Sheared Fastener
Sheared Fastener
(OP)
Folks,
I'd really appreciate some metallurgy input on the fastener pictured below. It's a Rolok self threading fastener. The application is a bus basement bulkhead. The bulkhead is at the rear of the basement and is made up of 1.5"x1.5"x0.125" thin wall box tubing and 14 gauge sheet steel. At the base of the bulkhead there's a 2"x2"x0.25" angle iron member running transversely. There are 15 Rolok bolts clamping the angle iron, at the base of the bulkhead to the bus basement floor. The bulkhead serves as a fixture for the drive axle articulating control arm pickup joints. We're experiencing shear of these bolts. The buses have been in service for 10 years or more, but with infrequent use, typically less than 150K miles. Thanks.
I'd really appreciate some metallurgy input on the fastener pictured below. It's a Rolok self threading fastener. The application is a bus basement bulkhead. The bulkhead is at the rear of the basement and is made up of 1.5"x1.5"x0.125" thin wall box tubing and 14 gauge sheet steel. At the base of the bulkhead there's a 2"x2"x0.25" angle iron member running transversely. There are 15 Rolok bolts clamping the angle iron, at the base of the bulkhead to the bus basement floor. The bulkhead serves as a fixture for the drive axle articulating control arm pickup joints. We're experiencing shear of these bolts. The buses have been in service for 10 years or more, but with infrequent use, typically less than 150K miles. Thanks.





RE: Sheared Fastener
RE: Sheared Fastener
Good luck!
RE: Sheared Fastener
You can look at bolts that have not broken for signs of the start of failure.
Bolts could be over tightened leading to tensile failure.
They could be under tightened causing either fatigue or overload of the remaining bolts.
Or it could be a corrosion fatigue mechanism.
Only partially failed bolts will tell you for sure.
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Plymouth Tube
RE: Sheared Fastener
They are breaking and therefore not fit for purpose.
You say that they have infrequent use - only 150k miles!!!! What is the design life you expect?
You can carry out as much analsyis as you like but they will still shear if they are not strong enough.
It may be difficult and costly to prove but stress corrosion fatigue probably has a hand in these failures.
As they hold in drive axles the application does seem somewhat 'Safety Critical'
I suggest you use either larger screws or rivnuts and bolts.
I would also think that an A4 Marine Grade fastener might not be a bad plan.
The alternative is that if they have only failed after 10 years replace them every 5 years.
RE: Sheared Fastener
I have a couple of additional comments on such an investigation. First, I neglected to mention performing SEM fractographic examination after cleaning surfaces. This step is probably not necessary if the mechanism turns out to be fatigue as you should have all the clues you need from optical examination. The other point is the necessity to attempt to identify which bolt failed first if several have already broken in the assembly. The reason is the first bolt failure will automatically increase static applied load of the remaining bolts (and increase amplitude of cyclic loading if that applies), making those bolts more susceptible to subsequent failure. You will need to compare all of the fracture surfaces with each other to find that first failed bolt; that will be the one you need to subsequently focus on and characterize. (A recent example for me involved an assembly with 8 bolts; my ground zero bolt exhibited reverse bending fatigue and almost no final fracture area, indicating it had been insufficiently tightened).
As for corrosion: you should ask yourself if bolts are supposed to see a wet corrosive environment. If you determine mode of fracture as fatigue, you will also want to determine how much, if any, effect corrosion would have had. Just because the mechanism is technically corrosion-assisted fatigue does not mean failure occurred primarily from corrosion exposure.
RE: Sheared Fastener
RE: Sheared Fastener