Bandsaw drive

The speed range required for my combination metal/woodworking band saw is huge: 3300FPM down to 60FPM, a 55:1 speed ratio.  This is made more complicated by the fact that AC motors are single speed devices.  The transmission is going to have to provide all of the spread. I could use a Variable Frequency Drive, but the torque characteristics at low speeds aren’t ideal for a saw.

Most AC induction motors run at 1725RPM or 3450 RPM.  Any time you have to reduce or increase the speed of a drive you incur losses, so I will try to stick to a 1725RPM motor.

Bandsaws are typically categorized by their wheel size, i.e. a 14″ bandsaw has 14 inch upper and lower wheels.   This is useful since it gives you a good feel for the throat depth of the saw.  But, the larger the drive wheel, the slower it has to turn for a given blade speed.  Since making large wheels is out of the scale of my available machine tools I am going to buy drive wheels.  After considerable scrounging around on eBay I found a set of 16″ cast iron drive wheels from fifty year old saw whose manufacturer went out of business long ago.  Good for since few people will be looking for spare parts.

So given a 16″ drive wheel I need the following wheel RPM for the speeds I want:

Blade Speed (FPM) Wheel Speed (RPM) Motor/Wheel Ratio
60 14 120.43
300 72 24.09
1800 430 4.01
3300 788 2.19

The 1800 and 3300 speeds are easy to achieve with a single stage v-belt drive pulley arrangement. But achieving 24:1 and 120:1 ratios are outside the reasonable capability of single stage drives. A 3 inch motor pulley would require a 72 inch wheel pulley to get the 24:1 ratio.

Clearly this will require multiple stages. But I want to make speed changes relatively painless, require only belt shifts, not complete belt changes. Inexpensive drill presses use a combination of step pulleys to achieve a fairly wide speed range. If can find a satisfactory set of step pulley I can use those.

As it turns out commonly available step pulleys aren’t going to get me to the speed ratio I need. If I limit the design to 1 jackshaft and two stages of step pulleys I need a step pulley that goes from 1.75 inch to 18.75 inch diameter step pulley to achieve the lowest speed \left(\frac{18.75}{1.75}\right)^2=114, the ratio needed to get to 60FPM blade speed. Clearly not practical.

Using 2 jackshafts each with two step pulleys would allow use of pulley 8.5 inches in diameter. That is still larger than normally available pulleys, and larger than I can machine. Additionally, two jackshafts would be very difficult to setup and maintain.

The largest commercially available (but expensive) step pulleys found that give five steps, 6-5-4-3-2 inches. Subtract a quarter inch for where the v-belt actually rides. If we use four of those pulleys and limit to one jackshaft we need an additional 10.5:1 reduction to get down to the 60FPM speed we need. Even that is beyond a single stage pulley, so I need some mechanical help.

Another thing to keep in mind is that losses in drive trains increase with the speed of the components, so try to keep all the shaft speeds as low as possible. Practically that means not to put the additional 10:reduction at the drive wheel, put it at the motor output. Otherwise you will have to drive the input to the 10:1 gear reducer at 7500 RPM at the highest speed.

So the initial design will have a 1-1.5HP motor, a gear reducer of about 25:1 on the motor output driving a step pulley, then a jackshaft with two step pulleys, and step pulley on the driven wheel.

Time to start scrounging on eBay…

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