How to Keep Terminal Block Screws From Backing Out Under Vibration?

NEWSFLASH! the wheel has already been invented. look up binding head screws.

Add spring or lock washers if it doesn't already have it.

If you really want something more there are plenty of locking compounds that dry hard but can easily be removed with a screwdriver.

Don't bother with silicon or something soft, if the vibration loosens lock washers then silicon won't do anything apart from make a mess and hide the fact the screw is loosening!

Silicone, or at least some formulas of it, can be corrosive to metals. You need to check that.

Electronic grade silicone like ASE 388 should provide you what you need without corrosion.

Glyptal is another product we use because it is easier to apply in small areas.

Neither is easy to remove like an acrylic conformal coat, which can be removed by solvents, but I don't know if conformal coat would be the best choice for this application. There are several types of conformal coatings and it would probably be a good idea to research the various types and/or contact a few manufactures to see what their technical support engineers recommend.

You just reminded me of a technician I used to work with a while ago. He was always talking to the folks at Loctite and he came up with some good solutions because of it. I hadn't thought of him for a while.

I'll give them a call.

Just a bit of warning. That package from Loctite indicates it is for 1/4" to 3/4" screws. The normal screws in a terminal block are smaller and may be difficult to impossible to unscrew if Threadlocker Blue 242 is used.

As Guru goes on to say in his next posting, talk to Loctite, they know what will work.

I've used it on screws as small as #4 without issue.

Good suggestions. Thanks. I hadn't thought about silicone being corrosive. I can also imagine it might break loose and still appear to be tight.

I'm a little worried that loctite will ooze into the surface between the ring terminal and the screw and base and prevent good electrical contact. I guess if the screw is tight to begin with, there will be no space for the loctite to fill.

If the silicone smells like vinegar (acetic acid) it will corrode metal.

THREAD lockers go only on a very small portion of the male thread, before insertion and tightening. Any excess should be reomved.

"Break loose and appear to be tight" is where Glyptal comes in. When applied to screw heads, it fractures when the screw loosens and is an indicator of thread back-out. It's a QA tool when used to paint screw heads. It has very poor adhesion to most smooth surfaces.

"If the silicone smells like vinegar (acetic acid) it will corrode metal."

And if the silicone smells like 'airplane glue'/'plastic cement,' make sure you are working in a WELL ventilated area. I'm talking active cross-breeze if not actually outdoors. All the solvents that smell like that will make you hear strange colors(1) if you take in too much, and those silicones pump out a LOT of fumes as they cure.

Also, if it smells like plastic cement, it'll likely corrode/dissolve the terminal strip base, if it's a styrene or abs resin.

Notes:

1) Yes I deliberately uses a synestasia reference. Those solvents do wacky things to your brain. That's why he have people addicted to sniffing airplane glue and huffing spray paint.

"And if the silicone smells like 'airplane glue'/'plastic cement' " it isn't silicone.

Most silicones used for adhesive/staking/bonding are 98% solids.

Silicones will not melt thermoplastics.

There are silicone coatings that contain solvents, but they would never be used for securing a screw.

I'm not sure what it was then, but a few years back, my officemate was setting something with a tube of 'caulk' (which, back then, I assumed to be silicone, it was in the same type of tube, and seemed to be intended for the same uses, based on the labeling) which stank up the officw with this 'airplaine glue' smell (actually, it reminded me more of Tolune than of the typical styrene solvents) and I quickly opened windows and doors to try and give the vapors time to dissipate.

Once that smell hit (and triggered a bit of a headache) I had absolutely ZERO incentive to get near that tube again. I've had bad luck in the past with idiots overexposing me to solvent/caustic fumes, and I don't wish to repeat the experience.

May have been some concoction with acrylic and silicone. I'm not saying silicones can't smell, just that "silicone" is usually not solvent loaded.

Check this out for a much more knowledgeable explanation and see that some can evolve acetone as a product of cure.

Silicone Cure Systems - Silicone Technologies Uncured ...
I'm not as smart as I thought.

Lyn-

"I'm not as smart as I thought"

I'm Shocked! Are you really sure?

Good Luck, Old Salt

Sad, but true.

It could be what was used to be called "Duco-cement", which emits a very strong scent of acrylic or acetone... It was a common gluing material and adheres to almost anything way back in the 60's.. It dries up fast and hardens when it sets..

consider replacing the screws with tri-lobular thread forming screws, they are by their very nature less prone to back out under vibration.

Deformed-thread screws may work, if you can find them in the right size.

Use a "thread-locking pan head machine screw". They are available in either slotted or Phillips head. Many distributors, supply houses and even the big boxes carry them. They are the machine screws that have a small nylon insert in the side of the screw, towards the bottom, and are made for the type of vibration you are confronted with. McMaster # 96562A833 is an example of a #10-32x1" machine screw with this insert. I have used them numerous times for any situation when vibration might occur. They are often used on cars, trucks, washing machines, motor driven devices, airplanes, motorcycles, trailers, industrial machinery and numerous other locations.

Some of the biggest advantages for them are readily available, inexpensive comparatively, require no extra preparation labor to apply thread-lock compounds, are not destructive to the mating threads, only require minimum extra torque to loosen and remove, don't leave residuals on the mating threads and many more. They are also available in bolts up to 3/4-10x4" for steel. Also available in 18-8 stainless steel.

Good Luck, Old Salt

while some suggestions deal with preventing the screw from backing completely out of the relatively thin sheet stock used in normal terminal blocks none deal with maintaining the initial contact surface pressure. The one exception being the binding head screw head which was specifically designed to withstand the extremely high vibration environment of piston engined aircraft. last time I checked all the makers of commercial terminal strips used this as their standard. Still do as far as I know.

The underside of the head has a slight hollow so when tightened normally, the head deforms and acts as a belvedere vibration proof washer. No locktite, no extra washer and if you drop one replacements are readily available from any industrial supplier in all sizes. BTW if you intend to use ring terminal, chances are you will drop at least one screw you cannot find. Milspec wiring standards require ring terminal and binding head screws precisely because they have worked so well for over half a century. The connections are damn near gas tight. Just disassemble some WW2 electrical equipment and see for yourself. I have seen bright metal under some of these screw heads after decades of use.

I'll have to have a closer look at the screws. I've used this style terminal block before and I have noticed that the screws seem to lock, at least more so than a normal pan head. This is the first application I've found where they'll undergo significant vibration so I'm not sure what to expect. Of course the boss wants no problems at all.

Thank you for the information. I am very familiar with the binding head screws and their uses but have found the nylon insert screws work equally as well on all the items I have used them for. My shift to them was because the binding head are so hard to find readily especially when working in the field. Also when ordered from a supply house or by mail order you can't be sure the supplier understands the difference and sent you the right type. The nylon inserts are also more readily available.

I checked about 25 of my barrier strips, 2 to 20 terminals and from new to over 60 years old and found some of them have binding heads and some don't. I was unable to find a pattern in the type and age of them. Most of them are about 40-70 years old removed from NMR7 and NMR8 ILS glide slope and marker ground stations. They were made for the military and the FAA so they should have been acceptable. Bold to indicate should have but not necessarily. Unfortunately I can't determine the age of each one. Perhaps some of the screws were "borrowed" and replaced with pan head screws.

Thanks for your input.

Good Luck, Old Salt

For some of my purposes (not an electrical box) I have used plain old fingernail polish. Always enjoyed the look from my Customers when I pulled out my bottle of fire engine red nail polish. Red because it was then very visible where applied, and in my cases it would break when I put the screwdriver on it again.

I don't know how much vibration you're talking about - maybe that would break it.

Didn't LOCTITE have a promation years ago, you could get a T-shirt stating "keep your nuts tight"

I've used green fingernail polish to tack down wires on PC board mods. Then there was the tech that bought green polish with glitter in it. Had to throw it out, the glitter was conductive!

It may not be the screws backing off. You have multi-stranded cable to cope with the vibration. The strands pack more tightly at first due to the vibration, the joint becomes loose, and this gives the impression that the screws have backed off. If this is a new installation just re-tighten a couple of times and the problem will go away. If it is a long standing problem, Change to spring clamp terminals. Locking up terminal screws is a bad idea. What happens when you want to change a component or modify the installation. Your screwed.

Re: How to Keep Terminal Block Screws From Backing Out Under Vibration?
I can tell you from experience that Glyptal will work best on terminals that are already installed.Use the brush on type in the can, not the spray, and a small spot at the junction of terminal and block will do it.
While commisioning a factory with Wire Guided Vehicles we had a problem with terminals loosening due to the vibration of rough concrete floors,expansion joints in the concrete and hard composite tires.
A real nightmare when you have 50 vehicles running at the same time.The final enduring solution was the Glyptal.(And we tried almost everything).
It also gives evidence of tampering.(We added a certain amount of UV sensitive additive to ours to make it distinctive and detectable under UV light).
It is hard,but still pliable enough to tolerate the vibration without loosening or breaking.
It is still relatively easy to break a screw loose if needed.
The spring type connecting blocks are good but one must be very careful not to get a false connection;one that feels tight,but has missed the conductor; or one that has crimped the insulation above the stripped wire.
The crimp on ring terminal is very visible and secure if the proper crimp tool is used.
The spring blocks are very good for saving space and reducing labor cost,(no crimping terminals needed),but the downside is the learning curve for installers to ensure a proper connection.
Please bear in mind that I am speaking from years of experience,and have been retired for quite a few years.
I am quite sure that there are more,and probably better, options available now with automation becoming the norm.

Another thing to consider is vibration damping the terminal strip to prevent it from vibrating to begin with. perhaps a mount from Berry Controls or EAR composites might fit the bill. EAR's C1000 is amazing how well it damps mechanical vibration. used to have a bit of swag from them that was a golf ball molded from that compound. felt like about 55 durometer rubber, but if you threw it to the ground instead of bouncing sky high it plopped like wet modelling clay....

finger nail polish-dries hard-still removable-cheap-available.

CAUTION Don't let wife/mistress/girlfriend catch you using her/their nail polish for mechanical projects...........next thing, she/they will in your fridge taking out a bottle of beer for cooking.........What sacrilege.

Be warned.......all is not fair in love and war!!!!!

"The Boss wants something hard" . If I were in your shoes I will just do and follow his advice as requested.. After all he is the boss! A lot of good other's advice were given that you can accept that will also satisfy what the Boss wants!

"If I were in your shoes I will just do and follow his advice as requested.. After all he is the boss!"

Being the boss doesn't automatically make the guy an expert.

Look up 'Pointy Haired Boss' in your favorite search engine for details, or on www.tvtropes.com to jump right to the punch line.

Had that issue on similar terminal blocks used on electromotive equipment(electric Lift trucks, narrow isle stand ups and pallet trucks). Which take a lot of vibration. I found finger nail polish work very well to retain the fasteners from loosening from vibration. Just paint it on after tighten. So you don't have to go and loosen them all to apply a thread locker on the threads. Also like most new electronics today with painted screws it shows tampering. Cheap too most the dime stores have plenty of ugly colors to pick from. I was partial purple.