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PRELOK Fasteners
Direct Screw Fixings provide a fast, economical, production line solution to
fixing plastic and light alloy components where the need for re-use and high
assembly torque is not a major factor.
The unique patented thread forms of the PT® Screw range have been
engineered to maximise the advantages of Direct Screw Fixings with forming
screws for thermoplastics and light alloy materials and cutting types
for use with thermosetting materials.
The PT® Screw
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.
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.
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 Prelok Fasteners for specific advice.
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
- Allows for smaller bosses
8º Optimum Pitch Angle
- Lowest installation torque/highest tripping torque
- Maximum resistance to vibration oosening 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 torquesa high shear and
stripping torque
30º 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
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:
1 Amount of torque required to form the first thread
2 The amount of torque required for the screwhead to come into
contact with the mating component.
3 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.
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 Prelok Fasteners should be undertaken for
optimum results.
 PT Screw Data Sheet (pdf)
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