July 28, 2012
Worm Gear Drive Experiment
Making a worm gear drive presents some new challenges. The first is how to create the worm gear itself, and this was covered in this blog entry. With the worm gear made, the next problem is the gear that will properly mesh with it.
Math was never my strong suit, and I expect I'm not alone in this. It is needed, occasionally, for some of the trickier things we would like to do. Luckily, we now have the internet with a wealth of knowledge available, and there is also the real possibility that someone has gone ahead and created a calculator or an application that does the "heavy lifting". Don't get me wrong: it's not that I lack the capacity to understand, it's more that I lack the time to devote to something that I will only use once or twice. I know the basic procedure for designing an involute gear, but I don't see the point in doing it the hard way when there is an application that will do it for me.
There are many sites online that cover how to design gears and there is even a plug-in for SketchUp:
It took very little time to draw the helical gear above using the plug-in. This is just a visual representation of how it should look and it wasn't necessary to go further than the basic 2D layout for this operation.
There were some manual calculations involved since the gear that this meshes with is a worm gear, and not another involute gear. Using the thread pitch from the worm gear (1/2") to calculate the pitch radius of the driven gear:
Pitch Radius = Pitch Diameter/2
Pitch Diameter = (Thread Pitch of the worm gear x Number of teeth on the driven gear)/Pi
Pitch Diameter = (.5 x 10)/3.14 = 1.59"
Pitch Radius = 1.59/2 = .795"
The plug-in uses this pitch radius to produce a gear that should mesh correctly with the worm gear.
One other thing to figure, and that's the angle the worm gear teeth are cut to produce the thread pitch. This is the lead angle, and there are a few ways to calculate it. Since this is a triangle, the angle can be calculated using the solve an SAS triangle method. I did it another way:
Again, using SketchUp. The angle is measured from the drawing. This angle is the angle the teeth on the driven gear will be cut at to make it a helical gear.
Since I was on shaky ground and not entirely trusting my calculations, I did a test. I printed the gear full size from SketchUp, cut it out and glued it to a piece of 1/4" hardboard. I then quickly cut it out on the band saw to try:
Seems to be a very good fit. I made the threaded wooden shaft from the thread cutting demo into a worm gear by cutting it to length and turning the ends of it smaller on my lathe.
To make the cuts at the 9.5 degree angle, I made a ramp that was cut on my miter saw at that angle. First step is to drill the bottom lands between the teeth:
This makes it easier to cut out the waste. I'm using the ramp to drill at the correct angle, paying attention to the orientation of the gear. I put a black marker "witness mark" on the edge to show which way the teeth are angled.
The ramp is then double side taped to the band saw to make the cuts:
A 3/8" hole is drilled in the centre and it's mounted on a threaded rod to do a quick demo:
Works well, and in spite of how roughly the worm gear was cut the coupling has very little backlash.
I made a short video of the gears in action:
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I expect I will find a few uses for this arrangement of gears in upcoming projects, possibly another router lift design. Of course there are other applications, such as speed reduction to a belt feed for a drum sander project.
Thanks for reading,