The PT® SCREW is a direct screw fastener for use in thermoplastics. The key design aspects include a profiled thread root to assist plastic flow, a 30° flank angle to reduce boss bursting forces and an 8° pitch angle which combine to give low installation torques with high stripping torques. For those applications not requiring the high levels of thread re-use and high clamping torque (properties associated with threaded inserts) PT® screw offers an economical solution to plastics assembly and has distinct advantages over other direct screw fixings.
Advantages
- Reduced installation torque
- High stripping torque
- Greater safety margin between installation torque and stripping torque
- High parent material shear strength
- Minimises material flow resistance
- Designed to reduce stress within the thermoplastic to an absolute minimum
- High resistance to vibration and relaxation
Design Guide
When choosing a direct screw fixing, one of the major criteria is to obtain the maximum margin between driving torque and stripping torque. Screw design, material characteristics, moulding practice, hole size and installation speed all have an influence.
The specialised equipment necessary to measure the effect of these factors is available at P.S.M’s Technology Centre. This allows customers to obtain the best design possible. Pre-production testing is therefore strongly recommended.
Please note the inclusion of a counterbore – this is important for the following reasons:
- – Prevents damage to the top of the boss
- – Helps alignment of the screw in the boss
- – Assists in repeated assemblies of the screw
- – Acts as a relief for any material extruded to the head of the screw and allows flush finishes.
N.B. If moulded tool design or application into an existing boss renders the recommended boss design impractical, please contact P.S.M for specific advice.
8° Optimum Pitch Angle
- Lowest installation torque/highest stripping torque
- Maximum resistance to vibration loosening and material relaxation in the component
Profiled Thread Root
- Improved plastic flow reduces stress, providing long term reliability
- Greater surface area contact between screw thread and plastic, to give a high shear and stripping torque
- Lower installation torques
30° Combined Flank Angle
- Lower radial stresses, reducing boss bursting force
- Greater thread penetration into the plastic material, permitting enhanced clamping torque
- Reduced centre of pressure, producing lower installation torque
Design Recommendation
Dimensions
PRODUCT CODE WN 14
HEAD STYLE 11 Pan Flange
HEAD STYLE 12 Pan
HEAD STYLE 13 Countersunk
Standard Recess Styles
Other available recesses are shown on optional features page.
Material
Through hardened & tempered steel
Standard Finishes
- Zinc Plate and Clear Passivation (Z)
- Zinc Plate and Chromate (ZC)
- Zinc Plate and Black Chromate (ZBC)
- Stainless Steel (SS)
- Stainless Steel 316 (S6)
Other Finishes
Available to order
Size Range
How to Specify
Assembly Data
To determine the optimum performance of direct screw fixings there are a number of influential factors which have to be considered if problems on the production line or in service are to be avoided. In order to establish the optimum clamping torque it is first necessary to establish the following:
- The amount of torque required to form the first thread.
- The amount of torque required for the screwhead to come into contact with the mating component.
- The amount of torque required for the joint to fail.
These figures should be obtained by the use of specialised equipment which simulates production line conditions since testing by hand omits the important effect of friction due to installation speed.
PSM’s Technology Centre is fully equipped to conduct these tests for you.
The repeat torque accuracy of the installation tool can vary and should always be considered when specifying the recommended clamping torque.
Recommended Installation Tool Speeds
The speed at which screws are installed should be carefully considered – speeds which are too high can destroy the material into which the screws are being driven.
Although a speed of 600rpm is recommended for most materials, pre-production testing and consultation with PSM should be undertaken for optimum results.
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