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In bulk mass-production situations, thread damage can come from a variety of sources most commonly due to improper handling, especially as tubs are dumped into loading/vibratory conveyance machinery and at the pre-wash station when fasteners are still soft. It is not uncommon for an operator to clear jams by using metal rods or to employ shovels or steel rakes to smooth out loading on a mesh belt. Improperly used, thread damage is inevitable.
In some instances, fasteners falling or tumbling down a quench chute will strike the sides of the chute or even a refractory ledge or hit a deflector plate at the bottom of the chute, causing deformation while they are in a plastic condition. It has also been reported that fasteners striking the flights on the quench-tank unload belt in a poorly designed quench system contributed to thread damage.
It is not uncommon to find bent fasteners when the length-to-diameter ratio exceeds 12. In these instances, fasteners must be straightened after processing. Fine threading might best be done after heat treatment.
Tips for Avoiding Thread Damage
1. Watch the loading operation 24/7 to identify potential sources of damage to the fastener size being run.
2. Issue clear instructions to operating personnel on what is and what is not allowed when loads jam.
3. Understand the quench chute design and look for potential problems with hot parts striking objects in the quench area.
4. Take advantage of maintenance downtime on the equipment to look for areas where parts would naturally “hang up,” and eliminate these areas.
5. Reduce loading as the length-to-diameter ratio becomes greater than 10.