Nylon-Insert Lock Nut Material Qualification 1

[wheaney]

I'm working to design a test to represent field use of a fastening tool, which includes a section to simulate prevailing torque. One idea I've been developing in the short term is to use Nyloc nuts in a rundown application to provide a consistent prevailing reaction torque for the tool to work against. However, I've run into some issues characterizing these nuts, specifically the insert. So far, I know of at least two varieties of the Nylon insert. One supplier's, which is purple Nylon, starts with a lower prevailing torque but maintains that level better over repeated use. The other supplier's is white Nylon, which starts at a much higher prevailing torque but drops off quite quickly in repeated use when compared with the purple. This led me down a rabbit hole on Nylon nuts - manufacturers (I've reached out to several) don't seem to list any material properties that might be used to qualify these inserts. Granted this is out of my field, but I'm assuming things like Shore hardness, friction coefficients, or any coatings over the Nylon must have an effect.

Is there any extant standard that characterizes Nylon inserts in terms of dynamic prevailing torque and/or its decay over repeated applications? If not, do manufacturers record this information? My current fallback approach would be to obtain samples from every manufacturer I can find and characterize their prevailing torque/decay individually. Seems to me like this may be a bit of reinventing the wheel, hence the above request. I appreciate any advice here that may save me some time.

 

[btrueblood]

Well, yes, I think:

https://www.asme.org/codes-standard...iling-torque-locknuts/2017/nondrm-enabled-pdf

 

[wheaney]

Thanks for the quick reply, btrueblood. I consulted the standard, and found some relevant information:

MATERIAL, MECHANICAL, AND PERFORMANCE PROPERTIES:
"10.1.1.1 Nylon Rings. The nylon insert material shall be manufactured of nylon sufficient to meet the prevailing torque requirements of Tables 12, 13, and 14 when tested as specified in para. 10.3.1. Nylon rings are only functional up to 250°F."

In the tables for loads and torques, prevailing torque is only constrained by maximum first installation, minimum first removal, and minimum third removal.

This sort of leaves an open question about Nylon nut properties - what if three manufacturers were ASME B18.6.6-compliant, but with differing Nylon performance? By the ASME standard, one manufacturer could make a 1/4"-28 nylon lock nut that initially installs at 40 in-lb, then removes at 5 in-lb, and by the third removal is down to 1.5 in-lb of prevailing torque. Another could (hypothetically) make a lock nut that remains at near 40 in-lb forever. The last one could manufacture a lock nut that remains just above 5 in-lb forever.

This doesn't really help me, because all three of these hypothetical lock nuts are extremely different and cannot be substituted for one another. I can confirm this by noting that my team has characterized the aforementioned white and purple nylon inserts, which behave far differently. My issue is that we have no clue what we're receiving in terms of torque/decay until we order samples and test them - including periodic AQL checks for any continuous quantity.

 

[TugboatEng]

Have you looked into Fuji locknuts? If you have a Japanese motorcycle take a look at the axle but or motor mount nuts.

 

[Tmoose]

Sounds like this is for testing in a "lab" ?
How many tests per nut would be acceptable ?

You mentioned looking for "a consistent prevailing reaction torque" for your test.
Is there a maximum torquage lb-ft requirement, or are you just looking for a consistent torque value?

Cordless drills often have adjustable clutches in addition to variable speed. As I recollect the few I've handled released abruptly in a series of jumps.
Maybe there are some with smoother release?
$75 at Harbor freight or $150 at Home Depot might buy the cordless drill and sockets to be your test rig

What is the interface with your fastening tool?
3/8" square like a mechanic's ratchet ?
1/4" hex drive as is common on cordless impact drivers?

I picture an adjustable fairly consistant smooth acting clutch could be made from thin steel plate conveniently shaped to be gripped in a vise, and with a 5/8" hole drilled in it . A fully threaded 1/2" bolt could be inserted with a sandwich of a few large washers (steel, brass or leather or ??) and a coil spring. The assembled stack could be adjusted and locked with a coupled of nuts.
If you tool does not easily accept a 9/16" socket then reduce the bolt (and nut) size.

https://upload.wikimedia.org/wikipe...bsorber_(Autocar_Handbook,_13th_ed,_1935).jpg

The head of the bolt might need to be welded or epoxied to the first large flat washer.

 

[geesaman.d]

Nyloc torque will vary widely based on temperature and manufacturing batch. I'm sure they don't have any reason to get deep into controlling durometer. Maybe there is a mil-spec version somehwere, but that seems unlikely that a mil spec both exists and the parts are commercially available.

I'd explore more mechanical options like a double-nut. Maybe if you use an accurate low-torque wrench to snug them against each other, you'll get consistent torque when you drive them together.

I would also start by lubricating the threads or using coated threads to help even out the mechanical imperfections. The threads will need to be quite accurate.

 

[wheaney]

All,

Thanks for the insightful replies. A quick update on the test - I did some work to characterize the performance of a few Nyloc samples as well as some deformed-thread, Fuji, and steel insert locknuts. In the short term, the white Nylon supplier will tide us over until we can implement a longer-term solution: a brake. Whether to use a magnetic or pneumatic brake is as yet undecided, but the idea is to control the load on the tool by way of guess-and-check with the tool's current draw. This way, we can match the load profile to field data and get an exact match from the tool's perspective, while bypassing the fastener question altogether.