Fisher Price Gearbox Torque

[zaphodp.jensen]

Does anyone know the torque of the Fisher Price Gearbox?

[Collin Fultz]

Stall torque is listed as 0.45 N-m for the motor.

[zaphodp.jensen]

For the gearbox, not the motor itself.

[BHS_STopping]

To find the torque output of a gearbox, multiply the stall torque of the motor by the inverse of the reduction in speed the gearbox provides.

For example, if the FP motor stalls at .45 N*m, and the gearbox provides a reduction of 1/5, multiply .45 by 5 to get the gearbox torque.

[kramarczyk]

A. Last year's gearbox was ~139:1. I haven't taken this years apart to verify yet.

B. This is a great learning opportunity, learning how to do it yourself is going to be the fastest and most reliable route to your answer. Take it apart and count the teeth.
Stage Gear Ratio = Driven Gear / Driving Gear
Multiply the 4 Stage Gear Ratios of the FP trans to get the overall ratio.
The white paper at http://www.chiefdelphi.com/media/papers/2284 should provide some guidance for getting the gearboxes apart and such.

C. 0.45 N*m is the torque for stall at 70A, don't plan on having that much torque available for any length of time. You can scale it to the 30A torque by multiplying 0.45 N*m by 30A / 70A. There are numerous white papers on this subject.

D. When you get it figured out, post it back here.

[zaphodp.jensen]

The gearbox ratio is approximately 125:1.
This would put the F-P motor with gearbox at around 41.5 ft/lbs. Unless I'm crazy. . .

[Vikesrock]

Originally Posted by zaphodp.jensen

The gearbox ratio is approximately 125:1.
This would put the F-P motor with gearbox at around 41.5 ft/lbs. Unless I'm crazy. . .

Nope, that would be the stall torque of the motor/gearbox assembly. You don't really want to stall the Fisher-Price motor however, it will quickly go up in smoke.

In FRC applications, half of stall torque is usually a good number to design around. You can use the sheet here to figure out how fast the motor spins at that torque.

[kramarczyk]

Originally Posted by zaphodp.jensen

The gearbox ratio is approximately 125:1

How did you come up with this number?

[Vikesrock]

Originally Posted by kramarczyk

How did you come up with this number?

He opened up the gearbox and counted the teeth. My team mentors/shares a shop with his and I saw him hard at work counting teeth.

[kramarczyk]

Originally Posted by Vikesrock

He opened up the gearbox and counted the teeth. My team mentors/shares a shop with his and I saw him hard at work counting teeth.

Awesome! I just came up with a slightly different number, 122:1. Attached is my counting work from this evening for comparison.

[Andy A.]

Originally Posted by kramarczyk

Awesome! I just came up with a slightly different number, 122:1. Attached is my counting work from this evening for comparison.

An exact ratio isn't very important for most FIRST applications. There are a lot of factors in play that make those pristine speed/torque calculations only a rough suggestion. You won't get either the speed or torque that you expect due to frictional losses, voltage drop during a match, manufacturing inconsistencies, directional bias etc. By all means, do the math and use it to design with, but always include a healthy skepticism about your numbers.

I've just used 128:1 in the past to make the math a little easier. I think we arrived at that number many years ago by hooking the motor and gearbox combo up to a charged battery and counting the number of Revs of the output shaft observed in 30 seconds. Then compared that to the specified no-load RPM and rounded off.

[kramarczyk]

Originally Posted by Andy A.

An exact ratio isn't very important for most FIRST applications. There are a lot of factors in play that make those pristine speed/torque calculations only a rough suggestion. You won't get either the speed or torque that you expect due to frictional losses, voltage drop during a match, manufacturing inconsistencies, directional bias etc. By all means, do the math and use it to design with, but always include a healthy skepticism about your numbers.

I've just used 128:1 in the past to make the math a little easier. I think we arrived at that number many years ago by hooking the motor and gearbox combo up to a charged battery and counting the number of Revs of the output shaft observed in 30 seconds. Then compared that to the specified no-load RPM and rounded off.

I am not disagreeing that a buffer is required to accommodate variation. How much depends on what you are trying to do.

I am curious if we have the same gearbox with the rumbling about different motor pinions that I have heard. At least one team claims to have received two different gearboxes in the same KoP.

In past seasons I have heard the FP gearbox provided anywhere from 122:1 to 144:1 so I was trying to take the additional steps to back up the numbers provided by including the spreadsheet.