Spacers with 2 different thread types

[bradyle]

Hi,

At the moment we have a process where a spacer is screwed on to a stud (stud is through a metal sheet) and then a tray is located onto the spacer using a standard screw. However we have an issue where the tray needs to be removable and when we go to undo the screw holding the tray in place we find that the spacer unscrews from the stud as well. We have resolved this by putting glue into the stud end of the spacer but this is creating quality issues of its own so we are looking for another soloution.

One thing we are looking at is placing standoffs in the part instead of the stud + spacer, however there is an issue sourcing standoffs in the lengths we need.

I was thinking about getting a spacer where one side has a reversed thread and one standard so unscrewing one side wouldn't undo the second, however i cant seem to find any spacers that have this after my morning of searching. Does anyone know if these do exist? Is there a term i should be using in my search? Would this work as I envision?

Can anyone think of other soloutions that could be used.

Thanks for your help

 

[drawoh]

bradyle,

Check out PEM's website for self clinching standoffs. This is the best way to attach PCB standoffs to sheet metal. Another solution is to specify thread-locker, a high strength screw and the highest possible screw torque on the standoff screw, and a softer screw (mild steel, brass or nylon) on the PCB side.

--
JHG

 

[LittleInch]

Can you add a sketch or a drawing to this please. I'm sure your description makes perfect sense to you, but not unfortunately to me...

I get the spacer onto a stud but don't understand "a tray is located onto the spacer using a standard screw." or how you can have "a spacer where one side has a reversed thread and one standard" eh??

Where is this screw in relation to the stud or the spacer?

What does the spacer look like?

Are there any other nuts on the stud?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MintJulep]

I'd guess these guys could make them if you really want them.

https://mbs-standoffs.com/need-custom-products

But you just create a different problem.

With opposite hand threads on each end tightening one loosens the other.

What do you mean by "glue"? I was going to suggest Loctite.

If you have quality problems at assembly then perhaps a pre-applied thread locker.

https://www.vibra-tite.com/product-category/threadlockers/removable-reusable-threadlockers/

http://www.longlok.com/s.nl/it.A/id.42696/.f

 

[bradyle]

Hi Dawlo,

Thank you for your link, unfortunately the standoff we'd need would be up to 70mm in length and PEM seem to end at 25mm.

Hi LitttleInch,

Ive attached an image, sorry its a bit terrible but hopefully helps explain it. At the minute we use a self clinching stud which is pressed into the metal sheet and then we screw the spacer onto that, then a bolt is screwed through the tray and into the spacer and this holds the tray the correct distance from the sheet. Does that help?
Sorry explanations are not my greatest skill.

For the spacer I was looking for it would look similar to the image attached, however the end going to the stud would have the right handed thread and the end with the bolt would have left handed so in theory unscrewing one side wouldnt undo the other.

Hi Mint Julip,

Thanks, I never thought of how Id manage to tighten the bolt, that does definitely raise an issues. We use locktite at the moment but I will look into the threadmate link you sent! Should help with the control of it

Thanks

http://files.engineering.com/getfil...346-88ed-5102fe77e3be&file=Current_set_up.png

 

[LittleInch]

That helps enormously.

So you have a few mechanical options.

Can you / have you space or time to insert a grub screw into your spacer to grip / lock the stud?
Could you insert a pin through the spacer and nut?
Could you not just extend the stud but with an internal threaded hole for the screw and then have a thin nut to hold it to the sheet?
Solder / CAD weld the stud to the metal sheet? they do this a lot for cathodic protection studs onto pipelines and similar example -

https://www.erico.com/category.asp?category=R2721

or search CAD weld




Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rb1957]

so I think you want to fix the red box to one end and allow the other end to be unthreaded ?

If the fixed bolt was drilled then a cotter pin trough the red box would fix in place.

another day in paradise, or is paradise one day closer ?

 

[drawoh]

bradyle,

Look at my second suggestion. Use a high strength screw, like an alloy steel cap screw. PCB standoffs are made of soft material, so you will need a lot of thread engagement. Maybe PEM studs are strong enough. Specify thread-locker and high screw torque. The thread-locker can be permanent! Use a softer screw and lower torque to attach your PCB. Stainless steel screws are soft, and easily acquired. Now, you can take your screw on and off without disturbing your standoff.

--
JHG

 

[rb1957]

or you could machine the red box spacer with an exposed thread and then nut on the outside.

many ways to skin cats ...

another day in paradise, or is paradise one day closer ?

 

[btrueblood]

Use an unthreaded spacer, run the stud out long enough to go thru the tray, and fix the tray with a nut instead of a screw?

 

[btrueblood]

Okay, second thought. We have an assembly with two stacked pc boards (separated by a male/female threaded standoff), and use hex standoffs between them. The box is pretty snug fitting, so we made up some bent wrenches out of sheet metal, so the techs can hold the standoff when tightening/loosening the last screw. Hex standoffs shouldn't cost much more than cylindrical ones.

 

[Tunalover]

bradyle, if you don't find an unusual size in the PEM catalog or on their website, they DO have odd lengths that don't show in either place. Some could be in stock with distributors. If you call PEM they can tell you what they have and I think they even tell you what distributors may have. There will probably be a high min buy!

BTW you didn't use the term "spacer" properly. A spacer is just a hollow cylinder. A standoff is a cylinder with internal or external threads.

ElectroMechanical Product Development
(aka Electronic Packaging)
UMD 1984
UCF 1993

 

[kjoiner]

I would think that if you put Loctite 271 on the stud threads, the standoff would never loosen. What size threads are you working with?

Kyle