More Mower Musings

I’ve had some more time to think about our mower blade and the system that will be required to power it. To summarize what we discovered last time:

1. A 21in blade is the maximum size we can use.
2. A larger mower will be more efficient in general because a smaller portion of the mower width will be used to overlap the previous pass.
3. Typical brushed DC motors are probably not a great way to power the blades.

Two Cutting Blades or One?

I think I need to retract statement (1) above. It turns out there are larger blades out there, the only caveat is that they have to be rotated at a slower angular velocity to maintain a blade tip speed of less than 19,000ft/min.

Using the math I outlined previously, a 30in blade needs to be rotated at just over 2,400RPM to achieve a blade tip speed of 19,000ft/min. This turns out to be a good thing too, because this slower speed can be achieved by using different sized pulleys on the drive motor and spindle, which will allow us to trade higher RPMs for more torque, which will be needed with a larger blade.

I was really hoping to avoid a mower with two blades for a few reasons:

1. Complexity: two blades means two spindles, some idler pulleys, and more moving parts.
2. Blade interference: the travel paths of the two blades need to overlap a little so you don’t miss any grass, forcing us to make sure the blades don’t get out of sync and crash into each other
3. Vibration: I imagine two blades will result in more vibration, but this is speculation.
4. Real estate: the mower deck needs to also house (4) 12V 35Ah batteries and a bunch of control boxes. Two spindles and a large belt with idler pulleys eat this space up really quick.
5. Robustness: More parts means more opportunities for failure.
6. Cost: More parts means more cost.

So seeing as we can use one blade while still achieving a cut width of about 30in, I think this is the preferable solution. There are mowers out there that use 30in blades, but the torque required for such a blade size is pretty high. This mower is equipped with a Briggs and Stratton engine that outputs 10.5ft-lbf of torque at 2,400RPM.

Keep in mind that’s engine torque output, not torque at the rotating blade, but that still means our electric motor needs to output at least 5.25ft-lbf at 4,800 RPM (see the pulley ratio math below). I’m guessing that will be a difficult number to achieve.

Motor Alternatives

Regarding (3) above, I think a brushless DC motor may be the way to go. I found this motor on an electric scooter parts store which seems promising, but I need to see a torque to RPM curve to be sure. Coincidentally, that motor is rated for 4800RPM, which results in a pulley ratio of about 2:1. Not bad.

On the down side though, most of those motors are designed to be used with sprockets and chain, which isn’t a great idea for mowers. There’s a reason mowers use pulleys and V belts. I suspect it is because they allow for a more fluid transmission of torque to the blade and also slippage if the blade hits something rigid, resulting in less strain on the drive motor or engine.

This will mean I’ll probably need to fabricate a custom drive pulley to adapt to the motor shaft and accommodate the V belt required by the blade spindle.

What Do The Other Engineers Think?

This is the 21st century. Has anybody designed an electric mower with similar parameters to the ones we’re looking at here? Turns out the answer is yes. The specs for the Ryobi RY48110 riding mower:

1. Looks like they’re using a few 12V 25Ah lead acid batteries for power.
2. They use two BLDC motors directly connected to the cutting blade.
3. Deck width is 38in which would imply a ~19in blade length.

Funny story, if you pull up the parts list and google their replacement motor part, you come up with this motor rated at 48V, 32A, 1800W, 4500RPM. So we’re at least in the neighborhood.

A few comments about this mower:

1. They’re using this motor to drive a ~19in blade. We’re looking at using a motor that is actually rated at 1600W to drive a blade that’s more than 50% larger. Not good.
2. Direct drive is interesting, it’s definitely simpler. I would be interested to see what kind of connection hardware they use.
3. A 19in blade implies a maximum blade tip speed of 3800RPM. The motor is rated for 4500RPM. I wonder if the mass of the blade results in a equilibrium rotation speed of about 3800RPM. If not, the mower isn’t in compliance with ANSI B71.1-1990.
4. How do the two motors stay in sync? I wonder if the blade paths overlap or if there is some kind of chain or belt between them to maintain their angular offset.
5. Being BLDC motors, I also wonder what the controller looks like. I imagine it shoots for a fixed rotation speed. I would be interested to know what that rotation speed is.
6. The Cadillac version of this electric riding mower only offers 100Ah of charge at 48V. That’s only 4.8kWh of energy. Our system with four 12V 35Ah batteries can store up to 6.72kWh of energy. This is interesting because not only does the riding mower need to cut grass, but it also has a possible 250lb person it’s driving around too. So I think we’re on the right track as far as power requirements go.

In Summary…

1. I need to get some torque speed curves for these motors. Otherwise we’re just guessing about motor capability here.
2. I think our power system is appropriately sized based on the RY48110 specifications.
3. Torque output is still the big unknown here. The RY48110 gives us a baseline to work from though.
4. It looks like BLDC motors are going to be the device that powers our blade, seeing as the folks at Ryobi arrived at the same conclusion we did.