Grade 9 Hardware

[Schwartz]

Has anyone come across good design information for grade 9 hardware, such as endurance limits or even Goodman diagrams? I also need to find reference material concerning the tightening of fasteners to near yield and past yield, and what effect cyclic loading may have on these fasteners.

I am in the process of trying to get information from ARP, and I will see what SPS has to say. They both manufacture high-strength hardware. Unless there is a reference book out there that covers fastener strengths above grade 8, I imagine the information I need will have to come from a manufacturer.

My problem is with a wheel joint on a diesel powered 50 tonne mine truck. Grade 8 bolts have broken at an alarming rate, and a quick trial with grade 9 bolts torqued to 85% proof revealed that they broke in half the time or less. We have since beefed up the wheel mount flange to try and stabilize the joint, but we do not know it's effect yet. We still believe our fasteners are marginal at grade 8, but we are hesitant to go with grade 9 again because the joint may continue to move, and they seem to be much more brittle. Testing with load washers has revealed cyclic loading of the grade 8 bolts, and fatigue may be a factor.

Anyone have experience with off-the-shelf grade 9 hardware such as that from PFC?

Any help would be appreciated.

Greg Schwartzenberger
Atlas Copco Wagner, Inc.
Portland, OR

 

[MikeMilburn]

Greg, Not having any experience with mine vehicles just regular on road tandems, singles etc., we had a similar problem with bolt piloted wheels and ended up changing to hub piloted. Don't know if the circumstances are even near the same, just had to butt in and add my 2cents worth...Mike

 

[butelja]

One thing to be wary of is that most "better than Grade 8" fasteners are a brittle as glass. The only manufacturers of these types of fasteners that I've heard any good things about are the 2 you mentioned in your post.

You may want to retorque the fasteners periodically. Many times broken bolts are caused by joints that have loosened, which greatly exacerbates fatigue. If you find that bolt tension is being lost, then you must try to determine what the cause is. It could be embeddment of the nuts into a comparitively soft hub, bolt yeilding due to excessive torqueing of the bolts, joint movement under load, fretting of the joint surface, a bad original design, or many other possibilities. Good luck.

 

[Schwartz]

Overall, our problems stem from a bad joint design, and there isn't much we can do about it. Plus, heavy equipment does not receive the best of maintenance, so designs must be made as "idiot-proof" as possible. If the wheel joint was serviced and maintained here at the factory, it may survive. Underground, the joint may go together with dirt and corrosion everywhere, and the impact wrench would be doing the job.

Anyway, about the grade 9 hardware. Speaking with ARP, I was told that grade 9 and even grade 8 hardware is generally heat treated following thread rolling. Thread rolling is greatly preferred when manufacturing threaded fasteners because the cold working and smooth formed surfaces greatly improve fatigue life. It is far from ideal to heat treat an already threaded part since the threads will have a much higher cooling rate than the rest of the bolt. This results in hard, brittle threads, and hardness transitions are just another form of stress raiser. I was also told that fatigue life could increase perhaps 60% when comparing rolled threads before and after heat treat. Tempering of a fastener after thread rolling is also undesirable because you will loose the benefits of cold working.

Therefore, it is no suprise that the grade 9 hardware in our application failed so quickly. We have thickened our wheel mount flange to stiffen up the joint, and it is a stronger material which will also have a higher hardness. We are also switching to G8 flanged bolts, eliminating the washers, which will distribute the load better. If this isn't enough, we will look to ARP or SPS to manufacture custom bolts with reduced shanks and all that fancy stuff.

 

[diamondjim]

Are the bolts failing in shear. If not
I would suggest using heavier washers thereby
lengthening the joint. By eliminating the
washers, you are shortening the clamping length
which seems like the wrong approach. If they
are failing in shear, then some pilot must be
made between mating parts to ensure that the
bolts do not see any radial load. Grade 9 bolts
are great in static conditions but not so hot
in dynamic. I would try an extreme thick washer
and see if the problem goes away. We recommend
in extreme cases the use of grade 9 bolts. How
are you tightening the bolts and to what percentage
of yield? Fat washers are cheap and seems an easy
fix, rather than increasing the hub thickness.
Keep us informed.