Wind Plant Blade Balancing Mick Sagrillo ©1989 Mick Sagrillo Many different ways of balancing wind generator blades and rotors have been used over the years. Some of these methods work, although often on a hit or miss basis. There is more to rotor balancing than just screwing some lead onto the blades. An unbalanced rotor will cause unnecessary vibration and stresses resulting in premature wear and tear on the generator's bearings, governor, blades and tower. A properly balanced rotor will give the wind generator long life and its owners years of problem free power. Here's a review of the most popular balancing methods. Ancient History The oldest way of balancing is to mount the blades on their governor and then hang the entire assembly from a wire attached to the ceiling. The wire passes through the generator shaft hole in the governor and acts as a fulcrum or balance point. The entire rotor is then hung horizontally a few feet from the floor. The wire must pass exactly through the center hole of the governor. The point where the wire passes through the rotor becomes the fulcrum about which the rotor teeters. Weights are added to the lighter of the two blades (for a 3 blade rotor) until the entire rotor is horizontal. The primary problem with this system is that the fulcrum is below the rotor's center of gravity. This is because the assembly is supported from beneath by the wire. Balance is best achieved when the fulcrum point is above the center of gravity of the entire rotor/blade assembly. It is virtually impossible to balance the rotor accurately by hanging it from a wire. Better But Not Perfect Another old timers method involves mounting the entire rotor on the generator shaft. The generator can be on the tower or preferably on a stub tower on the ground. The rotor is lightly spun and allowed to come to rest. The heaviest blade will always come to rest in the bottom position. On a three blade rotor, weights are added to the lighter of the two and the rotor is spun again. When the blades come to rest randomly, the rotor is considered balanced. This method is tedious and time consuming but with patience a somewhat balanced rotor can be achieved. Two Blade Balancing Individual blades can be balanced, one against the other or against a known weight, on a fulcrum. This only works with a two blade system and then only if the governor or hub that the blades are attached to is perfectly balanced. If the governor is not well balanced, the case with most two blade Windchargers, or the rotor has three or four blades, this method doesn't work well. Balancing with a Fulcrum A common and ineffective way of balancing blades with a fulcrum is to find the center of gravity of the heaviest blade and transfer this dimension to the lighter blades. The lighter blades are placed, one at at time, on the fulcrum at the marked point and balanced with weights. Fulcrum Balancing Problems One major problem with fulcrum balancing is the assumption that the blades are identical in gross weight and in weight distribution or density along their length. We have received blades from manufacturers that have varied by as much as six ounces from the heaviest to the lightest. And these have been considered quality blades. One of our customers bought blades from a well know supplier that varied by three pounds! Once you've individually balanced all three blades, the rotor assembly will still not be properly balanced if the blades don't all weigh the same. As an exaggerated example, let's say that you used two 2 X 4's and one 2 X 6 instead of blades. Obliviously, the 2 X 6 is going to be the heaviest of the three. If you find the center of balance of the 2 X 6, transfer this center distance to the two 2 X 4's, then balance the two 2 X 4's on your fulcrum, all three will have the same center of balance. You'd have plenty of problems if you tried to use these three pieces of wood to drive your wind generator. Wood Density One of the reasons for the large discrepancies in blades is wood density. Trees vary in density, as do parts of individual trees. Ideally, the densest wood should be at the butt of the blade. The butt is the blade end closest to the governor. I know that our blade supplier takes these things into consideration. But someone who is buying just one one set of blades to finish for his own wind generator is really stuck with whatever the manufacturer sends. Blade manufacturers sell their wares either prepaid or C.O.D. Once a customer has paid for the blades, he has to be content with what's been shipped. Blades and Rotor Together The blades should not be individually balanced using the fulcrum method. Here at Lake Michigan Wind & Sun, we do not balance individual blades. We treat the entire rotor (the blades and governor) as a single unit. The first step in system balancing is to break down all structures to fundamental units. The entire rotor assembly is a fundamental, rotating unit. In theory, the rotor can be balanced by balancing individual blades, then mounting them onto what is supposed to be a balanced governor. In practice, all sorts of assumptions come into play that are givens in theory. These assumptions are not even considered when someone is screwing weights to blades, and therefore become variables. And variables can become vibrations. How We Do It The way we balance rotors at Lake Michigan Wind & Sun is very similar to the way tires were balanced before computerized spin balancing. We bubble balance. INSERT PHOTO 1 Our balancer (photo 1) consists of a shaft machined to fit the center hole of the governor, in this case a Jacobs or DWS blade-activated governor. Different shafts are machined to fit different styles of governors. The shaft is bored out nearly to the top end. A pivot rod is machined to a point which fits inside the bored shaft. An inexpensive bubble balance, available from any local tool shop, completes the hardware required. INSERT PHOTO 2 The assembled balancer is shown in photo 2. The governor is placed on the bored shaft and the blades are then mounted onto the governor. At this point, the blades, governor hub, and governor spider are numbered. This is done so the rotor can always be reassembled in exactly the same way that it was balanced. If this is overlooked, the balancing is for naught. It's a good idea to place sawhorses under the first two blades that are mounted onto the governor to keep from placing undue stress on the pivot rod. Once the third blade is installed the sawhorses can be removed because the rotor will, more or less, maintain its own balance. INSERT PHOTO 3 The rotor mounted on the balancer is shown in photo 3. The primary advantage to this system is that the pivot point of the rod (the fulcrum) is above the center of gravity of the governor and blades, see figure 1. INSERT FIGURE 1 This makes balancing very accurate as well as very easy. The bubble balancer is placed directly over the center of the top of the bored shaft that is sticking out of the governor (see photo 4). INSERT PHOTO 4 The Balancing To balance a Jacobs or DWS blade mounted on a blade-activated governor, place gram weights on the eyebolt brackets of the two lighter blades until the bubble balance reads level (see photo 5). The eyebolt brackets are there to tension the governor springs. They provide a convenient place to attach weights along the blade's center of gravity. INSERT PHOTO 5 Adding The Weights Once the rotor is balanced, weigh the gram weights on a beam balance. Measure out the same weight of lead for casting. Add several grams extra to compensate for the mounting bolt holes which must be drilled in the weight. Melt the lead, pour it into a mold and set aside to cool. Drill the bolt holes and mount the weight to the blade by sandwiching it between the eyebolt bracket and the blade (see photo 6). INSERT PHOTO 6 This way, the weight cannot work its way loose from the blade. Now, recheck the balance of the rotor to see how good a balancing job has been done. If you've removed too much or too little lead during the drilling of the bolt holes, make corrections if possible. If not, then do it over. A perfectly balanced rotor is one of the most critical parts of your wind generator. Once you're satisfied with the balancing, move on to finishing the blades, and taping the leading edges. Finishing Up Before you balance the rotor, prime and finish sand the blades and mount all the hardware. Don't apply the finish coats of paint before balancing for two reasons. First, several ounces of paint spread more or less evenly over the entire surface will not affect the balance of the rotor. Second, it's very important to attach the weights to the blade before painting so the finished blade is completely protected by the final coats of paint. Then there are no breaks in the paint's skin for moisture to enter. Moisture entering the wood will not only affect balance, but also eventually cause rot. Inner Workings- Centers Problems often arise because of where the weights are placed on the blades. As can be seen from the cross section of a blade in figure 2, the thickest part of the blade is near the leading edge of the blade. This is called the aerodynamic center. Also shown in figure 2 is the pivot center. The pivot center is where the blade is attached to the rotor. INSERT FIGURE 2 The aerodynamic center is where most people would probably attach weights because it's the thickest and presents the least danger of the screws going all the way through the blade. The Aerodynamic Center vs. the Pivot Center In the Modified Clark Y Airfoil used on Jacobs and DWS wind generators, the aerodymanic center is 28% of the chord length measured from the leading edge of the blade. The chord length is the distance from the leading edge to the trailing edge of the airfoil. In the case of a Jacobs or DWS blade used on a blade-activated governor, the hole bored into the blade's butt for the governor blade shaft is the pivot center. The pivot center of the blade is 43% of the chord length from the leading edge. The three blade shafts of the blade-activated governor are 120¡ apart. The center of gravity of the blade lies along a line that passes through the pivot center of the blade (Figure 3). INSERT FIGURE 3 Therefore, the center of gravity of each blade is 120¡ apart. By placing lead weights beneath the eyebolt brackets, you add weight along the center of gravity of the blades. Since each blade's center of gravity remains the same (120¡ apart), the rotor will balance nicely. If you add weights along the blade's aerodynamic center, then the center of gravity is shifted forward, towards the leading edge. This means that the centers of gravity for the three blades are no longer 120¡ apart (figure 4). INSERT FIGURE 4 While a rotor assembly whose blades are weighted along the aerodynamic center will balance as a unit on the bubble balancer, it will vibrate in actual use. Care must be taken to locate weights along the center of gravity of the blades (the pivot center if a pivot is used) and not along the aerodynamic center. Weights On or Within the Airfoil? Never add weights on the outside of the airfoil. The airfoil of the rotor is a most important part of the entire wind system. The last thing you want to do after painstakingly preparing and finishing a set of blades is bolt some globs of lead to the outside of the airfoil. This creates all sorts of turbulence which will adversely affect the airfoil's performance. However, some rotors do not have governor hardware attached to them (like the Jacobs or DWS blades) and therefore lack convenient mounting points for weights. There is a method of attaching weights that does not compromise the airfoil. This technique involves countersinking the weights into the blades. We use this method quite often with different airfoils and it works well. Countersinking Weights INTO the Airfoil In this technique, weights are placed along the center of gravity line of each blade requiring additional weight. Countersink the weights into the blade with a Forstner bit. A Forstner bit drills a perfectly clean hole in wood and leaves a flat bottom in a blind hole (the hole doesn't go all the way through the wood). You'll find an example of a blind hole on chair legs. These blind holes are drilled into the legs and accept crossrails. The best results are achieved with a drill press. I use a 7/8" Forstner bit, although most any size will work. After determining how much weight is needed to balance a blade, the lead is melted and poured into a form. The form I use is a short piece of heavy wall tubing that has been bored to exactly 7/8" to match the holes. The weights are never more than 3/8" high. If more weight is needed for the blade, more holes should be drilled and the lead made into several weights, one weight for each hole. The holes are about 1/2" deep for a weight that is 3/8" high. Epoxy is applied to the bottom of the weight and the weight is pressed into the hole. The hole is then sealed with automotive body putty, such as Bondo or fiberglass. I have found that the 1/8" of body putty or fiberglass that covers the lead weight nearly equals the weight of the wood removed from the hole. No allowances for the additional weight of the body putty or fiberglass are needed. Once the body putty or fiberglass has hardened, it can be sanded to match the contour of the blade. If you have done a good job, it will be very difficult to find the weights once the finish coats of paint have been applied. You have now balanced the blades without destroying the integrity of the airfoil. The Bottom Line As you can see, rotor balancing requires looking at the entire rotor assembly as a unit. Balancing individual blades is approximate at best. If you take time and consideration with the balancing process, you'll have a happy and long lived wind plant. Access If anyone has any comments on these ideas or any other ideas concerning rotor balancing, I would appreciate hearing from them. Feedback can be sent to: Mick Sagrillo, Lake Michigan Wind & Sun, E 3971 Bluebird Rd., Forestville, WI 54213, (414) 837-2267. Mick Sagrillo rebuilds, buys, and sells wind generator systems, from pre-REA to present. He makes and sells a complete line of Jacobs replacement parts, as well as parts and blades for most other wind machines.