to increase wheel mass. It may be unnecessary in your case with hardwood.
Best wishes. Jim

    Shannon

    I do plan to crown the wheel Jim as I am using a belt. Though most of that work is already done when I shaved it smooth.

Jim Morris

If there is any vibration, it has been isolated to the flywheel section (water glass test). The good news is that the vibration won’t affect other parts of the lathe. I think you don’t have a compelling case yet to make any change. Adam Maxwell called out the biggest issue – if there is unbalance, it will begin to deform the components in the sub assembly albeit very slightly. I can’t picture that would be anything major in a few months of use. Try it out … see what you get and it would be good to determine an objective way to analyze if it is getting worse.

Snakeye

Shannon, I agree – if it’s minor enough with a massive, beautifully built frame that dampens any significant vibrations, then I think you’re good to go unless you’re a straight-up perfectionist. Moreover, I think at the velocities that the wheel is spinning at, it would take a LOT of time for other parts to start to wear down.

On the physics side of things: despite what you are feeling, that slight hump/deviation does slow the wheel down ever so slightly (mainly due to extra friction from the the slight off-balance). What the wheel is really doing is accelerating every time the hump goes down, and then decelerating every time it goes up – the accelerations induce a little added friction into the prongs that hold the wheel… (2 steps forward, 2.3 steps back?). Shaving the hump off will get you zero acceleration (unless you do it with your foot) which will take out the extra forces against the prongs reducing the friction. Instead what you’ll get is a wheel that constantly decelerates at a slower pace than before.

Either way, it looks minimal to me (ie, I think you can get away with either/or). The sheer size of that wheel is what offers up some pretty good inertia to keep it going that long!

Hope I didn’t geek out too much.

Shannon

Here is a little update for you readers interested enough to make it all the way down here. I added a crown to the wheel to keep the belt centered and did that by actually turning the wheel. This confirmed that there is indeed a “lump” in one area. I ignored it and just focused on adding the crown. To do this I have to remove part of the lump on each side and bring it into line with the rest of the wheel. It runs even smoother now which leads me to believe removing it completely won’t hurt. But then I got impatient and strung up the belt anyway and it cuts like a dream. It is everything I was hoping for when I started thinking about this lathe 4 years ago. So now it may be a while before I take it apart again and address the half a lump still left.