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GearRatio

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

1,12,3,14,5,16,7,18,9,20,11,2,13,4,15,5,16,7,17,8,19,1

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.

Gear Ratio

GearRatioAs 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:image05

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 example from Wikipedia

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.

Five Tooth Gear

As the spurs come into contact, rotate, and disengage, it is important that the rotation speed of the gears remains constant. This is needed for smooth transmission of power. The fundamental law of gearing essentially states:

The angular velocity ratio between two gears of a gear set must remain constant throughout the mesh.

 Norton, R.L., 2006, Machine Design: An Integrated Approach, 3rd Ed, 148190-8

To achieve this, an “involute gear” profile is generally used to design gear teeth. An involute curve is created as you grab the end of a string on a cylinder and unwind it. The end of the string traces out an involute curve. The edge of the gears teeth will have this same type of shape resulting in a kind of budging, rounded tooth. Read more

PeteLinforth / Pixabay

He lives with his creativity in high gear.

John Travolta

On the surface, gears seem pretty simple.  Create a couple short cylinder, put some teeth on it with more or less the same spacing and you are done. I’ve seen a lot of makers do just this and get some workable designs.  So why do mechanical engineers spend so much time optimizing them?

If you have done your own gears, you may have noticed that they can wear out quickly or they may wear unevenly. They may not turn as smoothly as you want. Reversing the gears may create a backlash while the teeth “catch up” to the teeth on the opposite gear. They may push to hard on their mounting axis.   The gears may turns faster or slower than you want.  Here is some information to help you raise your game:

Read more

beear / Pixabay

A good designer must rely on experience, on precise, logic thinking; and on pedantic exactness. No magic will do.

– Niklaus Wirth

Those of us creating designs to be 3D printed can face many of the same design challenges.  Some of us have solved them though hours of trial and error with our 3D printer.  Sometimes we have a friend that knows a good solution and we can leverage that.We might even find a good blog post from someone good enough to have documented their solution.  Over time, the best of us become experienced designers armed with a nice set of solutions to problems faced when starting a new design but there is always more to learn. Read more

yourschantz / Pixabay

It sounds very 2005… starting a blog.
In 2015, creating a blog is hard. Now you need the right URL, feeds, Facebook connections, Google plus connections, badges, Twitter profiles, logos, and themes. You need to #master #tags.

Sure, it is easier to get comments today, but 99% are from some evil troll trying to point search engines to their scam site.  I was half tempted to create a dummy blog, wait for the comments to come in, then rewrite the posts to match them… but I digress. Read more