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Rotor Strikes and Rotor Bearings - do not fly after a rotor strike.

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  • Rotor Strikes and Rotor Bearings - do not fly after a rotor strike.

    Hi,This really should just happen, but it doesn"t.Rotor head bearings are inexpensive!!!If your rotor system suffers a strike, no matter how trivial you think it is, replace the rotor head bearing, check all tolerances in all the controls as the feedback shock loading from a rotor strike is huge.I"ve just pulled down a friends rotor head after a strike and found the bearing had a "click" in it on every revolution... his blades scuffed the ground once or twice each, like a golfer having a bad day, and suffered a nick in the back edge from the prop which was repaired.After I flopped my machine over, with rotor revs at about 10 RPM my rotor head bearing has issues, I"m replacing my rotor head bearing as well as it is not smooth.Rotor head bearing failure in-flight has one outcome, both you and the machine die.So, replace your bearing after a strike.Happy aviating :-)Nic.

  • #2
    Good advice Nic.... a lot of energy goes thru everything with a fair strike....That bearing noise you found reminds me of why it is good to remove the rotors often, as opposed to leaving them on, hangared all the time. Apart from the obvious continuous strain on the hub bar, you never get to free spin and listen to the rotorhead bearing and check it for up and down play if the rotors are always on the machine.....

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    • #3
      Howdy all,I completely agree with changing the bearing in these circumstances. Glancing rotor strikes are comparatively rare because - as we all know - the rotor will either experience a "last millisecond save" and remain just clear of the ground or whatever obstacle, fence or tree branch it"s being endangered by, or else it"ll hit full-on leading to instantaneous destruction of the rotor. So, a situation where the "save" involved the tips making a couple of glancing scrapes of the ground is - in my opinion - rare. In any event, what Nick is describing is the defect called "Brinelling", where the ultra-high-G shock working it"s way through the rotor system and bearing results in the minute surface area contact of the balls microscopically denting the hard inner and outer race surface. The dents are usually only microscopic but once the"re there the bearing has to be trashed. Also, inadequate grease packing can sometimes allow for condensation to form inside the bearing in humid or coastal conditions, or if actual water has got in somehow, leading to spot rust and corrosion and roughness. That"s another instant reason to trash the bearing and replace.There are 2 things to watch with bearing replacement. (1) that the bearing is seated according to the manufacturer"s guidelines, especially taking care that it is "correct side up" because the deep grove ball bearings we use normally have a residual "filling notch" used for putting the balls in before the cage is riveted. It is absolutely crucial that the bearing therefore be seated so that the lifting load of the rotor corresponds with the side of the ball races that are opposite the "filling notch". You must make absolutely sure that the lifting load path running from the outer race (on the bearing block) via the balls to the inner race (on the spindle) is clear of the filling notches (although some brands don"t have filling notches).(2) that the aluminium block that the bearing is seated into has either a lower lip or flange, or a lower cover plate, that will prevent the rotor bearing block from pulling up and away from the bearing which is captive on the spindle. This last point is very important, because a beautiful green Dragonfly gyro crashed fatally last year in the US when the new owner did a rotor bearing replacement and put the bearing block in the rotor hub upside down, meaning the retaining lip or flange of the rotor bearing block was on top, rather than on the bottom of the bearing. The rotor came completely off the gyro when the new owner was about 300 feet up. See pictures.Cheers,Mark RMelbourne

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      • #4
        Those are high impact pictures Mark

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        • #5
          Here is a thread on the US forum regarding rotor head bearing correct placement, with pictures of a cutaway head to see how it works, and some facts about what the letters mean on bearings.... the most important thing to understand is which way the bearing is being pulled as it rotates (thrust path) .... once you understand this, it is easy to work out which way a one way (filling notch) bearing needs to go... there is some info in the gizmology thread on how the bearings balls are loaded. The last link just explains more about bearings in general, it is from a skate board forum.http://www.rotaryforum.com/forum/sho.../bearings.html

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          • #6
            Thanks Muz. This is an important topic.Mark R

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            • #7
              Yes, after Steve Weirs accident where he had the alloy bearing housing up the wrong way, they cut one up to show people.... it is quite amazing that steve did 15 hours flying before the rotor actually parted company.... 15 hours with only Loctite holding the rotating part of the rotorhead on

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              • #8
                Good commentary here!I"m glad for the mention of the filling slot, I"m waiting for the replacement to turn up, but on the last one I received there aren"t any warnings on the box in relation to the filling slot or the required thrust vector for the bearing. I will be sure to check this as some are filling slot & some are non-filling slot.I think the number is a 32052RS which has no specs for filling & non filling.Here"s an interesting link - with an "all up weight" of 400kg and prop thust of 200kg, such a bearing should last 1990 hours, I think mine lasted about 40 hours from new and didn"t survive the rotor strike at 10rpm, albeit a bad strike.http://webtools3.skf.com/BearingCalc...&grease=MT33My friends bearing may have lasted a little longer in service than this.Wal Flaks recommended to me at the very least to replace the bearing annually, at about $30 each bearing, it is well worth it.As to our unfortunate "Peter" in America, I cannot understate the importance of "before" & "after" & "step by step" disassembly photos. Even better - use a go-pro to see what has happened and what it should look like at the end - just rewind the disassembly footage to see how it goes back together.

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                • #9
                  Yes, he was Steve Weir.... and yes, many people know that the hub bar goes up there and the big bolt goes thru the towers and hub bar, you do it up so tight and put this little pin thingy in the hole... great, all done, lets fly

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                  • #10
                    Yeah thanks Nick,It is worth a few minutes to have a think about the precious bearings we all stake our lives on.I reckon that its good policy to only buy bearings from the huge international bearing manufacturers that put out full specs on their websites. With these big multi-nationals (SKF, etc), you can keep drilling and drilling into their Tech Information to get lots of really interesting information and guidance.Cheers,Mark R

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                    • #11
                      A very good point Mark, the difference between ordinary bearings and quality bearings is only a few dollars. Perhaps people could share the brand and numbers of the bearings they find successful and where they get them? All of the bearings I have are inside rotorheads, so I am no help there, but perhaps the guys here can add to a list that might even become a sticky?

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                      • #12
                        It is the same part number no matter what brand you buy.Then you may want to look at country of manufacture, for example an SKF bearing can come from Japan or Malaysia or Mexico or Brazil or Indonesia or Italy or China etc.Graeme.

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                        • #13
                          One thing that I"ve not worked out yet is why we don"t use an arrangement of Tapered Roller bearings... as are found in the front wheel hubs of cars, and trailers. I couldn"t think of an assembly that cops a huge amount of abuse and soldiers on regardless. I suppose the size of the arrangement is what does it..

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                          • #14
                            it"s not the size at all nic, the tapered cup cone set up with vertical load would cook the bearing in 1 flight alone. the best design is what we already have . many years ago [about 30] a boatie asked to have thrust load design set up on his inboard engine

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                            • #15
                              For Tony, With respect I completely disagree with you on this. Tapered roller bearings are the mainstay of 2nd generation helicopter transmissions and rotor main shaft lift arrangements. Some have a mix of deep groove ball races, or roller bearings, and/or thrust bearings working in conjunction with tapered roller bearings. The Hughes (now MD) 500 series use opposed taped roller bearing pairs for their static-mast rotor hubs, as does the Apache. I note in passing that the upper taper roller bearing in the Hughes 500C and D uses rollers that are twice the size of the lower bearing rollers - see picture below. Tapered roller bearings are also found in Iroquois, Blackhawk and Chinook transmissions and rotor hub installations.The bearing that Igor Bensen settled on using is the equivalent of the modern day SKF 3206A, a double row angular contact ball bearing, with an outside diameter of 62mm, a bore of 30mm, and a height of 23.8mm. This design originally was used in industrial washing machines. It can take very high radial (side-on) loads as well as not-quite-so-high axial (along the shaft direction) loads. They have an internal "contact angle" of 32 degrees, which translates to the load path running from the outer race through the balls to the inner race is angled through the balls at 32 degrees (this is relevant to the "filling slot or notch" issue). Used in gyrocopters, where the axial loads are relatively modest (400 to 600 kg) they can have an almost indefinite life. However, I support Nick"s calculation estimates and as best I can figure, the bearings (in gyros) if you want to be ultra-ultra conservative or cautious, shouldn"t be let run more than 1000 hours. Perhaps being even safer and more cautious still, replacement at 500 hours might seem a sensible replacement time limit, but that"s just me thinking out loud.They are speed rated to 7000 rpm in grease (but in gyros only spin at 300 to 420 rpm), but have a surprisingly low "fatigue load limit" of 850. The "850" is not hours in service, or Newtons or Kilograms. It"s a figure arrived at for comparison purposes between bearings and to give users an idea of whether a bearing will have a long, medium or short life. A similarly sized taper roller bearing will usually have a "fatigue load limit" 10 or more times greater (no doubt simply a result of the hugely greater roller-to-race contact area that rollers have)."Fatigue Load Limit" is an international-industry-wide calculation defined as:"The stress of the raceway fatigue depends primarily on the internal stress distribution at the highly loaded rolling contact point of the bearing. In order to simplify the calculation, the fatigue load limit Cu was introduced. The fatigue load limit is defined as the load below which no material fatigue will occur under laboratory conditions.""This factor takes into consideration:bearing type, size and internal geometryprofile of balls and racewaysmanufacturing quality of the bearingfatigue limit load of the bearing raceway material"What all this boils down to is that a number or inter-relating considerations can have a dramatic influence on ultimate bearing life. However, because the rpm in gyros is so very low compared to industrial uses, and because the axial load caused by the rotor lifting the gyro frame through the bearing is also comparatively low, then the fatigue load limit of 850 becomes rather meaningless in contrast to the 24hr a day torture that some of the same type of bearings are exposed to in manufacturing and industrial installations.The fact that almost every gyro flying today still uses the industrial washing machine bearing Igor Bensen selected doesn"t really establish much other than demonstrating that - deep down - gyronauts are a rather deeply conservative bunch, unwilling to stick their neck out and try new configurations. We keep endlessly repeating the same designs for safety"s sake, comfortable in the knowledge that "Well, it"s always worked before".Getting back to Nick"s question of why don"t we use tapered roller bearings, I can"t provide a definitive answer. Obviously it would take much more care to seat and fit a pair of taper roller bearings in lieu of the double row ball bearings we all use, but I can"t see that there is any real impediment to their use. After all, Hughes, McDonald-Douglas, Bell and Boeing use them in rotor transmission lifting arrangements, so Tony"s argument just doesn"t stack up.Cheers,Mark RMelbourne

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