Countersunk Socket heads binding on initial removal

[behindpropellers]

I had a complaint yesterday that countersunk screws on some machines we build are hard to initially break loose. Some of these screws have loctite on the threads. I am not sure if loctite is also getting under the head. This only seems to be a problem after a few years.


I understand that once these screws are torqued the torque creates great friction between the countersunk head and the host material. I am sure there is some corrosion, but when we are using steel screws on steel parts I am ruling out dis-similar corrosion.


Is there any way of getting around this?

My initial thought is that this is not a problem, just the way it is. Will never seize under the heads help?

Thanks

Tim

 

[KENAT]

Wouldn't loctite on the thereads cover why they're difficult to get lose, or am I missing something?

Are these patterns of countersunk screws going through a part with countersinks into tapped holes in another part?

Because of the self centering nature of the thread & the countersink, then unless match drilled this can cause alignment issues which may introduce stress/deformation in the threads/parts which might cause part of the problem.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[mcgyvr]

loctite = hard to initially break loose.

don't apply loctite = bolts loosen due to vibration,etc... customer complains because machine just fell apart during operation..

apply loctite = bolts are hard to break loose... customer still complains.

can't make them all happy.

 

[drawoh]

behindpropellers,

Are these hex socket flat head cap screws?

My experience with small ones has been nightmarish. On anything smaller the #10 or M5, the sockets strip. Anything else will work better. It has been pointed out on various posts here that even when hardened, the flat head cap screws are not suitable for high strength applications.

Regardless of the socket type and the material, if you are driving a flat head screw into a tapped hole, you have no allowance for positional error. Flat head screws are self-centreing. Depending on all your materials and tolerances, your screw heads could be doing all sorts of weird things that would affect loosening torque.

What kind of threadlocker are you using? My experince has been that the medium strength Loctites (242?) are removable. The high strength Loctites are, high strength![ ]

These are just some possibilities in addition to the ones listed above.

JHG

 

[ornerynorsk]

This is the nature of all flat head screws, particularly FHCS. It is a result of the tensioning of the head being out of perpendicularity with the axis of the screw. What you are experiencing is nothing unusual. Loctite generally not needed on FHCS, unless of the medium strength or less just as a precaution.

 

[williedawg]

I agree with using Loctite 242... never use 609 for fasteners. You could also call on a Loctite rep to go over your application...they should be willing to help out.

Cleanliness is of utmost importance when using Loctite, or other brand of anaerobic compound. Use something like de-natured alcohol to clean the fasteners and tapped holes; blow dry with air nozzle before applying compound.

Anti-sieze under the heads will help reduce the force needed for initial break-away, but seems to be at odds with trying to retain the FHCS "semi-permanently" with a thread-locker and would certainly be difficult to implement in the field by people that don't understand the use of both.

 

[MikeHalloran]

You might consider Phillips recess heads.

They are preferred over hex recess heads on, e.g. dirt bikes, for several reasons:
1. They are plenty strong. So long as nobody uses the wrong size driver on them and damages the recess, you can put a lot of torque on them before cam-out becomes an issue.
2. If you need still more torque, or somebody _has_ damaged the recess, they respond nicely to an impact driver and a big hammer.
3. You don't have to use dental tools to get all the crap out of the recess before you can even engage the driver.



Mike Halloran
Pembroke Pines, FL, USA

 

[behindpropellers]

Thanks for the replies.

I think some of them are using loctite 242, and some without.

Yes they are typically going into a threaded hole on the second piece of material.

This was just a complaint from one of our field engineers.

You have confirmed my idea that the initial break away is just a fact of life. I will see what tools they are using on these fasteners. It might be wise to buy some higher quality tools for this job that are specific to the application...IE the initial breaking loose of these fasteners.

Thanks

Tim

 

[OTHGWayne]

Loctite or no loctite is an arguement that has raged for years, you will need to make up your own mind on that. One thing that I will caution you on as I have seen it many times in my past.

As you know there are many loctite materials of varying degrees of strength, i.e. 242 vs. 609. They all have their place. What many assembly type people don't seem to understand though is that all "red" loctites are not the same, or all "blue" or, well you get the picture. I can't tell you how many times I have heard in the assembly shop some yell "Hey Ed, what loctite goes on this?" And Ed yells back "use the red one". Laugh, but it happens and it happens a lot.

Make sure your people are using the right stuff and are picking the right stuff for the right reasons.

 

[MikeHalloran]

Partially because of the red/blue/green confusion, I have taken to mostly using Loctite PST on threaded fasteners.

I think there are actually several grades, but none are strong enough to seriously interfere with removal.
All are strong enough for my purposes.
A thin white line is arguably more readily inspectable than a thin transparent red/blue/green line.





Mike Halloran
Pembroke Pines, FL, USA

 

[dinjin]

What do the loctite folks say about this?

 

[MikeHalloran]

I didn't ask.



Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

MikeHalloran,

Loctite 242 does an excellent job of dissolving electronic components like microswitches and PCB components. I noticed that on American Choppers, the elder Tuetle (sp?) warned someone not to get the stuff anywhere near the paint.

Now is PST? Does it attack plastics? The 492 we use around electronics is not very removable.

JHG

 

[MikeHalloran]

I think _all_ anaerobics, including PST, attack most thermoplastics.

For threadlocking in plastics, I use RTV silicone.


;---
On a separate issue, there is one thing that will weaken cured anaerobics enough to loosen a screw; a really big soldering iron.



Mike Halloran
Pembroke Pines, FL, USA

 

[byrdj]

Proper fitting, quality tools make a lot of differance in the percieved breakaway of a tighten fastener, whether a blade, torx, or key. It seems to me, finding proper fitting phillips is a challange

 

[drawoh]

... It seems to me, finding proper fitting phillips is a challange

Not as challenging as selecting the correct hex key out of a tool kit that is accessible to people who do not put tools away. This is even more challenging when you have English and metric screws lying around.

JHG

 

[TheRocketScientist]

"My initial thought is that this is not a problem, just the way it is." - there you go, it's only a problem if defined as such. The flip side of that coin is, it's a FEATURE that prevents loosening during normal use.

Perhaps the field service guy needs the right tools. The handheld hammer driven impacts work great, as pointed out above, commonluy used for dirt bike engine service.

http://cgi.ebay.com/1-2-DRIVE-HAND-...5|66:2|65:12|39:1|240:1318|301:0|293:1|294:50

These kinds of tools also work well:

http://cgi.ebay.com/Panasonic-EY720...5|66:2|65:12|39:1|240:1318|301:1|293:1|294:50

when used with a high quality bit. I use the Dewalt version every day, and twice on Sundays, and it has earned the title "The Godsend".

However, the Hammer Driven type has the great advantage of forcing the bit into the screw and also shocking and axially loading the screw simultaneously.

 

[NomLaser]

Be sure that you aren't using Loctite 242 on screws under 1/4", it will make them really hard to remove.

http://www.loctiteproducts.com/products/detail.asp?catid=10&subid=48&plid=153

For anything under 1/4" you should be using Loctite 290 (Green)

http://www.loctiteproducts.com/products/detail.asp?catid=10&subid=48&plid=695