What is the justification for left-hand threads?

Question

I'm familiar with left hand threads on bicycle pedals. The action of pedaling for that side of the bike would un-thread a right-hand threaded pedal, although some people have hacked a solution.

I have a chain saw that uses a left hand threaded axle to tighten the bar which tensions the chain.

The image below shows the cover over the bar, and also the legend embossed in the plastic.

The right hand "wing nut" is an ordinary thread and serves to secure the cover and the bar, once the tension is set with the left hand knob. Clockwise rotation for the locking nut works as expected. The left hand knob turns a gear in a pinion arrangement, clockwise pushes the bar away from the motor, as per the legend.

The left hand knob has unrestricted rotation (within reason) and has gear teeth as part of the construction. The pinion gear resides under the knob, engaging the teeth and rotating the axle, which resides in the groove shown in the photo. Near the end of the axle, the threads push or pull the rectangular tab (pin) which in turn pushes the chain bar.

As can be seen in this photo, the end of the axle has a severe bend, preventing movement of the tab/pin, removing the saw from service.

Unfortunately, all the sources I've been able to find for the replacement part have indications of "no longer available." Pricing indicates a cost in the US$3.00 range, but that's not helpful without product.

I have a Harbor Freight mini-lathe (toy lathe) that can handle replication of this part, although I'd like to avoid the left-hand thread operation.

I see no practical reason to not use a right hand thread. There are no rotational forces as in the previous pedal example. Once the bar is properly tensioned, the locking nut secures the bar, rendering the axle and pin immobile.

I can easily cut a piece of steel to replace the tab/pin, cut and tap the hole to match the rod which would be cut in the lathe, threaded with an appropriately sized die and mill a flat on the gear drive end.

What technical/engineering aspects would prevent me from making this work?

I do not consider the human aspect of a reversed legend on the knob to be a factor.

Answer 1

This is a user experience/design consideration. Users are simply conditioned to expect that rotating clockwise causes a movement to the right, and anticlockwise to the left - this is how a car steering wheel works, or bike handlebars etc. etc.

The mark of good design is where you are able to intuitively guess what the operation of an actuator does, before you use it. If there were to be a RH thread, then you would have to rotate the knob in the un-intuitive direction.

As @Solar Mike explained, once this constraint is understood, avoiding the LH thread would cause other undesirable mechanical features such as an idler, or extended shaft. The LH thread is the simplest of these options, so it's the one that was manufactured.

You could replace this component with a RH threaded variant, but make sure to mark the outer cover to make the new operation direction very, very clear, particularly if it's possible you'd lend this out to a friend!

Answer 2

I would think the only reason was to avoid having to have an idler gear to reverse the rotation, so it was cheaper to specify a left hand thread compared to providing an extra gear or being able to extend the drive to the left side of the knob.