Electric Landing Gear Circuit Breaker ('67 M20F)

[Vance Harral]

1 hour ago, Ragsf15e said:

my 68 F emergency crank is like 40 revolutions to get the gear down

I'm wondering if there is some difference between the Dukes and ITT actuators here.  My understanding is they are of similar design, but 40 cranks in our 1976F would not be anywhere near full transition.  We have an ITT actuator.  Earlier airplanes had the Dukes actuators.  Both actuator types are subject to the AD that requires periodic inspection of the gear train, and I always assumed they were basically the same; but maybe not.

[N201MKTurbo]

1 hour ago, Vance Harral said:

I'm guessing 4200 RPM is the unloaded speed of the motor, but I don't think it turns anywhere near that fast under load.

Your math is about right except there is no gearing between the emergency crank cable and the motor shaft in the M20F vintage of airplanes.  The emergency engage just slides the crank cable spline directly into the motor shaft, it's a 1:1 hookup.  The motor shaft is geared to the drive mechanism by a worm gear arrangement, which originally had a 20:1 ratio.  J model actuators and "upgraded" early-model actuators have 40:1 gears.  We've had both in our airplane, so I'm pretty familiar.  Prior to the 40:1 upgrade, gear transit time was about 3 seconds, and emergency extension required about 70 cranks from full up to full down.  After installing the 40:1 gears, gear transit time is about 6 seconds and emergency extension requires about 140 cranks from full up to full down.

Doing the math for 40:1 gears, 6 seconds is 0.1 minutes to make those 140 turns.  That's about 1400 RPM at the motor shaft.

The gearing is in the crank by the pilots left knee.

[Vance Harral]

1 hour ago, N201MKTurbo said:

The gearing is in the crank by the pilots left knee.

Thanks for the clarification.  That crank unit is an integrated assembly in the parts manual.  It never occurred to me it had internal gearing.

[A64Pilot]

The more load you put on an electric motor the more amps it draws, I believe the 40 amps comes from that being the max load that you can put on the motor without burning it out, it’s surely not a continuous duty motor

‘The 4200 RPM should be a no load RPM.

‘Curious, what voltage is it rated at? 12 or 14?

On edit just looked at the picture, yes 40 amps gives you .20 HP, but as it’s a 12V motor and we have a 14V electrical system, it will actually give more HP at rated amps, assumption is 25 amps gives more than is needed so why wire etc for 40?

CB’s work off of heat buildup of course so they can actually pass much more than rated current for short intervals.

This may be an interesting read, yes it’s talking boats, but theory is the same and this is easily understandable

https://www.bluesea.com/support/articles/Circuit_Protection/95/Choosing_Circuit_Protection

Look at the trip delay curve here, this is for a blue sea 25 amp breaker 

https://www.bluesea.com/products/7354/C-Series_White_Toggle_Circuit_Breaker_-_Single_Pole_25_Amp

 

Rule of thumb is try to reset it once, but each time you reset it and it pops, it will pop at a lower amperage each time, the reason is it’s getting hotter and hotter and since heat is what trips it, it will trip faster or at lower amperage each time, plus you could overheating wiring

‘I’m surprised you got it to work, assuming it’s not broken of course.

Edited March 23, 2021 by A64Pilot