Power Wheels/Fisher Price Gearbox

[tjump]

Hi All,

There are several threads regarding the PW/FP gearboxes, but one thing I could not find was maximum tolerance on these gearboxes before shreading. Way back in 1999 we added steel gears to the output gear as we were all shearing off the teeth of this final output stage, but what can the inner gears handle?

I ask as we are designing other robots (non-FRC) and we are looking at motor options including the new craze with brushless motors. In general, the brushless 550 motors at 12V will be screaming at 60,000+ rpms (the Mabuchi motors that come stock with the PW/FP gearboxes usually run around 15,000 rpm). At what point would higher rpm smoke these gearboxes?

An additional direction with this query: how would running lower voltages equate to robot performance (torque) if we used lower V+ (brushless motor speed is related to input voltage and running at 6 volts would cut top end rpm to 30,000 or less depending on Kv of the motors).

Anyway, we are just trying to dig deep into motor/gearbox/battery power design to understand the relationship in all of these (several posts on other robot building sites spend a great deal of time discussing higher voltages equaling better robot motor performance... i.e. 24V systems vs. 12V systems). We want to understand how to put all of this together.

I appreciate the input.

Cheers,
Timothy Jump

[Jeffy]

Originally Posted by tjump

An additional direction with this query: how would running lower voltages equate to robot performance (torque) if we used lower V+ (brushless motor speed is related to input voltage and running at 6 volts would cut top end rpm to 30,000 or less depending on Kv of the motors).

Anyway, we are just trying to dig deep into motor/gearbox/battery power design to understand the relationship in all of these (several posts on other robot building sites spend a great deal of time discussing higher voltages equaling better robot motor performance... i.e. 24V systems vs. 12V systems). We want to understand how to put all of this together.

I appreciate the input.

Cheers,
Timothy Jump

I'm not sure about the quality of the FP gearboxes, but I can answer some questions about brushless motors.

However for your torque question, that is easy. Power output of any motor is going to be: rpm X torque X a constant (changes based on units). So, half the voltage is half the rpms. When you give half the voltage, the max amperage becomes half also (Ohm's law). So, half torque. In turn, doubling voltage gives twice the power.

This can be a bonus in overall performance (velocity, kinetic energy, ect.) of the system if you can use batteries with similar capacity at 12 and 24volts because you get 4 times the power at twice the weight. However, your batteries will need to put out twice the amps and will then run out twice as fast.

When selecting a motor you will need to have some idea of what you need it to do. Power output is the real important one. You can gear it however you like, BUT more gearing will cost you more in weight. This is sometimes important.

If you are looking for applications under 8k rpms, you may consider a brushless outrunner instead of an inrunner like you mention above. They provide fewer rpms per volt, but more torque, giving similar power outputs as inrunners. And they will need less gearing.

If I may make a suggestion on motors, the Great Planes rimfire series are good quality but won't break the bank like Axi brushless motors can. Found here: http://www3.towerhobbies.com/cgi-bin/wti0093p?&C=GHG

I have heard good things about Turnigy series motors too. Not sure where they are bought though. I haven't shopped for a brushless motor in a while, but the above principles still apply (physics doesn't change).

Best of luck,
Jeff

[bradjordan111]

thanks a lot for pretty much useful information but Does anyone have experience converting the Power Wheels basic on/off control to some sort of more sophisticated variable-speed motor controller? Can you recommend a circuit/parts? Does this improve efficiency? Does it provide dynamic braking?

[Bruceb]

If you are planning on using a brushless motor you wil need to buy an appropriate brushless motor controller. You cannot power it directly from a battery.
Bruce