To kick off this Labor Day weekend I did some shopping. Wichita has a cool little place aptly named The Yard, which sells everything from screws, casters, foam, you name it. And because most of it is surplus, the prices are great, too.
When I design things, I start out planning to source everything new, and I record the price of every component I call out on the design. This information was immensely helpful as I dug through bins of screws. I had a price to beat on every component I was shopping for.
And as usual, The Yard came through. For example, a 5in long socket head cap screw, 5/8-11 thread was $5.80 through McMaster. The same screw at The Yard was $1.99. I found some 5/8-11 lock nuts for $0.68 a piece. McMaster quoted me $0.92 a piece. This was typical of all the hardware I was able to find there. Shopping at The Yard also saves me shipping, a cost I’d incur buying through McMaster or some other online source.
The Yard even cut some 3/16 chain for me. They charged me for two feet at a total cost of $2.50. I even got to keep the extra links. Not too shabby!
Those pennies add up. There are some components I’m going to have to shell out a lot of money for, like the hubs for my drive wheels. The bad boys below cost just shy of $50 a piece, and after shipping it was $120 to get them.
Harbor Freight had a sale on their cheap 10in diameter wheels, and I picked up four of them for $3.99. However, I think Harbor Freight is always having a sale on those tires. Their regular price is still dirt cheap.
Each tire comes with two hub pieces. One hub piece has two flanged bearings pressed into it, the other has no bearings. You can see the two hub pieces from one tire in the picture below.
I took two hub pieces with no bearings in them and made a “drive wheel” tire. I took the remaining two hub pieces with the bearings in them and popped one of the two bearings out. You don’t need a total of four bearings for one tire, and I have plans for these extra bearings.
I had to drill a hole in the hub piece with the bearings so the nipple on the innertube had a place to protrude from. You can see the hole in the hub without the bearings in the picture above.
After I drilled the holes and put the wheels back together I had a pair of “caster wheels”, a pair of “drive wheels”, and four extra flange bearings. I had to bend the innertube nipple on the caster wheels to get a bicycle hand pump on the nipple so I could inflate them.
The four extra flange bearings are going to be used to mount the stem of the casters into.
The Yard also has a vast selection of steel and aluminum raw stock. They didn’t have the 2in X 2in X 0.25in square aluminum tube I was after, but they did have some 2in X 2in X 0.1875in square tube. I think this will be easier to weld to 0.125in thick aluminum sheet metal anyway, so I had them cut several sticks for me.
My only complaint about The Yard is that they won’t do angle cuts for me. I’m either going to have to get my miter box and hack saw out, or find someone with a nice band saw to get them cut.
Overall, I think this is a good start to getting the prototype robot lawn mower fabricated.
The Mower Project 
Hello Mr. Mower, It would be nice if you had a name like Craig or something. My name is Willem but since I can remember my mother called me Eric. Now at nearly 70 years of age it is suddenly becoming a problem. Like why are you calling yourself Eric? You can imagine. Nevertheless I am chuffed to meet you. I am no engineer but I did a 5 year apprenticeship as an electrician and I have worked on high tension, low tension, AC and DC. elevators and escalators air conditioning and process automation. 1/4 and 1/3rd scale model aircraft. Built full size KR2. Toured through Europe twice on a Honda Gold Wing and built a 3 1/2 inch gauge live steam locomotive. In short I have not got much theory but miles of experience. Whilst reading your report on progress on your mower since 2018, I became aware that you are suffering from burnout. Hence the consuming of motors and blades spinning with a linear speed of 14130 ft/sec or something like that. But you got my interest going and now suffer the consequences. The only commercially viable thing I ever designed and manufactured was the very first(in South Africa) as far as I know, battery powered garage door operator in 1998. Then the Chinese got better and cheaper. Put me out of business.
Getting back to your mower, keeping in mind that I have mowed the lawn in my life more than I care to remember, you need to know the enemy! Which in this case is the grass. Now in South Africa we have a tough grass called Kikuyu. This is really tough and sometimes makes shoots as thick as my small finger which runs horizontally below the green leaves. That can play havoc with a lawn mower blade if you happen to run too close to the ground. The important thing is, using a disc type mower, it gets thrown off direction and subconsciously you auto correct whilst walking behind the mower. Not wishing to bore you to tears, I want to cut this short having dribbled on for so long. I leave you with 2 thoughts. 1). The cylinder mower is so efficient that you can propel it by hand. Powering it you can tow an articulated little trailer wit all your software and batteries and I’m sure it will consume far less power. Also going back to the dark ages where I come from, a “series wound D.C. motor theoretically generates more power the faster it runs. Without a load it can self destruct although I have no first hand experience of such. 2) You are dealing with a variable power factor in terms of the grass. The length, the density and the moisture content. These factors have little effect on a Cylinder mower. I hope my thoughts have at least given you some more food for thought. I am very impressed with your achievement to date and also your tenacity in pushing forward. In parting I leave you with this thought. You have to build a prototype! Weather it works or not you can modify the prototype at will.
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Thanks for your kind words Eric. I’ve got some things in the works to get the prototype built, that’s partly why I’ve been quiet lately. Not much to show right now.
Regarding the DC motor comment, you’re correct, if they stall for even a short amount of time you can damage them badly. I think good fuses can mitigate this risk. You run into a trade-off doing that, though, because you are essentially picking the point at which the motors are under too much load. Too small a fuse and thick patches of grass will blow it. Too large a fuse, and you run the risk of damage to the motor.
However, I have two reasons to think these DC motors will work acceptably under typical residential/commercial lawn mowing circumstances.
1. You can find a lot of electric lawn mowers out there running DC motors much smaller than the ones I’m using. I know that’s anecdotal, but I’ve also seen some even larger 24V DC motors I can substitute in place of the motors I currently have on the design if they prove to be lacking.
2. In my research, I’ve discovered that the lion’s share of power required to run a rotary blade mower comes not so much from cutting the grass, but from the fan power the blade consumes creating suction under the mower deck. Most rotary blade mowers use this fan power to pull the blades up so they’re cut evenly, and also to evacuate the clippings to a bag or discharge chute. Depending on the geometry of the blade, you can directly influence the amount of power the motor has to consume to spin the blade.
So I’m hopeful some DC motors can do the trick. You’ll know here before long when I make a blog post testing how much power the mower deck consumes, and whether or not it will work.
Thanks again for the encouragement Eric!
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Hey there Mr Mower, I realize this is an old entry, and possibly not monitored anymore…But one never knows.
I am VERY interested in those hubs you have pictured…Any chance of sharing your source?
I’m 75% into my 30″ RC mower build and I have got to find a way to cut costs. I’m using the same the same wheels you have, for my first build, but had to have custom keyed “sleeves” made to accept the 17mm shafts… ($$$)
I’ve been tinkering away…(waiting on parts mostly) for over a year now…Sigh
Hope this finds you well, & that your mower vision has become reality!
Dave
Email information provided
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Hi Dave, the part number is NPC-PH817, you can get it from NPC Robotics. They offer a few different sizes based on shaft size. They were about $35 a pop when I bought them back in 2019. Appears they’re a bit more now.
Hope this helps!
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