Helical Gears Gear Ratio

If we run the drill motors at 450RPM and put the kit together like the blowout on usfirst. Do the helical gears change the rpm at the wheels, or do we still have 450 rpm? What would be the gear ratio for the helical gears?

One gear is 1" and the other is 2" so you will have a 2:1 gear ratio with the helical gears.

Would it make a difference that the 2 helical’s are set up like a worm gear speed reducer?

you really should do ratios based on teeth.

Each gear has a tooth count stamped on it. I think it is 24 and 48, the gear ratio is still 1:2 or 2:1 depend on which gear is the drive and which gear is driven.

Here is an excellent Gear ratio spread sheet that I got from another team on Long Island in New York.

It was created by Frank Saladino - Team 353 - www.pobots.com

It has been a very valuable tool for our team. (yes I know this is a bit off topic for helical gears, but you’ll love it when you see it) :slight_smile:

drivetrain notes.xls (41.5 KB)

drivetrain notes.xls (41.5 KB)

With the stock 9" wheels and the 2/1 reduction of the stock helical gearbox supplied in the kit, the drill motor will push your robot at 8.8’/sec. in low gear. This is pretty fast. ( In high gear, your speed will be 29’/sec!!!)

It appears that you can swap the 24 tooth gear for a 36 tooth gear, reducing your speed to 5.9’/sec, much more manageable. The 36 tooth gear has a pitch diameter 1" larger than the stock 24 tooth gear, so you will need to slide the plastic drill motor shaft bearing blocks 1/2" farther away from the large gear. If you file out the end of the plastic holder on the gearbox end plates, you should be able to do this. You can then drill new 1/4" locating holes through the side plates to hold everything in the new position. When we receive our new gears and have made this modification, I will post some photos.
Note that MSC does not carry this size of gear. You can get it from any BostonGear distributor.

Rather than changing the gears, you could cut down the rubber on the stock 9 inch wheels. Reducing them to 6 3/4 inches will reduce the speed to about 6.6 ft./sec. It also reduces rather than increases the weight.

This is pretty fast. ( In high gear, your speed will be 29’/sec!!!)

5mph is fast??? Which is what I calulated as the speed of the robot with first’s configuation. I’m really confused with all these units.

generally, when speaking about robots, we use feet per second(fps) and not mph =)

5.5fps is not very fast… for low gear it is, but as a top speed it is not


Ahhh 5mph is 7.335fps. Fast but still a whee bit slow.

*Originally posted by Justin Stiltner *
**Would it make a difference that the 2 helical’s are set up like a worm gear speed reducer? **

Helical gears are not the same as a worm gear. Helical gears are like straight gears, it’s just that there are right- and left-hand twists. Depending on the mix of twists, the shafts will either end up being parallel or, like this year’s gearbox, at right angles.

A worm gear has the teeth parallel to the straighht gear and acts like a screw, so the pitch diameter is used

See this Georgia Tech mechanical engineering site for good basic inffo on gears, springs, motors and the like: http://www.srl.gatech.edu/education/ME3110/design-reports/RSVP/DR4/catalog/direct.htm

If you want a lot of info: http://www.bostongear.com/pdf/gear_theory.pdf

Does anyone know the percent effiency of the helical gears that came with the kit? Does that effect speed , torque or both?


The helical gears are still efficient, although there is a limited contact point between 2 gears when the shafts are perpendicular. They should still more efficient than a worm gear. Just keep them lubricated. And make sure that when the spacer tubes and spring washers are installed, the gear is dead-center with no slop, but it spins easily. Your main concern with friction is that you will draw a lot more power, even if you don’t see much reduction in speed. You can pop the breaker during a match or damage the motor from too much heat. Either one is worse than losing a bit of speed.