Greater than 3/4" Fastener Application design

[alliance]

I'm searching for resources (web or personal visit to my plant) to work through an existing fastener application design that is failing.
Currently a quantity of (8) 3/4" grade 8 fasteners 6"long clamping 3 plates (1", 2", and 5/8" horziontal) togeather. Application is on a vibrating machine (rotational) and is holding a 50,000 lb vertical force at the plate face.
Bolts are torqued to 350ft-lb (90% proof).

How much improvement will I get by going to a 1" bolt?
How many problems will I introduce by:
1. Misaligned holes in the plate.
2. Plates with surfaces that are just mill finished.

I know this is a lot of scatered information, I'm just trying to give an idea of the environment to get a starting direction.

Thanks

 

[diamondjim]

You have approximately 3.875 inches clamping length
assuming you are using hardened heavy duty washers at each end as well as nuts? You might want to try very thick washers to increase your clamping length. You said that you have 350ft-lbs clamping force measure how? Lubed threads or dry threads.
I assume these are failing dynamically. The 1 inch bolts should give you more than 50 percent improvement but since you would still have the 3.875 inch clamping length and you want to have something like 5 times the clamping length to the diameter of the bolts. What is the area of the plate interfaces? Why are you torquing so high? I am used to
seeing 75 to 80 percent of the yield unless you have a
very accurate method of tightening the bolts.
More info would be appreciated. Are the bolts in a circular
pattern around the axis?

 

[evelrod]

To start off , I agree with Jim. However, the bolt torque you specify is a bit of a mistery. 350# seem a bit off for a grade 8 bolt(thats about right for grade 5). Thicker washers are good, line reaming bolt holes in place to a close tolerance is good, going to 1 inch ??????? I think grade 8 @ 3/4 inch is more than sufficient for your load. The plate surfaces must be clean and free of rust or paint. Obviously, NO threads in the shear plane. You might also go to NAS grade, a bit expensive , but---
Try

http://www.boltscience.com/

Might be able to think of more with a few more details. Rod

 

Here are my thoughts

Currently a quantity of (8) 3/4" grade 8 fasteners 6"long clamping 3 plates (1", 2", and 5/8" horziontal) togeather. Application is on a vibrating machine (rotational) and is holding a 50,000 lb vertical force at the plate face.

> is the joint a symmetrical or unsymmetrical group ?
> I am having probs. visulaizng this is the load along the axis of the bolt or in the joint plane ? Diagram is required

Bolts are torqued to 350ft-lb (90% proof).

> in my experience SAE material properties are alays quoted as a minimum. I doubt that 350 ft-lb will take you to 90% proof in practice.

How much improvement will I get by going to a 1" bolt?

> almost double the clamp load but 2.5 times the torque (820 ft-lb)

How many problems will I introduce by:
1. Misaligned holes in the plate.

> can cause problems in shear. One bolt can end up taking all of the load.

2. Plates with surfaces that are just mill finished.

> The higher the coefficent of friction between the plates the better the joint is in shear. Just make sure it is oil free and clean.

I know this is a lot of scatered information, I'm just trying to give an idea of the environment to get a starting direction.

> are you in the US or UK. I might be able to help you further if you give me more details. Contact me at fea.services@virgin.net.

Derek

 

[diamondjim]

I have a Lamson & Sessions bolt data sheet
and for grade 8 bolts 3/4-10unc stating that
350 ft-lbs if dry or 280 ft-lbs if lubed.
Again I must ask what kind of washers are you
using. I hope you are using hardened washers
and that they are plain and not lock washers.
The above values for tightening are for 75 per
of Yield. Are these bolts failing in shear?
Is the load inside or outside of the bolt arrange-
ment? They are failing after how many hours?
Again how are you tightening these bolts? The above
values assume a coefficient of friction of .20 and
.15 respectively. If they are lubed, you may be
using too much torque. I assume from vibration,
you may be seeing some unusual settling or imbedding
of the bolts. You have so many unaswered question,
it is hard to determine a final solution. Are you
retightening the bolts after x amount of hours?
The basic equation for amount of proof clamping force
is 120000 psi for grade 8 bolts times (D - .9743/N)squared times pi divided by 4. N is the number of threads per
inch and D is the bolt diameter.