Questions regarding adjustment screw method and push nuts

[John2004]

Hi Everyone,

I would like to ask if anyone could please offer some advise with the following situation & questions.

Has anyone ever used the type of pushnuts sold by

http://www.mcmaster.com

as part number 94800A710 ? Do they work good ?

You can see pictures of this style of pushnut at this link...

http://www.aaronspushnuts.com/pushnuts.htm

Mcmaster sells an installation tool as part # 93633A030 but they are$61.93 each. Is it necessary to use this Installation tool or can you just use a simple steel tube & a hammer for installation ? The price of the tool seems high for what just looks like a hardened tube with a plastic knob on it.

I have a # 10-24 socket head cap screw going through a clearance hole in a flat steel plate. The head of the screw is on one side of the plate with a delrin thrust washer between the underside of the screw head and the plate. On the other side of the plate, I have a hex nut on the screw with a delrin thrust washer between the nut and the plate. I just snug the nut down to take up any axial play in the screw & when I install the nut, I put loc-tite on it so that after the loctite dries, the nut "rotates with" the screw as the screw is rotated, but the screw cannot translate axially. The screw extends past the nut & is threaded
into a part that moves up and down in response to the clockwise or counterclockwise rotation of the screw.

The space is very limited & I am using a 5/16" wide small pattern hex nut on the screw that is only 7/64" high. I could probably fit a 1/8" high nut in there, but a nut that is any higher would be difficult. Space is limited both height and width wise. I also need to use a round nut.

I am hoping I can use the .065" thick push-nut and a delrin washer to replace the existing hex nut. This way,installation will be faster, I won't have to fool with applying loctite or waiting for it to dry. Do you think this will work well ?

If anyone has a recommendation for an alternative method or a type or brand of nut please let me know. I already tried to use small profile lock-nuts, called "top-lock" nuts by mcmaster carr, but they don't work well in this application. I could not find any other type of lock nut that was small enough. The nut presently fits into a flat bottom blind hole or pocket that has a .453" diameter. I could go to a little larger diameter, but not much.

Thanks for your help.
John

 

[MikeHalloran]

I used to work in Ford's axle plant. We bought those pushnuts by the ton, literally, and used them to retain brake drums on axles shipped without wheels.

We didn't push them on, which is pretty difficult to do, and generates shavings. We used impact wrench sockets with shallow tangs on the face to engage the double-d outside of our nuts. They spin down pretty fast. They're not real strong. They're easy to spin off with the right wrench until they rust, then you have to cut them off with dykes or a chisel.

They were actually engineered for our application. No need to remove them to install the wheels; they just flatten out or fit within a counterbore in the back of the wheel. Some people reinstall them when servicing brakes, but it's not necessary; they serve no purpose on the finished car.

I wouldn't use them in your application; too easy to spin off, and too delicate/ frangible, despite considerable hardness. AND, they're intended as _shipping_ fasteners; they don't really grip hard until they're flattened by a second fastener.

As for your application, you could use custom round o.d. nuts with the Loctite. Ask your Loctite rep to come in and talk to you about their primers; drying time does not have to be a limitation.





Mike Halloran
Pembroke Pines, FL, USA

 

[John2004]

Hi Mike,

Thanks for your reply.

When I mentioned needing a "round nut", I was referring to the round push-nut, as opposed to the rectangular push-nuts that are available.

The stock small pattern hex nut is fine, I was just looking for a way to avoid using the loc-tite.

I talked to loc-tite on the phone and they recommended a #2760 threadlocker that already has the primer mixed in it. I tested the loc-tite with a black oxide finished screw and a stainless nut, a stainless screw and stainless nut, a black oxide finished screw and brass nut, and a stainless screw and brass nut. It seems to work in the application but I am not confident with the strength of the bond after 24 hours. The strength seemed about the same with all of the material combinations listed above. I still have to test a zinc plated nut & screw.

The # 2760 is supposed to be a high strength premanent theradlocker, but after 24 hours drying time, I could loosen the nut fairly easily with a small 5/16" wrench. I have used loc-tite before on larger bolts and have not been able to remove the nut unless heat was applied.

I also tested with #262 loc-tite and no primer and got approximately the same results after 24 hours drying time. Actually, the #262 with no primer on a black oxide finished screw and a brass nut seemed to produce the strongest bond.

Perhaps the screw is so small and thin ( # 10 screw that is only 7/64" high) that this is why the bond is not that strong. Perhaps it would get stronger with time. I could try a # 271 loc-tite & prime the screw first, but I think the strength is supposed to be about the same as the #2760. The loc-tite rep told me to avoid using primer with the #2760 because it already has primer mixed in it. The rep also said that the #2760 is some of the strongest threadlocker they make. It's even supposed to work on threads that have a little oil on them. My threads looked clean & dry, I got the screws right out of the box brand new.

I would like to have the nut locked so that it is extremely hard if not impossible to remove with a small 5/16" wrench unless heat is applied. The nut does not need to be removable. I thought of deforming the threads just above the nut after it is tightened down, but there is only about .04" of length to play with. Maybe peening the inside top portion of the nut with a center punch where the nut/screw threads mate would help to lock the nut to the screw.

Perhaps some type of quick drying epoxy would be the way to go instead of the threadlocker, but then I would probably have to fool with mixing it.

I may not need as much strength as I think because the delrin thrust washer reduces friction on the nut as it is rotated and helps prevent the nut from backing off. There should not be much back-off torque on the nut since the delrin washer reduces friction.

I just wanted some way that I could lock the nut and be nearly 100% sure that it was never coming off without a lot of work.

Please let me know if anyone has any other suggestions or feedback.

Thanks again,
John

 

[kboy]

Hi John
I have used simlair pushnuts in automotive applications, In fact we are in mass production today with a simlair fastener.
In this application a high retention force was required.
I can't recall who we used in the end put they are all very simlair and manufactured from a fairly high carbon steel.
The things you will need to consider is keeping the pushnut square to the shaft during insertion, depends what you want out of the design, if not high load it may not be such an issue, for me, I had to have the shats annealed to ensure the pushnut "bites" into the spindle and retains load, pushnuts will not work very well if the shaft is too hard..
I am not sure putting a pushnut on a threaded stud is a good idea, I suggest you talk to the rep but I would shy away from that..

 

[handleman]

At $7 for a pack of 100 you may just want to buy 'em and try 'em.

 

[John2004]

Hi everyone,

Thanks for your replies.

>Kboy wrote:
>I am not sure putting a pushnut on a threaded stud is a >good idea, I suggest you talk to the rep but I would shy >away from that..

Actually, the push-nuts shown at the link I gave are made for threaded studs. Some pushnuts are made for smooth shafts and some are for threaded shafts.

>Handleman wrote:
>At $7 for a pack of 100 you may just want to buy 'em and >try 'em.

I am going to try the nuts but I wondered If I had to buy the $61.00 tool that mcmaster carr sells to install them ? It just looks like a tube with a plastic knob on it. If it's not necessary for installation, I don't see how mcmaster can sell them for $61.00. On the other hand, I don't see why I can't just use a small steel tube and press or even tap the pushnut on with a hammer.

I was not sure if the tool was necessary for proper installation. Do I need the tool(part # 93633A030)? Since the nut is going in a bind hole and the OD of the nut is close to the ID of the hole, I don't think I can use Mike's method of spinning the nut on the screw, but perhaps I can try to hold the nut somehow and run the screw through the nut.

Thanks again,
John

 

[handleman]

From everything that has been posted so far (and by looking at the picture of the thing on line) it looks like these will not fit your requirements at all. You need something permanent, high-strength, and non-removable. These push-nuts may be available in permanent, non-removable versions, but they are not intended to be load-bearing at all. They're really nothing more than screw retainers. As far as the installation tool goes, it's likely not necessary. From the write-up, it's just a tool to help you hold onto the thing and push it down. Normally you'd have a guy whose job it was to put on a bushel of these a day on a production line. Not something you'd want to do with a hammer and a piece of pipe. If you just want to test it out then you certainly don't need to buy the tool.

 

[GregLocock]

If you use 600 grade loctite then you will have to physically destroy the nut to get the joint apart, or heat it to red heat. Why does it have to set within 24 hours?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[John2004]

Hi everyone,

Thanks for the tip on the 600 grade loctite Greg. Is that a threadlocker or a retaining compound ?

Loctite just recommended the #2760 threadlocker or a 2 part epoxy # E30CL.

I will call loctite and check into the 600 grade, I have never used it before. I usually use the 262 or 242 grades.

Thanks again,
John

 

[TVP]

Loctite 600 is a retaining compound (green) not a threadlocker (red).

 

[John2004]

Hi everyone,

>TVP wrote:
>Loctite 600 is a retaining compound (green) not a >threadlocker (red).

Thanks TVP. FYI, threadlockers and retaining compounds both come in red and green color (blue too).

I see no reason why a retaining compound would not work as a threadlocker, unless the retaining compound needs less space to dry for some reason. As long as the thread gaps are not too big, it seems it should work to me.

Perhaps loctite did not recommend the 600 grade retaining compound because they don't like to recommend using something that's designed for one purpose, to be used for another purpose. It's not a big stretch to use a retaining compound as a threadlocker, but perhaps the guys manning the phones don't think outside of the box that much.

I had wondered if a retaining compound might be stronger. I think my problem is just that the screw is so small. I have used loctite 262 threadlocker on larger 1/2" nuts and have not been able to remove them without heat.

I'm going to pick up a small bottle of the 600 grade and test it to see if it does better than the "threadlockers". I may call loctite one more time just to see what they say.

Thanks again guys
John

 

[tlee123]

We use Loctite 680 for production threadlocking with good success.

Just because your test showed that a prototype cured without primer doesn't mean you don't need primer. We learned the hard way. We originally tested some sample stainless-to-stainless parts and had good bonding. However in production we had occasional assemblies where the Loctite didn't cure. We finally switched to use primer and the problem was fixed. Check with Loctite regarding the need for primer based on your materials.

 

[John2004]

Hi everyone,

Tlee123, thanks for the feedback on your experience using the 680 loctite for production threadlocking. It makes me feel better knowing someone else has had success using the retaining compound as a thread-locker. I will contact loctite and get some of the recommended primer so that I can use it on my prototypes.

I never knew about all the various "active" and "inactive" materials and the need for primers until I started checking into loctite for this job. In the past I usually just applied some loctite 262 without primer and never really had any problems. Perhaps I was just lucky.

This is the first time I have tried to securely & permanently lock a nut on such a small screw with loctite.

I'm not sure if there would be any significant difference in bonding or holding power between a #10-24 course thread versus a #10-32 fine thread. Anyone had any exprience with the holding strength of loctite on course versus fine threads? My #10-24 nut is only 7/64" high and I am sure that is the source of some of the problem.

Thanks again,
John

 

[MikeHalloran]

This may be a good time to re-engineer the assembly.

I'm guessing that currently it's designed for low production volumes, based on use of generic parts that are not optimum.

Suppose you were making 10,000 units a day. How would the design be different? Take a shot at that design, using custom parts, and assuming a decent tooling budget.

Now, take that second design and adapt it for low volumes.



Mike Halloran
Pembroke Pines, FL, USA

 

[John2004]

Thanks for the additional feedback Mike.

If I were making thousands per day I would would probably see if a robot welder could be used to spot or tack weld the nut or put a small weld bead around the top of the nut.

I could try to weld the prototype nuts on, but I only have about .044" or so of room on top of the nut to play with. That would take a very small weld bead, plus the nut fits down into a .453" OD hole, with the top of the nut being .272" deep into the hole.

As long as I could get to it, I guess a tack or spot weld would do the job.

I wold also need to make sure that the heat does not damage the Delrin washers that are being used between the nut and the steel plate. I may be able to use stainless steel washers, but Delrin washers are nice because they allow for less friction and noise when turning the screw.

Thanks again,
John

 

[John2004]

Hi everyone,

I tested some 270, 271, 262, 2760, 638, & 680 loctite as a threadlocker on some 10-24 and 10-32 screws & nuts. Whether the thread was course or fine seemed to make no significant difference as far as strength.

I tested with and without primer using black oxide finished screws and zinc, brass, and stainless nuts. The stainless nuts had the weakest bonding, and the brass nuts seemed to have the strongest bonding although the zinc seemed close. I let the loctite dry for exactly 24 hours.

The best bond seemed to be the 680 using a brass nut, black oxide finished steel screw, and 7471 primer. I think it will be plenty strong for my application, but it did not destroy the threads when taking the nut off the screw & did not require heat to remove the nut.

The 271 was also good, but I think the 680 was a little better, and it seemed to dry faster than the 271 using the brass nut.

The 2760 does not seem very strong at all on small screws.

I just thought I would report my findings to you in case anyone was interested.

John

 

[MikeHalloran]

Thanks for the feedback, John.



Mike Halloran
Pembroke Pines, FL, USA