# Who can make this calculation for me please?

Discussion in 'Lawn Mowing' started by FrankenScagMachines, Feb 25, 2005.

1. ### FrankenScagMachinesLawnSite Platinum Memberfrom INMessages: 4,739

I know some of you are capable of making these calculations, and at one time I probably knew, but have forgotten now

If a two bladed 36" mower has a 13hp Kohler, which spins at 3,350 RPM and turns the blades at 15,000 FPM, then how fast would the blades spin at 3,600 RPM? How fast at 3,750 RPM?

I am getting a John Deere GS30 walk behind (fixed deck belt drive) with the above specs and am quite sure it would perform much better with higher blade speed. This machine is unique from other commercial 2 blade walk behinds (which have overlapped blades) this one has timed blades! They sit side by side, no overlapping. The only thing preventing them from hitting each other is a timed cog belt. Don't see that as a problem unless you hit something immovable (which rarely happens for me). Actually there is a good side to it too- the trim side blade is closer to the drive tire for much better trimability, especially on a belt drive.

Thanks if anyone can help me!

2. ### lsylvainLawnSite Senior Memberfrom Sarasota/Bradenton, FLMessages: 779

15000 FPM/3350 RPM =4.477.....FPM/RMP

3600 RPM * 4.477....FPM/RPM = 16119.4 FPM

3750 RPM * 4.477...FPM/RPM = 16791.04 FPM

There ya go, although probably not 100% Accurate because the formula to calculate this is most likely exponential due to the loss of energy from crankshaft to blade tip, but it should be fairly close.

3. ### cwardLawnSite Memberfrom Wichita, KSMessages: 190

FrankenScag,
If you just bump up the engine RPM, your blade tip speed will increase to 16119.4 FPM. Is your initial data from Scag? Just curious, the numbers don't quite work out as listed. However, the 16K FPM is close. Take care.

4. ### FrankenScagMachinesLawnSite Platinum Memberfrom INMessages: 4,739

Thanks guys. Cward- no its from the JD website specs page for the GS30 w/b w/36" deck. So, in order to get anywhere near 18000 FPM I will have to change pulleys. I'll just speed her up a little and see how that does before swappin pulleys.
Thanks!
Eric

5. ### packerbackerLawnSite Bronze Memberfrom Overland Park KSMessages: 1,433

I got a question, if you increase the HP on your mower wont your blades spin faster?

I wanted to increase my 48 to a 17 and some clown that works at my dealer said it wont,

But this also the same guy who tried to sell me 2 cycle to put into my mower...

6. ### FrankenScagMachinesLawnSite Platinum Memberfrom INMessages: 4,739

Horsepower does not increase blade speed, it only increases the amount of power. Only changing pulley sizes or engine speed (RPM) will increase blade speed.

7. ### packerbackerLawnSite Bronze Memberfrom Overland Park KSMessages: 1,433

How would you change engine speed?

8. ### cwardLawnSite Memberfrom Wichita, KSMessages: 190

FrankenScag,
I don't blame you for wanting to up your blade-tip speed. However, be careful. I thought the Fed's? set limits on blade tip speed. I'm sure that the manufacturers have a nice safety cushion on the performance specifications of their parts. I'm not sure what the actual tensile or flexural strength of the spindles would hold up to. I wouldn't get to crazy though. Hugh liability problems if something ever went wrong and someone could prove you changed a factory setting.

9. ### KathysLGCLawnSite Bronze Memberfrom Wilton, CTMessages: 1,345

You couldn't prove the owner changed the settings. I'm sure everything would hold up fine. Spindles are very strong.

10. ### cwardLawnSite Memberfrom Wichita, KSMessages: 190

Probably not. Always remember....Big brother is watching you!