Tightening Torque Issue with Plastic

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.

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I'm no expert but in my understanding plastic "flows". That's why they call it plastic. I wouldn't expect plastic to maintain a compression force over time.

[QUOTE=jboggs;10989]I'm no expert but in my understanding plastic "flows". That's why they call it plastic. I wouldn't expect plastic to maintain a compression force over time.[/QUOTE]

Compression Set...

The property of your plastic you want to check is termed "[FONT=arial black][FONT=arial]creep".[/FONT][FONT=arial] It characterizes the long tem "flow" property.

Timelord[/FONT][/FONT]

I work at a company that specializes in manufacturing assembly and material handling, especially tightening fasteners, and what you have going on is a lot of what we refer to as "joint relaxation"; this behavior occurs in virtually all joints, although in many cases it is not a noticeable or measurable effect. Obviously, when you tighten the nut down it compresses the plastic piece, but since plastics can be very fluid, they will slowly be pushed aside by the compressive force and creep.

The first thing I'd look into in your situation is re hitting the fasteners a second time after allowing sufficient time for relaxation. This is very common when there is significant torque loss due to relaxation. Many times, the second hit will hold more torque than the first.
If this doesn't help, I'd look at your choice of plastic and try to figure out the compressive loads that will result in an elastic type of behavior. A very soft plastic won't be able to hold much force without creeping, while a very hard plastic will. Is changing plastics or fasteners an option?