In the last post, we discussed some useful information fore designing and building gears. Lets add a few more ideas to help create useful 3D printed gears:
Gear Teeth Count and Wear
Another point to consider when deciding the number of teeth to use on each gear is wear. In our initial discussion on mechanical advantage, we chose ten tooth and twenty tooth gears in the example. While this gives a clear doubling of mechanical advantage, it also means the same teeth will meet up over and over again. In other words, tooth one on the ten tooth gear will hit tooth one and tooth eleven on the twenty tooth gear with each rotation.
Compare this to an 11 tooth gear. The first spur will hit
While this does not give exactly the same mechanical advantage, it does give a more consistent wear on the gears. Dust, dirt, and imperfections in the spur spread across the whole gear. For this reason, using prime numbers of teeth or avoiding teeth counts that are factors of each other might be a good idea for your design.
Spur Gear thickness
Spur gear thickness ultimately depends on the application, but there are some practical considerations in 3D printing. A good general rule is for the thickness to be about three to five times the circular pitch of the gear.
One consideration is stress and fatigue of the teeth. Doubling the width of the gear essentially doubles its strength. A second issue is that 3D printed gears cannot be made as precise as machined gears. There might be some play in the shaft of the gears that allows them to twist and disconnect making thicker gears a better option.
As stated earlier, mechanical advantage and velocity modification is achieved by pairing gears with different numbers of teeth. For spur gears, there are some practical limits on what can be done with just two gears.
Generally, the tooth ratio between the two gears should be between 0.2 and 5. That is, the large gear generally shouldn’t have more than five times the number of teeth on the small gear.
If you need a much a much greater mechanical advantage, there are a several options. One option is to use more than two gears:To get larger mechanical advantage, there are a several options. One option is to use more than two gears:
In the example above, a large gear drives a small gear on the same axis of a second large gear, resulting in the second large gear spinning five times faster than the first large gear. This large gear then drives another small gear, resulting in this second small gear turning 5 times faster than the second large gear. In total, there is a 25 (5×5) times increase in speed of axial rotation from the first gear to the last gear.
Another option is to look at non-spur gear options which we will cover in a future post.
Backlash and Tolerance
You will notice in most of the examples here that gear spurs match up regardless of direction. There is no ‘play’ in the mechanism. Reversing the rotation of the gear would instantly cause the attached gear to reverse as well.
Backlash occurs when there is some gap or play so that reversing one gear does not cause an immediate reversal of direction in the other gear. The reversed gear must first make up some distance for the reversed teeth to again make contact.
It would seem that Backlash would be a bad thing. The gear train would be loose and reversing direction would cause gear impact rather than a smooth application of force. In reality, designing for some backlash is often a good thing in 3D printing. As gears are created in 3D printing, there are usually some changes in the part due to thermal expansion or contraction of the material. There may be other artifacts left over from the printing process, a layer that is slightly off, or dust from the teeth interacting that will affect the meshing of the gears. Designing for a small amount of backlash to provide some tolerance around the manufacturing process is a good idea.
In a future post we will talk about some other types of gears.