The following general rules should be followed in most cases for thermoplastic thread design: 1. Maximize root radius (internal and external threads)
2. Avoid tapered (pipe) threads 3. Select an appropriate thread type
This screw thread is recommended by the British Standards Institute for all screws smaller than 0.25 in (6.35 mm) diameter. This thread design is well-suited for plastic applications due to its specified root radius of 0.18083 P.
Although this thread is being phased out in industry by the unified thread, it is an excellent thread design for plastic applications. This is true because of the generous radius at the root of each thread which reduces the stress concentration effects. The root radius is specified as 0.1373 P.
The unified thread is currently the most common thread design for general use.
The buttress thread is somewhat unique in that the threads are not symmetric about a cross-sectional center line. Buttress threads have advantages in load bearing applications where the load is in one direction only (along the axis of the screw). Because the load bearing face is nearly perpendicular to the axis of the screw, the loads are transferred almost entirely along the axis rather than in the radial direction. The American National Buttress Thread has a standard root radius of 0.0357 P to 0.0714 P. The largest possible root radius is suggested for plastic applications.
Although it is the easiest to tool, the V-thread is one of the worst thread designs for plastic applications. The sharp notch at the root of the thread often results in extreme stress concentrations which can cause catastrophic part failure. The stress concentration factors associated with this thread type are often between 3.0 and 5.0. Thus, three to five times the average stress values (resulting from simple calculations) are often seen by the part.
A square thread is often used for power transmission applications, such as the power screw which drives a lathe, vice or jack. This is done because the square thread is very efficient at transferring force and power. The square thread type is not commonly used for plastics due to the stress concentrations induced into the threads at the base, which can cause the threads to strip or shear off.
The acme thread has a flat root and crest and is generally used for power transmitting applications. This thread type is not well-suited for plastic applications due to the stress concentration effects of the sharp corners.
Product Assembly
Fasteners
Mechanical fasteners are a popular means of joining thermo- plastic parts. When surface appearance is not of great importance or when disassembly may be required, mechanical fasteners are an excellent assembly option. Mechanical fasteners provide a strong, inexpensive joint in a relatively short time.
The least expensive and most common family of mechanical fasteners for plastics are self-tapping screws, which can be classified as either thread cutting or thread forming. Self-tapping screws create their own threads as they are driven. This aspect helps to decrease production times and costs by eliminating the need for molded-in threads or tapped holes. Self-tapping screws are available in a wide variety of sizes and configurations, each having properties suited to specific applications.
When surface appearance is not important, or when disassembly is required, mechanical fasteners are an excellent option.
Definitions
When screws are discussed, several basic terms are used. The following are brief descriptions of these terms.
Thread Angle – The angle included between the sides of
the thread measured in the axial plane.
Crest – The top surface of the thread.
Root – The base surface of the thread.
Pitch – The distance measured from crest to crest parallel
to the axis.
Pitch Diameter – A diameter measured at the point on the
thread where the width of the thread and the groove are equal.
Minor Diameter – The smallest diameter of a straight
thread screw.
Major Diameter – The greatest diameter of a straight
thread screw.
Flank – The load bearing surface of a screw thread located
Product Assembly • 5-21 Product Assembly • 5-21 There are six attributes that determine the performance of
screw/part assemblies. Drive, strip, seating and prevailing torques are measured in foot-pounds and pullout force is measured in pounds.
Drive Torque– The torque necessary to drive a screw into
an unthreaded pilot hole. Lower values of drive torque are desirable as they produce lower assembly costs and help increase the strip/drive differential.
Strip Torque– The torque that causes the threads in
the plastic to fail in shear. High values of strip torque are desirable as a safeguard against over-torquing.
Strip/Drive Differential– The difference between the
strip torque and the drive torque for a given screw. High strip/drive differentials are desired to allow for some margin of error in the settings of torque wrenches used in production.
Prevailing Torque– The torque required to remove a screw
from the threaded receptacle after the clamp force is released. High values of prevailing torque are desirable as they are an indication of resistance to vibrational loosening.
Pullout Force– The axial force that must be applied to tear
the screw out of the mating material. High pullout forces indicate a strong bond.
Seating Torque– The amount of torque necessary to produce
a desired clamping force. Suggested seating torques are usually the drive torque plus 50 to 80% of the strip/drive differential.