Inserts prevent stripped threads in marine application

The knob in question had been designed in plastic with an internal thread used to mount it to the mating shift lever. This plastic thread did not provide adequate resistance to rotational torque and also introduced wear problems. A reusable thread was required that would satisfy the torque requirements and eliminate wear associated with repeated cycles of removal/installation. Spirol recommended standard Series 29 brass inserts as a cost effective solution. For many reasons, plastic threads are not appropriate in demanding applications requiring repeated tightening and loosening. For example, the threaded steel shaft is much harder than the plastic and will cause wear during installation and removal. Cross-threading can also occur in a soft plastic host. Assuming the knob could be threaded on to the shaft without damage, there is still the issue of plastic creep – whereby plastics will creep or move opposite an induced stress until the stress is relieved and material normalised. This results in the loss of clamp load over time. A Spirol post-mould installed brass insert would eliminate these problems by providing a reusable thread that would maintain a clamp load and keep the knob in the correct position relative to the lever. The knob could be removed and installed without fear of stripping the thread or excessive thread wear.