proper bolt length and hard copy information

[ironworker]

I have an inspector whom wants my crew to install longer bolts (because the contractor does not have the correct length) and double or triple washer them. I am looking for hard copy printable specifications to prove to him this is a no no. I have been tought to put 1 bolt, 1 washer, 1 nut per 1 hole. the formula i have been tought is add 1 1/4 inch to the thickness of meterial and you answer is the length of bolt you need for a 3/4 and 7/8 diameter bolt shank and 1 1/2 for 1 inch diamter bolt shank. if i am wrong then so be it. but I do not want to waste time an materials.
jeff spoor
email at flaminganvil@yahoo.com

 

[Billybaru]

You might check the Research Council site and their handbook.

http://www.boltcouncil.org/

 

[nashjp]

Longer bolts can be beneficial. - The longer length being stretched allows for more relaxation of the joint after it has been tightened. This means you have more clamp load after the joint has set. A single thicker washer is recommended (but probably harder to find than your original bolts) each additional interface in a joint allows for more setting.

Multiple washers could hurt. - On structures exposed to heavy corrosion, I could see how multiple interfaces could make a minor corrosion problem much worse.

If your application needs future disassembly, some standards recommend a limited number of threads extending past the end of the nut. (This is so you have a hope of unscrewing the nut throut the corrosion).

My awareness is not focused on the structural field, if their are limitations put on the number of washers I am unaware of it. I hope someone else can tell you if there is. I recommend to always use a washer under the nut and bolt head to prevent abutment yield.

Good luck with your project

 

[butelja]

This web site probably has what you are looking for:

http://www.boltcouncil.org/download/RCSC Specification 6-23-2000.pdf

 

[evelrod]

Well, IW, I can tell you that I have used multiple washers on many occasions over the years. In the LA area, inspectors usually do not object to engineering specs that require multiple washers on overlength bolts. When bolts first started replacing rivets (yes, I AM that old) we used TWO washers on each bolt, one on the head and one on the nut. I think $$$ caused the current one washer setup. At any rate, I've never had a job rejected for using more than one washer.
From a long time 'raising gang pusher', be sure that you get that RFI in your hand , anyway!


Rod


PS---That 1 1/4 rule is just a 'rule of thumb'. Most A325 and A490 bolts are acceptable these days with a 'flush nut' after proper torquing ie, 100% thread engagement. I rather like a thread or two sticking out, but that is NOT the general case here.
Check out

http://www.boltscience.com/pages/tighten.htm

and the FAQ of this forum.

 

[WKTaylor]

ironworker...

Standard aerospace DESIGN practice for a tension rated BOLT/NUT joint is to allow: (1) thick WASHER under the BOLT-head [chamfered ID-lip to clear head-shank fillet radius of bolt]; and (1) thick WASHER under the NUT .

Most Repair and Liaison Handbooks allow up-to [b](1) additional[/b] thick WASHER under either the NUT or the BOLT-head WASHER to accomodate the next-longer grip-length BOLT.

Washer-stack-ups beyond the repair allowable are discouraged for several obvious, and not-so-obvious, reasons, such as:

a. Increased corrosion potential and sealing problems.
b. Increased BOLT elasticity [elastic strain is larger].
c. Stack-slippage and joint relaxation [washer IDs are traditionally 0.008"--0.016" larger than the bolt shank, so some slippage can occur within the ID].
d. Clearance problems with adjacent structure or systems elements.
e. Increased weight and parts count [co$t].
f. Potential increase in bolt-bending loads due to shear.

WARNING: WASHER compressive strength MUST be matched to the BOLT/NUT combination!!! I have seen low-yield strength washers score/crush in-service under high strength BOLT-heads or NUTS!!! A classic [sad] example is a 50KSI-Yield WASHER under the head of a 220KSI BOLT that was head-torqued into position. The WASHER scored on installation due to high torque values; and then "crushed" [yielded] when subjected to high vibratory shear-loading. The Joint would deteriorate very quickly in-service!! My field-tests with high yield-strength washers eliminated this problem... but the "owning engineering agency" would not listen to my recommendations to use a higher yield-strength WASHER because it hadn't seen this situation on acft comming to depot for overhaul. I found-out later most units changed the components and installed NEW BOLTS/WASHERS just before flying to depot maintenence!!!! Regards, Wil Taylor

 

[evelrod]

Ironworker, I don't recall any data on A307 washers, etc. but in my previous post I assumed the use of A 325 or better bolts AND washers. Soft washers are, and have ALWAYS been, a no no on hard bolts.

Will, at one time my oldest worked for Falcon and it was rather common practice to replace bolts and washers that were incorrect for their application on customers acft.
On the older vintage stuff I fool with, you would not believe what we find. I guess 'out of site, out of mind' was the credo. Perhaps these acft were set up for static display and later returned to flying status without proper rebuild???


Rod

 

[nashjp]

Hello, decided to add two more cents worth

I usually am encouraging designers to have longer bolt stretch lengths. Bolt joints often fail from bolts that have to short a stretch length and almost never from bolts that are to long. Washers 'to me' are a must under both the nut and bolt head. I know some specs might allow only one, but the compressive force that a bolt can generate without a washer to help distribute the load over more area, can yield almost all commonly used steels.

wktaylor, please explain your reason for not liking washer stack ups (b. Increased BOLT elasticity [elastic strain is larger].) Did you mean increased abutment elasticity? >Abutment stiffness lower so more cyclic load is seen by bolt. Would the additional length/unit length mean cyclic stress amplitude in the bolt was lowered?