Lathe vibrations fixed...sorta

Since I've set up my Logan 816 it's had more vibration that I thought it should (although I've never been around a running metal lathe....). It has the three speed cones. High and low speeds are pretty smooth, but on middle speed it vibrates like the washer on spin cycle with a tennis shoe in there. Enough that if I put a tool on the pulley cover (which I imagine you shouldn't anyway) will quickly rattle off. I'm pretty sure it's a motor harmonic thing, as when the high speed spools up, it vibrates as it speeds up, then is smooth. Upon power down, it shudders again when passing through that RPM.

I've isolated it to the drive side. The motor, with the v-belt off, runs smoothly. If I lift the pulley cover, move the flat belt off to the side so it doesn't engage, engage the v-belt and run the motor, the countershaft spins smoothly until I put pressure on it, simulating lowering the cover to tension the belts. Then the vibration starts.

I've replaced the v-belt, the countershaft pulley, as well as the countershaft and the bushings. No change. I tried every combination of belt tension and no change. I've been living with it, but it's annoying and it will leave marks in the work sometimes.

Today I thought maybe the motor position somehow had something to do with it. This is a bench model that was converted to a stand, with the motor supported by a wood post. The motor mount is about 1/4" from the chip pan, so with the lathe running I stuck a screwdriver in the gap as a lever and slightly lifted the motor. The vibration decreased! So I spent some time raising and lowering the motor with the screw on the back, but the vibration continued. Then I figured out that it was actually sticking the screwdriver in that gap. If I wedged it in there, the vibration lessened to what I guess is "normal."

So here is my "fix", which seems to be working:



My point is to ask if anyone has had this issue, as well as to ask if I'm missing something obvious, or if this solution will somehow cause some harm to something. I guess the wedge puts some pressure on the motor and dampens the vibrations. If it's not a big issue I suppose I could come up with a strap to bolt between the two parts.

I'm no expert in shock and vib, but it sounds like the countershaft assembly is out of balance and the force doesn't move through the machine until the assembly gets restrained a little bit.

I believe the driver cone is locked to the countershaft with 2 setscrews. You may want to try marking where it presently sits with respect to the shaft, and rotating it an 1/8 of a turn at a time to see what happens.

Also, is the entire countershaft assembly sitting "square" in the drive box? Maybe check the position and tension on those four screws that position the countershaft.

Let us know how it goes.

TomKro

Could it just be that the entire motor mount flexes and causes the vibration?
Perhaps your wedge is just giving the motor some extra support against the drip pan.
-brino

Is the 816 one of those that's supposed to have the peg leg under the motor?

eeler1 said:

Is the 816 one of those that's supposed to have the peg leg under the motor?

Click to expand...

Originally it was a bench model, but someone converted it to free standing. When I got it there was no post, so I've been using a 4x4 wood post, which appears to be working ok. I took some photos in an earlier thread.

When searching for the cause, I moved the countershaft pulley all around and when I took it off found the countershaft was buggered up from the setscrews coming loose at some past time; as well the oillite bushings were worn, so I replaced the shaft and bushings with new. I also replaced the flat belt then, and worked a lot on getting all the drive pulleys square to everything so the belt wouldn't spin off, but the vibration continued. I tried attaching a bit of weight to each spoke of the countershaft pulley, thinking perhaps it was out of balance (even though it was a replacement with no change) and that didn't help. I could however REALLY make it vibrate that way!

I think brino is right, it's the motor mount flexing. and wedging dampens it by "connecting" it so to speak to the chip pan and thus the mass of the rest of the lathe. This would seem to negate Logan's design to isolate the motor from the rest of the lathe with rubber bushings (which I've also replaced). I'm sure no expert either, but I'd think if the countershaft pulley was out of balance, the vibration would greatly increase at high speeds, but it mostly goes away, which leads me to the harmonic vibrations from the motor. I've read that can happen with machinery when the motor vibrations at a certain rpm somehow match the vibration from the turning equipment, which then magnifies the vibrations? Sounds good anyway....

I think this was an issue with the prior owner, since he had put rubber gasket where the pulley cover rests, I would assume because the little rubber bumpers there were not enough to stop the rattling.

Unless I come up with something else, I guess I won't mess too much with success and see how it goes, and do some work with the lathe instead of on it. I need to find someone in my area who knows lathes and how smoothly they are supposed to run and have them actually see it run.

What I found helpful on my old Barnes 4-1/2 inch that also had the motor on a pivoting arm behind the lathe, was to add two bolts; one to the motor arm, and the other to the lathe leg. Between those bolts was a turnbuckle used kinda backwards to push the two apart. It not only braced the motor, but also acted as the tensioner for the flat belt.

-brino

IF what you have is a harmonic, changing the harmonic frequency of the assembly might be possible. Try some things like adding weight to the motor mount, changing the wood post to something different, or if you have another suitable motor, change it out. Turning a heavy spacer and pinning it to the countershaft with a set screw might help. Parts have natural frequencies at which they vibrate; (think tuning fork). Changing their mass will often change the frequency
enough to move the frequency outside the working range of the machine.

I will give that a try. The reverse turnbuckle brino mentioned is worth a try too. A new belt is up first. The stainless steel wire I laced it up with has started ripping through.