balancing weight position?

[adelaide]

I am still unclear if the position of the balancing weights on props is important or not! eg light weight on the tip will do the same as a heavy weight on the root area.

Which will balance it ? say a string in the middle or on free bearings.

I have heard mixed opinions in the past and have noticed anomalies on props that are balanced, I have heard you should balance the tip then the base and go from there is that important ?

Would you think that this will maybe affect acceleration and the yaw.

I am  not shore if this effects the balance at a fixed rpm.

Also does a 1 kg weight doing 100 rpm/say 1 meter from shaft weigh less or the same centrificly as a 1 kg weight doing 100rpm say 2 meters from the shaft CENTRIFICLY. I KNOW IT WILL NEED MORE ENEGY TO SPEED IT UP .

Sorry for opening the worm can.

http://nnw.berlios.de/docs.php/centrifuga may help above me thow

[Flux]

A perfect prop will not need balancing. If it is symmetrical, blades are the same physical size and material ( wood) is of the same density throughout.

What you have to balance is any difference from perfection. That should be small and any balance weight you add will have no effect on acceleration or yaw. If it is far enough out to affect any of these it must be a monstrosity.

Let's think of it as a disc so you will not have couples to worry about( it's too thin).

Adding a weight at the centre will have no effect but you can add the weight at any other point. The out of balance acts as a couple ( weight x radius) and you will not know that weight or its radius. You can determine its angular position if it is freely supported on bearings or on a string at its EXACT centre. If it is on bearings the heavy point will rotate to the bottom. You need a weight diametrically opposite to produce an identical couple in opposition to the  one from the out of balance.

It doesn't matter whether you use a brick on the root of the blade or a washer at the tip as long as the product of weight and radius is correct to balance the original couple.

The force on the balance weight will increase as the square of the ROTATIONAL speed and so will that on the original un-balance. It will stay balanced at all speeds.

Generally it is easier to do it at the root of the blades, you don't want lumps causing drag at the tip, but if it is done without causing aerodynamic problems then it is perfectly possible to balance at the root , the tip or any other radius.

Now you don't have a perfectly simple disc, the prop has thickness ( especially at the root).  You don't know where the out of balance lies, but you can bet that it is virtually at the centre (front to back) of the blade. If you have to add a large weight at the root, you should avoid putting it on the front or back disc. It should be split into 2 parts of half the weight so the effective weight is mid way from front to back. It is not always easy to attach things to the back disc so a better place is likely to be on the root of the blade before the aerofoil is formed.

If you can not hit the exact spot (angle wise) you may again need two weights and the weights may need varying so that the combined weight acts at the correct angle.

If you have free bearings and no iron in the alternator you can balance on its bearings. If not you may need to make a balancing rig with free bearings or resort to the string method.

All methods work, but unless you use the machines bearings you will need to be absolutely sure that what you use is done at the exact centre that the final shaft will use. 1/16" error in the centre could make a big error with a heavy prop.

Finally you must make sure that the blade tips track in the same plane, or otherwise you will introduce a couple.

One final warning, out of balance is usually the cause of vibration, but there can be other factors and a perfectly balanced blade may still vibrate if the aerodynamic forces are not equal. Differences in angle or profile between blades and other factors could cause trouble, but it would need to be very badly made to show this.

This has got quite long, the issue of balancing seems to cause a lot of confusion and I hope this may clear up some of the confusion.

One final point, there seems to be some confusion regarding static and dynamic balance. This has all assumed a single plane balance and works for a disc ( thin object). It is static balance and will remain so if you achieve it with it rotating on a machine.

Dynamic balance concerns the removal of a couple on an object of significant thickness. It can not be done statically as there is no couple at standstill.

flux

[Flux]

I must correct an error in terminology.

In the earlier part I referred to a couple when I meant moment ( force x radius).

A couple is a different thing and when I mentioned it in the final part it was the correct use.

Sorry, bit of a nuisance not being able to correct things after posting.

flux

[Ungrounded Lightning Rod]

Sometimes explaining in different ways helps understanding, so I'll chime in.

Static imbalance amounts to adding an off-center weight to a balanced disk.

 - The amount of the imbalance is the weight times its offset.

 - ANY added weights whose combination of weight, offset, and offset angle adds up to canceling out the imbalance will give you dynamic balance.

This can be

 - an equal weight directly opposite,

 - a heavier weight directly opposite and closer in,

 - a lighter weight directly opposite and colser out,

 - two equal weights, either side of directly opposite,

 - unequal weights at various angles and distances from the center that add up right,

and so on.  As long as the vector sum of the weight * offset of all of them adds to zero, you get static balance.

Since a heavy weight close to the axis is equivalanet to a light one far from the axis, you can cancel out a static balance with weights at any distance from the axis that is handy.

Static balance can be checked by spinning the mill on a horizontal axis and seeing if it stops in a preferred position.  Or by supporting it on a point at the center (i.e. a nailhead) with a bubble level on top of it and seeing if it levels.)

One hack with balancing wheels is to add TWO weights, summing to more than the amount needed to achieve balance, at the point opposite the heavy side, them move them apart equally until balance is achieved.  They act like one weight of twice their individual weights, centered between them.  As they are moved apart this combined weight stays between them, which means it stays at the same angle but gradually moves toward the axis.  A large movement of BOTH the weights produces a small movement of the combined weight - at least at first - so very fine tuning is easily achieved.  This can be approximated by adding bolts at the "best" stud until you've overcompensated, then moving pairs of them to the adjacent studs to fine tune.

Another, used in fan blades, is to put weights on the blades and move them in and out to tune them.  First you find the blade where the weight minimizes vibration and move it in and out until you've gotten the best balance you can.  Then you do the same on another blade at a different angle.  If the two blades are at right angles you get balance right away.  If they're at some other angle they'll interact a little bit, so you need to go back and forth a couple times to fine tune.  But it converges quickly.  This can be approximated by putting a bolt on first one, then another, blade and sliding them in-and-out.  Once you've achieved balance, measure the distance from the axis to the hub and from the axis to the studs.  Multiply the weight of each bolt by the ratio and add that much weight in bolts and washers to the appropriate stud.

Wind turbines have long, relatively thin, blades.  So a very slight error in angle from exactly equal angular spacing can create an imbalance.  Accordingly, a very slight adjustment in angle, if the mounting allows for it, can achieve balance.  (You really want your mount solid, so the angles don't change in service and unbalance the turbine.  So balancing by tweaking blade angle is probably out - especially since it would unbalance the thrust on the turbine.  But angle is something to pay close attention to when assembling the blades - especially if tearing down and reassambling the mill - to avoid creating imbalance.)

If you have to add a significant amount of weight to the hub to balance things, you want to add half of it to the front and the other half to the back, to avoid creating a dynamic imbalance.

- - -

Dynamic imbalance amounts to adding two, equal and opposite, off-center weights to a balanced disk, at different distances along the axis.  It also amounts to tilting the disk from being at right angles to the axis.  (As the disk spins, it tries to spin on its own axis - pushing to get to right angles with the line between the bearings.  This makes the actual shaft want to move in a cone around the line between the bearings rather than spin nicely on its axis, creating a rotating outward force on each bearing - but only when the mill is spinning.)

Because the mill is effectively a very THIN disk, with a lot of weight far from the hub, getting the tips of all three blades to be the same distance in the along-axis direction virtually assures good dynamic balance.  It's easy to measure once the blades are mounted.

The blades are thin, so trying to compensate for dynamic imbalance by putting weights on the front and back of them is impractical.  (Also:  They flex.  So adding along-axis weights would make them bend and goof up what you are trying to do.)  The only place to compensate with weights is at the hub - and you can only compensate a little there.  Fortunately you probably will not have to do this if your blades aren't tilted, and if you do it will only be a little you'll have to compensate for.

With dynamic balance you need to spin thing to figure out what's up.  And without fancy sensing equipment and a computer it's tough to work out where to put them by measurement.  Fortunately it's easy to estimate the AMOUNT of imbalance and cancel it by trial and error.

If you've got a dynamic imbalance, put a pair of bolts on studs on opposite sides of the hub and on studs on opposite sides of the axis.  Keep changing studs until you find the pair that gives you minimum vibration.  Then add or subtract weight to reduce vibration further.  If you go through a non-zero minimum set your weights for the minimum, then start the hunt for angle over with another pair of bolts and tune for minimum again.  Two passes should do the job.

If you have an odd number of studs, put two bolts on a stud on one side and one bolt each on the two most-opposite on the other.  (This will give you a slight static imbalance, so you may need to retweak that if you end up adding a lot of compensation for dynamic.)

If you have long studs, you can fine-tune the amount of dynamic balance compensation by moving the nuts along the studs.  Farther out, more compensation.  (Using pairs of nuts as "jam nuts" will keep them from moving once they're installed.)

I hope that long-winded exposition is helpful rather than just redundand with what was already posted.

[DanB]

I really prefer to keep it at or near the hub in the strongest area of the blade.  Even if things are well balanced, if one blade is heavy due to a weight near the tip then the forces bending the blade when the machine yaws or furls will be much greater on that one blade and I worry that it could snap the blade or - bend it enough to cause a dynamic balance issue.

even if it seems an obnoxious amount of weight to do it near the hub - I prefer to keep it there rather than put a small weight out near the tip.