Steps to Calculating Gear Ratio of a Cyclo Drive

In the steps below we walk through how to calculate gear ratio, in hopes that if you encounter this problem in the field, you be able to quickly determine what size Sumitomo gearbox fits your application .

Step 1: Find the Motor RPM

First, you need the motor RPM. With the demo we brought, we gave you the motor RPM, which was reading right at 1800 rpm at the VFD.

Step 2: Find the Final Output RPM

The next most important piece of information is the final output RPM of the gearbox, as measured at the Output Shaft. This is where it can get a little tricky. Without a tachometer or strobe device, you were left to calculate this manually. You can do this one of two ways:

  1. Determine how many rotations per minute, which will then give you your rotations per minute or RPM
  2. Calculate how long it takes for the shaft to make one revolution.

If you take the latter approach, which we saw many of you do, and which can be a little easier given the situation, there is some additional math that is needed. If you timed it right, you would have calculated about 36 seconds for every 1 rotation of the shaft. Knowing this 36/1 relationship, and knowing there are 60 seconds in 1 minute, we can answer our RPM question with the equation 36/1=60/X. By solving for X we get a figure of 1.667, meaning 1.667 revolutions of the shaft per minute.

Step 3: Calculate a Rough Ratio

Now, with this information, you can easily calculate a rough ratio by taking the input speed divided by the output speed, or 1800 divided by 1.667, which gives you an approximate number of 1080. This is where it gets a little tricky, and where you either need to know the gearbox, or need to open a catalog. I'll explain

  • If you look at the catalog or know our product, you'll know that 1080 is not a standard double reduction ratio. It's not even a possible combination. Remember, the ratio for a double reduction is the multiplication of ratios from the first and second stages. When looking at the standard double reductions offered by the Cyclo, you will see 1003 and 1247, so both of these can be ruled out. But we do have some unique ratios, and you may think 1080 could be the answer. To check this you'd have to go through a series of guess and checks, by either multiplying our single reduction ratios until you get 1080 or dividing our single reduction ratios into 1080 until you get a whole number. If that whole number equals a single reduction ratio we offer, then you have your answer. But, you will not get a whole number, and there is a better way.

Step 4: Check Cyclo Disc for Ratio Stamp

Again, if you know our product, you know that we stamp our ratios on the Cyclo Disc. Since the demo was a cutaway, you were able to see the second stage, and if you looked hard enough, you would have seen the disc was stamped with a "25". This means the second stage has a ratio of 25:1. With this, you can now figure out what is the true ratio.

Step 5: Multiply Primary and Secondary Stage Ratios

Knowing the second stage ratio is 25, and the assumption that the final ratio is 1080, some quick division of 1080/25 will return a 43.2 figure which you could assume is the first stage. Except this is not a whole number. Checking the catalog will tell you that we do offer a 43:1, which is pretty dang close to 43.2. So, given the second stage is a known ratio of 25:1, and a newly assumed ratio of 43:1 for the primary stage, we multiply these numbers to get our final ratio. 25 x 43 = 1075. A quick output RPM check dividing 1800/1075, gives us a 1.674 number, which again is pretty dang close to the original 1.667 number we calculated earlier.

As you can see, you can get pretty close with only a stopwatch and calculator. Unless it's critical to the application, getting pretty close will often be ok. But to calculate an accurate ratio, you will need a little knowledge of the product, and you might even need to refer to the catalog.