Tightening Torque Issue with Plastic
Tightening Torque Issue with Plastic
(OP)
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.
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.





RE: Tightening Torque Issue with Plastic
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
RE: Tightening Torque Issue with Plastic
see page 2 of this link it suggests using a compression limiter:-
http://www.spirol.co.uk/library/main_catalogs/cmpl...
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Tightening Torque Issue with Plastic
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.
RE: Tightening Torque Issue with Plastic
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
RE: Tightening Torque Issue with Plastic
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.
RE: Tightening Torque Issue with Plastic
RE: Tightening Torque Issue with Plastic
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
RE: Tightening Torque Issue with Plastic
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?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tightening Torque Issue with Plastic
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.
RE: Tightening Torque Issue with Plastic
RE: Tightening Torque Issue with Plastic
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tightening Torque Issue with Plastic
Ted
RE: Tightening Torque Issue with Plastic
What was the material used in the 50s. and since the 50s?
RE: Tightening Torque Issue with Plastic
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