While I know many lawn mowers have the cutting blade connected direct to the engine's crankshaft, several others may not have the blade mounted direct on the shaft, but rather on a spindle or a pulley.

The Toro 23267 25" commercial mower with its 6 hp Tecumseh engine may be an example of a mower whose blade isn't mounted directly on the crankshaft, because the engine turns 3,200 rpm, which coupled with the 25" blade length gives out a blade tip speed of 20,944 fpm (nearly 2,000 fpm above the government-mandated maximum that is 19,000 fpm), until you figure out some math which would have the spindle traveling at no more than around 90% of the engine's speed to bring the blade tip speed on this unit below the CPSC maximum of 19,000 fpm.

So while the engine may be running 3,200 rpm, the spindle diameter and other underdeck mechanicals on this mower would be designed thus that the blade will travel at 91% or less of the engine's speed to stay within the limit.

This issue also baffles me in regards to certain mowers produced in the 1960s, when the ANSI (then the USASI) had then imposed a 21,000-fpm max limit.

Some engines have a 4,000 rpm governed speed, which was the case on some Allis-Chalmers mowers (among other manufacturers); this speed was OK on the WB1930 19" cut model (19,897 fpm, which back then was below the maximum; after August 1, 1968, however, the engine speed on mowers with this blade length had to be less than 3,820 rpm), but it becomes a problem on both the WB2235 and WB2235S models, where coupled with the 22" blade length, the blade tip speed is 23,038 fpm (2,000 over limit) unless these two particular mowers have spindles that are designed to travel at no more than 91% of the engine's speed.

Sources: Briggs & Stratton Engines and Short Blocks for Replacement of Allis-Chalmers Original Equipment Engines, form MS-5741-48 (May 1, 1968).

~Ben

Thus my 21 inch R7070 running at 3300 Rpm is

3300 x 21 x 0.262 = 18156.6 feet per minute blade tip speed.

PS. I typed this really slow so some people could follow along.

On machines where the blades are belt driven (ride-ons) I've noticed large variations in blade rpm. Which seems to imply that certain manufacturers are leaving themselves a large margin for error.

Blade RPM is meaningless unless you know the length of the blade. An OEM running 2 blades to get the same deck width compared to one using 3 blades must run at a much lower RPM to remain within safety regulations. Don't get RPM and linear speed at the blade tip confused.