paragliding training center
By Had Robinson
Propeller vibration is usually caused by a hub that has been damaged and has excessive face runout. The hub must be checked with a dial micrometer to be sure that the runout is not more than 0.013mm (0.0005").
The propeller hub should not easily slide off the shaft. This is by design and why a loose hub must be replaced. Loose hubs are usually caused by propeller strikes against the cage or the ground. Severely unbalanced propellers can also cause damage to the hub.
DO NOT HIT THE HUB WITH A HAMMER, USE A PULLER, or PRY IT OFF. SEE THE INSTRUCTIONS BELOW.
Removing the aluminum hub in this way will likely bend or warp it. It must have a tight fit on the shaft or it will quickly be ruined by the vibration transmitted from the engine to the propeller shaft. If you can pull the hub off the shaft by hand, it must be replaced.
Use of a loose fitting or a damaged hub could cause catastrophic failure of the propeller and other parts. Putting threadlock, RTV, or silicone cement in the hub bore or on the propeller shaft will not fix this problem. I am not sure exactly what happens when the hub bore wears past a certain point but it probably would not be good to find out.
It is extremely rare that the shaft or bearings are damaged. Propeller strikes, improper propeller installation, or propeller disintegration can damage the aluminum hub. If a balanced propeller vibrates excessively, the hub is likely bent or warped.
Because the hub is aluminum and the shaft is steel, it is an easy job to remove it but only if you have these special tools and follow the correct steps here. These tools can be purchased at Harbor Freight or an auto parts store and are good enough for occasional use.
You will need these tools if you had a propeller strike and you experience vibration, no matter how carefully you have balanced your propeller
The redrive should be removed from the engine. (It is harder to reinstall the hub properly if the redrive is not removed.) Spray copious amounts of WD-40 in and around the center screw of the hub and let it sit an hour or more, overnight is best. This screw is usually installed with threadlock, making it difficult to extract. The WD-40 will help ensure that you will not destroy the head of the screw or break it off -- a difficult thing to fix.
Check the face (or circular) runout (FR) of the hub and the total indicated runout (TIR) of the propeller shaft before taking it all apart. Excessive FR will cause the propeller to vibrate, sometimes severely, and can lead to failure of the redrive and other parts of the paramotor including engine mount failure and frame fatigue cracks.
The hub below had a FR of > 0.025" and had to be replaced. This was indicated by severe vibration the pilot experienced when the paramotor was run at full speed. Tools of sufficient quality needed to measure this are a magnetic base tool holder and a dial indicator and are available together for under $30 from Harbor Freight. (Note: the tools pictured here are not from HF.) After setting up the dial indicator as shown, press firmly down with a finger on the center of the hub. This will eliminate any vertical movement of the shaft while rotating it. Slowly rotate the hub from the clutch bell side and note the FR -- the difference between the high and low values. A good hub should have a FR less than 0.0005".
In the photo here, the magnetic base is fixed to the anvil of the vise. If you do not have a vise, a piece of steel must be attached to the engine upon which the base can be attached.
In the photo here, the propeller shaft runout was checked and was less than 0.0001". It is extremely rare that the shaft might be bent from an accident but it should be checked, nonetheless. Thankfully, the hub is aluminum and not steel. It is much less expensive to replace a hub than rebuild the entire redrive.
Press down HARD on the 6mm hex bit as you turn the screw CCW. This helps keep the screw head from being deformed. If the screw does not budge, heat the hub as in step #4 below. If the WD-40 starts to smoke near the screw, things are too hot and you should stop applying heat. If things still do not move, you may have to repeat this step after more WD-40 is applied and allowed to soak for an hour. Removing things that are stuck requires patience. Of course, you can twist the screw head off but then you will have to send it to us or a machine shop and it will cost $100 for us/them to remove it.
The aluminum hub is easily damaged by the improper the use of a puller so it must be carefully done. Attach the puller to the hub and tighten the center bolt on the puller by hand. You want just enough force to slide the hub off the shaft after it is heated up. The puller applies uniform force to the hub which is difficult to do without it, especially if the hub is hot. If the puller is tightened excessively, it will bend the hub. A wheel puller, bolt type, (below) is always safer to use because it pulls closer to the center of the hub and is less likely to damage it.
Use the propane torch to heat the hub for 30-45 seconds as shown in the photos below. Rotate the hub as the torch is applied so that the heat is evenly distributed around the center of the hub. Be very careful to not get the flame near the base of the propeller shaft where the neoprene seal is.
The aluminum hub will expand and the joint between it and the shaft will loosen. When this happens, the puller will suddenly become loose and its job is done. Remove heat and turn the center bolt of the gear puller by hand to complete the removal of the hub from the shaft. As the hub cools, it may grip the shaft and you will have to apply heat again. If you see the slightest bit of smoke, STOP IMMEDIATELY and let things cool down. DO NOT OVERHEAT THE HUB or you will wreck it.
Note the position of the seal in this photo. It had been jammed into the casing bore by glider lines getting caught in the propeller. The turning propeller wrapped the lines around the shaft, like a winch. The smooth lines worked their way along the shaft, pushing the seal in and ruining it.
Or use a wheel puller which is safer to use, especially if you have less experience as a mechanic.
Preheat the oven to 177ºC (350ºF). Let the temperature stabilize for at least 15 minutes. Be certain that the oven temperature is correct. When aluminum is heated to 205º C (400º F), it enters the danger zone for its metallurgical properties.
The redrive must be placed close to the oven. During the next step, the hub will cool down too much if you have to carry it to your work area in some adjacent building or room.
While the oven is heating up, thoroughly clean the hub and shaft with solvent, using a brass wire brush. There must not be the slightest bit of threadlock, silicone, corrosion, etc. on the shaft or in the bore of the hub. Use a pick tool, if needed, to remove stubborn bits of whatever, just like the hygienist does to get plaque off your teeth. If this is not done, the hub will not go on smoothly during the next step.
Place the hub in the oven for 15 minutes minimum. If you have an infrared beam thermometer, measure the temperature. When it is close to the oven's temperature, it is ready.
Do not put any RTV, threadlock, or any type of sealant on the propeller shaft or in the bore of the hub.
Remove the hub from the oven and IMMEDIATELY align it with the shaft and let it drop home. It will make a nice "click". The shaft end should be no more than a few millimeters below the hub face. If the shaft is flush, above the hub face, or the hub is loose after it cools, it is ruined and must be replaced. Using washers and spacers WILL NOT FIX a damaged hub. If the hub does not drop home with a nice "click", it is either damaged or you did not clean it thoroughly. Immediately remove the hub and let it cool off. Examine the shaft and hub again as you may have missed something. If things are clean, the hub is damaged and must be replaced.
Allow the joined hub and shaft to cool. If you do not get the hub fully seated, it will loosen and the movement of the steel shaft will eventually destroy the hub.
Reinstall the center screw with BLUE threadlock and torque to 9-10 Nm. DO NOT OVERTIGHTEN THIS SCREW. The hub is held to the shaft via a tight fit, not as a result of this screw. The screw only prevents the hub from sliding in and out on the shaft.
IT IS USELESS TO FIX A DAMAGED HUB BY OVER-TIGHTENING THE SCREW OR BY USING RTV, EPOXY, SILICON, ETC.
Optional: Measure the face runout of the hub again to be sure things are properly installed and not damaged. The FR should be less than 0.001".
Reattach the redrive to the engine according to paragraph [B. 1] given in these directions. The redrive must be aligned properly to preserve the life of the clutch, to adjust the low speed system properly, and to minimize engine vibration.