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Old 23-06-2007, 06:08   #1
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Jam Nuts

JAM NUT ON TOP OR BOTTOM?

When a nut is torqued a deformation of the bolt threads take place. You can visualize that one nut, fully torqued will tend to deform the threads upward.

Now a half of a nut (the jam nut) will have less overall force than a fully torqued full nut. So if the full nut is placed on first, the threads are deformed upward. The jam nut, placed on top will bear against the full nut, tending to reverse the deformation. However since the full nut applies more force, the thread deformation under both the full nut and the jam nut is in the same relative direction.

If the jam nut is torqued first, the threads are still deformed upward. But when a full nut is fully torqued, the bearing against the jam nut will tend to reverse the thread deformation. The threads under the full nut are deformed upward, while the threads under the jam nut are deformed downward. This thread deformation reversal provides the locking mechanism.

Two full nuts have occasionally been used in an attempt to provide locking. If both nuts are torqued to the same value, this is the worst of all worlds. Theoretically the bottom nut will impose no deformation on the thread leaving it useless.

The jam nut should be installed on the inside or bottom.
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Old 23-06-2007, 10:43   #2
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Yo Gord,

the small nut goes against the prop, with the large nut aft of that.

best, andy
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Old 24-06-2007, 10:52   #3
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Quote:
Originally Posted by Terra Nova
the small nut goes against the prop, with the large nut aft of that.
If true, then every boat I've ever seen has it wrong. Even right out of the factory.
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Old 24-06-2007, 11:33   #4
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Quote:
Originally Posted by fstbttms
If true, then every boat I've ever seen has it wrong. Even right out of the factory.
Yo FST,

then every boat you have seen had it wrong.

best, andy
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Old 24-06-2007, 12:23   #5
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In the Machinist world we were taught to torque the long not against the shoulder and then to just snug up the narrow nut. If the long nut tries to back off the narrow nut just get tighter. At least, keeping the hub in place and not falling off. But the illustration above IS the correct method!

The ideal set up is to have a keyway with a spur washer between the lock nuts, as what's use on lathe spindles.

This copy from Illinois Institute of Technology-Modern Shop Practice-Vol. 8-1943
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Old 24-06-2007, 13:33   #6
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Yes fsttbttms is correct. Del has it spot on. The "big" nut is the main nut that takes all the loading. This is the one that is torqued to the correct tension. The small "locking" nut is simply that, it "Locks" the main nut in place. It takes no shaft load on the thread at all, other than hold that main nut in place.
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Old 24-06-2007, 14:42   #7
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Quote:
Originally Posted by Alan Wheeler
The "big" nut is the main nut that takes all the loading. This is the one that is torqued to the correct tension. The small "locking" nut is simply that, it "Locks" the main nut in place. It takes no shaft load on the thread at all, other than hold that main nut in place.
Yo Alan,

this is counterintuitive, but I am afraid you (and others) are mistaken regarding the installation, as well as how the nut-pair works together. (Alan, you had an even 50/50 chance of getting this one right ).

Here is shown a proper installation, as described by marine propeller experts at Michigan Wheel:

http://www.miwheel.com/MIWheel/pdf/install.pdf

scroll to diagram.

best, andy
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Old 24-06-2007, 15:19   #8
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Gee, I thought this thread would be another personal injury story (slipped, jammed nuts).



Steve B.
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Old 24-06-2007, 15:48   #9
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Does this apply to peanuts?
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Old 24-06-2007, 16:13   #10
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And then of course there is Nord-Lock

Nord-Lock - Home
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Old 24-06-2007, 17:54   #11
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Quote:
Originally Posted by Terra Nova
Yo Alan,

this is counterintuitive, but I am afraid you (and others) are mistaken regarding the installation, as well as how the nut-pair works together. (Alan, you had an even 50/50 chance of getting this one right ).

Here is shown a proper installation, as described by marine propeller experts at Michigan Wheel:

http://www.miwheel.com/MIWheel/pdf/install.pdf

scroll to diagram.

best, andy
To avoid confussion!

Your installation diagram is showing it with the short nut inside, as we have been discussing. Which is the correct method!!!

The proper grade short nut can be torqued to spec's, the long nut insures it's reliablity.
It's a common mistake that happens a lot (long nut first) in the industry. Some times I see two short nuts but usually not in a torqued situation.

Double nutting is used a lot for just keeping the first nut from falling off in high viberation situations. People that see the wrong way or just install out of ignorence follow suit.

BTW Short nuts should always be replaced after use, just like a Nylock.
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Old 24-06-2007, 17:56   #12
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Quote:
Originally Posted by DeepFrz
And then of course there is Nord-Lock

Nord-Lock - Home
Those are nice but not cheap!
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Old 24-06-2007, 20:10   #13
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Quote:
Originally Posted by delmarrey
To avoid confussion!

Your installation diagram is showing it with the short nut inside, as we have been discussing. Which is the correct method!!!
Yo Del,

just so. I agree, and was having fun with Alan, who said Fstbttms was correct (not so). The industry standard is the one you describe, for which I have provided the link.

best, andy
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Old 24-06-2007, 22:43   #14
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Quote:
Originally Posted by Terra Nova
Yo Del,

just so. I agree, and was having fun with Alan, who said Fstbttms was correct (not so). The industry standard is the one you describe, for which I have provided the link.

best, andy
A quick Google search comes up with illustrations refuting your position:



LOCKING DEVICES, KEYS, SPRINGS, RIVETS AND WELDING

However, there seems to be more literature supporting the "jam nut first" theory. I will contact my favorite propeller repair shop tomorrow and ask which they recommend. I will post their response here.
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Old 24-06-2007, 23:20   #15
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Quote:
Originally Posted by fstbttms
A quick Google search comes up with illustrations refuting your position:

However, there seems to be more literature supporting the "jam nut first" theory. I will contact my favorite propeller repair shop tomorrow and ask which they recommend. I will post their response here.
Yo Fst,

you'll have to do better than a phone call to your local repairman to convince anyone here. While you're Googling, try ANY other marine propeller manufacturer if you won't believe Michigan Wheel.

best, andy
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