My Tractor Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
376 Posts
Discussion Starter #1
http://books.google.com/books?id=9Q...g#v=onepage&q=blade tip speed 216 mph&f=false

This link lists two ideal blade tip speeds for grass types:
137 MPH (12,056 FPM) for "most common grasses"
216 MPH (19,008 FPM) for "tough grasses (Bahia, etc.)"

My 1984 Toro 20672 has a Tecumseh TNT100 engine that is governed for 3000 RPM. With a 21" blade, this equates to 16,493 FPM or 187 MPH. The engine has a handle-mounted throttle lever which can allow the engine to run as low as 2250 RPM, which translates to 12,370 FPM or 140 MPH which is close to the first figure listed in the April 1970 issue of Popular Science that the above link takes you to.

We all know there is a maximum blade tip speed - 19,000 FPM (216 MPH) - but is there also a minimum blade tip speed (so as to provide satisfactory cutting results)?

~Ben
 

·
Registered
Joined
·
2,534 Posts
As long as your blade is sharp, I would cut at full speed all the time. Mowers should be set up for maximum torque at that blade speed.
 

·
Registered
Joined
·
376 Posts
Discussion Starter #6 (Edited)
What happens if you cut common grass too fast?[/QUOTE

:dunno: Less seat time?
The grass may look ragged after you're finished, especially if you don't follow the "1/3 rule" which indicates to cut the grass no more than 1/3 of its original length, which varies by mower deck height range coverage. If the grass you intend to cut is about 4" long, only shave off about 1-1/3" of it.

Also, no speed beyond 19,000 FPM is considered safe for any mower per ANSI B71.1-X1968, which formally became law as of May 1, 1970, and thus first implemented on 1971 model year mowers (production start: August 1, 1970, engineering freeze August 1, 1969). Prior to that date, the mandate called for 21,000 FPM (or 239 MPH) maximum.

The old days: to run a typical 20" cut, 3.5 HP mower at 4000 rpm or less
Since 1971: to run a 20" cut, 3.5 HP mower at 3600 rpm or less

~Ben
 
1 - 6 of 6 Posts
Top