How to make a rotating rod turn the axle of a small motor?

[unseeingdog]

I'm building a small wind turbine and I'm trying to find a way to make the metal rod where the propellers are rotate the axle of a small motor. The diameter of the rod is 1/4" and the diameter of the axle of the motor is about 2 to 3 millimeters. This is more or less the design of the turbine I'm trying to make. I know it says "coupling" there, but I have no idea what that actually is, and the pictures I've found seem to be all for bigger stuff. Any ideas are appreciated.

 

[Baluncore]

The simplest solution is to use a very short piece of rubber tube that is a tight fit to both shafts. It is flexible, not size critical and will slip if something breaks. Look for small engine fuel hose or car emission hose.
https://en.wikipedia.org/wiki/Coupling
google or eBay 'model boat coupling'

 

[Asymptotic]

The motor isn't mounted to the support base. How will it maintain alignment with the fan input shaft, and what will prevent the motor enclosure from rotating?
Shaft couplings are available from dozens of manufacturers in a wide variety of designs.

A solid coupling could be used, but alignment would need to be dead on, and a flexible coupling is often used. High precision isn't a requirement (as it is when coupling a motor to an encoder, for instance). 1/4 inch shafting tells me not very much torque has to be transferred. A double loop coupling is a low cost solution that fits the application so long as torque is low enough. A three piece 'insert' coupling can handle more torque.

Browsing through industrial parts catalogs (this is McMaster-Carr's) is one way of getting an idea what couplings are in common use.

 

[Baluncore]

A 1/4” =6.35mm to 3 mm is a big change. You might need to drill out one end of a smaller bore coupling. Take a look at the adapter choices here.
http://www.ebay.com.au/itm/D19L25-Flexible-Shaft-Coupling-CNC-Stepper-Motor-Coupler-Connector-3-10mm-BYWG/252517006329?ssPageName=STRK%3AMEBIDX%3AIT&var=551416232198&_trksid=p2055119.m1438.l2649

 

[CWatters]

Perhaps take a trip you your nearest model aircraft/boat shop or go online for collets designed to mount propellers onto small motor shafts...

Examples...
http://www.gliders.uk.com/Aeronaut-Collets-Prop-Adaptors/products/124/

 

[unseeingdog]

Is tightening the set screws on the coupling really enough to hold both shafts in place during rotation? Sorry if it's a stupid question but I feel as if I'm missing something here.

 

[Asymptotic]

Is tightening the set screws on the coupling really enough to hold both shafts in place during rotation? Sorry if it's a stupid question but I feel as if I'm missing something here.

If torque is low enough, and carbon steel shafts are used (stainless tends to be harder, and doesn't deform as easily) set screws will bite into the metal enough to prevent rotation, however, it doesn't take much torque to overpower their gripping ability and allow the shaft(s) to spin in their couplings.

If this occurs (or, as a proactive measure, to prevent it from happening) a flat (or flats, if each coupling half has multiple set screws) can be machined onto the shafts at the set screw locations. It may be necessary to use slightly longer set screws depending upon how deep the flats are.

One caveat is this unbalances them, but not by much, and it shouldn't be an issue unless very high speed rotation, and/or very low vibration is required.

 

[CWatters]

Try and find one with a hole that's a close match to the shaft diameter and with the set screws on the same side.

I suspect the rpm for your set up might not be very fast so this probably isn't a very critical application.

Try and pick a motor with a low "motor constant" (eg one with lots of turns).

 

[Baluncore]

When a DC motor is used as a generator, without a load, the voltage it produces is proportional to RPM. The diagram in post #1 shows a multimeter, which I assume is being used to measure voltage as a measure of RPM. There will be less than 1mW needed to drive the multimeter, so there will be little torque in the shaft and the turbine will spin freely in the airflow. For testing now, you do not immediately need a purpose made coupling, you only need a short rubber tube to couple the two shafts.

It is important to maintain the turbine blade “angle of attack” and so prevent aerodynamic stall of the turbine blades. When significant power is drawn from the generator, you will need to regulate the power extracted so as to maintain an optimum load on the turbine.

The diameter of a shaft is related to the maximum torque it must transfer. The fact that you are coupling two very different diameter shafts suggests you have a torque mismatch. The generating motor and propeller may be matched as far as power is concerned, but since power is torque multiplied by RPM, the motor is probably designed for a higher RPM than the turbine. For serious power generation an efficient gearbox might be needed to better match the torque to RPM ratio. You will need different couplings if you change the ratio with a gearbox or roller chain.