Here’s an email from today:
Subject: Pulley selection help
Message: Hi George! Thanks for a wonderful site!!!
I’ve got a question on an order I’d like to submit…
here’s the specifics:
We have a Metro 12/1 and an ST 12KW gen head using one of
your drive pulleys to run the motor at 1,000 rpm. We’d like to slow it down to
650-750rpm and have no idea what pulley to select for a 12/1. Are there other
RPMs available in appropriate pulleys?
Robert glad to help, but you my style, and I’ll ramble a bit in answering to assure I cover a few topics on the fringes of this one.
The 12/1 clones may be different from Erector (Builder) to Erector, they may be different bores, and even different RPMs, so be careful in assuming your is just like someone else’s 12/1. We’ll assume this one is rated at 1000 RPMs.
My past advice has been to test run your specific engine on a study engine base like the designs like I describe on the Utterpower CD. Run the engine at rated speed and assure you like the sound and feel of the engine at that speed, a lot fo DIYers are not happy with the vibration, sound or feel, and choose to back off the RPM some.
We all know that reducing RPMs reduces horsepower since the formula for Horsepower is Torque X RPMs, you can get an idea of what you give up by figuring the percentage of power pulses ber minute you give up, this is no exact, but a good swag.
In Robert’s case, he has a 12KW generator head that will require around 24 REAL horsepower to produce a real 12KW. We’d expect about 6KW close to Sea Level if the 12/1 were running near it’s rated speed.
Utterpower provided three pulleys for these engines, the 11.25 inch pulley was provided for those who wanted near full output. The other Standard pulley we stocked was the 8.4 inch pulley used for the 6/1s with 22.5 to 23.5 inch flywheels, these also worked well with the higher speed GM90s and smaller diameter flywheels designed to run faster.
Since I had a few 50HZ customers, I stocked a few 10 inch pulleys, these work well for the 6/1s and 4 pole heads, and it also gives the 60hz world another option to slow the RPMs a bit if they have one of those thumpers that it a bit too rough at 1000 RPMS.
The 10 inch pulley will drop the RPMs by 100 or so over the 11.25 inch pulley.. you need supply an accurate circumference of the flywheel and do the math for your own machine to be sure.
So, there’s the two standard options, and of course Allmand will machine you a custom, but at added cost, and no returns,so it’s best you discover that RPM you like first.
all the best,