Torque values for plastic screws into plastic threads

Is something off on your units? 5 in-lb sounds awfully low to break a 3/8" screw, but it really sounds low if, as you say, it's from the spec for metal.

Hi,

Give me the mechanical properties of the polycarbonate and Delrin you use and I shall give you a torque level hope to be safer than the one you use.

Roughly you can consider the torques as being proportional to the elastic limits of screw material. But better if you send the values and get a correct value.

Do you use any lubricant on the thread? If yes which type?

Does the screw have a hex head or Allen head? or an enlarged head?

All those informations are important.

Hi all,

Bolted unions are usually measured/specified by the torque you need to apply.

Plastic materials have some different behaviour of metallic ones (shear strength among others), but mechanics are the same: You really need a tensile force on the bolt, and this is achieved through a torque given which produces in the thread. The factor which relates torque and axial load and have length dimensions (Torque= Thrust * Factor) is dependent on the thread geometry (mean diameter, pitch, lead...) and usually taken as constant for each thread type and the friction factor between male and female, which is dependent on the specific materials, lubrication, etc and which may vary in a great manner.

This is the main point because with the same torque and the same thread, the axial load (thrust) can be double or a half of calculated due to variations in the friction factor.

Formulae for calculating the factor can be found in the literature (Roark...) and I'm sure that in the net.

Good luck

Guest,

I woke at 3:30 this am with the word "craze" ringing in my ears. Are you exposing these polycarbonate screws to something like octane? I'm too far from my organic chemistry days (I may have taken it before Kekule was even born) to remember all of the chemicals which cause crazing in polycarb, but perhaps one of the sharp chemist types on here will pick up and tell you exactly. I used to use octane as an incoming inspection tool to make sure polycard pieces had been stress-relieved after machining. If they hadn't been, they'd craze like crazy.

The other thought I had was whether you are lubricating the assembly. A quick, back of the envelope calculation says you need a friction coefficient of maybe 0.04 or less to make your screws yield and that sounds like you might be using something really slick.