sjbrownie4x4
Active Member
- Location
- SLC, UT
I have a rear bumper (Fabbed by AFW in Montana) that recently had the tralier axle shear off on the highway. After studying the failure it looks like there was a small fatigue crack that propogated into a large split and finally the catastrophic failure.
I have seen this failure on other boards and it looks like it is starting to happen to more people. Luckily, the latch system (large 3/4 bolt with T handle) held the thing from actually bouncing down the highway and killing someoone!
I will try and post some pictures, but I would advise all of you to take a look at the motion you get in your carrier. If there is any motion, you WILL start a fatigue crack, its only a matter of time. Those trailer axles that everyone uses are so brittle there is no resistance to fatigue!
So its off to the drawing board, and I am trying to think outside the box. First of all double shear will help, second - a higher, pivot or attachement method (as EZ and others have mentioned) would for sure help. Is the high screwdown method enough?
I have a new design I would like to get some comments on. I have posted a picture of the solid model. Take some box tube cut one side off and and cut a 1 1/2 hole in the ends.Weld a huge 1 1/2 -6 Grade 8 Nut and cut a clearance hole for the nut in the top of the bumper. Now weld the box tube all around the base for a very large footprint weld for stability. Now take the arm tube and machine some shoulders (or just the ID) for some high capacity bronze bushings. Slip a huge 1 1/2 -6 Partially threaded Grade 8 Bolt through the top of the channel, the arm and bushings and into the welded nut.
The grade 8 bolt has a huge double shear strength and unlike the trailer axle that has been surface hardened to the brittle point and is less resistant to fatigue than a grade 8 bolt.
http://www.rockcrawler.com/techreports/fasteners/index.asp
Toughness is an important feature of a fastener. It is the opposite of brittleness and gives you an idea of how it will handle abuse without being damaged and eventually weakening the fastener or can cause fatigue to appear much earlier than normal. One way to “measure” toughness is by looking at the hardness rating of a fastener. The higher the number (Brinell, Rockwell …) the harder the material is and the tougher it is to damage. According to Marks’ Standard Handbook for Mechanical Engineers, Grade 5’s typically have a core Rockwell hardness of C25-C34 whereas a grade 8 typically has a core Rockwell hardness of C33-C39. Based on this, grade 8’s are tougher than grade 5’s.
Now you have a simple, cheap, double shear system that you can actaully replace the bolt either on the trail in event of failure or at some regular maintence interval every year or two.
You can do the bolt for ~$10
Nut for ~$5
two bushings for ~$13ea
and the Box tube ~$15
I have seen this failure on other boards and it looks like it is starting to happen to more people. Luckily, the latch system (large 3/4 bolt with T handle) held the thing from actually bouncing down the highway and killing someoone!
I will try and post some pictures, but I would advise all of you to take a look at the motion you get in your carrier. If there is any motion, you WILL start a fatigue crack, its only a matter of time. Those trailer axles that everyone uses are so brittle there is no resistance to fatigue!
So its off to the drawing board, and I am trying to think outside the box. First of all double shear will help, second - a higher, pivot or attachement method (as EZ and others have mentioned) would for sure help. Is the high screwdown method enough?
I have a new design I would like to get some comments on. I have posted a picture of the solid model. Take some box tube cut one side off and and cut a 1 1/2 hole in the ends.Weld a huge 1 1/2 -6 Grade 8 Nut and cut a clearance hole for the nut in the top of the bumper. Now weld the box tube all around the base for a very large footprint weld for stability. Now take the arm tube and machine some shoulders (or just the ID) for some high capacity bronze bushings. Slip a huge 1 1/2 -6 Partially threaded Grade 8 Bolt through the top of the channel, the arm and bushings and into the welded nut.
The grade 8 bolt has a huge double shear strength and unlike the trailer axle that has been surface hardened to the brittle point and is less resistant to fatigue than a grade 8 bolt.
http://www.rockcrawler.com/techreports/fasteners/index.asp
Toughness is an important feature of a fastener. It is the opposite of brittleness and gives you an idea of how it will handle abuse without being damaged and eventually weakening the fastener or can cause fatigue to appear much earlier than normal. One way to “measure” toughness is by looking at the hardness rating of a fastener. The higher the number (Brinell, Rockwell …) the harder the material is and the tougher it is to damage. According to Marks’ Standard Handbook for Mechanical Engineers, Grade 5’s typically have a core Rockwell hardness of C25-C34 whereas a grade 8 typically has a core Rockwell hardness of C33-C39. Based on this, grade 8’s are tougher than grade 5’s.
Now you have a simple, cheap, double shear system that you can actaully replace the bolt either on the trail in event of failure or at some regular maintence interval every year or two.
You can do the bolt for ~$10
Nut for ~$5
two bushings for ~$13ea
and the Box tube ~$15
Attachments
Last edited:
