Crankshaft - Blade protective assembly for lawn mowers?

Question

Hi, I'm trying to put together my own lawn mower for pulling behind my 35hp old tractor. I'm from India and nobody makes powered tow-able mowers here. I have a 12hp diesel engine from an old pump which I can mount onto a deck. My family's property has very tall grass but also has hidden rocks. I wanted to know,
1. what's the best way to make sure that the blade or crankshaft do not get damaged when the blade hits a rock?
2. I'll import 20"+ blades, what are the best mulching and regular cutting blades you recommend?
3. How do you adjust cutting height? In some areas we want it tall, at others a close cut. Is this done at the crankshaft spindle, or
4. What's a recommended cutting RPM? I'll need this to decide if I have to gear or pulley down the speed from engine RPM or take a direct drive.

Answer 1

You pose some very good questions. I'll do what I can to answer them.

1. Not knowing how many blades you are using, the best way to keep the crankshaft of the engine protected is belt driving the mower. As far as protecting the blades and arbor shafts from rocks, maybe shear keys in the pulleys? But that could be extremely time consuming.

2. Get the ones that will fit your arbor shafts. Don't get the fancy curved ones, they are a pain to sharpen. Get ones that the cutting edge is flat.

3. The easiest way to adjust would be to adjust the deck its self. That way when you have the blades even, you don't have to mess with them. So adjusting the wheels holding the deck up would be the best.

4. Most small engines run about 3400 RPM. Here in the US the highest speed the tip of the blade can go is 19,500 feet per minute. So that means a 20 inch blade can go 310 RPM, a 21 inch blade goes 295 RPM, and a 22 inch blade goes 282 RPM.

I hope this helps.

Answer 2

Usually in the late winter. At the end of summer I have had my fill of lawn mowers, hoes, weeds, grass seed, compost, etc. and once the grass stops actively growing and the rainy season begins (October in the Pacific Northwest), I'm done with everything outdoors with the exception of raking leaves. I use a push mower -- good exercise, no gasoline required, no pollution, don't have to have my blood pressure go off the charts trying to get the thing started after six months of idleness. The grass stays where it falls unless there is a lot of it and then it goes into the compost bin.

Answer 3

Check Norther tools, they have several tow behind selfpowered mowers. The best way to protect the crankshaft from damage is to not direct couple it to the blades. The belt/pulley will protect it from sudden stops.