Nuts, bolts and washers. Help please.

[flying dog]

I recently watched the ACI about the China air plane which caught fire after landing and AFTER engine shut down.

 

(No spoilers.)

 

But it has raised this query from me:

 

What is the function of a washer?

 

Apart from the possible exclusion of a spring washer which is used to keep nuts tight.......

 

I don't get it. As a kid, I accept that I knew "jack" about how that kind of thing worked. There were simply "nuts and bolts".

 

It seems weird that a washer is needed for the function mentioned in that episode.

 

Actually if I remember I will put my idea on a better solution at the end.

 

Also, the word BOLT is somewhat vague and general, as really what most people call a BOLT is not a bolt, but something else which I now can't remember.

 

But AFAIK, the idea is that two things are to be joined at a point, a "nut and bolt" combination facilitates that.

 

Similar to nailing two pieces of wood together. I won't start on SCREWS.

 

As there are "now" nylock nuts and crown nuts, I am really stumped to the mechanics of a washer - as in THAT episode.

 

But, I am interested to learn and expand my knowledge on their functions if anyone is willing to share.

 

Thanks in advance.

 

(spoiler space)

 

So the "problem" was the nuts were coming off the bolts. The solution was to put locktight on them.

 

Why not use a crown nut and lock wire?

 

Yeah, ok, they would still need to remove the existing one, blah blah. AND the bolt too.... Ok.

 

But also the "practice" of not having a good view of the area seems silly. It is leaving a whole new vector for faults - as shown - to happen.

 

That seems very strange.

 

 

[jetboy]

I havent seen the episode you refer - so maybe some context is missing

 

Its important for a Jabiru owner to know what washers and loctite can or cant do. think flywheel bolts where they now use Nordlocks but the plain type not the belleville Nordlock X series which the Brits I think started using first.

 

for aircraft bolts there is a 'washer' profile under the head so a washer is not normally used there but one is normally placed under the nut presumably to spread the contact area and prevent scuffing of the clamped parts when tightening. also the washer material and coating is important for corrosion control

 

There is plenty of reading available about this in AC 65 manuals from the FAA so an experienced LAME would probably know more.

 

star washers, spring washers, tab washers and belleville. AN series have thick and thin for adjusting the amount of thread clearance remaining inside the bolt as well as penny washers for spreading loads across wood or for shielding rod eyes from disconnection failures.

 

 

[M61A1]

From the AC 43-1B.... I've noticed that it doesn't mention those soft copper or aluminium washers which act as seals on banjo fittings and the like.

 

9/8/98 AC 43.13-1B Par 7-85 Page 7-15 (and 7-16) SECTION 5. WASHERS 7-85. GENERAL. The type of washers used in aircraft structure are plain washers, , and special washers. Typical washer types are shown in table 7-14

 

7-86. PLAIN WASHERS (AN960 AND AN970). Plain washers are widely used with hex nuts to provide a smooth bearing surface, act as a shim to obtain the proper grip length, and to position castellated nuts in relation to drilled cotter pin holes in bolts. Use plain washers under lock washers to prevent damage to bearing surfaces. Cadmium-plated steel washers are recommended for use under boltheads and nuts used on aluminum alloy or magnesium structures to prevent corrosion. The AN970 steel washer provides a larger bearing surface than the plain type, and is often used in wooden structures under boltheads and nuts to prevent local crushing of the surface.

 

7-87. LOCKWASHERS (AN935 AND AN936). Lock washers may be used with machine screws or bolts whenever the selflocking or castellated type nut is not applicable. Do not use lock washers where frequent removal is required, in areas subject to corrosion, or in areas exposed to airflow. Use a plain washer between the lock washer and material to prevent gouging the surface of the metal. CAUTION: Lock washers are not to be used on primary structures, secondary structures, or accessories where failure might result in damage or danger to aircraft or personnel.

 

7-88. BALL SOCKET AND SEAT WASHERS (AN950 AND AN955). Ball socket and seat washers are used in special applications where the bolt is installed at an angle to the surface or when perfect alignment with the surface is required. These washers are used together as a pair.

 

7-89. TAPER PIN WASHERS (AN975). Taper pin washers are used with the threaded taper pin. NAS143 and MS20002 washers are used with NAS internal wrenching bolts and internal wrenching nuts. They may be plain or countersunk. The countersunk washer (designated as NAS143C and MS20002C) is used to seat the bolthead shank radius, and the plain washer is used under the nut.

 

Here is the link to the whole of chapter 7 (aircraft hardware)

 

http://www.faa.gov/documentLibrary/media/Advisory_Circular/Chapter_07.pdf

 

 

[Bruce Tuncks]

Good question, flying dog.

 

The washer is to spread the load and to stop the rotating nut ( when its being tightened) from damaging the stuff underneath. Think of a wooden aircraft... in fact there are extra large washers ,called penny washers, for this job.

 

Now the same effect applies to aluminium although not so much. The washers also can be used to adjust the effective length of the bolt.

 

Good point about the loctite too. I reckon this was overused by Rod Stiff. I think he wanted to make assemblies permanent and tamper-proof.

 

Tie-wire is not an alternative to loctite, tie wire will not prevent the first little movement of the nut. In some situations, that first little movement means the thing has failed ( think cylinder hold down studs ). What it will do is prevent the whole thing coming undone, which is good for things like a sump-plug.

 

Now nuts will not undo themselves without an off-torque, so it is only in certain situations ( think wheel-nuts) where anything at all is needed.

 

There is a great chapter in "Sky Ranch Engineering Manual" where Schwaner tells the story of how this major engine maker tried everything on their big-end bolts ( split-pins, tabbed washers etc) till they eventually found that nothing at all gave the best reliability.

 

 

[Bruce Tuncks]

Good stuff Jet and M6, done while I was writing. This sure is a good forum for technical advice.

 

 

[M61A1]

The AC 43 is a good bit of reading.

 

 

[DrZoos]

Flat washers are used mainly to ensure the nut presses against a full 360° smooth surface, or to stop the surface being damaged by the nut or bolt as they compress. But washer as stated by others here can also be used for many other reasons, ranging from space fillers when a bolt is too long or an internal space needs packing , to increased surface area for pressure distribution, crush washers for a seal, some act as a lubricant against another surface eg nylon washers, some act to reduce friction and let a nut and bolt spin, some act as a friction plates to increase friction eg in a Foxbat throttle, to stop the throttle sliding open, but allow it to move when pushed.

 

They basically give engineers a lot of options in how to attach items with different characteristics.

 

 

[facthunter]

It is . No mention of "tab" washers for locking nuts and things like filters. Some nuts/bolts include a washer captive but rotating or just have a flange built into the underside of the head. Most "plain' washers you buy are just stamped out of soft steel plate and pretty crummy. A washer to spread load on say a wooden spar, or an area of fibreglass would have to be designed for the job. IF a spring washer breaks there is instantly a 'play" situation created, so they are not a thing I like at all. On aircraft today use the "AN" system. You are paying through the nose for anything else above "RUBBISH quality and the AN system is not that much more expensive, relatively and you can specify correct thread lengths etc (critical for untensioned shear situations). Shortening your own "auto" bolts, it is difficult to remove burrs, the unthreaded section is well below the spec dimension. Thread fit and form is critical to achieve specified performance. (tension figures). Don't use crap bolts and nuts. (Poor economy) Too critical a part. Nev

 

 

[flying dog]

I'm then taking it no one actually has seen said episode of ACI.

 

:(

 

(Should I?)

 

SPOILER SPACE!

 

The problem was a nut/bolt assembly fell "off" the slat mechanism and when the slats were retracted, the bolt punctured the fuel tank, and so when the engines were shut down: Blam-o!

 

After extensive research, they discovered that the washer had fallen off the bolt and so the entire nut/bolt unit would "fall through the hole which would otherwise be stopped by the washer.

 

The nut was taken off because of a "design flaw" where the nut could come off anyway and cause such a problem.

 

In taking off the nut and applying locktight, the washer fell off and so negated the nut being fixed on.

 

(How much do the people who design these things get paid? It seriously worries me when you see the problems.)

 

So: Although the bolt couldn't fit through the hole, the nut could. So a washer was used.

 

BUT HANG ON! WTF??!! If the nut can pass through the hole: Isn't the bolt too thin?

 

That's a pretty simple question.

 

I just can't see HOW this happened - in real life - that no one actually thought: "Maybe we should make the bolt a bit thicker, or the hole a bit smaller."

 

Using the washer there seems........ stooopid.

 

And that they didn't put a crown nut rather than a normal nut seems just ......... (I give up.)

 

 

[old man emu]

I cannot recommend highly enough to you that you obtain a copy of this: Mechanic's Toolbox CD, John Schwaner, Sacramento Sky Ranch, Aircraft Technical Book Company, Aviation Books and Video Resources if you do any work on an aircraft. Alongside your copy of AC43, this is as essential to aircraft maintenance as a 1/2" AF open-ended spanner.

 

The CD has been produced by John Schwaner, and is regularly updated, which he cannot easily do with "Sky Ranch Engineering Manual".

 

Washers: Don't forget those nylon washers NAS1515H*** which are used under the heads of the screws securing panels. The nylon washers prevent the screw head from damaging the painted surface of the panel.

 

Castellated or non-castellated nuts?

 

Nuts are used in two ways in an aircraft:

 

1. To maintain clamping forces that hold two or more components together.

 

2. To retain a metal rod in place, but to allow movement of components around the metal rod, which acts as an axle.

 

Taking the first case. Consider fitting an engine mount assembly to the fuselage.

 

 

The assembly has to be firmly attached, so the bolts have to be torqued up nice and tight. (There are tables which give the recommended toque values for various diameter bolts) It is not possible to get the toque correct if you are also trying to line up a hole in the bolt with the castellations of the nut. In this case, you use a normal hexagonal sided nut (and flat washer). You can't use a Nyloc nut because they are not to be used in hot areas ie. forward of the firewall.

 

In the second case, consider joining two assemblies which have to be able to move relative to each other, such as landing gear.

 

 

Here, the bolt acts as an axle, letting the shock strut move in an arc around the U-fitting. In this case, all that is required is that the bolt be kept in place. Therefore, a castellated nut, secured with a split pin will do the job. Often, thin washers are fitted between the strut head and U-fitting as sacrificial surfaces which are allowed to wear out instead of the fuselage components.

 

OME

 

 

[M61A1]

It is . No mention of "tab" washers for locking nuts and things like filters. Some nuts/bolts include a washer captive but rotating or just have a flange built into the underside of the head. Most "plain' washers you buy are just stamped out of soft steel plate and pretty crummy. A washer to spread load on say a wooden spar, or an area of fibreglass would have to be designed for the job. IF a spring washer breaks there is instantly a 'play" situation created, so they are not a thing I like at all. On aircraft today use the "AN" system. You are paying through the nose for anything else above "RUBBISH quality and the AN system is not that much more expensive, relatively and you can specify correct thread lengths etc (critical for untensioned shear situations). Shortening your own "auto" bolts, it is difficult to remove burrs, the unthreaded section is well below the spec dimension. Thread fit and form is critical to achieve specified performance. (tension figures). Don't use crap bolts and nuts. (Poor economy) Too critical a part. Nev

Actually, Table 14 in the Chapter 7 of the AC 43- b,(link provided in post #3) shows several varieties of tab washer, including the aptly named "chickenhead" washers.

 

 

[Bruce Tuncks]

Thanks OME. your endorsement was enough for me to order the CD. As you must have noticed, I really like Schwaner's stuff.

 

 

[geoffreywh]

Tab washers?... I have, so often knocked back the tab and undone the nut by hand. Really miserable level of safety.....

 

 

[jetjr]

The problem with the nut pulling through attachment, I think, was that it was a swivel end fitting, IF the ball in center of fitting was to seize and pull out then the whole thing could pull through, a washer might keep it together even with center no longer in place.

 

Back up for what is a rare but possible failure.

 

 

[M61A1]

Tab washers?... I have, so often knocked back the tab and undone the nut by hand. Really miserable level of safety.....

Are you talking about the stuff you find on a mower or on a certified aircraft? I've seen some brilliant types of tab washers.

Here are some examples of washer I see on a daily basis.

 

The control rod washers are usually serated or notched so that they can be adjusted by a measurable amount, for example when adjust rotating control rods on a heli swashplate, the vibration analysis program will might tell you to adjust blade XX "two notches up", as part of a vibration solution. The rods look like turnbuckles, but aren't, they have a fine thread on one end and a course on the other and are usually both "handed" the same, so that they act as a Vernier when adjusting.

 

When the boss on the washer(which is keyed into the rod end) falls into the correct groove on the rod, the jam nut is torqued, then the nut is lockwired to the washer.

 

 

 

 

 

 

 

[facthunter]

IF after the tab is bent back the nut can be removed by hand means it wasn't tight. How can that be the fault of the tab washer.? IF despite it being loose it didn't wind off, the tab did it's job. Much more secure in those circumstances than lockwire. Nev

 

 

[geoffreywh]

That is , of course, possible, but not likely. Early motorcycles ( up to 70's) often used them. Guzzi's used many. on Jap bikes they were mainly used on the final drive sprocket. The nut would loosen, the tab would hold on and the gearbox shaft would be hammered to hell by the now loose sprocket or clutch drum. That was sometimes from new. loctite cured the problem. when i rebuilt a Guzzi. there would be a icecream container full of the things (almost). and i rebuilt lots.Never had a problem after you threw them away. get a hold of Carol Shelby's book on fasteners, re: NASCAR preparation it's an eye opener.. Really

 

 

[facthunter]

The applications you list are the most often noticed. They are also very hard to hold while tightening and have often been tightened with a hammer and chisel.(probably not very successfully). It's a splined drive which if it has any play will work in use.. Many Japanese bikes just use a fancy circlip and they put up with the increasing play as the bike wears. I don't "write off" tab washers. Some are very clever and effective. Most auto applications use nothing and just rely on the tension being enough to lock the bolt.( Earlier on they tabbed or split pinned everything.) Just relying on the tension isn't the usual approach to the aircraft situation, where it's more belts and braces and often "appearing" to have done something, rather than guaranteeing anything. All of the locking devices make it hard(er) to retension or check it. Nev

 

 

[Bruce Tuncks]

One place on the Jabiru where nothing is used is the tappet-adjuster. These nuts sure move a lot, but they don't come loose. Why not? Because there is no off-torque to give even the smallest impetus to unwinding. But there are many similar nuts and bolts in the Jabiru where loctite is called for.

 

Control circuits can be deceptive though and there can be off-torques where you might not expect them, so when in doubt, a locking device is a good idea.

 

If I was more brave, I would use a lot less loctite on my Jabiru. In the meantime I am following the official factory advice but welcoming it when they drop a loctite instruction. The cylinder hold-down nuts and the flywheel bolts come to mind.

 

 

[geoffreywh]

I agree. I never saw Loctite used on a crankcase bolt, big end , main bearing bolt ect., until I saw a Jabiru...I thought it was highly suspect and to say the least, unusual....Although I realise that the aviation industry is hamstrung by "Certification". I was rebuilding my 0-200 a while ago, The guys in the engine shed said" come and see us when you are ready to assemble the c/case and we'll show you how to use silk thread and permatex." I didn't laugh out of respect. But politely declined and used Yamaha Three Bond. Never leaked a drop. whilst Certified Engines leak like a sieve. Certification locks things into the 19th century.Unfortunate at times.

 

 

[facthunter]

The silk thread (cotton is OK too) is an old one for sealing faces of metal to metal as well as a good sealant . As to the actual sealant. I used Yamabond for ages. I use a Loctite product (sealant). CARE and beburring, and remachining if it's out of true. Nothing will work if the surfaces are fretting.. Nev

 

 

[M61A1]

Threebond is a wonderful thing.....never had leaks when using it. Whether car, motorcycle or aircraft.

 

On that subject, one group I know of uses grease on their aircraft that was superceded years(decades) ago by automotive standards, but you have to use what the publication says.

 

 

[M61A1]

I'm then taking it no one actually has seen said episode of ACI.:(

 

(Should I?)

 

SPOILER SPACE!

 

The problem was a nut/bolt assembly fell "off" the slat mechanism and when the slats were retracted, the bolt punctured the fuel tank, and so when the engines were shut down: Blam-o!

 

After extensive research, they discovered that the washer had fallen off the bolt and so the entire nut/bolt unit would "fall through the hole which would otherwise be stopped by the washer.

 

The nut was taken off because of a "design flaw" where the nut could come off anyway and cause such a problem.

 

In taking off the nut and applying locktight, the washer fell off and so negated the nut being fixed on.

 

(How much do the people who design these things get paid? It seriously worries me when you see the problems.)

 

So: Although the bolt couldn't fit through the hole, the nut could. So a washer was used.

 

BUT HANG ON! WTF??!! If the nut can pass through the hole: Isn't the bolt too thin?

 

That's a pretty simple question.

 

I just can't see HOW this happened - in real life - that no one actually thought: "Maybe we should make the bolt a bit thicker, or the hole a bit smaller."

 

Using the washer there seems........ stooopid.

 

And that they didn't put a crown nut rather than a normal nut seems just ......... (I give up.)

I cant speak with absolute certainty, but, perhaps the reason that the bolt was that small and the hole that big, is that the slat track stop is often an eccentric, designed to be adjustable, so that the place the slat stops is perfectly adjusted. I can't recall if the 737 is designed that way, but some aircraft are.

The image suggests that they probably are, if you use a bigger diameter bolt, you lose some adjustment, and as the image shows, the nuts are already flanged, and sometimes, room is tight. Why not just assemble it correctly?

 

 

When building and maintaining aircraft, is doesn't help to scrape too close to the bottom of the barrel when hiring employees.

 

 

[Garfly]

A Nordlock promo video:

 

 

[rgmwa]

My prop bolts are secured with Nordlocks.