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View Full Version : Blade Tip Speed....Does It Really Matter That Much?


Tonyr
09-25-2005, 10:04 PM
Just wandering....and yes, I have seen this topic discussed before, but curious for fresh opinions.

How important is it that blade tip speed be 18500 fpm plus etc?

I ask because, I have a new J.D 997 Rear Discharge Fabricated deck, it's tip speed is only 17480 fpm and it can cut full speed 11.5 mph and not leave one stringer, not one uncut grass leaf, cuts cleaner and much faster than my Toro with TFD ever could and it's blade speed was 18500 fpm.

And the Toro would have much higher vacume than the rear discharge deere, hmmmmmmmm

So is blade speed a selling point....or does it really matter?

obviously different decks require more/less vacume to perform best, just curious.......

SproulsLawnCare
09-26-2005, 10:14 AM
I run John Deere products and I see no problem with the cut either. Maybe faster blade speed would perform even better, but I think there is a point to where it wouldn't be worth extra speed. Then it would only create more wear on the moving parts. I am quite sure that all of the manufacturers who design these machines have tested them with different blade speeds and decided which speed works best with the particular design of the deck. Either that, or built the deck around the blade speed that the machine will produce. Either way, the blade speed surely matches the deck design to produce the best cut possible for each machine regardless of the manufacturer.

PMLAWN
09-26-2005, 03:12 PM
I always felt that the blade speed gave you more passes with the edge to the blade of grass so if the first pass missed or hit a nicked spot the next one would get it.
I felt that the deck design and depth with the blade lift affected the grass lift.
But I may be wrong.
I am still very pleased with the cut of the turbo force deck and only have problems if the blades get nicked bad.

By the way are you happy with the Deere.
What made you switch as I know that you really liked the Toro.

Precision
09-26-2005, 05:31 PM
blade tip speed is a function of the length of the blade.

I don't know/want to do the math, but a 10" blade spinning at the same RPM of an 18" blade will have a much lower blade tip speed. So if blade tip was the only issue, then the single blade 30 and 32" mowers would give the best cut around.

Now we all know that isn't true.

There is a relationship between blade tip speed and ability of the blade length to keep that up. Obviously you can bog down a 32" blade much faster than a 10" blade.

I know it isn't apples to apples, but I have a toro 36" WB with 2 18 inch blades and 15 hp. I also have the new scag rider 36" with 3 12" blades and 19hp. Guess which one bogs quicker. Big hint it isn't the one I sit on. And yes I know the rider has 4 more hp but it also is dragging around 350 or more extra pounds. and it can cut at 7 mph.

My very unscientific view on blade tip speed. Marketing idea with no merit.

mowtech
09-26-2005, 06:13 PM
Just wandering....and yes, I have seen this topic discussed before, but curious for fresh opinions.

How important is it that blade tip speed be 18500 fpm plus etc?

I ask because, I have a new J.D 997 Rear Discharge Fabricated deck, it's tip speed is only 17480 fpm and it can cut full speed 11.5 mph and not leave one stringer, not one uncut grass leaf, cuts cleaner and much faster than my Toro with TFD ever could and it's blade speed was 18500 fpm.

And the Toro would have much higher vacume than the rear discharge deere, hmmmmmmmm

So is blade speed a selling point....or does it really matter?

obviously different decks require more/less vacume to perform best, just curious.......

This can depend on deck size. The difference between 17480 fpm and 18500 fpm is not that significant, about 6%. Per the fan law air flow is proportional to the rotational speed. So in this case, if you have the same blade size, the Toro would move about 6% more air. On the other hand, if you are comparing a 72-inch deck to a 60-inch deck, then you have to look at the blade size. Given the same rotational speed, the air flow is a cubic function to the blade diameter. But, we are talking tip speed not rotational speed, so to get the same tip speed the larger blade of the 72-inch deck would be spinning slower than the 60-inch blade. But that would be offset by the cubic function which means is that a 72-inch deck could move a lot more air at the same tip speed as the 60-inch.

As for the number of passes the blade makes, at 11.5 mph a 60-inch deck at 18500 would travel about 0.27 inches during one revolution of the blade while at 17480 it would travel about 0.29 inches. Again, not that significant. On the other hand with a 72-inch deck at 18500 you would travel about 0.36 inches per blade rotation. At 17480 you would travel about 0.38 inches. 3/8th inch is starting to get up there!

Higher tip speeds are generally better because more airflow promotes more vacuum to lift the turf, but there are other factors that can significantly effect the quality of cut: sail height and style, general design of the deck to promote vacuum, and the location of the casters. Higher tip speeds for side discharge decks also promotes better dispersion of the grass, but with rear discharge you don't care how far it is thrown.

Tonyr
09-26-2005, 06:43 PM
Thanks everyone, we all see it the same....

good replies guys!


PMLAWN
By the way are you happy with the Deere.
What made you switch as I know that you really liked the Toro.

Mate, the deere has some issues with the rear screen that j.d are looking at a cure for due to our dusty conditions, and rear discharged dust gets sucked up and clogs the radiator.

I bought the j.d as dealer is 20 mins away, toro 1.5 hrs, I needed/wanted rear discharge for liability, safety, versatility etc, and j.d had just released a 60'' ztr.....and hustler, but their 60'' deck model wouldn't be here till next month. The toro was great, but was getting a bit tired needing some work, and my dealers mechanic is just not good enough, he botched the last job, not again.

plus, i was keen to go bigger hp, and diesel....I'm very happy with the power and torque and feul consumption of the j.d.

why rear discharge? These are faster, cut stringers, weeds, stalky stuff clean as, where my toro struggled, they eat long rough ferral jobs so easy, my jobs are about 20% - 30% faster with the deere.

the j.d is a lot better built than the toro, the finishing touches are great, unlike my toro, better paint etc, ya pay more for a deere, but ya get more machine....

sorry for rambling....answer is, rear discharge deck on a diesel ztr.

dvmcmrhp52
09-26-2005, 07:17 PM
Tony,
I believe the blade tip speed is much more important in a side discharge deck than it is in a rear discharge due to the movement of cuttings from one side of the deck to the other. With rear discharge there isn't near the work to be done by the blades as with side discharge or a mulching set up.

Tonyr
09-26-2005, 07:21 PM
G'Day Dave,

I think you would be right mate.

Love to try meg mos on it, but total cost shipped to hear is around $600, ouch! lol.

doubt I would get a set sent to demo 1st.....bummer lol.

dvmcmrhp52
09-26-2005, 07:24 PM
G'Day Dave,

I think you would be right mate.

Love to try meg mos on it, but total cost shipped to hear is around $600, ouch! lol.

doubt I would get a set sent to demo 1st.....bummer lol.




Why not just try some doubles and see what happens? It sure beats buying meg mos and finding out there's no real improvement after (ouch!) $600...............If the doubles help you out then maybe consider the meg mos.

Tonyr
09-26-2005, 07:37 PM
good point mate....

mowtech
09-26-2005, 07:46 PM
blade tip speed is a function of the length of the blade.

I don't know/want to do the math, but a 10" blade spinning at the same RPM of an 18" blade will have a much lower blade tip speed. So if blade tip was the only issue, then the single blade 30 and 32" mowers would give the best cut around.

Now we all know that isn't true.

There is a relationship between blade tip speed and ability of the blade length to keep that up. Obviously you can bog down a 32" blade much faster than a 10" blade.

I know it isn't apples to apples, but I have a toro 36" WB with 2 18 inch blades and 15 hp. I also have the new scag rider 36" with 3 12" blades and 19hp. Guess which one bogs quicker. Big hint it isn't the one I sit on. And yes I know the rider has 4 more hp but it also is dragging around 350 or more extra pounds. and it can cut at 7 mph.

My very unscientific view on blade tip speed. Marketing idea with no merit.


Most decks are designed based on tip speed, not rotational speed. Per ANSI standards the maximum allowable tip speed is 19000 fpm. Usually, whatever rotational speed gets you in that ball park with a factor of safety is what's used. So 18-inch blades will be turning slower than 12-inch blades to get to that limiting tip speed. However the faster turning shorter blades still cannot move as much air as the larger blades due to the fan laws: air flow is proportional to rotational speed, shorter blades spin faster, but air flow is a cubic function of the diameter so even though the larger blades are spinning slower they are moving a lot more air. This costs some power--so this is partly why your wb behind bogs down (my guess is most of the bogging is due to the fact that this deck is shallow). Anyway, it's all a balance, quality of cut vs. power and so on, so everything must be considered in the design. Given all that, it has been found that the optimum blade diameter from a quality of cut stand point is in the 19 to 21 inch range. Units with shorter blades and longer blades will generally provide sub optimum cut. As you may have found, the best cutting decks generally are 60-inch or 21-inch walk behinds. As I stated before, faster tip speed is generally better, but it isn't the only thing that determines quality of cut and no it isn't just a marketing gimmick.

CutInEdge Lawn Care
09-26-2005, 07:52 PM
Precision; I also have the 36" Z-Cat, 19hp. Blade tip speed is aprox 19000, from what I have read. I have a little over 200 hrs on unit.

Precision- call or email me. www.cutinedge.com

Envy Lawn Service
09-26-2005, 10:46 PM
Tonyr,

The ability to cut clean is hinged much more so on the design of the deck and the setup of the machine than tip speed... because today most are close enough not to matter... and chances are, if the deck must be running at 19,000 fpm tip speed, most likely this is a compensation for shortcomings in deck design.

Also building off of what mowtech said.... whom I can tell is very obviously knowledgeable.... I bet you have not cosidered the fact that your new John Deere probably runs longer blades than your Toro did. This builds upon what mowtech was explaining and also provides additional blade overlap that the Toro did not have.

My Lesco for instance, is a 60" cut, but runs three 21" blades. So the blade is longer than your typical 60" blade and also provides a whopping 1.5" of blade overlap!!!

Tonyr
09-26-2005, 11:52 PM
Hi Envy,

good points.

the standard blades on my little plastic lawn mower lol, are 21'', the blades on my old toro were 20.5''.

But, funny thang is....I'm running my old toro spares on the little j.d, yes, I'm using spacers to fit them below the 23mm shoulder which sets them close to where the curved j.d blade tip is.

I'm having great success with the toro notched high lift, 20.5''

and now the toro low sail blade is awesome in light mowing, 20.5''

when I wear these out, I'll use my spare sets of 20.5'' raptors, can't wait, loved this blade on the toro!

Blades can run all week and be sharp due to the no re cutting etc, good thing!

The 20.5'' blades leave no streaks etc, cut clean at full speed.

I don't know for certain what the standard blade overlap is for this deck, but the standard blades miss each other by only 6mm.

Like said mate, deck design.....like a rear discharge isn't feeding material from one end to the other as well as cutting fresh material, which seems primative when you think about it, where R.D decks cut it and discharge it each blade individually, so the deck isn't working hard at all, no need for the ultra high transfer of clippings to allow for it to perform...no need for the fastest blade speed....

But in saying all of that, the 7 iron side discharge decks run at the same blade tip speed as mine, so in theory compared to faster blades on other decks, it should perform slightly worse....should, not do....?

Then again as you guys say, deck design....is the key to a good cut, not tip speed on it's own.


Thanks everyone, I understand more now and agree.


Can I just add, I've previously complained a lot about my mower's cooling system and cut, please realise I was trying to get j.d's attention, not rubbish this mower or turn you guys off it.

They say they are working on a fix for the cooling to suit my conditions.

I have got the deck cutting very neat now.

Please remember, I'm in Australia, our cutting is very different here, the machines were designed in your country and Europe so they won't have the issues I've before mentioned.

This model, 997 is an awesome machine, extremely well built and finished, ultra smooth hydro, very quiet engine, very clean cut, etc, etc.

All I want to say is, for the sake of it, go demo one, you will like it, and it will be great on your grasses! Just use it like a mulcher.....that's the idea!

Lastly, only a couple of weeks ago J.D released upgrade specs on how the decks should be set up, make sure the dealer has done this, it makes all the difference, somehow!

PMLAWN
09-27-2005, 12:09 AM
Thanks for all the replies on the J D. A little bummed that you had problems with the Toro as you were so helpful and up on them when I bought my first one. ( I do know that you had some problems with the distance that it and parts had to travel). I also know that you cut stuff that we don't even know in the states. Just hoping that that was the problem with the Toro as I have a fleet now. So far, so good for them.
Again thanks for the info and good luck with the Deere.
Thanks Mate,

Tonyr
09-27-2005, 12:31 AM
The Toro was good, it was the back up that was the problem, and dealer stocked no parts, and crappy mechanic, but the Toro mower was awesome.

Toro are releasing a rear discharge model soon, I'll even look at them in the future, Toro make good mowers, if I had a good local dealer I would of kept mine....it was in great condition, it hurt to trade it!

I also know that you cut stuff that we don't even know in the states. Just hoping that that was the problem with the Toro as I have a fleet now.


No, I didn't do any modifying to the Toro, it was fine stock standard. Only thing that was a bad design was the plastic clip that hold the spindle covers down, nothing else went wrong with Toro, the big thing was a Strator and Rectifier on the EFI Kohler, under warrantee.

I did remove a baffle under the J.D deck, which has since been resized and now by fluke is awesome.

I may in the future modify the ROPS as they are dangerous being so tall, but not until after deere fix my cooling screen problem....


All is good, Toro has some great things coming out.....this company is on the up and up, and I would not hesitate to buy another toro.

just wish they had their rear discharge deck model ztr out, it will be the best deck out of all I believe, they seem to of figured everything out well since the turbo force.

I'm keen to see it, will be awesome!

Envy Lawn Service
09-27-2005, 12:34 AM
Tonyr,

Yup... the Raptors are a tough blade. A lot of the reason is they have marbain steel material in them. Not super sure about the exact specs, but it makes them last and stay sharp. The highlift marbain cutter blades that came stock on the Lesco were still pretty much "stock sharp" when I took them off at 50 hours!!! These are the real deal no doubt. Beats the heck out of daily sharpening!!!

Anyways, I'm going to see if I can get some Raptors and other Stens blades to fit it, as well as a set of OEM Gator Magnums. I gotta find something that chews these tough leaves a little better...

Back on the rear discharge topic... no that deck doesn't have to work as hard... and since that is the case, the 'actual working BTS' probably stays closer to no-load specs than the side discharge counterparts. In the end though, I can and have dropped my engine down 500 rpm via the CrewBoss meter, and I'm not sure what the BTS reduction is, but cut quality is not effected.

Anyways, once I am in the market for another ZTR, the new rear discharge models will be a consideration for me. I'm hoping for quick-tach mulch plugs for them by then... so I can mulch and also hit the rough stuff rear discharge. I have severe doubts that the rear discharge decks will properly handle our wet sappy growth though....

Tonyr
09-27-2005, 12:39 AM
Mulch plugs/kit for a rear discharge....now that would be the ultimate deck!

I think it would be very easy to do on my deck actually as part mulch baffles are already there as the J.D cuts the clippings up more than the Hustler, so the positioning and bracing is in place, just need to figure depth as these decks taper back....

Good Idea Mate!

Ric
09-27-2005, 12:52 AM
My 2 Cents

CLIP RATE is the actual number that includes all of what is basically said above and then some. This is the travel of the blade over the surface of the turf being cut. It is calculated by Ground Speed and blade revolution and tip speed. The more times the blades cutting surface crosses the area to be cut the better change it has of giving a clean cut. When Ground Speed decreases Clip rate increases and you can cut heaver grass with better results, because you are cutting less grass with each revolution. Only when Ground speed and Blade rotation gives sufficient Clip rate will a mower cut right. The Idea of high Tip speed is a manufactures Marketing tool to tell the Operator that he can cut at a faster speed. However Tip speed is not the true Clip Rate only a factor in Clip Rate. Therefore a mower with a very high Tip Speed and low clip rate will not give a good cut. A 36" single Blade will have a low RPM and High tip speed, Add in a high ground speed and you have a low clip Rate. How Before someone tells me the old Snapper 30" single blade gave the best cut, Let me remind them the Ground speed was about Zero. Yes the 30" Snapper also had one of the best vacuum decks for bagging also.

Envy Lawn Service
09-27-2005, 01:21 AM
Clip Rate....

Now that is a GREAT term to help explain things.

However, I still maintain that deck design and machine setup have the greatest influence among alike-size decks in today's market.

Doubt that?
Apply the Clip Rate principle to cutting low density tall wirey grasses.
Flushes Clip Rate right out the window in the cut quality equation.

Plenty of mowers out there with screaming BTS and high clip rates that can't cut their way out of a brown paper bag in the thin wirey stuff, despite the fact the clip rate, ect, is very high and the number of blades to actually 'clip' are very low.