[TowerTalk] Biggest, strongest rotor that fits a Rohn 45 rotor plate?

[jimlux]

On 2/28/19 12:41 PM, Patrick Greenlee wrote:
> Re-purposed prop pitch motors are definite contenders in the 
> strength/robust department but fitting in a small space???
> 
> On 2/28/2019 1:58 PM, Chuck wrote:
>>
>> On 2/28/2019 1:22 PM, RVZ via TowerTalk wrote:
>>> Pretty hard to find a better, more reliable rotor than the BIG RAK.
>>
>> Chuck  KØTVD
>>
>> www.dxham.net
>>
>>>
>>> Please advise the rotor that will handle the biggest windload and is 
>>> likely to be the most reliable that will fit a standard Rohn 45 rotor 
>>> plate?


What will the ham world do when the last prop pitch motor dies??

Some years ago I was pricing big DC (and AC) motors with suitable 
gearboxes and they are *really* expensive.

The BIGRAK as an example, is 4000 in-lb torque at something like 0.5 
RPM, but as noted elsewhere prop-pitch tend to be somewhat higher torque.

The PP has a reduction gear of something like 7000:1 or 9000:1 - so the 
basic DC motor is running at 5,000-10,000 RPM or thereabouts.  It's 
probably something like a 1/3 HP motor (240W @ 24V is 10A...not having 
run one myself, I don't know if this is a typical current)
1/3 HP at 1 RPM is about 28,000 inch pounds.
(1700 foot pounds)

Typical AC electric motors have speeds around 1800 RPM, so an 1800:1 
reduction is needed.  Getting that in one step is unlikely, so figure 
you need two gearboxes with 30-40:1 reduction.




So, a $500 3/4HP C-face TEFC electric motor, and a pair of $1000 speed 
reducers will set you back about $3000. Plus maybe $500 for a decent 
Variable Speed Drive.

At least, you're now using something that is a standard industrial 
product and if you need to replace it, you can order the parts anywhere.

Will it fit inside the tower?
A 56C size motor is 6.5" in diameter.  I don't think that will fit 
inside Rohn 25 (barely).



(There are better solutions out there. using, for instance, a 
worm&pinion reducer, but I didn't happen to have my catalogs handy).