How much to tighten bolts?

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rtfm

Well-Known Member
Hi,
A quick question - how much pressure is required to adequately tighten a bolt through Spruce? Recommendations re: washers to spread the load - required?

Regards,
Duncan

Hi tdfsks,
Thank you.

Duncan

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Gets a little more involved when working with wood. You can calculate (approximately*) using this formula:
Bolt torque/pressure

Using a AN-4 bolt and AN-970 washer you get around 1300 psi at the minimum torque of 50 in/lbs. That is fine for Spruce parallel to the grain at 4700 psi but a bit high for normal to the grain at 750 psi.

*torque is in reality has a pretty poor correlation with clamping pressure, but it is so easy to use in the field that we still use it.

tdfsks

Well-Known Member
For a 1/4" bolt (AN4) I agree with your calculations: i.e. min torque is 50 in.lb for a tension nut. At 50 in.lb the preload on the bolt is: F = T/(kd) = 50/(0.16*0.25) = 1250 lb (where k = 0.16 and is the value for cadmium plated bolts). OD of a AN970-4 is 1.125" and ID 0.265" so area is (pi/4) * (1.125^2 - 0.265^2) = 0.939 in^2. So compressive stress is 1250/0.939 = 1331 psi.

For those wondering where the allowables for Spruce came from, they are from ANC-18 for Stika Spruce at 15% moisture.

Note that a lower torque of 30 in.lb is specified for a shear nut and that lower torque would be relevant in many (most ?) cases.

Generally bolts in wooden structures will have their heads and nuts bearing on steel fittings and hence the steel fitting will likely distribute the load over a larger area than a AN 970 washer.

However, in the case where there is a washer under the head / nut bearing directly on the wood, normal practice is to glue a plywood pressure pad to the spruce for the washer the bear on. I cannot think of too many cases where the washers bear directly on the spruce (although I admit I can think of a couple of examples). Plywood will have higher bearing capacity than spruce. Also for highly stressed bolt joints in wood (say an engine mount attachment to the firewall) you will often see a hardwood block (Ash or similar) used to improve the bearing capacity. Finally, the bearing pressure through the wood thickness is not constant but reduces as you move through the thickness away from the surface where the washer is bearing.

The numbers in AC43.13-1B are a good starting point ... if they designer feels that they need something different, they can always put it on the drawing. There is no rule that says the values in AC43.13 have to be used.

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Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Generally bolts in wooden structures will have their heads and nuts bearing on steel fittings and hence the steel fitting will likely distribute the load over a larger area than a AN 970 washer.
Agree.
The reason I added to the conversation is that RTFM, a long time member, may be in the design stage. From the simplicity of his question, and his history, he may not be aware of some of the details needed for a sound joint/fitting.

@ OP Evans Light Plane Designers Handbook has an easy to use condensed section on bolts in wood/ply.

maya.ayoub.32

Well-Known Member
HBA Supporter
Hi Hi - another quick question related to bolts.

How much should the thread be touching the metal, if at all? We're using a bolt through our aileron torque tube which is 1.5 inches in OD and the options for grip length are 1-7/16 or 1-9/16.
Also wondering how much we should worry about the bolt sliding back and forth; aka how close will the cotter pin be to the end of the smooth part of the bolt?

WonderousMountain

Well-Known Member
Follow the Manual Duncan, or you're designing a whole new Flea.

maya.ayoub.32

Well-Known Member
HBA Supporter
Follow the Manual Duncan, or you're designing a whole new Flea.
Would do! But we're designing our plane from scratch. But we can check out other planes' manuals and see what they do- nice idea thank you!

Sockmonkey

Well-Known Member
Don't forget to varnish or provide some kind of sealant to the inside of the bolt holes as well.
Everyone seem to forget that and so they get moisture infiltration at a high stress point because metal bolts in a wood structure are a place where condensation wants to form.

Dana

Super Moderator
Staff member
Hi Hi - another quick question related to bolts.

How much should the thread be touching the metal, if at all? We're using a bolt through our aileron torque tube which is 1.5 inches in OD and the options for grip length are 1-7/16 or 1-9/16.
Also wondering how much we should worry about the bolt sliding back and forth; aka how close will the cotter pin be to the end of the smooth part of the bolt?
Use the longer one, you never want the threads bearing on the structure. Use two AN960 washers to bring the bolted thickness to 1-5/8 (better yet, a thin washer under the head and a thick washer under the nut) so you're not bottoming the nut against the shank of the bolt. Tighten it just snug. If the bolt isn't subject to rotation and it's inspectable (visible) I'd probably use a nylon locknut instead of a cotter pin.

maya.ayoub.32

Well-Known Member
HBA Supporter
Use the longer one, you never want the threads bearing on the structure. Use two AN960 washers to bring the bolted thickness to 1-5/8 (better yet, a thin washer under the head and a thick washer under the nut) so you're not bottoming the nut against the shank of the bolt. Tighten it just snug. If the bolt isn't subject to rotation and it's inspectable (visible) I'd probably use a nylon locknut instead of a cotter pin.
Perfect! Washers are a fantastic idea - I think we'll use the AN960 in the front to protect the metal near the head, then the AN970 between the nut and the metal since it's about double as thick. The bolt itself is going to be under quite a bit of vibration and rotation since it's controlling the aileron, so I think we'll stick with a castle nut for this bolt in specific. Thank you for answering our questions and amazing suggestions!

cvairwerks

Well-Known Member
AN960 and NAS1149 washers are available in various thicknesses, so you have options depending on total material stackups.

Dana

Super Moderator
Staff member
AN 960 and 970 are the same thickness, 1/16. The 970 is a larger OD, mainly used where where the increased surface area is important, like when bolting wood. But the 960 also comes in the thinner "L" version which is 1/32 thick.

cvairwerks

Well-Known Member
Depending on - number, 960’s come in .016, .032, .049 and .063” thickness. Not all variations are available for each - number. For example, a -9 comes only in .049” thickness, others only the .016” and .032”. There are so many possible combinations, one needs to reference the specs to select the correct ones.

Alan_VA

Active Member
Perfect! Washers are a fantastic idea - I think we'll use the AN960 in the front to protect the metal near the head, then the AN970 between the nut and the metal since it's about double as thick. The bolt itself is going to be under quite a bit of vibration and rotation since it's controlling the aileron, so I think we'll stick with a castle nut for this bolt in specific. Thank you for answering our questions and amazing suggestions!
If I might add a couple of hints - Recommend this article: Nuts and Bolts - KITPLANES and this product:
In the course of building nearly anything, but especially aircraft, we often temporarily assemble something to check a fit up and then ask ourselves later "Did I just spin things up hand tight or really torque the fitting properly?" Dykem torque seal can quickly resolve that problem. I am not associated with the company in any way, but I believe in the process/product. After you have torqued down a fitting, and used a jam nut (you got the jam nut tight, right?) or nylock or whatever, you apply a thin bead of torque seal across the bolt and nut. Torque seal provides no actual holding properties; but if the nut vibrates loose you will see a crack in the dried film and know that things have shifted. Very simple idea, but can save you wondering and worrying. Best of luck.

maya.ayoub.32

Well-Known Member
HBA Supporter
AN 960 and 970 are the same thickness, 1/16. The 970 is a larger OD, mainly used where where the increased surface area is important, like when bolting wood. But the 960 also comes in the thinner "L" version which is 1/32 thick.
Ah yes gotcha! I scrolled a little too quickly on the Aircraft spruce website and thought 960 only came in the L sizing. We just bought various thicknesses of both the 960 and 970 to protect the bolt from the metal and use as spacers between the metal and castle nut. Thanks!

cvairwerks

Well-Known Member
For those that don't know, everyspec.com is one of the best free sites for looking up specifications. They don't have all of them, but do have a considerable library. One thing to remember is that quite a few of the AN/MS specs are being converted to NASM and C specifications.
To find a spec, hit their website and use the pull down menu under the Library tab.