This episode is a two-fer: The first part will explore changing speeds on the Shopsmith and the restoration of the Shopsmith Speed Changer.
The second special bonus part is a reader challenge to see how well you paid attention in high school math classes.
The steam locomotive ran on, well, steam. A throttle is used to regulate the amount of steam to the piston/cylinder hence increasing or decreasing the power to the locomotive drive train. Yet even with this capability, Dr. Emmett Brown invented green, yellow, and red presto logs in order to sequentially increase the speed of the locomotive to reach 88 miles per hour, enabling the Delorean to enter temporal displacement (Great Scott!).
However, the Shopsmith has no use for presto logs...at least at the moment.
Ten speed bicycles use a series of gears/sprockets to adjust the speed of the bike. To change speeds, you change the chain to a different sprocket through the use of a Derailleur.
Similar to the bicycle, the early Shopsmith included the ability to adjust its speed using a belt driving multiple size pulleys. To change speeds, you changed the belt to a different pulley combination. For example, Belt position “3” increases the speed of the Headstock spindle (the motor pulley is large than the headstock pulley), while belt position “1”slows the headstock speed (the motor pulley is smaller than the headstock pulley).
To change the headstock speed, you loosen the motor brackets, and relocate the belt to a different pulley, and reposition the motor. This is like changing gears on your bicycle by stopping your bike, removing the rear wheel, manually moving the chain to a different sprocket, then reassembling the wheel, and resuming your bike ride.
Doable, but not very efficient...and not very much fun.
The Speed Changer
The people at Magna Engineering invented one of the most clever devices I have ever seen, called the Speed Changer (the cleverness is in the design...not the name). The SC mounts on the Way Tubes between the motor and the Headstock pulleys. Two belts are used: one from the motor to drive the SC pulleys, and a second belt from the SC to drive the headstock pulley. The knob on the right turns which raises or lowers the center pulley assembly, and thereby adjusting the speed of the headstock.
Unfortunately, Rusty’s Speed Changer doesn’t slide on the Way tubes, doesn’t raise or lower, and doesn’t rotate around its own shaft with much enthusiasm.
After removing Rusty’s Speed Changer assembly and with some soaking, cleaning, polishing, and some waxing, the pulleys now spin about their shaft (with much more enthusiasm), the knob rotates nicely moving the pulley assembly up and down, and the entire assembly slides on the Way Tubes. I reinstalled the Speed Changer, and began dreaming about the next step: which will be the motor and its mounting bracket. Then alignment of all the parts, add the belts, and complete the drive train.
Later that evening....
Lying in bed around 3:00 in the morning, relishing in my accomplishment of getting the Speed Changer restored without much bother, it began to gnaw at me that just moving the pulleys up and down doesn’t necessarily change the speed of the spindle. All it would do is make one belt tighter, and there other one looser. SOMETHING'S NOT RIGHT!
Later that same day....
I re-read the Speed Changer Set-up paper (provided by MICKYD of the Shopsmith10ER Users Group) as well as other user forums on the Speed Changer, and sure ‘nuf: their’s more. The middle part of the SC pulley system is also supposed to slide back and forth (hence the term “floating sheave”). This is where the cleverness comes in (about the Speed Changer...not me).
As the SC pulley system is raised or lowered, the floating sheave slides to the right or left allowing the belts to come closer to or farther away from the center part of the SC in perfect harmony. This has the effect of increasing or decreasing the radius of the belts around the center pulleys, and hence adjusting the speed of the headstock spindle. Clever, no?
I went out to the shop to check on Rusty’s Speed Changer to see if the center part slides back and forth....nope. Remove the SC, back on the shop table, and proceed to disassemble the center pulley system. Unfortunately, I could find no set of instructions on any forum group which would help at this level...so I am on my own. The outer sheaves have set screws, which I removed, but still nothing moves. Everything is frozen onto the center shaft (after 20 - 30 years of no use).
Using a gear puller, WD40 to help loosen the parts, and a few choice words of encouragement (or were they threats?), I was able to finally get all three sheaves disassembled from the center shaft.
And it is no wonder that the floating sheave doesn’t float. There is a ton of anti-floating rust and corrosion everywhere.
More cleaning, polishing, and viola!...the center sheave floats like a butterfly.
Mount it back onto Rusty. NOW I am ready for the motor and its mounting bracket, then finally new belts and alignment.
READERS CHALLENGE:
For no extra charge, a second bonus part has been added to this blog episode which, to be honest, requires the use of basic algebra and geometry which we all slept through studied in high school. I know, you thought when you were taking this stuff in high school, that you would never really use it in real life. Well here’s your chance! And you can tell your grandkids that you actually had to use this in real life, and to pay attention in school.
Good Luck!
The challenge: The drive train on the Shopsmith is set up with four pulleys: One on the motor, one on the speed changer driven from the motor pulley, a second on the speed changer which drives the headstock spindle, and finally the headstock spindle. Given the diameters and radius of all four pulleys (see diagram), and the speed of the motor at 3600 rpm, can you calculate the speed of the headstock spindle?
Hint: It is based on the ratios of the circumferences of the pulleys.
This will be open book...but please do your own work. And no fair peeking at the section which follows. Good Luck!
To begin: the basics. For every complete rotation of the motor, the motor pulley drives the belt so many inches based on its circumference. To determine the speed of the second pulley, we divide the circumference of the motor pulley by the circumference of the second pulley (to determine how many rotations there will be of the second pulley for every one rotation of the first pulley). We then multiply that ratio times the speed of the motor to obtain the speed of the second pulley.
If you remember from your geometry class, the circumference is equal to 2 times Pi times the radius. Since we are dividing one circumference by the other circumference, we needn’t care about the "2"and Pi...they cancel themselves out.
So let’s start by determining the speed of the speed changer. The motor pulley has a diameter of 2”, which gives us a radius of 1”. The diameter of the speed changer pulley being driven by the motor is 1.75” giving us a radius of 0.875”. The motor is running at 3600 rpm. We divide 1” by 0.875”, and multiply that by 3600 to get a speed changer speed of 4114 rpm. A slight speed increase.
As a personal note: I would like to thank user 'daldrich' from the Shopsmith 10ER Users Group for correcting my formula for the circumference. I guess I forgot to check with my own grandkids!
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