A crack in a screw element rarely starts in the steel. Look at the load path first, then at the material.
Most cracks begin at the spline or at a sharp corner next to it. A loose fit lets the element rock, and the rocking loads one tooth pair harder than the rest. A crack starts at the tooth root and grows under every cycle.
A tight fit spreads the load. It also needs clean surfaces. Grit between the teeth creates a point load that no hardness can survive.
Cold start-ups crack more elements than any filler does. A screw that starts with cold, hard granules in the barrel sees a torque spike. The spike travels through the spline, and the weakest section takes it.
Warm the barrel before you feed. Bring the screw up to temperature with the heaters, then start at low speed and let the torque settle.
A bolt, a piece of screen or a tool left in the hopper will jam the screw. The screw stops, the drive keeps pushing, and something gives. Usually the element closest to the jam.
Put a magnet in the hopper and check it every shift. It costs little and it catches most of these events before they reach the screw.
A cold plug left in the barrel during a stop becomes a hard block. On the next start the screw hits that block and the shock loads the whole shaft.
Purge before a long stop. If you cannot purge, turn the screw by hand before you apply torque.
A crack that runs across the flight suggests a bending load. A crack that runs along the flight suggests thermal fatigue. A crack that starts at the bore suggests a fit problem.
Take a photo with a scale next to it, and note the running hours. That record helps the next diagnosis more than any opinion.
When one element cracks, look at the elements on both sides. A worn neighbour changes the load path, and the good element pays for it.
Check the barrel too. A scored liner grips the flight and adds load that the design never expected.
A part that cracks in a clean polymer line may carry the wrong grade. Through-hardened elements crack under shock, while nitrided parts bend. If the fracture face looks bright and brittle, ask what grade the supplier used.
Ask for the heat treatment record with the batch. A hardness number alone tells you little about toughness.
Write down what the line did before the crack. A start-up after a weekend stop, a feeder blockage, a torque alarm. The sequence usually points to the cause in a way the part cannot.
Send the position on the shaft, the running hours, the polymer and the filler. Send a photo of the fracture face. We read cracks every month, and the pattern usually points to one cause.
We machine replacement elements in grades that suit the load path you describe. If the crack came from the fit, a new element alone will not fix it, and we will say so.
