# How to accurately calculate the rpm from a gear train

I'm trying to build a self-regenerating door lock as a side project in college and i'm having some difficulty accurately calculating the gear trains output (in rpm), the purpose of this gearing system is to operate a AC generator. Any help at all would be much appreciated!

I've estimated that a torque of about 1.12 Nm will be used to turn the door handle down 50 degrees. This torque will be transmitted through the shaft which is attached internally to a one way gear. The one way gear is attached to both a fly wheel and the gear train.

My goal is to get an output of about 100 rpm to operate the generator. My question is what factors must i include in the calculation? do i need to include things such as the weight of the gear train and how do i factor in the fly wheel? Thanks!

• This is about basic mechanics and not at all about electrical engineering. The output speed of a gear is completely determined by the gear ratio. Torque is lost in the gear because it is not 100% efficient, but no speed is lost. When the speed of the flywheel is increased, energy is being stored in the flywheel, so torque is required to increase the flywheel speed. The gears also behave like flywheels, but that is usually not significant. You probably need to find a way to learn basic mechanics and come back here if you have an electrical engineering question. – Charles Cowie Jan 18 '17 at 2:45
• I'm voting to close this question as off-topic because it is about basic mechanics, not electrical engineering. – Charles Cowie Jan 18 '17 at 2:46
• Check out the following image search for ideas - google.com/search?q=gear+ratios&tbm=isch – KalleMP Jan 18 '17 at 7:48

Obtain a cheap plastic hand crank generator or several and play.
That will show you much of what you need to know.

If you turn the handle 1/8th turn (45 degrees) in 1/4 second then its
RPM is 60/time_to_turn x fraction of turn =
60s/0.25s x 1/8 turn ~~= 30 RPM.
In engineering 30 ~~~= 100 in many cases :-).
But if you want closer to 100 RPM then a 3 or 4 to 1 step up gear ratio will give around 100 RPM.

SO coupling the shaft to the alternator with a one way clutch and a say 4:1 speed up gearbox gives you about what you want. One stage of the epicyclic gearbox in many battery electric drills may be about right. These may have multiple stages to get higher ratios.

Just turning the chuck on a battery electric drill and measuring motor voltage produced may be worthwhile. Slightly tidier than this ideally.

A useful rule of thumb is Power is ~~~= RPM x kgm Here RPM = 100 and torque = torque in / gear ratio = 1.12 Nm/4 = 0.28 Nm ~= 0.028 kg.m
So power = kg.m x RPM = 0.028 x 100 = 2.8 Watts. This is produced only during 1/4 second of turning so energy
= power x time = 2.8W x 0.25s = 0.9 Watt seconds = 0.9 Joule.

A useful amount of energy.

My experiments with cheap plastic squeeze type hand alternators shows they make about 1 Watt with very enthusiastic cranking so the above calculation is "in the order of right".

Flywheel does not alter energy delivered but can modify user handle-feel and rate of delivery - eg if a flywheel is spun up by a 1 way clutch it may spin down over 1+ seconds and deliver energy at a less peaky rate.

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Bonus: Obtaining energy from the opening or closing door is liable to be rather more productive if suitable to purpose. The door can eg have a spring closer and drive the alternator on one or both halves of open/close cycle. Energy taken from this source is liable to less noticed and easier to obtain in greater quantity.

Many ideas here
and here - each image links to a related page.

Cheap plastic hand crank alternator. Mechanical efficiency is usually poor. Note gear ratio is probably higher than apparent as another gear is probably not shown. ie their alternator spins MUCH faster than yours. The plastic Z-ish shaped piece in the flywheel acts as a one way clutch/pawl.
From Wikipedia

Patent imahge
from here hand squeeze generator

You can buy these for far far too much \$ on ebay OR pull apart old CD or DVD drives.

Stepper motors also may serve. Old large ones from 5.25" floppy drives (remember them :-) ) are usually better.

• Thank you for the reply, very helpful. In relation to the fly wheel, i was hoping to that it would make the handle more rigid and thus increase the input force required to operate the handle. – Tom Jan 18 '17 at 15:05
• A flywheel adds inertia - you have to rotationally accelerate the flywheel and it stores energy in the rotating mass.It will and the perception of "stiffness" to the handle. As noted, buying a very cheap plastic unit will give you a reasonable feel (literally) to forces involved. It's somewhat hard to add mass to the existing flywheel (see image above) as the axles fit in the two case halves and operation requires that the two halves be assembled. The very keen could make support brackets for one side so it is "open frame" – Russell McMahon Jan 19 '17 at 13:57