Marine Shift Lever Knob
The manufacturer of a marine shift lever knob was experiencing quality issues due to stripped threads. The plastic knob had been designed 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. Mold-in installation was not an option so any solution would have to be post-mold installed. The customer required a reusable thread that would satisfy their torque requirements and eliminate wear associated with repeated cycles of removal/installation.
There are many reasons why plastic threads are not appropriate in demanding applications requiring repeated tightening and loosening. The threaded steel shaft is much harder than the plastic and will cause wear during installation and removal. It is also important to note the ease with which cross-threading can occur in a soft plastic host. Assuming the knob could be threaded onto the shaft without damage there is still the issue of plastic creep. To maintain joint integrity the thread must be capable of holding a clamp load. Plastics will creep or move opposite an induced stress until the stress is relieved and material normalized. By nature this will result in the loss of clamp load over time. In this case the end user would then be required to continually tighten the knob to re-establish a clamp load and the thread would eventually fail or strip in rotational torque. A post-mold 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.
Engineering recommended the customer incorporate SPIROL’s standard Series 29 brass Insert, a cost effective solution readily available from inventory. Installation would be performed post-mold with heat installation equipment produced and supplied by SPIROL Automation. SPIROL Engineering performed heat installations and testing in the customer components to validate the recommendations. This solution provided the customer with the reusable thread they desired while surpassing the application’s performance requirements.
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