Novice Question

Being somewhat of a novice with motors/gears/transmissions I have a question. When doing the calculations to determine the correct reduction ratios to achieve a given speed, which rpm number do you start with - no load, normal load, @ max efficiency, or @ max power?

Also, is the “drill motor” everyone keeps talking about needing specs for the one that is in a small box with Bosch on it? Thanks in advance for any help.

When we all say “drill motor” yes that’s the one we mean. But we usually refer to it with the gearbox attatched (longer brown box same bag) as the motor is almost never used by itself. The gearbox has a high and low speed shift selector. For a rookie team, I would recomend locking it in place and not shifting during the match. Check out these links
Gear Ratios
FIRST Drill Motor Gearbox Exploded View

There is also the option of using the Chiaphua (Atwood, CIM) motors which have a coupler provided but no mount.

The rpms and gear reductions for the drill motor can be found in the Kit section of the Robot Rules. The Chiaphua motor specs have been posted as an attatchment in another thread here on Chief Delphi.

If you want to use all four motors. Match their free (no load) top speeds. Your rpm will be th number you match them at. The number you will actually get in performance will be a little lower so design with that in mind. You will get the torque of both motors combined plus whatever you get through a reduction.

Also, look thorugh the white papers on this site as they are very helpful. Just remember, I would not recomend shifting for a rookie team. (This is our third year and we are still too scared to attempt it)

Hope this helps,
sanddrag

Thanks for the great information sanddrag. I have a follow-up question. Are the rpms that are quoted applicable when the gearbox is attached to the motor i.e. if free load rpm is 20000 then that is the on the “shaft” of the gearbox? Or is free load actually on the shaft of the motor and the gearbox reduces that number (20000)?

20000rpm is the free speed without any gearbox. I forget the exact reductions, but it is approx 450rpm in low gear or 1500(?) in high.

There was a thread on this just yesterday talking about the claimed RMP not matching a real world effieciency. Anyway here is what the manual says about the drill motor and gearbox

Motor
19670 RPM
0.87 Nm Stall torque

Gearbox
12.07:1 reduction in high
42.62:1 reduction in low

The Chiaphua from last year had a free speed of 5500 RPM and a stall torque of 2.22 Nm

Also check this thread http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=15916

we’ve decided to use the drill motors (no gearbox) with direct drive to the wheels, and design a very lightweight bot. At 20,000rpm, our bot should go about mach 19.:smiley:

*Originally posted by f22flyboy *
**we’ve decided to use the drill motors (no gearbox) with direct drive to the wheels, and design a very lightweight bot. At 20,000rpm, our bot should go about mach 19.:smiley: **

Nice try. You’d actually be gearing up the motors with a wheel of any size that you use. Plus, you would never have enough torque to accellerate to such a high speed. If you were already going that speed, and then your motors kicked in, it probably would keep going really fast if all friction is minimal.

PS dmellich,
Don’t mind my post and the above one. We’re just having fun.

*Originally posted by dmellich *
**Being somewhat of a novice with motors/gears/transmissions I have a question. When doing the calculations to determine the correct reduction ratios to achieve a given speed, which rpm number do you start with - no load, normal load, @ max efficiency, or @ max power?

Also, is the “drill motor” everyone keeps talking about needing specs for the one that is in a small box with Bosch on it? Thanks in advance for any help. **

When calculating the reduction, you should calculate the speed at free speed, because that’s how fast your robot would go when driving around the field with no obstacle. Plan on losing 20% or so of free speed of your motors just because of friction inside the system.

Another way to approach the gearing is figure out how much torque you need to do certain tasks, such as going up the ramp. I mean, you can always gear however fast you want, but its useless if you don’t have enough torque to go up the ramp. So, figure out the minimum torque that will get you up the ramp, and that will limit how fast you can go already. Try to gear it such that the amount of torque needed to go up the ramp is about half your stall torque.