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Bolt preload vs failure in shear
thread404-438548 Forum Search FAQs Links MVPsForum
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MVPs
(Mechanical)
(OP)
2 May 18 15:47We 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!
(Aerospace)
2 May 18 16:46In the analysis world, the shear loadpath between the plates is the bolt. bolt allowable is tension-shear interaction. So higher tension = less allowable shear.In the real world there are two shear loadpaths ... the bolt and friction between the plates. then practically higher preload means more shear carried by friction !?
another day in paradise, or is paradise one day closer ?
(Mechanical)
2 May 18 17:31Chiwahua,Let'signore friction. If the bolts are clamped down hard enough, plates are held by friction, and there is no shear load on the bolt. If the bolt has come loose or is deliberately assembled loosely, there is no friction, and the joint is held entirely by shear. There is a transition where there is some friction clamping and some shear, but for most joints, this is a very tiny range of nut movement. Would you really try to design for this state? This is mostly a failure condition.Your bolt torques have no meaning unless you tell us the size of your bolts and what they are clamping.
--
JHG
(Mechanical)
2 May 18 21:49If a bolt is used properly it induces enough friction that the bolt is only in tension. Your observation is a good example of why bolts should be properly torqued.
I used to count sand. Now I don't count at all.
(Materials)
2 May 18 22:24And if they are not torqued enough you might load them in bending also, then the whole thing is short.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
(Mechanical)
3 May 18 11:32" a bolt that fails in shear."One bolt in a pattern of 6 holding a single component?One bolt holding a single component, like serving as the axle for a pulley or wheel.What is the evidence it is a shear failure ?Does your bolt's fracture surface look like any of these?Please post twenty seven eight-by-ten color glossy photographs with circles and arrows and a paragraph on the back of each one explaining what each one was to be used as evidence.
(Mechanical)
(OP)
3 May 18 13:05Thanks for your answers so far.At first my post was more about the theory that the higher the tension in the bolt, the less it would take to shear it.But as I understand you want to know more about the reason why I ask the question.So here is an image.It's a load-lifting mechanism (cylinder is black and the mechanism is yellow) attached to a chassis (grey) and the pivot is orange and you see the locking mechanism that is a 5/8-11 Grade 8 bolt that is running through the "locking ring" welded onto the chassis. The shaft is 2"OD, the locking ring is 3" OD.At the installation, the torque on the bolt is not controlled (no torque wrench). The bolt is installed with an impact wrench. From my research, the impact could give a torque up to 600lbs.ft. Not that I have reasons to believe it was torque this high but I just don't know how each mechanic "insists" on the impact.. The suggested torque for this fasteners is somewhere between 170-220 (lub-dry).We've had failures in service that looks like double shear I believe:So, obviously we will need to specify the torque, and since I suspect over-torquing being a part of the problem, I would tend to keep the torque minimal to allow more room to withstand higher shear but I know that doing this, I'm removing the friction from my side but from what FEA have shown me is that a 200 lbs.ft would not squeeze the shaft enough that the bolt would not see any shear.Also, If you have good shaft-locking mechanisms you want to share, I'll be glad to consider.Thanks again!
(Mechanical)
3 May 18 13:17Chiwahua,Okay. That has no resemblance to anything I was conceiving, above. Regardless of how your bolt is tightened, you have a full shear load on it. If you tighten the bolt hard, you have combined stresses, and a rather complicated analysis. You need Von Mises stress (Tresca stress?), not a fastener manual.Can you drill the bolt and use a castle nut and cotter pin?
--
JHG
(Aerospace)
3 May 18 13:20I don't understand the pictures relative to the drawing. The drawing shows a hinge pin joining a bunch of pieces together with a cross bolt at the end (to prevent travel of the hinge pin). The pictures look to have a plain slug in lieu of the bolt?That said, what's being tightened by the impactor ? From the drwgs I assume the cross bolt ? So if the failure is these bolts are being Way over-tightened (nothing being clamped up, just a nut being tightened on shaft) and then there's some lateral load applied in operation, shearing the cross bolt. In this case it is a classical tension-shear interaction, Rt^2+Rs^2 = 1. My suggested solution is to torque the nut with a wrench; if you want to control the torque, then use a torque wrench. Or else add a spring element under the nut, so there is some compression available and some visible sign about how tight the nut is.Do you understand where the lateral load is coming from ? Is it expected ? Or is it a load not intended by the original design. If so, then you should re-design the joint to account for this lateral load.
another day in paradise, or is paradise one day closer ?
(Mechanical)
3 May 18 13:28Chiwahua,Have you analyzed this thing at all? For a 5/8-11UNC Grade 8 bolt, at 90% of yield stress and no lubrication, I am getting about 250lb.ft of torque maximum. That bolt looks long enough that you can control the tension by calculating turns of the nut. What is pulling your pivot out of the hole?
--
JHG
(Mechanical)
(OP)
3 May 18 14:16So from your comments, having a minimal torque (let's say 50 lbs.ft) would kind of make sense? To have plenty of room for the shearing.Here is a non-exploded view:The pictures above showed a shaft half-removed (man-removed, and not a sideways load) and remainings of bolts. That looks to have double sheared, right?rb1957, the equation Rt^2+Rs^2 = 1 puzzles me a bit.Rt would stand for (tension stress ratio) andRs would stand for (shear stress ratio),Rt^2+Rs^2 = 1 should be <1 instead, right?drawoh, I know the bolt would not torque up to 600 lbs.ft It would break much before. I'm just saying the impact could torque up to 600, meaning that maybe the bolt was over-torqued but not knowing at what point.
(Aerospace)
3 May 18 14:43yes ... MS = 1/sqrt(Rt^2+Rs^2) -1the parts (the small pin) in the pic just don't look like a bolt.part of your problem is that you've got a "gorilla" with an impacter and he's torquing the cr@p out of the nut ... surprised you're not finding the bolt in two pieces (on installation). Find the picture about "who decides the torque on a bolt". As currently designed I think the joint is too rigid for the simple tools (and tool holders?) that you want/need to use. I thinkyou need to introduce some soft element (copper washer ?) that'll collapse under load.
another day in paradise, or is paradise one day closer ?
(Mechanical)
3 May 18 14:46Chiwahua,I repeat -- you need to do an analysis.
--
JHG
(Mechanical)
3 May 18 15:06I see no reason for there to be any shear load. Use a lynch pin instead of a bolt and nut.
Ted
(Mechanical)
(OP)
3 May 18 15:14Drawoh:
yes, I have started an analysis on this. It's just, that in the mean time, I'm asked to specify a torque for this bolt before I got all the results and I obviously want to be on the safe side.
yes it makes sense that the pivot is forced to turn (and shear the bolt, the lifting mechanism rotates around the pivot and friction is present.
so far, the FEAs give me around ~11 000 lbs shearing force at each interface.
rb1957: what do you mean the "joint is too rigid"?
(Structural)
3 May 18 15:18You have a problem that is different that what you suspect. The keeper's, i.e., the bolt, purpose is to keep the pin from working out when the machine is in operation. The various components,i.e., the hydraulic cylinder should be rotating freely on the pin. If there is insufficient or no lubrication on the pin, the hydraulic cylinder is forcing the pin to rotate and shear both ends of the keeper (as seen in the photograph). I suspect the pin and the hydraulic cylinder are galled and roughened. Now, no amount of lubrication is going to help. The clevis end of the hydraulic cylinder needs to be repaired and a new pin with grease fittings installed. The pins do have a grease fitting right?
Best regards - Al
(Structural)
3 May 18 15:19Almost forgot to complement Tmoose for the photographs. Excellent my friend!
Best regards - Al
(Mechanical)
(OP)
3 May 18 15:31gtaw: There is grease that should be applied through a grease fitting in the pivot pin and you can see the greasing holes in the pivot. We have no way of knowing if the grease is applied regularly at our customer's.
(Mechanical)
3 May 18 15:41Chiwahua,I have another thought here. Is your bolt failing at 1.5×capacity, or at 15×capacity? Perhaps someone has unwittingly designed a shear pin, and it is preventing a more catastrophic failure. You need to thoroughly understand how your pivot works. This is not FEA. This is mechanics of machines and machine design.
If you want to learn more, please visit our website TRM.
--
JHG
(Aerospace)
3 May 18 16:13"so far, the FEAs give me around ~11 000 lbs shearing force at each interface." but this shear is carried by the hinge pin; this is not the shear that is causing the failure (which looks to be axial along the pin, yes?) or is this force acting along the hinge pin axis ?rb1957: what do you mean the "joint is too rigid"? ... it looks to me has though you are putting a bolt into a hole in a solid shaft (as a cross bolt) then torquing the nut against the shaft. There is nothing to deflect under the tension from the torque, not flexibility in the joint, you're tightening the head of the bolt with the nut on the opposite side of the solid shaft. Without careful control of the torque it is easy to overload the bolt, it is difficult to know the torque you're applying.as others have noted, there are many possible failure causes or contributors
another day in paradise, or is paradise one day closer ?
(Mechanical)
3 May 18 17:10I 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.
(Mechanical)
3 May 18 17:44This study found that the shear capacity varied no more with preload than between bolt sample material variation.
Ted
(Mechanical)
(OP)
3 May 18 17:55Tmoose: 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.
(Mechanical)
3 May 18 18:57Chiwahua,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
(Mechanical)
3 May 18 19:57I 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.
(Mechanical)
3 May 18 20:13 ---(Mechanical)
4 May 18 19:18Quote:
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.ORUse 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.
(Mechanical)
4 May 18 19:45EngineerTex,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
(Mechanical)
4 May 18 21:26Look 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.Ted
(Mechanical)
7 May 18 13:50hydtools,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
(Structural)
7 May 18 15:01Can 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
(Mechanical)
7 May 18 17:07drawoh,
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
(Mechanical)
7 May 18 17:22hydtools,What is shearing the bolt?
--
JHG
(Aerospace)
7 May 18 17:22we'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 ?
(Mechanical)
7 May 18 17:47drawoh,
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
(Mechanical)
9 May 18 06:56When 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.
(Aerospace)
9 May 18 11:59'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 ?
(Mechanical)
(OP)
9 May 18 12:10I 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.
(Mechanical)
9 May 18 21:32How 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.
(Structural)
11 May 18 16:51A 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 atultimate load there is little preload left in the bolt. The shearingdeformations that have taken place in the bolt prior to its failure have the effect ofreleasing the rather small amount of axial deformation that was used to induce thebolt preload during installation."Pre-load may not be a limiting factor.
Thanks,
Mark
(Aerospace)
11 May 18 21:37that feels odd to me ... I see the bolt under tension, initially from preload then increasing with applied tension until the joint gaps (consistent with relaxed preload) and all the applied tension is now carried by the bolt.
another day in paradise, or is paradise one day closer ?
(Mechanical)
11 May 18 21:52rb1957, the bolt failed in shear.I don't understand how you see additional tension being applied to the bolt. Unless you see the pivot pin seriously bending and prying the collar into an ob-round shape through which the bolt runs.
Ted
(Aerospace)
11 May 18 22:14I was replying to the general situation, as the previous post discussed.In this case, of course, there is no additional tension from the load. In this case I see no mechanism to relieve the preload (as suggested by the previous post).
another day in paradise, or is paradise one day closer ?
(Aerospace)
12 May 18 02:09My first guess is that it is fatigue failure. Since the holes and the pivot are not reamed for a close fit one end will see the majority of the load and partially fracture, allowing enough motion for the other end to take the load and then fracture. Eventually they even up, taking turns and advancing the crack until one end completely fails, and then the other is too weak and it also fails.
It looks like side loading on the arm could provide a significant prying load, overcoming any lubrication. The outer bores should probably be the bearing surfaces, allowing the pin to be fixed to the arm and simplifying lubrication.
Having seen similar pin applications, I can say that the grease passages are very likely inadequate to clear the thickener when attempting to introduce new grease, that the volume of grease involved is inadequate to store enough oil, and that what grease does manage to get in will not be well distributed. The grease to the center will end up as solidified thickener and the grease jockey will see grease take the short path right out the ends, making a mess and not lubricating the joint.
(Structural)
14 May 18 14:19My apologies rb1957, I did not include enough context for the Kulak quote.The explanation is in regards to pre-loaded bolts under pure shear testing. Tension pre-loading to levels near yield did not reduce the pure shear failure loading.
Thanks,
Mark
(Aerospace)
14 May 18 15:02'k; I wonder why the preload relaxes under shear loading ?
another day in paradise, or is paradise one day closer ?
(Mechanical)
14 May 18 15:56It could be that the shear deformation increases the effective length of the bolt enough to reduce the preload stretch.
Ted
(Mechanical)
14 May 18 21:47While I glossed over some of the middle of this thread, I caught the end of it and wanted to throw something out there.Some time ago I had a discussion with a structural engineer on anchor bolts / anchor rods who mentioned that under a shear load the effect of Poisson's ratio was to elongate the bolt, thereby relieving a bit of the preload / pretension in the bolt. I hadn't thought about that until our discussion.Somewhat related, I found a paper that address lap joints and the effect of longitudinal loads on the preloads. I didn't read the entire article, but it seemed like the Poisson's effect of pulling at the lap joints reduced the thickness slightly enough to reduce the preload.
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Bolt preload vs failure in shear
thread404-438548 Forum Search FAQs Links MVPsForum
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FAQs
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MVPs
(Mechanical)
(OP)
2 May 18 15:47We 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!
(Aerospace)
2 May 18 16:46In the analysis world, the shear loadpath between the plates is the bolt. bolt allowable is tension-shear interaction. So higher tension = less allowable shear.In the real world there are two shear loadpaths ... the bolt and friction between the plates. then practically higher preload means more shear carried by friction !?
another day in paradise, or is paradise one day closer ?
(Mechanical)
2 May 18 17:31Chiwahua,Let'signore friction. If the bolts are clamped down hard enough, plates are held by friction, and there is no shear load on the bolt. If the bolt has come loose or is deliberately assembled loosely, there is no friction, and the joint is held entirely by shear. There is a transition where there is some friction clamping and some shear, but for most joints, this is a very tiny range of nut movement. Would you really try to design for this state? This is mostly a failure condition.Your bolt torques have no meaning unless you tell us the size of your bolts and what they are clamping.
--
JHG
(Mechanical)
2 May 18 21:49If a bolt is used properly it induces enough friction that the bolt is only in tension. Your observation is a good example of why bolts should be properly torqued.
I used to count sand. Now I don't count at all.
(Materials)
2 May 18 22:24And if they are not torqued enough you might load them in bending also, then the whole thing is short.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
(Mechanical)
3 May 18 11:32" a bolt that fails in shear."One bolt in a pattern of 6 holding a single component?One bolt holding a single component, like serving as the axle for a pulley or wheel.What is the evidence it is a shear failure ?Does your bolt's fracture surface look like any of these?Please post twenty seven eight-by-ten color glossy photographs with circles and arrows and a paragraph on the back of each one explaining what each one was to be used as evidence.
(Mechanical)
(OP)
3 May 18 13:05Thanks for your answers so far.At first my post was more about the theory that the higher the tension in the bolt, the less it would take to shear it.But as I understand you want to know more about the reason why I ask the question.So here is an image.It's a load-lifting mechanism (cylinder is black and the mechanism is yellow) attached to a chassis (grey) and the pivot is orange and you see the locking mechanism that is a 5/8-11 Grade 8 bolt that is running through the "locking ring" welded onto the chassis. The shaft is 2"OD, the locking ring is 3" OD.At the installation, the torque on the bolt is not controlled (no torque wrench). The bolt is installed with an impact wrench. From my research, the impact could give a torque up to 600lbs.ft. Not that I have reasons to believe it was torque this high but I just don't know how each mechanic "insists" on the impact.. The suggested torque for this fasteners is somewhere between 170-220 (lub-dry).We've had failures in service that looks like double shear I believe:So, obviously we will need to specify the torque, and since I suspect over-torquing being a part of the problem, I would tend to keep the torque minimal to allow more room to withstand higher shear but I know that doing this, I'm removing the friction from my side but from what FEA have shown me is that a 200 lbs.ft would not squeeze the shaft enough that the bolt would not see any shear.Also, If you have good shaft-locking mechanisms you want to share, I'll be glad to consider.Thanks again!
(Mechanical)
3 May 18 13:17Chiwahua,Okay. That has no resemblance to anything I was conceiving, above. Regardless of how your bolt is tightened, you have a full shear load on it. If you tighten the bolt hard, you have combined stresses, and a rather complicated analysis. You need Von Mises stress (Tresca stress?), not a fastener manual.Can you drill the bolt and use a castle nut and cotter pin?
--
JHG
(Aerospace)
3 May 18 13:20I don't understand the pictures relative to the drawing. The drawing shows a hinge pin joining a bunch of pieces together with a cross bolt at the end (to prevent travel of the hinge pin). The pictures look to have a plain slug in lieu of the bolt?That said, what's being tightened by the impactor ? From the drwgs I assume the cross bolt ? So if the failure is these bolts are being Way over-tightened (nothing being clamped up, just a nut being tightened on shaft) and then there's some lateral load applied in operation, shearing the cross bolt. In this case it is a classical tension-shear interaction, Rt^2+Rs^2 = 1. My suggested solution is to torque the nut with a wrench; if you want to control the torque, then use a torque wrench. Or else add a spring element under the nut, so there is some compression available and some visible sign about how tight the nut is.Do you understand where the lateral load is coming from ? Is it expected ? Or is it a load not intended by the original design. If so, then you should re-design the joint to account for this lateral load.
another day in paradise, or is paradise one day closer ?
(Mechanical)
3 May 18 13:28Chiwahua,Have you analyzed this thing at all? For a 5/8-11UNC Grade 8 bolt, at 90% of yield stress and no lubrication, I am getting about 250lb.ft of torque maximum. That bolt looks long enough that you can control the tension by calculating turns of the nut. What is pulling your pivot out of the hole?
--
JHG
(Mechanical)
(OP)
3 May 18 14:16So from your comments, having a minimal torque (let's say 50 lbs.ft) would kind of make sense? To have plenty of room for the shearing.Here is a non-exploded view:The pictures above showed a shaft half-removed (man-removed, and not a sideways load) and remainings of bolts. That looks to have double sheared, right?rb1957, the equation Rt^2+Rs^2 = 1 puzzles me a bit.Rt would stand for (tension stress ratio) andRs would stand for (shear stress ratio),Rt^2+Rs^2 = 1 should be <1 instead, right?drawoh, I know the bolt would not torque up to 600 lbs.ft It would break much before. I'm just saying the impact could torque up to 600, meaning that maybe the bolt was over-torqued but not knowing at what point.
(Aerospace)
3 May 18 14:43yes ... MS = 1/sqrt(Rt^2+Rs^2) -1the parts (the small pin) in the pic just don't look like a bolt.part of your problem is that you've got a "gorilla" with an impacter and he's torquing the cr@p out of the nut ... surprised you're not finding the bolt in two pieces (on installation). Find the picture about "who decides the torque on a bolt". As currently designed I think the joint is too rigid for the simple tools (and tool holders?) that you want/need to use. I thinkyou need to introduce some soft element (copper washer ?) that'll collapse under load.
another day in paradise, or is paradise one day closer ?
(Mechanical)
3 May 18 14:46Chiwahua,I repeat -- you need to do an analysis.
--
JHG
(Mechanical)
3 May 18 15:06I see no reason for there to be any shear load. Use a lynch pin instead of a bolt and nut.
Ted
(Mechanical)
(OP)
3 May 18 15:14Drawoh:
yes, I have started an analysis on this. It's just, that in the mean time, I'm asked to specify a torque for this bolt before I got all the results and I obviously want to be on the safe side.
yes it makes sense that the pivot is forced to turn (and shear the bolt, the lifting mechanism rotates around the pivot and friction is present.
so far, the FEAs give me around ~11 000 lbs shearing force at each interface.
rb1957: what do you mean the "joint is too rigid"?
(Structural)
3 May 18 15:18You have a problem that is different that what you suspect. The keeper's, i.e., the bolt, purpose is to keep the pin from working out when the machine is in operation. The various components,i.e., the hydraulic cylinder should be rotating freely on the pin. If there is insufficient or no lubrication on the pin, the hydraulic cylinder is forcing the pin to rotate and shear both ends of the keeper (as seen in the photograph). I suspect the pin and the hydraulic cylinder are galled and roughened. Now, no amount of lubrication is going to help. The clevis end of the hydraulic cylinder needs to be repaired and a new pin with grease fittings installed. The pins do have a grease fitting right?
Best regards - Al
(Structural)
3 May 18 15:19Almost forgot to complement Tmoose for the photographs. Excellent my friend!
Best regards - Al
(Mechanical)
(OP)
3 May 18 15:31gtaw: There is grease that should be applied through a grease fitting in the pivot pin and you can see the greasing holes in the pivot. We have no way of knowing if the grease is applied regularly at our customer's.
(Mechanical)
3 May 18 15:41Chiwahua,I have another thought here. Is your bolt failing at 1.5×capacity, or at 15×capacity? Perhaps someone has unwittingly designed a shear pin, and it is preventing a more catastrophic failure. You need to thoroughly understand how your pivot works. This is not FEA. This is mechanics of machines and machine design.
--
JHG
(Aerospace)
3 May 18 16:13"so far, the FEAs give me around ~11 000 lbs shearing force at each interface." but this shear is carried by the hinge pin; this is not the shear that is causing the failure (which looks to be axial along the pin, yes?) or is this force acting along the hinge pin axis ?rb1957: what do you mean the "joint is too rigid"? ... it looks to me has though you are putting a bolt into a hole in a solid shaft (as a cross bolt) then torquing the nut against the shaft. There is nothing to deflect under the tension from the torque, not flexibility in the joint, you're tightening the head of the bolt with the nut on the opposite side of the solid shaft. Without careful control of the torque it is easy to overload the bolt, it is difficult to know the torque you're applying.as others have noted, there are many possible failure causes or contributors
another day in paradise, or is paradise one day closer ?
(Mechanical)
3 May 18 17:10I 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.
(Mechanical)
3 May 18 17:44This study found that the shear capacity varied no more with preload than between bolt sample material variation.
Ted
(Mechanical)
(OP)
3 May 18 17:55Tmoose: 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.
(Mechanical)
3 May 18 18:57Chiwahua,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
(Mechanical)
3 May 18 19:57I 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.
(Mechanical)
3 May 18 20:13 ---(Mechanical)
4 May 18 19:18Quote:
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.ORUse 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.
(Mechanical)
4 May 18 19:45EngineerTex,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
(Mechanical)
4 May 18 21:26Look 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.Ted
(Mechanical)
7 May 18 13:50hydtools,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
(Structural)
7 May 18 15:01Can 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
(Mechanical)
7 May 18 17:07drawoh,
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
(Mechanical)
7 May 18 17:22hydtools,What is shearing the bolt?
--
JHG
(Aerospace)
7 May 18 17:22we'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 ?
(Mechanical)
7 May 18 17:47drawoh,
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
(Mechanical)
9 May 18 06:56When 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.
(Aerospace)
9 May 18 11:59'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 ?
(Mechanical)
(OP)
9 May 18 12:10I 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.
(Mechanical)
9 May 18 21:32How 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.
(Structural)
11 May 18 16:51A 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 atultimate load there is little preload left in the bolt. The shearingdeformations that have taken place in the bolt prior to its failure have the effect ofreleasing the rather small amount of axial deformation that was used to induce thebolt preload during installation."Pre-load may not be a limiting factor.
Thanks,
Mark
(Aerospace)
11 May 18 21:37that feels odd to me ... I see the bolt under tension, initially from preload then increasing with applied tension until the joint gaps (consistent with relaxed preload) and all the applied tension is now carried by the bolt.
another day in paradise, or is paradise one day closer ?
(Mechanical)
11 May 18 21:52rb1957, the bolt failed in shear.I don't understand how you see additional tension being applied to the bolt. Unless you see the pivot pin seriously bending and prying the collar into an ob-round shape through which the bolt runs.
Ted
(Aerospace)
11 May 18 22:14I was replying to the general situation, as the previous post discussed.In this case, of course, there is no additional tension from the load. In this case I see no mechanism to relieve the preload (as suggested by the previous post).
another day in paradise, or is paradise one day closer ?
(Aerospace)
12 May 18 02:09My first guess is that it is fatigue failure. Since the holes and the pivot are not reamed for a close fit one end will see the majority of the load and partially fracture, allowing enough motion for the other end to take the load and then fracture. Eventually they even up, taking turns and advancing the crack until one end completely fails, and then the other is too weak and it also fails.
It looks like side loading on the arm could provide a significant prying load, overcoming any lubrication. The outer bores should probably be the bearing surfaces, allowing the pin to be fixed to the arm and simplifying lubrication.
Having seen similar pin applications, I can say that the grease passages are very likely inadequate to clear the thickener when attempting to introduce new grease, that the volume of grease involved is inadequate to store enough oil, and that what grease does manage to get in will not be well distributed. The grease to the center will end up as solidified thickener and the grease jockey will see grease take the short path right out the ends, making a mess and not lubricating the joint.
(Structural)
14 May 18 14:19My apologies rb1957, I did not include enough context for the Kulak quote.The explanation is in regards to pre-loaded bolts under pure shear testing. Tension pre-loading to levels near yield did not reduce the pure shear failure loading.
Thanks,
Mark
(Aerospace)
14 May 18 15:02'k; I wonder why the preload relaxes under shear loading ?
another day in paradise, or is paradise one day closer ?
(Mechanical)
14 May 18 15:56It could be that the shear deformation increases the effective length of the bolt enough to reduce the preload stretch.
Ted
(Mechanical)
14 May 18 21:47While I glossed over some of the middle of this thread, I caught the end of it and wanted to throw something out there.Some time ago I had a discussion with a structural engineer on anchor bolts / anchor rods who mentioned that under a shear load the effect of Poisson's ratio was to elongate the bolt, thereby relieving a bit of the preload / pretension in the bolt. I hadn't thought about that until our discussion.Somewhat related, I found a paper that address lap joints and the effect of longitudinal loads on the preloads. I didn't read the entire article, but it seemed like the Poisson's effect of pulling at the lap joints reduced the thickness slightly enough to reduce the preload.
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