Bolt preload vs failure in shear 1

[Chiwahua]

We have a case where we have a bolt that fails in shear.

Am I wrong to think that:

the higher the tension in the bolt, the less it takes to be able to shear the bolt?!

For example:

Bolt 1 is torqued at 600 lbs.ft
Bolt 2 is torqued at 200 lbs.ft

Let's pretend the friction due to the clamping force doesn't exist, it would takes less force to shear the bolt 1, right?

Thanks a lot for your help!

 

[Tmoose]

I see no detailed close up pictures of the bolt fracture surfaces yet.

I'd say there is way too much friction in the pivoting joints. Bad finishes, poor size and roundness, inadequate lubrication, undersized for the applied loads, distortion of the structures moving the bores out of alignement, etc,

The orange pivot looks galled and fretted. Do the bores in the grey chassis look equally torn up? How about the cylinder eye and yellow mechanism bores, where rotation/oscillation is expected to occur?


The bolt should be living an easy life on the beach in margueritaville, if pivoting surfaces are lubricated and not seizing.
The body of the bolt fragment is dirty and dark like it is being shoved hard this way and that in service.

Where are the bolt fracture planes?
Is the "shearing" happening axially, or from orange pivot rotation ?
On the OD of the orange pivot?
On the OD of the grey chassis boss, under the nut and bolt head ?

Only Twenty five eight-by-ten color glossy photographs left to go.

 

[hydtools]

This study found that the shear capacity varied no more with preload than between bolt sample material variation.

Ted

 

[Chiwahua]

Tmoose: it's not that I don't want to show you 25 glossy pictures with vibrant colors. I wish I would be able to show you that.

These failure happens on the other side of the globe we can't get all the information when we need to. We know it fails, not sure if maintenance is properly greased. So Margaritaville is best case scenario. The grease can collect outside ambiant dirt as well.

For sure, a great deal of friction occurs while the mechanism rotates.

Thank you all for your help. I know there is a lot of missing information (and pictures). But you've all been helpful.

 

[drawoh]

Chiwahua,

You still can make assumptions, and then analyze the resulting forces and torques. Assume your pivot is not lubricated, and that the pieces are rusty. You are looking for a shear force sufficient to break a 5/8-11UNC grade[ ]8 bolt. When you find it, you can contact your customer and describe to them all the things you think are happening. If you are right, then your assumptions and you analysis probably are good, and you can engineer a solution to your problem.

That fact that this place is on the opposite side of the world makes it that much more important that you do engineering, and not just hack with the thing.

--
JHG

 

[SandCounter]

I notice the hole in the orange pivot for the through bolt intersects what is presumably the grease hold on the right end of the pivot. If grease is intended to be introduced through that hole on the end, it may not be making it through to the cylinder clevis.

It seems that your bolt (here acting as a pin) is shearing because the cylinder is seizing on the pivot. Is it necessary for the pivot to remain stationary? As an alternative to the bolt, and if welding is an option, I might suggest putting a slip-on flange on the pin housing and capping the housing with a blind flange. That would allow the pin to rotate inside the housing.

I used to count sand. Now I don't count at all.

 

[tmschrader]

[tt][/tt]---


HELLO If Gtaw's interpretation (and others) is correct the shear load is not directly transmitted to the bolt by the Cylinder. So better lubrication should reduce the shear and less torque. ?? I do not see the whole mechanism.
So why can't you allow the bolt to rotate and reduce the shear load? A bolt and washer on either end of the pin would restrain the pin but allow some rotation reducing the shear stress.


The OP does say there is alot of friction above. So it more then a good assumption the the pin's rotation does not have to be constrained. Let it roll around. No more shear.

 

[EngineerTex]

Also, If you have good shaft-locking mechanisms you want to share, I'll be glad to consider.

A couple of users here have mentioned that there is no reason to keep the shaft from turning.

Make the shaft longer and put the through-bolts on both sides and NOT through the sleeve. Add a thick washer between the bolt and the face of the boss.

OR

Use an actual off-the-shelf high-strength bolt for this instead of the shaft you have now. I had had a conversation about a similar challenge a few years ago with another engineer. We could find no reason not to use an actual fastener, if you have a good supplier and they will supply you with high-quality products. Find a hex-head cap screw or a socket-head cap screw where the unthreaded part extends past the full length of the joint. Retain a castle nut with a cotter pin and you should be set. Just make sure that the threads do not extend into the area under the sleeve, or you will have someone tighten the nut way more than you want. Regarding lubrication, you would have to move the grease fittings to the sleeves as opposed to being in the shaft. But you have to do that anyway. Once you have a path for the grease to come out in one of the bearing surfaces, you will probably get no lubrication to the other two. This potential lack of lubrication is likely to be a link in the chain for the failures you are already experiencing.

Engineering is not the science behind building. It is the science behind not building.

 

[drawoh]

EngineerTex,

We know we have a pivot that moves with enough force to shear a 5/8-11UNC grade[ ]8 bolt. If we remove the bolt, are we certain that nothing else will break. The bolt failure is annoying. Shearing of the pivot could be catastrophic. The OP needs to study the joint, and thoroughly understand how it works (or not). It appears there is too much friction in the joint. Probably what is needed is a bearing design, reliable even with sloppy assembly and maintenance.

Consider the possibility that the OP has inherited something designed by an idiot. Maybe the forces were not understood. Maybe the materials were not properly selected. Given the size of this thing and the forces involved, there could be a number of safety hazards present. This makes engineering all the more important.

The very first analysis I ever did was the hub on a 2m[ ]long truck-mounted telescope. A worm gear was (not) held to a Ø.625" shaft by two 10-32UNF set screws. I looked up the motor torque, and I looked into the way it was manhandled around the shop. I wound up recommending a Ø1.250" shaft with two 1/4" keys. Later, an electrical engineer told me that she had worked out the inertia of the telescope and the effect of the worm gear so she could size the stepper motor. She was not sure about her calcs, so she showed it to the mechanical engineer, and "his eyes glazed over". Fortunately, no one got hurt.

--
JHG

 

[hydtools]

Look at how excavator buckets are pinned to dipper sticks. A pin all the way through and a lynch pin through a crossdrilled hole in each end of the pin. Let the pin rotate.

http://www.brokentractor.com/john_d...ckhoe_pins_deere_310e_backhoe_bucket_pins.asp

Ted

 

[drawoh]

hydtools,

We know the pivot is being forced to move, probably in rotation, with enough force to break a big bolt. If we replace with bolt with an external pin or retaining ring, will the pivot rotate freely, or will one end rotate and the other end non rotate due to friction? There is at least one very serious friction contact in there. One friction contact is harmless. Two friction contacts could result in a broken pivot.

--
JHG

 

[dik]

Can you use a Belleville washer or a castellated nut and cotter pin? The outer part is too stiff and there is no friction 'clamping'. You need to restrain the nut and the tension is only putting a load on the bolt to act with the shear.

Dik

 

[hydtools]

drawoh,
The pivot pin will rotate freely as it has two external pins, one in each end. Make the pivot pins long enough to accommodate a washer between the retaining pin and frame shoulder. All is loose fit axially.

Ted

 

[drawoh]

hydtools,

What is shearing the bolt?

--
JHG

 

[rb1957]

we've raised lots of questions that can only be answered by the OP.

1) is the hinge pin seized, corroded, deformed ?

2) how did this happen ? is the lubrication ineffective ? or not applied ??

3) is this cross bolt designed as a structural fuze, designed to protect the rest of the structure from overload ?

4) are the simple assembly tools eroding the structural margins ? I think we think they are, and possibly reducing the effectiveness of the structural fuze. If the latter then the simplest redesign would be (other than changing the assembly tools) to add a spacer to the bolt so you'd ensure some small clearence between the lug and the hinge pin.

another day in paradise, or is paradise one day closer ?

 

[hydtools]

drawoh,
Clearly the pivot pin is being rotated with enough torque to shear the bolt.
That is assuming there is no high axial loading on the pin.
Ted

 

[enginesrus]

When enough pressure is applied to the pin by way of the hydraulic cylinder, it holds it like a nice vise, and enough torque is applied to shear the bolt. The fix is to bolt a plate on the one end and on the other machine the pin with a flange that looks similar to the plate bolted on the other or small diameter end. That way the pin is free to rotate and the plate just stops the pin from falling out. Depending on the pin diameter 2 bolts should be enough and their diameter related to the pin diameter.

 

[rb1957]

'cept if you consider that the machine isn't working as intended (or so we suspect). it's been posed that maybe the cross bolt is a structural fuze, protecting the rest of the structure from overload. it's been suggested that the issue is a seized hinge pin, possibly by corrosion. The original idea was that the cross bolts were over-torqued and failed at a low shear load; this could well be the issue.

The fix starts with understanding the problem (which we don't know well enough). There are plenty of redesigns that'll remove the cross bolt, and other fixes for the possible over-torque on installation, but is that fixing the problem ?

another day in paradise, or is paradise one day closer ?

 

[Chiwahua]

I really want to thank you all for taking the time to talk about this issue.

Many valuable comments that I will be sure to consider.

I'm still missing a lot of info right now about the current case. I'll let you know when I get something new to show and tell.

 

[Tmoose]

How many customers do you have, and how many are experiencing bolt failure ?

Requesting all the broken bolts back doesn't seem unreasonable, barring issues like value added/ scrap parts/ warrantee type paperwork issues when sending large stuff from Canada back to the US.

 

[seaaggie]

A rational from Kulac, et.al. 2001, is consistent with the comment by hydtools above.

"Measurements of the internal tension in bolts in joints have shown that at
ultimate load there is little preload left in the bolt. The shearing
deformations that have taken place in the bolt prior to its failure have the effect of
releasing the rather small amount of axial deformation that was used to induce the
bolt preload during installation."

Pre-load may not be a limiting factor.



Thanks,
Mark