1 Large Nut vs. 8 smaller ones
1 Large Nut vs. 8 smaller ones
(OP)
I need some help deciding between two different methods of sandwiching 8 rods (axially) between two plates. Currently, we use ONE 8 1/8" nut on the outside of the plates, that's threaded onto the shaft. I'm thinking that we could try using EIGHT 1" bolts that would go through the plates and into the rods (ie, need to thread the ends of the rods). I 'feel' these bolts would be stronger than that one 8 1/8" nut...especially due to the fact that the nut only has about 1 3/4" to 2" of thread engagement.
Please Help!
Thanks :)
Please Help!
Thanks :)





RE: 1 Large Nut vs. 8 smaller ones
RE: 1 Large Nut vs. 8 smaller ones
From my experience of designing joints it has always been good practice to use smaller bolts or screws and increase the number of fixings, in preference to using larger and fewer fixings, one reason for this is that it spreads the resulting clamping load more evenly over the joint you are designing.
If you could furnish us with some more information about your plates and rods ie:- materials, torque loading on existing single nut etc as the previous writer has requested
I maybe able to assist you further in your quest.
RE: 1 Large Nut vs. 8 smaller ones
Blacksmith
RE: 1 Large Nut vs. 8 smaller ones
Good points mentioned by desertfox and TheBlacksmith. One disadvantage to multi-bolted joints is complicated/unknown fastener interactions (sometimes referred to as crosstalk). Tightening one fastener can unload another fastener. Remember to use a pattern that tightens bolts located across from each other.
RE: 1 Large Nut vs. 8 smaller ones
The nut is currently made from 1 1/2" thk, T-1 steel w/ a yield of 100k.
The shaft is made from 4150 steel.
The rods...that would be threaded to hold the 1" bolts that will go thru the plates and into the rods (this is what would hold/squeeze the plate together on the ends of the rods...proposed) is made from 8620 Cold Finish Round...and is then heat treaded to 44-46 RC.
The loads are VERY difficult to envision. It's a dynamic pounding/beating that this mechanism is going to be accomplishing. The 'hammer mill' turns at about 1800rpm and is meant to grind/shred wood waste, stumps, trees, etc.
ANY help that any of you can provide would be GREATLY appreciated!
Thanks very much,
Bill
RE: 1 Large Nut vs. 8 smaller ones
Your small 8620 shafts apparently have been case-hardened, or they were quenched in water or brine. Makes a big difference when you're going to be tensioning the whole cross-section.
More when these points are cleared up.
RE: 1 Large Nut vs. 8 smaller ones
And you're right on the rods...they're hardened to 44-46RC and quenched & tempered.
RE: 1 Large Nut vs. 8 smaller ones
The 'cross talk' would be an issue here b/c there will be a fair amount of disassembly and reassembly with this devise...it's rather high maintenance. Thus, one of the things we always try to incorporate into our designs is simplicity.
RE: 1 Large Nut vs. 8 smaller ones
I'd also check the ends of the 8620 rods-same story. Once you know what you REALLY have-strength wise- you can start to make an intelligent choice about what you need to do. It certainly sounds like you CAN use 8 smaller bolts/rods to achieve what you need.
RE: 1 Large Nut vs. 8 smaller ones
Actually, I'm calc.ing that the 8 bolts will be weaker than the one big one. I'm using the equation in the other tread that you and I are chatting on "tensile capacity/min engagement", given by CoryPad. Maybe I'm missing something here, but I show the eight 1" bolts holding quite a bit less force than the one big 8 1/8" one.
What do you think?
RE: 1 Large Nut vs. 8 smaller ones
Since you have so little thread engagement on the large shaft, it's full cross-sectional strength isn't going to limit anything. Measure the hardness near the OD threads and use 60% of the equiv. UTS (they relate fairly well) to determine the shear strength of the shaft threads. I suspect THEY will be your limit (the 1 1/2" length of them), not the nut-which is *probably* harder.
When you end up with the pounds/tons those shaft threads will withstand, you can then figure out how many bolts/rods of what ACTUAL strength will equal/exceed the shaft value.
RE: 1 Large Nut vs. 8 smaller ones
A very rough calculation would be
1 times 1.00 inch squared = for one inch bolt
8 times .125 squared = 8/64 inch squared,
they would be very weak in comparison.
It would not be as great as what I have
shown. But do the values suggest something
similar?
RE: 1 Large Nut vs. 8 smaller ones
Could you please explain how to relate the hardness that I would find, with the UTS...(that I would then take 60% of to get the shear strength).
Thanks
RE: 1 Large Nut vs. 8 smaller ones
The 60% value is the ratio of the ultimate shear strength to the ultimate tensile strength. Shear strength is often the limiting factor is threaded fasteners, not tensile strength.
RE: 1 Large Nut vs. 8 smaller ones
I just have never seen the relationship b/w hardness and UTS.
Thanks again!
RE: 1 Large Nut vs. 8 smaller ones
If you can directly measure the Brinell hardness (BHN) which presses a 10 mm dia. steel or tungsten-carbide ball into a surface with a load of 3,000 kg, you can take that number and divide it by 2. That will be close to your UTS.
Simple, huh? <g>
RE: 1 Large Nut vs. 8 smaller ones
Actually, in doing to calcs. previously, the material shear strength would have to be A LOT lower than the #'s I was using to get down to equivalent stength as the 8 bolts....
I think I'm going w/ the one big nut (which I feel like right now).
Thanks!