What Size Lawn Mower Blade Do I Need
At that place are a lot of variables to play with equally we design the prototype democratic mower from scratch:
- The deck dimensions and shape are entirely up to u.s..
- We get to choose the number of blades.
- The size of the blades.
- How the blades are driven: direct bulldoze, through a pulley, chain, or timing chugalug.
- What standard backyard mower components we attempt to utilize.
This is not an exhaustive list, only these are the main variables I find myself tweaking as I try to optimize the design.
Nosotros have a lot of freedom to do whatever we desire because nosotros are custom making the mower deck. But this as well creates a lot of questions. Every bit 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 whatever further, I desire to get into detail the advantages and disadvantages of a design with one, two, and 3 mower blades.
Unmarried Blade Mower Design
The single blade design has one hugeadvantage: i blade, one motor. The motor can even be coupled straight into the bract if information technology is sized appropriately. This could minimize cost and complexity in a big style for our design.
Unfortunately, it as well creates some disadvantages that aren't immediately obvious:
- A unmarried bract design results in the longest wheelbase, which will adversely bear on the agility of the mower.
- It also results in the largest mower deck. That ways information technology will be heavy and expensive compared to alternatives.
- The largest blade yous can get is ~30in. So in improver to the negative operation and cost impact from the points higher up, you can merely accomplish a cutting width of ~30in with this design.
- And on height of all these issues, you also have to become with a minimum 3kW BLDC motor and controller to get the power you need to rotate the blade. That'll prepare you dorsum close to $700 later on everything is said and done.
Yikes. Turns out a single blade actually creates more than problems than it solves. If our target cut width was in the neighborhood of 20in, this would exist the way to go. Just since we're aiming for closer to ~36in, this pattern is unacceptable.
Two Blade Mower Design
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 utilise one large motor and accept a caster drive system transmit power to each blade.
The biggest drawback with a two bract design is related to geometry. In the picture to a higher place 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 pattern as shown in that film, the blades will crash into each other during performance. If space them apart, the grass between them doesn't get cut.
There are two possible solutions to this issue:
- Use a concatenation or timing chugalug to link the two blade spindles together. This will ensure they are synchronized through their rotation paths and won't crash.
- Carve up the blades and then their paths don't intersect, but bending the deck. As the mower travels, it won't leave a small tract of uncut grass.
Using chain isn't a expert option to synchronize the rotation of the two blades in my stance. The blade drive system needs to exist designed for shock loading, and also to minimize vibration for the Pixhawk. Chain doesn't help whatsoever in this realm. Information technology likewise creates maintenance bug, although those are secondary concerns. I suspect this is why V belt is used on commercial mowers most usually, non concatenation.
A timing chugalug 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 made adapters to link an off the shelf timing belt sprocket to the mower spindle. And so it'southward a better solution than chain, just has it's own set of problems. Then option 1 is out.
Choice 2 is an elegant solution, and yous see information technology on commercial mower decks that feature two blades quite ofttimes. However, the tradeoff is that you lot increase the length of the deck because you are substantially moving i blade further forrard than the other. See the picture show beneath to encounter what I'thou getting at.
Dimensionally, option 2 results in the same wheelbase every bit a unmarried bract design, only with the headache of 2 motors. It may even be longer than a single blade design, because the swivel pulley assembly on the front needs clearance to swivel.
Plus information technology just looks funky. So a two blade design is out, as well.
Three Blade Mower Design
Initially I was hesitant to even consider a three bract design because of the number of parts it will crave. 3 blades, three spindles, 3 pulleys, a belt to connect all of the spindles, or three motors to directly bulldoze the blades.
To complicate matters, the smaller the bract length, the faster it needs to rotate to attain expert 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.
This is a problem considering most DC motors don't operate at those speeds with any significant corporeality of torque. In fact, virtually of the motors I've seen take 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 organisation constraints, the three bract pattern is geometrically very efficient. It results in the smallest wheelbase, and considering it is triangular in shape, you become bonus clearance for the front end casters to hinge. It requires the least amount of material to fabricate. That means cheap and lightweight.
The blades are separated past a small amount but because the center blade is slightly more than frontward than the outer two, you still get 100% cut coverage, similar to the angled ii bract design.
The three bract mower pattern is past 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 pattern it will by far be the best i of the three. Does such a motor exist?
The Holy Grail of DC Motors
We had previously considered BLDC motors to give us the ability and efficiency we need to spin the cut blade. But unfortunately that power and efficiency comes at a cost: BLDC motors crave a controller to run them.
And so even if you find a fairly cheap BLDC motor that meets your needs, tack on 50% of the motor price for the controller. Demand three motors? Looks like yous demand iii controllers, besides. And the space to mountain them somewhere. I'm sure they brand combo controllers out there, but I tin can't find them.
The other problem with these controllers is that you're paying for a ton of features you don't fifty-fifty need. Most are designed for electrical 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 lot get a agglomeration of these features, and boy practise you pay for them.
And so while BLDC motors fit our awarding requirements, they are costly. Ideally we'd like to use a simple brushed motor that operates at the speed and torque we demand. Plow it on with a $3 relay. Keep it uncomplicated.
In my adventures across the interwebs, I had problem 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 brand a really nice gear up of brushed, DC motors in the speed and torque range we need. They also appear to be The states 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 crawly.
Next time I'll go over the specifications for the E30-400 DC motor, which I call up is perfect for this awarding.
What Size Lawn Mower Blade Do I Need,
Source: https://mowerproject.com/2018/10/21/mower-design-how-many-blades/
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