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Correct max RPM for a 21" mower?

Jason Rose

LawnSite Fanatic
Ok, I know that mowers that run the blade(s) with a belt drive run at 3650 rpm to acheive the correct, and top blade tip speed... But with a 21" mower (any brand) isn't 3600 rpm too fast, or is it? Seems like I've read somewhere that 3200 is what a 21" should be set at...

I came across a 21" snapper with the Robin Suburu engine. Right now it's running at 3200, and it "sounds" right... I pushed it up, by hand, to 3600 and it really sounded too fast... (using a tach of course).
 

Valk

LawnSite Gold Member
Location
KS
3600 rpm's = 19782 ft/min

3457 rpm's = 19000 ft/min
 
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dutch1

LawnSite Silver Member
Location
Jayhawk
Jason,

I think you will find that most 21" OEM equipment will come from the factory set up at anywhere from 3000-3200, most likely closer to 3000. Yes, 3600 is too high for the small splash lubricated equipment. I always adjust the small stuff to a range of 3120-3160. The smaller hp . Briggs, in particular, always seemed to "hunt" when they got above 3300. Force lubed engines can be run a little higher although I don't do it. Although you don't see them around much anymore, the old 2 stroke Suzuki found on Toro 21's always seemed to perform better at 3400-3600.

Dutch
 

Shasta Lake Landscaping

LawnSite Senior Member
Location
Upstate, Ny
Ok, I know that mowers that run the blade(s) with a belt drive run at 3650 rpm to acheive the correct, and top blade tip speed... But with a 21" mower (any brand) isn't 3600 rpm too fast, or is it? Seems like I've read somewhere that 3200 is what a 21" should be set at...

I came across a 21" snapper with the Robin Suburu engine. Right now it's running at 3200, and it "sounds" right... I pushed it up, by hand, to 3600 and it really sounded too fast... (using a tach of course).
Boy I'd like to come across one, A guy I mow for has one but he just wont let it go.
 

GravelyNut

LawnSite Bronze Member
Location
Wet part of FL
Ok, I know that mowers that run the blade(s) with a belt drive run at 3650 rpm to acheive the correct, and top blade tip speed... But with a 21" mower (any brand) isn't 3600 rpm too fast, or is it? Seems like I've read somewhere that 3200 is what a 21" should be set at...

I came across a 21" snapper with the Robin Suburu engine. Right now it's running at 3200, and it "sounds" right... I pushed it up, by hand, to 3600 and it really sounded too fast... (using a tach of course).
A Gravely PRO21SCH or PRO21CH with a Honda GXV160KA12 has a max speed of 3350 RPM with a governed RPM of 3200 +/- 150. Other models with the same engine go as low 2900 and governed at 2850 +/- 50.
 
OP
Jason Rose

Jason Rose

LawnSite Fanatic
OK, thanks guys. I'm going to add all that together an make the assumption that 3200 is about right for a 21" mower.

This Robin engine is a pressure lubricated engine, with an oil filter. The oil capicity I need to research... I changed the oil and filter today and poured in the "usual" 20 ounces to start off with, figuring it would hold probably 24 to 28 ounces with the filter. With 20 ounces and after running it to fill the filter it was WAAAAAY over full! So I measured the oil that came out of it, which was right on the full mark before I drained it, and it was 12 ounces!? So I dumped the fresh oil I had put into it and refilled it, this time with about 16 ounces, and with the filler cap NOT threaded down I'm on the full mark. I don't know if you are supposed to thread them down to check or not... I will have to look up the model number of the engine from snappers website, IF they have suck a feature, and find out what the deal is.
 

dishboy

LawnSite Fanatic
Location
zone 6
I have ran every 21" @ 3600 for close to thirty years now and have yet to have a engine failure. Just keep a eye on your blade sails, but you usually wear out the blade before this is a issue.
 

madmower

LawnSite Member
Location
cornwall uk
this is the formula for safe blade speed


19000 divided by (blade length x 3.14 Divided by 12) = max engine rpm

Eg
18” x3.14 ÷ 12=4.71 =
19000÷4.71= 4033 rpm
This is the max rpm for a blade tip speed below
19000 foot per minute

so 21" max engine speed to fall under 19000 fpm is 3457 rpm
 
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OP
Jason Rose

Jason Rose

LawnSite Fanatic
Ok, so then 3600 is still WELL within the "legal" blade tip speed.

Thanks!

The mower turned out to be just like new once I got it all shined up and everything. One pull starts and runs perfect. Paid more than I should have, especially for the new "commercial" bag, but I spent about $350 overall, that's a LOT better than $850 for a new one and the Robin engine is awesome (and it's not offered anymore) and it's lighter. Even better than a new Toro or JD 21", those would have cost me $1,200, for a mower I really don't use much at all, lol.
 
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