# Need help determning pulley size for pto driven generator



## Dave1953 (Nov 1, 2015)

I am building a 25KW pto driven generator that turns at 1800 RPM. I am using a pto speed of 400 RPM and using 6" pulleys on the shaft being turned by the tractor pto. What size pulley do I need on the generator to turn it at 1800 RPM ? I know there are formula's out there that pulley size can be determined but I'm too old to do the math. I will be using two 1 1/8" shafts and 4 pillow blocks with dual belts to turn this thing. Thanks in advance.


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## ToolLover (Jan 13, 2020)

4.5 to 1 should increase the speed to give you 1800 rpm.
But this is not a pulley drive project.
All of the tractors I have dealt with and that has been many have a PTO HP rated at 540 rpm at the rear.
Both of the PTO generators I owned were gear driven and were run in oil.
We set the speed of the tractors to produce 60 hertz and that was not at full rpm where the HP is.
You surely are aware that a 25 KW generator needs at least 50 HP at the speed of 400 PTO rpm's to meet the load of 25 KW. You need a 68 HP tractor to drive the generator to power at 400 PTO rpm.
You might look around and find a used one to meet your needs or borrow one to go by.
Craigslist under generators comes to mind as I saw two in NC.
But believe you me you cannot pull 25 KW with belts unless you have strong ones.


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## drmerdp (Apr 10, 2018)

According to this site you’d need a 1 5/16” pulley.

I used the pulley 1 pulley 2 calculator. Enter the 3 known values and it’ll calculate the 4th.






Pulley Calculator. RPM, Belt Length, Speed, Animated Diagrams - Inch







www.blocklayer.com


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## ToolLover (Jan 13, 2020)

@ drmerdp: This is not a lawnmower he is dealing with.
I have a friend that powers a 60 KW PTO generator to use on water pumps when he looses three phase power to his system. The tractor is 130 HP and it is huge.


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## Dave1953 (Nov 1, 2015)

ToolLover said:


> 4.5 to 1 should increase the speed to give you 1800 rpm.
> But this is not a pull drive project.
> All of the tractors I have dealt with and that has been many have a PTO rated at 540 rpm at the rear.
> Front PTO's were faster for hydraulic power.
> ...


 I have 60 pto hp available. My goal is to run the tractor at about 1500 rpm instead of the 2100 to 2200 to achieve 540 rpm. I have gas appliances so I would probably never need over 10 or 12 KW at any given time. I just ran the entire house including the electric dryer off of a 5500 watt Generac that's 24 years old during out power outage here in NC. The only thing I did not use was the air conditioner. Im just trying to use parts I already have around here which is everything but the pillow blocks and 2 belts. I have 2 new Woods brand 6" pullies that go on some mowing machine they once made and can make the small pulleys on the lathe if I have to. Do you see a problem with using a 1 5/16" pulley on the generator head? Bigger would probably be better.


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## drmerdp (Apr 10, 2018)

ToolLover said:


> @ drmerdp: This is not a lawnmower he is dealing with.
> I have a friend that powers a 60 KW PTO generator to use on water pumps when he looses three phase power to his system. The tractor is 130 HP and it is huge.


Just providing an answer and the calculator for reference.... OP, Youll need a larger driven pulley, 1 5/16” pulleys are not practical. That means the drive pulley would need to be larger as well. 4.5-1 ratio as stated, if you maintain 400rpm at the pto.


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## Dave1953 (Nov 1, 2015)

drmerdp said:


> Just providing an answer and the calculator for reference. Clearly there’s holes in the plan...


If there is holes in my plan please let me know. I would rather know now than find them out later. I am in no sense of the word an experienced generator builder.


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## ToolLover (Jan 13, 2020)

Great, you are set up to run the 25 KW with that tractor.
I think after pondering your idea that I would find a junk yard that would let you rummage thru their truck transmissions so you can find a small transmission that fits your needs.
Just turn the input shaft until you find one tranny that has a ratio that gives you 1800 rpm with a 4-500 RPM input. Say 4.5 to 1 in low gear. 400 in and 1800 out.
Then turn the tranny backwards to drive the generator.
PTO shaft to the output shaft, and the generator to the input of the tranny.
Bingo, no spending your time engineering a pulley system that may or may not work and it will have a gear drive.


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## iowagold (Jan 22, 2020)

they make pto gear boxes made just for changing the ratio.
so dave do you own the equipment yet or is this still in the planning stage?
if you own the equipment.
post the make and model number as well as the serial numbers for both the tractor and the generator you wish to use.
*click here for northern tool pto gen page*
also see the pages below
*click here for winco pto generators*
*and this one*
*click here for pto gear box*
*also*
*and maybe this retailer for pto gear box for generator*

on large pto power you need direct drive...
belts and rubber couplers are for 20 hp and below...


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## ToolLover (Jan 13, 2020)

I am curious about the generator.
Where did Dave1953 get it?
Was it salvage or did he buy it new?
Is it two or four pole?
Is it self exciting?
@ Iowagold: You nailed it with you last internet check. 4.5 to 1 for 400 rpm PTO speed.
$345 is not a bad buy.
The one more comment.
25Kw (104 amps) is not a plug and play generator.
@Dave1953: Get yourself a frequency meter from Amazon ($15) and mount it on the generator. That will save you a lot of time setting the speed.


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## Ground Fault (Jun 9, 2020)

Just a suggestion here: Call a major mechanical power transmission product distributor in your area, one like Motion Industries. Ask to speak with one of their outside salesmen who calls on the heavy industries in your area. Those reps have almost daily experience in calculating the equipment needed to transfer power from BFPMers to BFE. Not only will they have the proper experience to correctly size your application, but will know the best products to transfer that power.


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## Ground Fault (Jun 9, 2020)

Two other things I just thought about:
1. I would refrain from trusting the recommendations and calculations of anyone who is working the Will Call counter. 
2. Ask the person who calculates your equipment needs what service factor they used. If they say anything below 2.0, find you someone else to do the calculations.


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## Dave1953 (Nov 1, 2015)

ToolLover said:


> I am curious about the generator.
> Where did Dave1953 get it?
> Was it salvage or did he buy it new?
> Is it two or four pole?
> ...


I removed the generator out of a Ingersol standby system with a 3 liter GM 4 cylinder gas engine which was the marine version, the same used in a 140 Merc Cruiser and sold the engine to a boat owner for what I paid for the whole rig. The computer was fried in it. The rest of it was like new. It is a Leroy Somer generator made in France. It can be set up in many configurations but is set up for 240 volts single phase. It is a single bearing unit. I have just about deceided to buy the 4.5 gearbox from Central Georgia the 400rpm to 1800 version for 345.00. It will act as the second bearing. It has a 1 3/8" splined shaft that will mate up to the pto driveshaft.


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## Dave1953 (Nov 1, 2015)

iowagold said:


> they make pto gear boxes made just for changing the ratio.
> so dave do you own the equipment yet or is this still in the planning stage?
> if you own the equipment.
> post the make and model number as well as the serial numbers for both the tractor and the generator you wish to use.
> ...


I have decieded to go with a 60 hp gearbox.


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## ToolLover (Jan 13, 2020)

Good move Dave


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## Dave1953 (Nov 1, 2015)

Thanks to all who responded to my post of yesterday. After sleeping on the matter last night I rejected the belt plan and I have already ordered the 4.5 60 HP gearbox from Central Georgia Generators, the 400 rpm to 1800 rpm version and do it up right.. My unit is a Leroy Somer 42.L I believe it is that I removed from a Ingersol standby generator that was powered with a 3.0 liter GM 4 cylinder gas engine. It was the marine version of that engine that was used in the 140 Merc Cruiser and wound up selling it to a boat owner who needed it for his boat for the same price I paid for the whole unit so I really dont have anything in the generator. Thanks again to all who responded and convinced me that the belts were not the best way to turn it. I will be able to machine a coupler to go between the 1 3/8" output shaft of the gearbox to the generator shaft. I have CGG's digital voltage and frequency meter to install on it so the rest of fabbing it up should be academic.


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## ToolLover (Jan 13, 2020)

Good move, but before you wind it up, add a outlet panel with circuit breakers to divide the amperage.
After you get the generator finished, take some pictures and return to show us the finished product. We would like to hear about your difficulties and challenges also.


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## iowagold (Jan 22, 2020)

cool move!
and yes to the hz meter to help with the rpm!
make sure to set the rpm with a basic light or space heaters as a load before connecting to house or shop loads.
you can use these to make power...

but for me i like the basic system i have...
I can do ok with smaller power.
pull the rope, or push the button and cut in..
and not have to worry about the load and rpm..
the little honda eu gens rock for me.


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