strenght of plascit bolt 1

[UchidaDS]

How do you calculate plastic bolt strength? Looking for tensile, shear... strength.
So far the formula that was found I believe it is for metal material.

 

[UchidaDS]

It is Plastic, not plascit...

 

[tbuelna]

The simple P/A stress calculations are the same. What is more important is what kind of plastic the bolt is constructed of. Some common plastic fastener materials are nylon, PVC or high strength PEEK. The problem with using plastic for threaded fasteners is that the plastic will creep over time if the fastener is subject to any appreciable level of sustained loading. So if relaxation of your plastic bolt due to creep is a concern, you'll need to check the creep characteristics for the particular material you have selected, and this will likely determine what level of stress you can design your bolt for.

Here is a brief discussion of creep in some common nylon compounds. Creep can be reduced by adding reinforcing fibers to the plastic compound, like the glass fibers added to the polyester/epoxy of these bolts.

Hope that helps.
Terry

 

[racookpe1978]

Crude and rude: Buy a few and test them to get an average. Plastic parts are notorious for being abused by "over-enthusiastic" customers/users and they will likely be damaged by over-torque even on hand-operated fasteners like thumbscrews, toys, clamps, or mating parts.

 

[MikeHalloran]

Plastic fasteners with machine screw threads are available in a fair selection of resins, but if they're stressed highly enough to need much analysis, you might be better off looking at plastic _coated_ fasteners, or redesigning the assembly to not use them at all.

Note that most successful high volume applications of plastics with threads, like toothpaste tube caps, radiator caps, radiator overflow tank caps, oil filler plugs, stuff like that, use fairly coarse buttress threads of some form, often in an interrupted thread configuration to facilitate molding.




Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

MikeHalloran- Your example of plastic threads on radiator and expansion tank caps is excellent. I believe these caps are usually made from a high performance nylon, and as you noted the threads are usually fairly coarse in cross section. These caps are exposed to operating temps of around 220degF and a wet environment, which is tough for most nylons. But the pressure loads on the cap threads are not sustained 100% of the time, and there is an elastomer seal under the cap face that can compensate somewhat for relaxation in the plastic threads.

 

[UchidaDS]

Not sure should I use brittle material or ductile material as for static loading - failure analysis. Anybody could help to send a link of related topic?

 

[MikeHalloran]

In the case of automotive plastics, they're ductile at operating temperature, but also must not fail in a subzero startup, where they're brittle.

Your application may have a more limited operating temperature range, but in general, you have to analyze plastics at all temperatures within both operating and storage conditions.



Mike Halloran
Pembroke Pines, FL, USA