Hooo boy, Bart, you are about to embark on an exciting adventure. If I could have my druthers, I would like to have a minimum speed of about 240 RPM and a maximum speed of about 1800 RPM. However, you will soon find out that there are so many deign constraints that once you look at the practical mechanical configuration, there is really not much leeway in what you are able to choose.
The first thing that I will recommend is that you get ahold of some engineering design manuals for belts and pulleys. Many of the manufacturers such as Gates Rubber Company have them available online in PDF format. Sometimes they are a part of their product manuals. Don't worry, the engineering stuff is not too difficult -- it mostly consists of a lot of tables and graphs along with a few simple equations and design guidelines. This will be invaluable information.
Here are a few examples of some design limitations and other considerations:
- All of the various types of belts have operating restrictions such as maximum linear feet per second velocity, minimum wrap radius as a function of velocity, maximum distance between sheaves, and minimum contact wrap angle to prevent slipping.
- Pulleys also have design limitations such as maximum rim speed to maintain a safe angular momentum. For the very expensive machined cast iron multi-V sheaves, this limitation is typically 6,000 feet per minute, if memory serves me right. Without actually doing the math, I think that keeps you below about eight inches diameter. Machined aluminum sheaves might allow a slightly greater rim speed, but they are harder to find and more expensive. As you can probably already see, my desired minimum spindle speed of 240 RPM is not achievable because of belt and pulley operating limitations. The flimsy cast aluminum pulleys that you see in hardware stores are not suitable for this type of project.
- Good pulleys are not cheap. They run at very high speed and so they must be perfectly balanced or else your machine will vibrate all to heck. Generally these pulleys are two piece affairs which have a tapered hole into which a hub is inserted to fit the arbor on which the pulley is mounted.
- Everything becomes a juggling act -- you pick a belt length and then size the pulleys for the minimum speed -- next you try to come up with pulley sizes for the other speeds that will work with that belt length -- next you change the belt length and start over, etc. This sounds like an exaggeration, but it really isn't. In addition, you have some practical limitations on how the motor is mounted and how much throw to allow for locking it down. Ideally, it would always lock down at the same position for each speed, but it won't quite work out that way. And remember that you can't have the belt too long or else the wrap angle will be too large.
I found that the best solution for me was to use an Excel spreadsheet and then create graphs to help me design the drive system. Having a CAD program is also invaluable.
This may be a case where you are better off with an electronic variable frequency drive because the cost of the mechanical components such as pulleys and hubs can get out of hand rather quickly unless you are able to machine them yourself (such as what Brent English does). I am sure that he probably looked at the cost of buying machined pulleys and hubs from some company like Gates and quickly decided that it was too expensive. Something else to consider is that having five cast iron pulleys is not possible on most motor shafts -- first there is not enough room and even if there was, the moment of inertia of the iron pulleys may be excessive. So far, I have not located a good source of machined aluminum pulleys.
Your project certainly is feasible and I would say to go for it, but I just wanted to let you know that it is a bit more than picking your five favorite speeds. That is fine for a starting point, but once you get into it, there will necessarily be some design trade-offs.
