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Spring washers


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Yes they do. :-)

On the Parajets, it was always a flat washer followed by spring washer.

Never had a problem with them.

SW :D

Sorry to ask silly questions, but is the spring washer touching the prop plate or the head of the bolt? In other words, which order do the flat washer and spring washer go in? And is the flat washer necessary?

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Flat washer against plate, then spring washer, then bolt.

So the spring washer is in contact with the bolt.

No such thing as a silly question, only an inquisitive fool. :-)

SW :D

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Before trusting anything to spring washers, search "Junkers spring washer test". There are several videos showing the effects of vibration on bolted joints with different locking methods.

It has been understood since the 1960s that commonly available spring washers don't work.

There are some that do work. They are made of hard steel and have a distinct sharp tang on the ends, which digs into the bolt head and the mating surface. They are for single use and will usually break when the bolt is undone. I have found these on military Landrovers. I believe these are available as 'aviation spring washers'

Adding a plain washer to the joint negates any locking effect of the spring washer, as the whole assembly of bolt, spring washer and plain washer has no interlock with the mating face.

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If you really need to keep things tight, have a look at Nord-lock washers. They're two piece washers, with sharp "teeth" on one side which lock in to the clamping surfaces, and then wedge-like cams one the internal faces. The cams have a greater angle than the thread, so any rotation causes a tightening of the fixing.

I did a bit about them at uni, we couldn't get them to come undone by vibrating them, and we tested them at all sorts of frequencies and amplitudes. Clever bits of kit.

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People tend not to thread lock the props on because they have to remove them each time for transportation.

Also not forgetting sods law. The day you thread lock the prop on, will be the day that you prang it and need to take it off again.

SW :D

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People tend not to thread lock the props on because they have to remove them each time for transportation.

SW :D

And if you do remove them for transportation, by definition you're checking them every one or two hours, in any case. In my limited experience I haven't found anything in the slightest loose on the prop when I come to dismantle.

The spring washers have been on and off many times and they still have some "spring" left I them , ie they're not completely flat.

I suppose a well balanced prop is pretty important, too, in this respect.

I think in these circumstances one should be worrying most about not over tightening the steel bolt/ ally hub connection and knackerring the threads.

It's a Parajet Macro V2 BTW

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With prop bolts I think a lot of it is down to the direction of prop rotation. The Parajet motors are likely to remain tighter than say a Polini which will be trying to undo the bolts whilst the engine is running. I never had a loose prop bolt with my Parajet XT engine but as soon as I got a Parajet Polini my prop bolts slackened regularly, even with locking washers (it was supplied with just plain washers). Since then I always put Loctite 243 on my prop bolts and mark the heads with my daughters metallic pink nail varnish - they stay put. They come out easily and clean up and go back in with fresh Loctite easily.

I don't understand why manufacturers like Polini don't use left hand threaded bolts.

ETA - I don't remove the prop for transportation.

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Whether the threads are left or right handed makes no difference. It is the vibration, i.e. the oscillating load, that causes loosening.

It was believed some time ago that wheels should be retained with 'handed' threads and I believe legislation still requires it on commercial vehicles. The legislation was based on the statistics for UK vehicles losing more left hand than right hand wheels. In reality, this was more likely due to running the left hand wheels into more potholes, kerbs, etc.

For fasteners that are regularly removed and replaced, Nyloc is useful, but has limited life. Mechanical methods, such as 'R' clips are preferable.

None of these locking methods will prevent the fastener from coming loose. A good locking mechanism will prevent the fastener from coming away completely.

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Whether the threads are left or right handed makes no difference. It is the vibration, i.e. the oscillating load, that causes loosening.

Surely direction of rotation plays a big part too (regardless of additional locking). Isn't that why specific handed threads get specified in engineering? I did an apprenticeship with Rolls Royce (admittedly over thirty years ago after which I left the industry) where any bolts used on rotating shafts had to have their threads opposed to the direction of rotation. I never questioned it, it seems so obvious. Please do re-educate me though.

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If a shaft is rotating continuously, the only thing that would make the 'wrong' handed fastener loosen would be friction between the fastener and something stationary. If the fastener is loose enough that friction with the air is enough, then it's already loose and it's too late.

As a rotating shaft accelerates and slows, the momentum of the fastener may exert slightly more torque. The 'wrong' handed fastener would experience a small loosening torque during acceleration and a small tightening torque during slowing.

All of the above torques are very small when compared to the torque to which we typically preload the fasteners. For example (approximated fag packet calculation), an M8 x 25 cap head screw might be tightened to 20Nm and weighs 13g with a radius of gyration of 2mm. To loosen it by acceleration of the shaft, the acceleration would have to exert 20Nm at 2mm radius = 10,000N on the 13g mass. To do this requires an acceleration of over 78,000G. This means 0 to 62,000,000 rpm in one second.

An F1 engine might accelerate from 5000 to 20000 rpm in 0.2 seconds (equivalent to 75,000 rpm in one second).

Air impact spanners produce the kind of acceleration required. To do this, they have a rotating 'hammer' which hits an 'anvil'

Accelerations due to vibration are much higher and likely to be of similar magnitude in either direction of rotation. These accelerations might be caused by impacts, e.g. a loose component colliding with another.

Both left and right handed fasteners have their own ratcheting effect. It is easier to loosen a threaded joint than to tighten it, so anything that can move it in either direction will result in loosening of the fastener.

Aerospace applications typically use prevailing torque fasteners (K-nuts etc.) which have a deformed thread. When assembled, the deformed area of the thread generates friction between the male and female threads, significantly more so than Nyloc nuts. This prevents fasteners from falling off. To prevent loosening, the fastener must be preloaded to generate a force greater than any load that will be applied to the joint.

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Are they single use?

Nope, they are entirely re-usable.

Of course, "uni" was ten years ago now so I have no idea if they still make them, or if you can get hold of them over here.

RS online sell them, coincidentally I have just found some on a bit of farm machinery I was modifying, being a tight fisted farmer I used them again!

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