RPMs: To Adjust or Not to Adjust, That is the question

[RhettWS]

See what the claimed blade RPM is suppose to be and adjust to that.. Speaking of which, did you put aftermarket blades on? Could be high-lift blades slowing it down.

I did put on a set of Oregon G5s. I thought of that. This weekend I will do the math first, then test with the OEM blades. Bert also has a point that I hadn't really thought about concerning emissions. You never know what the feds are up to behind the shadows regarding environmental issues.

I'll let you know what I find out. Thanks again.

 

[Mad Mackie]

G5s are 3" wide, thicker, heavier and will slow down the engine RPM just to turn them.
I tried a set on my Scag and went to G3s. However I am going back to the stock high lift blades as I haven't noticed any distinct advantage to the Gator blades.
When one has a blade strike with G5s, the force from the impact due to the heavier blades travels into the spindles, V belts and clutch more so than with stock blades.

 

[RhettWS]

I tried a set on my Scag and went to G3s. However I am going back to the stock high lift blades as I haven't noticed any distinct advantage to the Gator blades.
When one has a blade strike with G5s, the force from the impact due to the heavier blades travels into the spindles, V belts and clutch more so than with stock blades.

I'm not too worried about a blade strike but yes the inertia is considerably greater. I've used my G5s twice. First time the grass was very deep, second time it needed cutting, but was not excessively tall.

The cut is just fine although I don't know that it cuts any better than the stock when sharp. The discharge was finer for sure to my eye. I did notice a massive amount of blow back in that my deck was covered with clippings using the G5s while that was never an issue with the stock blades. I guess all the turbulence and suction has to exhaust somewhere. A large amount blows out particularly on the front right side of the deck. This blow back is not a deal breaker for me yet.

 

[RhettWS]

As promised I took some measurements and calculated the blade tip speed. I took RPM readings on the engine and then the right side deck pulley with the blades running. I used my laser tach to take the readings.

1. The rear pulley under the engine is 6" OD, at least to my eye. The deck pulley is 5.75" OD thus the deck pulleys turn slighter faster than the engine RPM
2. With G5s on the engine RPM was 3490 and the deck pulley was 3810
3. With stock blade (601124) the engine RPM was 3499 and the deck pulley was 3809

Thus there is little difference in blade tip speed between the two blades.

The blade tip speed is 18453 fpm. Thus the blade tip speed is 447 fpm on the slow side. In other terms, a little less than 2.5% off of the target value.:thumbdown:

For what it is worth the unit cuts pretty dang good as is. Now I have to decide if I want to adjust myself, take it to the dealer or leave it alone.

 

[Midniteoyl]

As promised I took some measurements and calculated the blade tip speed. I took RPM readings on the engine and then the right side deck pulley with the blades running. I used my laser tach to take the readings.

1. The rear pulley under the engine is 6" OD, at least to my eye. The deck pulley is 5.75" OD thus the deck pulleys turn slighter faster than the engine RPM
2. With G5s on the engine RPM was 3490 and the deck pulley was 3810
3. With stock blade (601124) the engine RPM was 3499 and the deck pulley was 3809

Thus there is little difference in blade tip speed between the two blades.

The blade tip speed is 18453 fpm. Thus the blade tip speed is 447 fpm on the slow side. In other terms, a little less than 2.5% off of the target value.:thumbdown:

For what it is worth the unit cuts pretty dang good as is. Now I have to decide if I want to adjust myself, take it to the dealer or leave it alone.

Sure you got those pulley sizes correct? Seems more like 6.25" PTO and 5.75" on the spindle with the rpms given... If the PTO pulley is 6.25", then 3600rpm engine will get you ~3913 blade RPM which will mean 18937fpm tip speed. Assuming I got your blade diameter correct @ 18.5".

So bump that engine RPM by 100 and gain ~500fpm tip speed. You could also just leave it. 18500 is pretty good.. though mine run @ 19500.. Max they are suppose to be is 20,000 by some stupid 'standard', but have seen faster.

 

[RhettWS]

Sure you got those pulley sizes correct? Seems more like 6.25" PTO and 5.75" on the spindle with the rpms given... If the PTO pulley is 6.25", then 3600rpm engine will get you ~3913 blade RPM which will mean 18937fpm tip speed. Assuming I got your blade diameter correct @ 18.5".

So bump that engine RPM by 100 and gain ~500fpm tip speed. You could also just leave it. 18500 is pretty good.. though mine run @ 19500.. Max they are suppose to be is 20,000 by some stupid 'standard', but have seen faster.

The blade diameter is correct. The PTO pulley could be off. I was laying on my back in a tight space to measure. Having said that I reported OD for each. Also the lip on the PTO pulley is larger in diameter on the top side than on the bottom. I guess they want insure if the belt jumps off it goes down instead of up. The depth of the pulley was not measured and I suspect that would be the best diameter to use in comparing them. For the blade tip speed, it hardly matters what the diameter are (in this instance) because I actually measured the spindle pulley RPM. I do get what you mean though in figuring out the mechanical advantage so to speak.

I think I just need to tweak the tension spring on the governor. What say you?? Thank for you input.:thumbsup:

 

[Midniteoyl]

The blade diameter is correct. The PTO pulley could be off. I was laying on my back in a tight space to measure. Having said that I reported OD for each. Also the lip on the PTO pulley is larger in diameter on the top side than on the bottom. I guess they want insure if the belt jumps off it goes down instead of up. The depth of the pulley was not measured and I suspect that would be the best diameter to use in comparing them. For the blade tip speed, it hardly matters what the diameter are (in this instance) because I actually measured the spindle pulley RPM. I do get what you mean though in figuring out the mechanical advantage so to speak.

I think I just need to tweak the tension spring on the governor. What say you?? Thank for you input.:thumbsup:

Well, the reason to know the pulley ratio is so better to figure the max blade speed you can get with out over revving the motor. In your case, the 3600rpm spec'd in your manual would mean a max of ~18900.

Go ahead and adjust the governor to get you to 3600rpm and give the mower a test drive.

 

[RhettWS]

Thank you sir. I will do that probably over the weekend. I have to psyche myself up to dig into a new machine that is under warranty.