Fastener failure
Fastener failure
(OP)
Does this look like a fracture failure in the fastener. 3 out of 5 bolts broke off of in a pressure washer pump head.
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RE: Fastener failure
Yep. Tensile overload/tensile bending fracture.
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams
RE: Fastener failure
Looks like a fatigue failure I can see some classic beach marks on the surface, how long was it in service.
RE: Fastener failure
Ted
RE: Fastener failure
RE: Fastener failure
The fractured bolt failed due to fatigue. Fatigue failures that have this specific type of appearance (crack initiation in the thread root, flat propagation area, final fracture due to tensile overload) are usually due to preload variation, fastener misalignment, or a manufacturing defect (thread lap in the root, etc.).
RE: Fastener failure
RE: Fastener failure
What method is specified to tighten the fasteners?
What method is actually used to tighten the fasteners?
RE: Fastener failure
My best guess is uneven torque on the bolts. These are head bolts, and should have a prescribed tightening sequence, probably torqued in steps, too (like 30%, 60%, then 100% of proper torque). If the head was replaced without the proper procedure, then the bolts would not be loaded evenly, when you add the cycling of the high pressure pump, the highest torqued bolt will receive the majority of the alternating load. Once that one goes, then next one, and on and on until it starts leaking so bad the operator has to turn it off.
rp
RE: Fastener failure
Don't jump to conclusions. The bolt in the picture has clearly experienced fatigue failure. Fatigue requires cyclic loading, which usually indicates insufficient preload in a fastener. I think redpicker has described a very likely scenario, and Tmoose asks some pertinent questions. Always compare fatigue fracture appearance with the matrix of sketches given in the ASM Handbook, which helps categorize the stress conditions.
The bolt also appears corroded, and this possible factor should be investigated. Cyclic loads + corrosive environment turns fatigue into corrosion fatigue, which requires lower initiation and propagation stresses.
I have no doubt the entire bolting SYSTEM (all components and the torquing procedure) needs a thorough review.
RE: Fastener failure
RE: Fastener failure
Ted
RE: Fastener failure
RE: Fastener failure
Whether or not it is a car has nothing to do with a multiple torque sequence. Any well-deigned manifold will have a prescribed torque sequence. Whether or not there are multiple steps is debatable, but even installing all the bolts "hand-tight" then applying the prescribed torque is a multiple torque sequence.
Whether or not the product was produced in China has absolutely nothing to do with, well, anything. If it bears the Briggs & Stratton name, then B & S engineering signed off on it as meeting their design requirements. I do have to say that I don't think I've every seen a 1/4-20 bolt on anything coming from China, but the last B&S pressure washer I worked on was built in the USA.
There is a 100% certainty that there are torque specs for those bolts. Every Briggs & Stratton service manual I've read had torque specs for every fastener on the product.
If the bolts are difficult to get to, then there is actually a much higher chance that the manufacturer torqued them properly. The manufacturer can afford special tools designed to specifically tighten those bolts properly, and it just makes good business sense to use the proper tools.
All three broke at the mating surfaces because that is where the bending stress in concentrated. Not really that surprising. The ratchet marks and beach marks on the fracture surface are clear evidence that bending fatigue was the mechanism. And, brimstoner is probably correct that corrosion-fatigue (corrosion-assisted fatigue, if you prefer) was an active mechanism.
If you've already decided on your remedial actions, this is probably a waste of time, but I think you'd be much better off getting a service manual from the manufacturer and following their procedures.
rp
RE: Fastener failure
Some big assumptions implicit there, for a chinese bolt....
RE: Fastener failure
Briggs and Stratton probably had specified the torque value
in some manual. I would guess the bolts to be grade 2 equivalent
if used with aluminum parts.
RE: Fastener failure
Before I did anything else I would have the bolts sent to a lab for analysis.
Further when a bolted joint is made the proportion of the external load seen by the bolt over and above its pre-load is determined by the stiffness of the joint material and the bolt itself.
Imagine a joint with a single bolt carrying 100N external load but the bolt is only preloaded to say 80N, the joint material then separates leaving the bolt to withstand the full external load, a bolt that is tightened above 100N pre-load will not separate and will only carry a portion of the external load dependant on the joint stiffness, therefore the bolt tightened to the highest torque does not see the highest portion of the external load which as been suggested in an earlier post.
regards
desertfox
RE: Fastener failure
At 4 lbs on a 1/4-20 the claming force looks to be 960 lbs x 5 = 4800. pressure at head is 2600. 3 pistons, swash plate design.
RE: Fastener failure
I took the values from the SPS literature. Their socket head capscrews are grade 8, 170,000psi yield strength alloy steel.
Ted
RE: Fastener failure
The tear on the far side is that due to tensile loading, probably the pre-load in the bolt. It is possible the bolts have an issue with them, you would need to see the manifesto for location, maybe plant number or machine lot.
I've seen bolting fail like this as the result of improper tooling when cutting the threads. May or may not be the issue here. Could also be chemistry for example, sulfide stringers embedded in the carbon steel alloy base or even some grain issues incurred during the manufacturing process.
But I agree with DesertFox and HydTool on their observations as well.
Regards,
Cockroach
RE: Fastener failure
Johnny Pellin
RE: Fastener failure
http://www.dalecompany.com/pdf/UNBRAKO.pdf
2.0 diameters minimum is pretty safe.
Note their comment on page 64/66 about attempts to compute lengths of tapped hole thread engagement and related factors "by formula" being "not entirely satisfactory". Thus what they chose to present in the technical section of their "engineering guide" were tables created using empirical data from tests done with non-digital threaded test pieces, real bolts, and torque wrenches of some type. Seems like something that could be confirmed pretty easily if I doubted UNBRAKO's credentials or intent, or feared typos had snuck in.
RE: Fastener failure
RE: Fastener failure