FTM 102: Why did a Grade 8 Flange Bolt break?

Fastener Training Minute 102: Why did a Grade 8 flange bolt break?

This edition of the Fastener Training Minute with Carmen Vertullo was originally published February 18, 2016 as “Why did a Grade 8 Flange Head Bolt break?” during episode 102 of Fully Threaded Radio.

Well hi everybody, I don’t know what time it is when you’re listening to this but it’s like zero dark thirty here in beautiful San Diego and going to bring you the Fastener Training Minute from the Fastener Training Institute and the Carver FACT Center here in El Cajon, California.

Today’s Fastener Training Minute has to do with an email that I received from one of our CFS graduates. I’m going to read it to you and let you see if you can figure it out. When I come back I’ll tell you what I told her.

The question is from a very smart quality engineer named Rachel. She said: I wanted to reach out with you with a question about a broken flange head bolt where the bolt broke and cracked a piece of a machine causing lots of damage, and parts cost, and lots of downtime. The customer is worried because there are seven more of these bolts installed like this one on the machine.

The customer installed a 5/8 x 1 3/4 USS (which means coarse thread) black phosphate and oil finish flange head bolts on a machine. The bolts are specified to meet SAE J429 Grade 8 and the bolts were installed to 600 foot pounds of torque using a combination of impact wrench and torque wrench to ensure accuracy. (She notes with skeptical punctuation marks). The bolts are in a threaded blind hole attaching 2 inch aluminum blocks to the machine. There are eight of these blocks on the machine. The OEM for the machine recommends grade 5 bolts with a washer, but the customer has been using grade 8 with a washer for years. The higher hardness prolongs the life of the bolts, but this time the customer decided to use flange head bolts to save the step of installing a washer, to give a higher head to the bolt and to prolong the life even more. No washer or threadlocker was used. The machine runs at room temperature with lots of vibration and impact on the fastener heads.

One of these bolts broke and cracked a piece of the machine causing all this damage. And the customer is worried that since there are more bolts on the machine, considerable losses in downtime and repairs are feared.

My question is: “could a Grade 5 flange bolt used in an application where Grade 5 is recommended break and cause the aluminum blocks to crack? any insight into this is greatly appreciated for any more information, please let me know. Thank you much. Have a great day Rachel“.

All right. This is a very interesting problem. There are some questions and there are some answers. So when we come back, I’ll tell you what I told Rachel.

You just heard my story from Rachel regarding some broken bolts and there are some interesting details here.

The machinery in question is a high vibration piece of machinery. Lots of impact going on. What they did was that in the past, they replaced a Grade 5 bolt with a Grade 8 bolt. No problem there though my first piece of advice was from an engineering perspective never change a fastener grade to a stronger grade without knowing enough about the application to know why that original grade was chosen. Even though Grade 8 is stronger than Grade 5, it’s also harder and less ductile, and in some applications that could lead to fatigue failure. But when you have a joint like that you really want to look at the joint because you really typically you shouldn’t have an issue replacing a Grade 5 with a Grade 8.

Secondly, I love the idea of changing to flange bolts. It’s one of my favorite fasteners and as a matter fact it’s my first favorite fastener episode which you will hear one day soon.

So they changed the grade from Grade 5 to Grade 8, but that was done years ago. No problem. So it’s unlikely that just changing the grade was the issue.

But what they did when they changed to the flange bolt was they removed to the washer. So this is bolting up against aluminum. Aluminum is probably a more or less rigid joint, but tightening a hardened bolt directly against an aluminum surface would deliver a much different torque tension relationship result, than by using a hex cap screw or even a flange bolt with a hardened washer. That’s a very significant piece of information there. I suspect they’re not getting the required clamp load due to the friction between the flange bolt bearing service in that aluminum block. That leads to the part loosening from vibration and eventually leads to fatigue failure.

There might also be something going wrong with the heads getting knocked around by the impact in the machine, but that’s not likely. So that is probably most likely their problem. The CFS was very right to be skeptical about the torque value of 600 foot-pounds, that’s much too high for that bolt. The torque should probably be in the 200 to 250 foot-pounds range depending upon what the real coating and any lubrication that might be used are, so she has got to find out about that.

They say that they’re concerned about damages and money. So, you know you want to really zero in on hey, you know, we gave you the part the part is a good part that actually caused the failure, so you don’t necessarily want to push back too hard on that but give them a solution based on the problem that they caused, not based on there being some problem with the bolt.

So I don’t know if this solution has been applied yet and the next time we talk we will find out if Rachel was able to solve this problem with this information.

I would also like to let you know that if you’re planning to come to San Diego for the NFDA Pac-West meeting in a couple of weeks, the long, dark, San Diego winter is finally over. It’s a good time to come here. The Carver FACT Center in or is doing a program on fastener graphics called The Illustrated Fastener . That’s on Wednesday. And the two days before that Monday and Tuesday, if you have not heard we have a hydrogen embrittlement training program here at the FACT Center. There is still room. There’s still time for you to get in. We have a seat for you. If you need any kind of hydrogen embrittlement training look at our website at CarverEM.com or Carverfact.com, and it will tell you about the program.

Thanks for listening to the Fastener Training Minute, and we’ll see you next time.

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