Propeller hub info, removal, and installation
By Had Robinson
updated December 16, 2020
DO NOT HIT THE HUB WITH ANY KIND OF HAMMER. DO NOT USE A PULLER (except as explained below). DO NOT PRY IT OFF. SEE THE INSTRUCTIONS BELOW.
This page is primarily about the Top 80 hub, other makes and models are very similar
Vibration in a paramotor is usually caused by a hub that has excessive face runout usually caused by a propeller strike or during shipping of the paramotor. (This is another reason why the shipping container must adequately protect the hub from damage due to handling.) The hub must be checked with a dial micrometer to be sure that the runout is not excessive (see below for more on this). Very rarely, a propeller strike can be severe enough to bend the redrive shaft. It should be checked for runout in the same way as the hub. (Note: To date, I have not yet encountered a bent shaft.)
Hubs on redrives made before 2004 are different (see red arrows) and they are NOT interchangeable. The shafts are shorter and there are no seals in the redrive which is why they were lubricated with grease. However, they are removed in the same manner.
The propeller hub should NOT easily slide off the shaft. This is by design and why a loose Top 80 hub must be replaced. Loose hubs are usually caused by propeller strikes. This bends the hub and the out of alignment propeller will vibrate severely and the resulting forces will chew up the area of the hub in contact with the propeller shaft.
Here is a photo of a hub that was so chewed up from being bent by a propeller strike that the propeller shaft extended 5+ mm above the face of the hub. The hub just rattled around on the shaft despite the presence of the center screw (removed in the photo). I am not sure what would have happened had the hub continued to wear and move towards the redrive case. It would eventually force the oil seal into the case or destroy the oil seal bore – all an expensive mess in one way or another.
Removing the aluminum hub with just a puller or banging/prying it off will bend or warp it. (Some paramotors have hubs that will easily slide off the shaft but, alternatively, use a beefy center screw to firmly hold the hub tightly against the shaft.) 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 (see photo above). If you can pull the hub off the shaft by hand, it must be replaced (Top 80 ONLY).
DANGER 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. I.e. if you want to find out if there is life after death, this might be a good way....
It is rare that the shaft or bearings are damaged. Propeller strikes, improper propeller installation, or propeller disintegration damages the aluminum hub. If a balanced propeller vibrates excessively, the hub is most likely the cause.
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.
- 3 Jaw gear puller or (best) a wheel puller, bolt type
- Propane torch
- brass brush or Dremel-type tool
- Kitchen or machine shop oven. (Get your woman's permission before sticking engine parts in her oven. It will not ruin the oven and you will have a happier life by getting permission first.)
You will need these tools if you had a propeller strike and now experience vibration and want to know if the hub is damaged. No matter how carefully the propeller is balanced it will not stop the vibration caused by a damaged hub.
- 1 In. Travel Machinist's Dial Indicator
- Multi-Position Magnetic Base with Fine Adjustment
- 4" x 6" piece of steel with a hole in it to attach to the engine to hold the magnetic base (not needed if you have a big vise).
1. Hub and shaft evaluation and preparation
The redrive should be removed from the engine and the oil drained. (It is harder to reinstall the hub properly if the redrive is not removed.) Spray copious amounts of penetrating oil 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 penetrating oil will help ensure that you will not destroy the head of the screw or break it off – a very difficult thing to fix. DO NOT REMOVE THE CENTER SCREW AT THIS TIME.
Optional (but recommended): drain the oil out of the redrive. To help prevent oil going everywhere, remove the bottom drain plug first and then the top fill plug.
If you have purchased a new hub and/or propeller shaft or choose not to evaluate the condition of the hub and shaft (you are convinced that any vibration of the propeller is not a problem), go to step #2 below and skip the rest of this step. Thankfully, it is rare that the shaft is bent. If you choose not to evaluate both the shaft and the hub, you will only know that the shaft is bad after you assemble everything and test fly the paramotor.
Check the face (or axial) runout (FR) of the hub. Then measure the total indicated (or radial) runout (TIR) of the propeller shaft before taking it all apart. A good hub should have a FR less than 0.001". New hubs that I have measured are often in the range of 0.0005". 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 TIR is less important but should also be less than 0.001". If it is more than this, the redrive will have to be rebuilt.
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. The 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 professional 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.
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 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.
2. Loosen the center screw 10 turns
The reason that the center screw should remain in the shaft is that the gear or wheel puller center bolt must not also push against the hub. If that happens, you can tighten the puller all you like and things will not move. Pushing against the center screw will ensure that the puller center bolt will only contact the shaft and not the hub.
Use an inexpensive oil filter wrench or some bolts with a screwdriver or a piece of steel to keep the hub from rotating. If you do not have a tool to hold the hub, remount the propeller in order to keep the hub from rotating.
Press down HARD on the 6mm hex bit tool as you turn the screw CCW. This helps keep the screw head from being deformed. If the screw does not budge, use penetrating oil and heat the hub, as in step #4 below. If the penetrating oil 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 penetrating oil is applied and allowed to soak for an hour. Loosening things that are frozen 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 to remove it.
3. Attach the gear or wheel puller
The aluminum hub is easily damaged by the improper the use of a puller so it must be carefully done and the directions here followed exactly.
Attach the puller to the hub and tighten the center bolt on the puller by hand so that the puller is JUST snug.
If the puller is tightened excessively, it will bend the hub and ruin it! This is why pullers must not be used in the usual way on aluminum parts.
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. A wheel puller, bolt type, (below) is safer to use because it pulls closer to the center of the hub and is less likely to bend the aluminum hub and damage it.
4. Apply heat (Top 80 only)
READ THIS SECTION COMPLETELY AND CAREFULLY BEFORE APPLYING HEAT. IT IS EASY TO RUIN THE HUB AND THE OIL SEAL.
Use the propane torch to heat the hub for 30-45 seconds, as shown in the photos below. Direct the flame about 1/2" (1.2cm) below the bottom face of the hub. Rotate the hub by reaching under the redrive and turning the clutch bell as the heat 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. Remove heat and pull UP on the puller's center bolt. The hub should come right off with little effort. With some hubs, you may have to tighten the puller bolt until the hub is completely off the shaft. This is often the case if RTV or threadlock was used on the shaft.
Sometimes, the hub may be corroded on to the shaft and may grip the shaft again as you are using the puller. 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 and/or the seal (if you are not removing a damaged seal). Once the hub is removed, immediately cool the shaft with a rag soaked with water so that the neoprene seal does not get too hot.
Note the position of the seal in this photo (not even visible). 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. The puller here is a jaw-type puller which is quick and easy to apply. I used it because the hub was ruined in the accident and I was not worried about damaging it. It was easy to get off, anyway.
You can use a wheel puller, bolt type, which is THE BEST AND SAFEST to use, especially if you have less experience as a mechanic.
4. Hub installation
If you are handy with a torch you may use it to heat up the hub enough so it slips on the shaft. (If not, use the oven method in the next paragraph below.) Place the hub on the shaft so that it just aligns with the spline i.e. so the hub will slip down on the shaft once it is heated up. Using the clutch bell to continuously rotate the propeller shaft, apply heat to the hub just below the flange for about 60 seconds. Around that time, the hub should just slip down on the shaft by itself. It may not drop down more than 75% of the way. If so, quickly use a dead blow or a plastic tipped hammer to gently tap the hub home. DO NOT FORCE THE HUB ON THE SHAFT. If it does not easily go the rest of the way, start over with step to remove the hub and tray again. As soon as the hub is installed QUICKLY install the center screw and torque to specifications (9-10 Nm or what is correct for your engine). If you wait too long the hub may not be completely seated.
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. I.e. if the hub gets this hot, it will become soft aluminum.
The redrive must be placed as near as possible to the oven if it is in your woman's kitchen. 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, grease, oil, 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. You must allow time for the hub to fully expand. If you have an infrared beam-thermometer, measure the temperature. When it is close to the oven's temperature, it is ready.
Have a dead blow hammer handy.
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. Immediately give the hub a light tap with the dead blow hammer. This will ensure that the hub is fully home. If the hub does not drop home with a nice "click", it is either damaged or you did not clean it thoroughly.
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 under the center screw WILL NOT FIX a damaged hub.
If you suspect there is a problem, 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.
As soon as possible, reinstall the center screw (do not use threadlock) while the hub is hot and torque to 9-10 Nm or to the value given for your engine. 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 mainly prevents the hub from sliding in and out on the shaft if it becomes damaged and starts to wear badly.
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".
5. Redrive installation
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.