Mower deck Blade tip speed.

Recently purchased a z Master model 74444 48 inch deck with a 21HP Kaswaski. Specs say engine speed maxed at 3600RPM (Probably adjusted via governor) and blade tip speed should be 18,500FPM. Measured engine speed at max throttle with a lazer tach and get 3570 RPM. Well within reason for a 2010 z master. Next measured the spindle RPM and get approx 4200RPM, and calculations indicate that the tip speed is 17,500. Toro specs for this mower says tip speed should be 18,500FPM. Somethings not right. Any ideas ?
(The PTO clutch was replaced in 2015, so is there a PTO that will fit a model 74444 z master that is set for residential use vs commercial use ?) IIRC government limit for residential mowers is 17,500FPM and 18,500FPM for commercial...

Are you sure you got the same diameter pulley on the PTO when you replaced the clutch?

cpurvis said:

Are you sure you got the same diameter pulley on the PTO when you replaced the clutch?

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I didn't do the PTO replacement, it was done at a Toro shop by the previous owner. I talked with the service dept and they shared the service record as far as they replace the PTO. When I look at the part drawing the PTO and pulley are one assembly. (It's buried under the mower and I have not done a complete inspection of that area yet.) I did just finish tightening the deck drive belt as it appeared somewhat loose, but tach reading is the same.
And thanks for the quick response and thought. It did cross my mind that possibly the wrong PTO with a residential diameter was used. But parts lists don't show 2 different PTO's, one for 17,500 and the other for 18,500 FPM.

ToroZ4003340 said:

I didn't do the PTO replacement, it was done at a Toro shop by the previous owner. I talked with the service dept and they shared the service record as far as they replace the PTO. When I look at the part drawing the PTO and pulley are one assembly. (It's buried under the mower and I have not done a complete inspection of that area yet.) I did just finish tightening the deck drive belt as it appeared somewhat loose, but tach reading is the same.
And thanks for the quick response and thought. It did cross my mind that possibly the wrong PTO with a residential diameter was used. But parts lists don't show 2 different PTO's, one for 17,500 and the other for 18,500 FPM.

Click to expand...

Figured out the problem, equation for tip speed is pi*dia*RPM/12. I was using 16inches for dia and the blades are 16.5inches. At 4200 RPM tip speed calculates at 18,147 which is close enough for me. Thanks for your response, it made me revisit all the measurements and actually do a blade diameter measurement...