Tightening Torque Issue with Plastic

[dcattena]

Hi, new to the forums I'm just looking for a place to discuss some manufacturing issues we are experiencing.

The assembly that this affects is basically a stack of plastic spacers slotted over two brass M3.5 threaded studs. We then place a small metal plate about 1mm thick on top of these spacers and fit a M3.5 spring washer then a M3.5 steel nut.

We torque these nuts to 1.6Nm which is quite high, but we have just recently found assemblies that have been sitting for around 4-5 days, the nuts can be loosened with a torque screwdriver set to around 0.5Nm.

What could be causing this loosening? And what can I do to stop it? Is the plastic shrinking over time?

Thanks.

 

[desertfox]

Hi

It sounds like the joints is relaxing after the initial tightening up, in addition are the assemblies subject to any vibration?
The first thing I would do is get rid of the spring washer because if there is any vibration then spring washers only help to loosen the joint.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[desertfox]

Hi Again

see page 2 of this link it suggests using a compression limiter:-

http://www.spirol.co.uk/library/main_catalogs/cmpl_us.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dcattena]

Hi Desertfox,

Thanks for your reply, yeah there can be vibration caused but the ones we are finding have not been subject to any. I will test some without a spring washer. We found that the nut is not loosening as our marks show its in the same position. Everything is still its just the distance the plastic takes up must be reducing.

 

[desertfox]

Hi dcattena

Maybe the torque you are placing on the fasteners is generating to much axial load for the plastic, have a read through that link I posted it's got some good information.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[ornerynorsk]

cold creep.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[dcattena]

Any suggestions how to fix besides compression fittings. I cant use them as there are electrical blades in between each of these spacers which can be shorted out by the compression fitting.

 

[desertfox]

Hi decattena

Well why such a high torque? According to my calculation the plastic is seeing 2285.7N per screw, I have no idea what the plastic is or what sort of area the load is working over but I hazard a guess the plastic can't take the load so I think you need to look at reducing that load if possible to a more acceptable level.

How do you prevent the bolt from shorting out the contacts in the present design?




“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[KENAT]

Is the torque just to generate pre-load to prevent the screws coming undone?

Are the these screws loaded in some other way during operation?

How about using thread lock, or nyloc nuts or some other method for retention if the clamping load isn't fundamentally required by the design?

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[dcattena]

The bolt has a silicon sheeth slotted over it and it sits inside. The torque is so high to stop these stacks rotating about the studs. They have some minor play so the high torque is to eradicate that. I dont know how this torque value was worked out, it is quite an old design, from the 1950's actually. Im going to do some tests at about 0.7Nm or 0.5Nm due to it being brass and see what the creep is like after about a week.

Hey Kenat, thats true the torque is set so they cant move, the studs are screwed in with a driver that has a female thread that just ends and locks them into place. The issue is the plastic deforming not the nuts actually turning loose, as i found with markings on tests we found the nut had not moved but the plastic had shrunk.

 

[btrueblood]

You might try ceramic spacers instead, or fiber-reinforced plastic spacers as a second choice. The ceramic should have "zero" creep, it either crushes or holds firm. The fiber reinforced spacers will creep much more slowly...

 

[KENAT]

So you need to keep the plastic loaded, but loading it too much and it will just Creep?

Maybe you weren't so wrong with the spring washer - you just need a real spring that will maintain force over a much greater distance. Though I'm wondering if eventually the plastic will still creep enough that clamping is lost.

Posting guidelines faq731-376

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

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[hydtools]

I like trueblood's ceramic spacer idea.

Ted

 

[Tmoose]

Have there been issues resulting from loosening?
What was the material used in the 50s. and since the 50s?

 

[MikeHalloran]

It sounds like the plastic is behaving, uh, plastically, as all thermoplastics do.

I'd bet the '50s version used Bakelite, which is a thermoset, and somebody got an attaboy for subbing a different, inferior, material.


Mike Halloran
Pembroke Pines, FL, USA