Mower Design: How Many Blades

There are a lot of variables to play with as we design the prototype autonomous mower from scratch:

  1. The deck dimensions and shape are entirely up to us.
  2. We get to choose the number of blades.
  3. The size of the blades.
  4. How the blades are driven: direct drive, through a pulley, chain, or timing belt.
  5. What standard lawn mower components we attempt to use.

This is not an exhaustive list, but these are the main variables I find myself tweaking as I try to optimize the design.

We have a lot of freedom to do whatever we want because we are custom making the mower deck. But this also creates a lot of questions. As we discussed previously, the deck geometry and the number of blades used on the mower lock down several of these variables. So before we go any further, I want to go into detail the advantages and disadvantages of a design with one, two, and three mower blades.

Single Blade Mower Design

dfsa
Dimensions of the top of the single blade mower deck. Length is 37.5in, width will 31in for a 30in blade.

The single blade design has one huge advantage: one blade, one motor. The motor can even be coupled straight into the blade if it is sized appropriately. This could minimize cost and complexity in a big way for our design.

Unfortunately, it also creates some disadvantages that aren’t immediately obvious:

  1. A single blade design results in the longest wheelbase, which will adversely affect the agility of the mower.
  2. It also results in the largest mower deck. That means it will be heavy and expensive compared to alternatives.
  3. The largest blade you can get is ~30in. So in addition to the negative performance and cost impact from the points above, you can only achieve a cutting width of ~30in with this design.
  4. And on top of all these issues, you also have to go with a minimum 3kW BLDC motor and controller to get the power you need to rotate the blade. That’ll set you back close to $700 after everything is said and done.

Yikes. Turns out a single blade actually creates more problems than it solves. If our target cutting width was in the neighborhood of 20in, this would be the way to go. But since we’re aiming for closer to ~36in, this design is unacceptable.

Two Blade Mower Design

2blade
Dimensions of the top of the two blade mower deck. Length is 25.5in.

The two blade design solves a lot of the issues that the single blade design faces. The deck length and resulting wheelbase are considerably smaller, and because there are two blades that need to be driven, two smaller, cheaper motors could be used. Or alternatively, you could use one large motor and have a pulley drive system transmit power to each blade.

The biggest drawback with a two blade design is related to geometry. In the picture above there are two dashed circles showing the path the tip of each blade with follow as it rotates. See how they overlap? If we leave the design as shown in that picture, the blades will crash into each other during operation. If space them apart, the grass between them doesn’t get cut.

There are two possible solutions to this issue:

  1. Use a chain or timing belt to link the two blade spindles together. This will ensure they are synchronized through their rotation paths and won’t crash.
  2. Separate the blades so their paths don’t intersect, but angle the deck. As the mower travels, it won’t leave a small tract of uncut grass.

Using chain isn’t a good option to synchronize the rotation of the two blades in my opinion. The blade drive system needs to be designed for shock loading, and also to minimize vibration for the Pixhawk. Chain doesn’t help any in this realm. It also creates maintenance issues, although those are secondary concerns. I suspect this is why V belt is used on commercial mowers most commonly, not chain.

A timing belt is a better solution, but this forces us to find a way to integrate timing belt sprockets into our design, which will invariably result in some expensive fabricated adapters to link an off the shelf timing belt sprocket to the mower spindle. So it’s a better solution than chain, but has it’s own set of problems. So option 1 is out.

Option 2 is an elegant solution, and you see it on commercial mower decks that feature two blades quite often. However, the tradeoff is that you increase the length of the deck because you are essentially moving one blade further forward than the other. See the picture below to see what I’m getting at.

offset 2 blade
The two blade mower deck, angled to where the blade paths overlap as the mower travels straight ahead.

Dimensionally, option 2 results in the same wheelbase as a single blade design, but with the headache of two motors. It may even be longer than a single blade design, because the swivel caster assembly on the front needs clearance to swivel.

Plus it just looks funky. So a two blade design is out, too.

Three Blade Mower Design

Initially I was hesitant to even consider a three blade design because of the number of parts it will require. Three blades, three spindles, three pulleys, a belt to connect all of the spindles, or three motors to direct drive the blades.

To complicate matters, the smaller the blade length, the faster it needs to rotate to achieve good grass cutting velocity. We previously discovered that a 21in blade requires 3500RPM in order to achieve a blade tip speed of 19000ft/min. For a 12in long blade, that number jumps to more than 6000RPM.

blade rotation speed
The relationship between blade tip speed and blade rotation speed. We’re aiming for a Vmax of 19000ft/min, or just below that.

This is a problem because most DC motors don’t operate at those speeds with any significant amount of torque. In fact, most of the motors I’ve seen have no-load speeds listed far below 6000RPM. That’s the bad news with a three blade design.

The good news? Other than these blade drive system constraints, the three blade design is geometrically very efficient. It results in the smallest wheelbase, and because it is triangular in shape, you get bonus clearance for the front casters to swivel. It requires the least amount of material to fabricate. That means cheap and lightweight.

three blade
The three blade mower deck design. It’s a very good combination of deck width and length.

The blades are separated by a small amount but because the center blade is slightly more forward than the outer two, you still get 100% cut coverage, similar to the angled two blade design.

The three blade mower design is by far the most efficient, and I like the way it looks, too. The only hurdle to making it works is finding a motor that won’t break the bank, but still get us close to the 6000RPM requirement for a 12in blade. If we can find a motor that works with this design it will by far be the best one of the three. Does such a motor exist?

The Holy Grail of DC Motors

We had previously considered BLDC motors to give us the power and efficiency we need to spin the cutting blade. But unfortunately that power and efficiency comes at a cost: BLDC motors require a controller to run them.

So even if you find a fairly cheap BLDC motor that meets your needs, tack on 50% of the motor price for the controller. Need three motors? Looks like you need three controllers, too. And the space to mount them somewhere. I’m sure they make combo controllers out there, but I can’t find them.

The other problem with these controllers is that you’re paying for a ton of features you don’t even need. Most are designed for electric scooters. I don’t need the ability to go in reverse, or to vary the speed. I just need a motor to spool up and stay there. With typical BLDC motor controllers, you get a bunch of these features, and boy do you pay for them.

So while BLDC motors fit our application requirements, they are costly. Ideally we’d like to use a simple brushed motor that operates at the speed and torque we need. Turn it on with a $3 relay. Keep it simple.

In my adventures across the interwebs, I had trouble finding anyone who makes a brushed motor that runs close to 6000RPM with significant amounts of torque, that also costs less than $400.

But that was before I found the folks out at AmpFlow. They make a really nice set of brushed, DC motors in the speed and torque range we need. They also appear to be US based, which is a plus. They post torque values in oz-in and dimensions in inches. And they have torque curves for their products. All around, these guys are awesome.

Next time I’ll go over the specifications for the E30-400 DC motor, which I think is perfect for this application.

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