Out of balance: Lathe weight or a simple fly wheel.

[Matt Mackinnon]

I have read these pages for a bit now and this seems to be a recurring question. I am no scientist and don't have much real world experience with the number of lathes that people have. I happen to own a rather big and heavy Oneway lathe so I can't say what other people's lathes spin like.

But I am perplexed at the number of times and amount of effort that is put into adding weight to a lathe to try and dampen the effect of an off-balance load on the headstock. Then I think to a great demonstration from Dave at DWay where he is talking about a grinding wheel and how to stop a matrix wheel from bouncing. He mentions about how if you put one of the CBN wheels onto one side of the grinding wheel, it will buffer out the bounce on the other.

Would that not have the same effect on an out of balance piece of wood turning on a lathe? So if I put a heavy in balance flywheel on the outboard side of a lathe, the rotational mass of the flywheel would counter act the off-balance piece of wood on the other. HP of the motor really effect the time it takes to get the mounted weight up to speed, but the flywheel in effect would help keep the turning spinning at a constant speed due to momentum as you cut/shape what you are turning.

Wouldn't a heavy flywheel be more cost effective than building a rock filled table as it eliminates the vibration at the source (head stock) than trying to dampen that vibration throughout the whole lathe?

I don't know the answer, so I am asking.

Matt.

[Dennis Nagle]

I don't know the answer either, but the issue with weight is to increase the RPMs and size of an out of balance piece can spin. Before I weighted my old tube lathe, an out of balance pen blank spinning at a moderate speed could make the lathe vibrate across the floor. After I added 4 bags of Quick-Crete I could spin a larger out of balance bowl blank at a higher speed and the lathe would not move.

[Matt Mackinnon]

Yes, I understand that adding weight to the lathe will help dampen the effect of an offset balance.

But, if you added 10 lbs to the weight of the lathe it would have little to no effect on offsetting a standing wave form vibration. But adding a 10 lbs weight at 24" flywheel will have a significant impact. The down side is there is a 24" dia flywheel on the outboard side of your lathe, so it limits the access to your blank from the head stock end, but when the blank is brought back into round, you can take the flywheel off.

Is dampening vibration better than eliminating at the source? I know that weight is a simple you put it on and then forget about it, where a flywheel needs effort to put it on and take it off. But what would work better?

[David C. Roseman]

Not an engineer, Matt, but a couple of thoughts. In theory, a well-balanced flywheel with enough mass would do as you suggest. May even be better for the spindle bearings, when you contrast it to the eccentric torque that an off-balance 150 lb bowl blank generates when damping is done through weight only on the frame. I'd sure check with Oneway, first. But while your big Oneway certainly has the clearance to mount a large flywheel, many lathes do not because of the way the power train is set up. Also, you mention cost-effectiveness. The 450 lbs of all-purpose sand for my Grizzly G0733 cost around $30, plus scrap materials to build the box and shelf. Hard to see coming up with a safe, well-balanced flywheel that will mount on your spindle for that and dampen as much energy. If you plan to do outboard turning, a heavy outboard flywheel would just be something else to deal with.

But it's a great question. Interested to see what the mechanical engineers have to say. :cool:

David

[Scott Hackler]

There was a vendor at SWAT that sells a counter-balance for the lathe. If memory serves me, it attaches to the back side of the headstock. There are a bunch of weights that you adjust to counter the out of balance blank. It is also very helpful for off axis turners. The demo was very impressive but the cost was VERY expensive. I can't remember for sure but I think it was pushing $1000.

[Reed Gray]

Well, not a trained engineer, but lots of experience. Sand is a great vibration dampener as in it absorbs a lot of vibration that a solid chunk of concrete won't, and adds weight to the lathe so it won't vibrate as much with an off balance piece. Some pieces of wood will never balance out no matter what you do. As your speed goes up, so do the vibration forces, but there are rare times when slightly out of balances will spin more true at higher or slower speeds. Harmonic vibrations or some thing like that. Even with the extra weight, vibration is still there, and it will add wear and tear to your headstock bearings. This is the nice thing about variable speed, turn it up till your lathe starts to wiggle, then back off a bit. Pretty simple, and way better than the old pulley lathes. I have seen where people will add weights to really out of balance pieces just so they can rough them out without having to turn at 50 rpm. Of course the weights must be VERY securely attached. Bolting your lathe to the concrete floor also adds a lot of weight to your lathe base, also making it more rigid. There is debate about it adding stress to the moving parts, and I have never done it, or felt the need.

robo hippy

[Dale Miner]

Adding a flywheel will have no positive effect on reducing the forces on the headstock bearings from imbalance. The forces will still be there, dampened to be less noticeable perhaps, but still there. The extra load on the spindle bearings from carrying the extra weight of the flyweel will have an effect of increasing the rate of wear of the bearing. I did field balancing of all sizes of rotating equipment for 25 years. Some items as small as 1/16 horsepower, some as large as 5000 horsepower. I have not read the statement regarding a single CBN wheel improving the running of a grinder so will not comment to strongly in rebuttal. I will say that having one very well balanced wheel and one marginally balanced wheel should make the grinder run smoother than having two marginally balanced wheels. Having both wheels well balanced is obviously the best condition.

As to putting a counterbalance on the outboard end of the headstock to balance the out of balance blank on the inboard end of the headstock; Not a good idea. What this does is introduce a "Couple Imbalance" into the machine. With the spindle stopped, and a counterweight on the outboard end to eliminate the tendecy for the heavy side of the blank to rotate to the bottom, the rotating assembly will be "Statically Balanced", meaning that the assmebly is balanced when it is 'static', or not rotating. When the assembly starts to spin, each end of the headstock will be out of balance, 180 degrees out of phase. When the imbalance on the inboard end is trying to push the headstock to the front, the counterbalance will be trying to push the headstock to the rear. When the imbalance on the inboard end is trying to push the headstock down, the imbalance in the counterweight is trying to lift the headstock. The two imbalances are essentially fighting each other, loading each bearing the same amount as if they were acting alone. In addition to the loading of the bearings, since the loads are out of phase 180 degrees, a twisting effect is introduced into the machine. As the force on one end pushes one direction, the force on the other end pushed opposite, increasing flexing/twisting of the machine and accelerating the eventual fatigue of the machine components. The ridigity of the machine may mask the loading of the machine, and since we tend to run the speed up until we feel the lathe vibrating, the loading of the bearings and other parts of the lathe will very likely be higher, accelerating wear and failure.

If a counterweight system is employed, it needs to be as close to the source of the imbalance as possible, i.e. on the inboard end near the blank. By having the counterweight as close as possible to the source, the couple imbalance is minimized, and the loading of the bearings and machine components is reduced.

[Ray Koberstine]

I am a mechanical engineer and I'd say you're somewhat right Matt. Balancing at the source would be ideal and eliminate the wear and tear on the bearings and lathe. However the counter balance really needs to be added in the same plane or you get dynamic imbalance. So if you added weight on the outboard side you would statically balance it but once you started running the two imbalances would manifest themselves. I think the answer comes down to how much imbalance vs. lathe size and how often you're loading it with imbalance. If it's something you're doing occasionally lathe ballast is probably sufficient. However if you seeing it daily or weekly maybe a system like Scott suggested is prudent.

Ray

[Richard Coers]

I think a better system than a heavy flywheel might be a tuned mass damper. I have read work about it on a lathe. An engineer friend uses one on his drill press and it is amazing. The trouble with a system on the lathe, is that you would have to tinker with it at the start of the job, and as the job progresses. Not very practical.

[Matt Mackinnon]

It is interesting that two different idea's have been floated. So if I had a 30lbs bowl where one side was 10lbs heavier than the other side (left side was 20lbs / right side was 10lbs). When you turn it would shake the lathe badly as one side is 2 times the weight compared to the opposite side.

1. To have a counter balance (like you have with the Oneway grinder balancing system, where you are adding an equal weight to the opposite side to offset the difference. So to counter that force with this bowl example, you would add 10lbs to the side that is lighter, so now both sides weigh the same.

or

2. To increase the total mass evenly so the percentage of offset weight compared to the total is much less. So add on a 30lbs perfectly balanced weight onto your spindle. Now with the bowl, one side would weight 35lbs and the other side 25lbs. Now the ratio is 7:5 where before adding a balanced flywheel it was 2:1


Now a flywheel has a rotational mass. Not being an engineer, I don't know the math of it. But from what I understand, if I had a 100lbs in a bar that is 4" in dia, it would have less rotational mass than a 75lbs bar that is 8" in dia. The weight might be more, but the moment of inertia is much less.

If the flywheel is totally balanced, then the stress on your bearings is much less, as all the forces wanting to push outward away from the rotational centre would cancel each other out. That is why adding a balanced fly wheel reduced vibration as it averages out the total mass with the offset out of balance mass.

But that is just my understanding of it. But as it's been pointed out. Not all lathe's allow for a flywheel to be added.

[Harry Robinette]

At one time Jerry Glaser had a vibration dampening thing that mounter to the head stock and was a pole that had weighted plates connected to the top. The pole held the weights about 3 foot over the head stock. Don't know what happened with it or if it worked but we all know Jerry thought out of the box. Just a thought.

[robert baccus]

This is cajun engineering--drill weight off the heavy side with a forster bit. Be careful of course. Works well for me when you have a blank thatis half burl. Forgot which engineering school taught this.