How can I design a gear train to meet specific torque and RPM requirements?

[OblongPea]

TL;DR Summary

Request for help on gear train design

Hello, I'm after some advice/ clarification on some gear train design. This may just be a simple clarification

Requirements
I have a lead screw which needs to be supplied power by a Motor.
This Lead screw must have 140Nm Torque to raise and lower the load, and must turn at 200RPM.
The motors I am finding are providing about 20Nm of Input Torque at approximately 1400RPM. So I'll assume for this I will be using one of these.
I wish to use Spur Gears within the gear train.

I know that Spur Gears should have a maximum gear ratio of 5:1. Therefore using 2 gears (One from the Motor, One on the screw) isn't possible as this requires a ratio of 7:1.

I looked at many options, and using a set-up similar to the attached image is my conclusion, where there are 4 gears.

The issue I have is determining what gears, no. of teeth etc. in order to get the required Torque AND Rpm from the Motor.

My idea was to use a ratio of 3.5 from Gear 1 to 2 (n1, n2 on the diagram). Giving a Torque of 70Nm on the second gear, and 400 Rpm.

Then as gear 3 will have the same Rpm and Torque (I think??) as Gear 2 as its on the same shaft, I thought to use a ratio of 2 from Gear 3 to 4 to give me a Torque of 140Nm and 200 Rpm.

Is this correct? I am questioning whether the number of teeth and diameter of the gears affect the torque and Rpm in any way?

Many Thanks in advance

 

[jrmichler]

Your sketch would work. There is a lot more to designing a gear reducer than just calculating numbers of gear teeth. Gear pitch diameters, gear tooth size, gear tooth form, shaft sizing, bearing sizing, housing design, lubrication, and much more all need to be calculated. A good book to read is Dudley's Handbook of Practical Gear Design: https://www.amazon.com/dp/1498753108/?tag=pfamazon01-20. If you read this book, you will understand why most engineers avoid designing their own gears if at all possible.

Are you sure that you need a gear drive? There are also synchronous belt drives available that work very well. One example is Gates Poly Chain GT belt drives. See this link to their catalog: https://www.gates.com/content/dam/g...y/catalogs/old-pc_carbon_manual17595_2011.pdf. They have the components to make a 7:1 reduction drive in a single stage.

If you need gears, the preferred method is to buy a gear reducer. There are many manufacturers of many different styles of gear reducers. A search will find them.

Or is this a homework problem? If so, it needs to be moved to the Homework Help - Engineering section.

 

[OblongPea]

Many thanks for your input here, I will read the book you mentioned as it seems to contain all the information I require. Looking through it briefly, I see why Engineers prefer to buy!

It's not necessarily a homework problem, just a small custom theoretical project I'm doing to upskill myself as I haven't had a lot of experience with gears.