INCREASING MOWER DECK RPM

[ronhatch 406]

I have a nice older 42" SD deck from about 1967 or so mounted on my 1979 C-101. The grease fittings are accessible through holes next to the spindles, so I think it's from that time period anyway. The center spindle's top pulley is  about 5" in diameter and the lower is about 3.5" in diameter. I would like to speed up the blades RPM for a better mowing job by decreasing the 5" diameter pulley.  Does the center double pulley from a 36" deck have a smaller top pulley then a 42" deck has? If so, has anyone tried to speed up their deck by installing a 36" deck pulley on their 42" deck?

Edited June 7, 2017 by ronhatch

[ronhatch 406]

And yes, I'm driving my deck using the larger clutch pulley.

 

[JimmyJam 542]

Interesting! 

[gregg'shorses 77]

I would leave it as is and just make sure the blades are sharp. The deck are designed for a certain wing tip speed . Excessive speed isn't safe.

[ronhatch 406]

I'm not one to leave things 'as is'. Today's lawn mowers have tip speeds far high then our machines of yesteryear. What's the difference? Nothing that I know of.  So, back to my simple original question; Is there a difference in diameter between the driven pulley of a 42" deck and a 36" deck? Yes or no? And if so, what is the difference in diameter of the driven pulley?   And yes, the blades are sharp. Thank you.
 

[EricF 589]

Higher blade tip speeds can help with modern mulching deck and blade combinations. The speed improves the ability for the clippings to be held under the deck and re-cut several times while they're suspended in the vacuum effect. But neither the decks nor blades from the last century were designed to work that way.

 

The older decks were meant to snip off the clippings and throw them out of the way as quickly as possible. In fact, from reading Red Square, I've come across several reports that modern "gator" mulching blades don't work all that well on the old-school Wheel Horse deck design -- they work best with the OEM blade profile which is a "high-lift" design -- it creates a wind tunnel effect to stand the grass up straight so it can be snipped off in one swipe by the blade and then ejected.

 

Trouble is, it's a little counter-intuitive but blade speed seems to reach a point of diminishing returns with the older high-lift design. You'd think that more speed = more lift because it will move the air faster. But grass blades will only stand up so quickly, particularly on a thick lawn. Not to mention that as the mower moves forward, there's a boundary effect as the moving wall of air hits the grass just ahead of the blade, which can flatten it down momentarily -- exactly what you don't want to happen. It'll just cause as many "misses" as it will "hits" between blade and grass at the front, and the back of the blade sweep (which is supposed to be set about 1/8" higher than the front of the sweep) will wind up doing more of the cutting. Since the back is 1/8" higher, the cut height will be off. In practical terms, some of your grass will get cut 1/8" higher than the rest, leading to an overall more uneven cut. The thicker the grass, the more noticeable it will be.

 

If any of your blades are out of balance, higher speeds will increase vibration and shorten the life of spindles and bearings. That's not a big deal on modern "throwaway" equipment. But eating Wheel Horse spindles could get costly.

 

You might get better results by leaving blade speed alone and stacking a second blade at a 90-degree angle on each spindle. Some folks have tried this and reported good results. The second blade does cut a bit taller than the one at the bottom of the stack, but it helps ensure that stubborn grass gets cut one way or another. The lower blade edge should have less work to do and just provides the "finished" cut height. Stacked blades do put more rotating weight on the spindles and bearings -- be sure to keep them greased. The starting load on the engine will be greater, but once the extra mass is rotating, it should have enough inertia that it ought to help keep the engine from bogging down on thick grass as long as you keep travel speed under control. It also goes without saying that keeping the blades properly balanced is critical, since a stacked configuration adds more potential balance problems.

 

The short of it is, other folks have experimented along these lines and it keeps coming back that Wheel Horse's engineers did an outstanding job -- there's not a lot of room for any noticeable improvement if a deck is properly leveled and the blades are sharpened and balanced. There's a limit to how fine a non-mulching deck will leave clippings. Grass that likes to bend down when mowed or has too many stalks for seed heads doesn't cut well with either a mulching or non-mulching deck in a single pass -- problem lawns have to be mowed with only 2/3 to 1/2 of the deck over the uncut grass, and leave more of the blade sweep to clean up the edge of the cut swath. And you'll probably have to adjust travel speed. The real time-saver is a wider deck in those cases.

 

Just my , but you'll probably have better luck working with the existing design rather than against it.

[Tankman 3,518]

Eric said it all. 

 

I mow frequently, never allowing the turf to get out of control. 

Mowing frequently allows me to use a faster ground speed and far less cleanup; small grass clippings, less thatch buildup.

 

Clean the deck often and grease the spindles more than necessary.

My decks are removed yearly, cleaned, paint touched up.

[EricF 589]

5 minutes ago, Tankman said:

Mowing frequently allows me to use a faster ground speed and far less cleanup; small grass clippings, less thatch buildup.

 

And more seat time!  

[Racinbob 11,041]

I've always thought that Wheel Horse decks spun pretty slow compared others and have tinkered with speeding them up but I never really saw an improvement in the performance. I never pondered it to the extent Eric has but there's some excellent points in his post. Go ahead and experiment ronhatch and see what you come up with but the spindle type will dictate what pulley you use. I just rebuilt a 36" and it had the double D shafts and is 4" in diameter. I'm doing a 48" now and it's just a plain shaft and 5" in diameter. Others have keyways. I've also got a 42" and I could measure it later but I doubt any of them will interchange.

[EricF 589]

I've mowed with, or been around a lot of machines over the years. I never payed much attention to the speed the decks run at -- only cared about the quality of the cut they turned out and the reliability. To my way of thinking, slower blade speeds probably result in less pulley and spindle bearing heat and wear over time. Possibly less wear and tear on belts, too. If the cutting performance of the deck is efficient and smooth at a lower blade speed, then that's fine with me. What matters is how the grass looks when you're done.

 

I kind of suspect that was what the Wheel Horse engineers had in mind as they designed their decks. I grew up near South Bend... there were lots of engineers and machinists employed in the area at one time. Even though Studebaker had closed down, other industries locally and further away relied on the area's machine shops and smaller industrial contractors. I learned quite a bit from the "old heads" -- while modern engineering focuses on designing things to meet a set of qualifications -- and not one bit more -- the older guys followed the principle that things had to not only do the work, but last reasonably while doing it. Downtime and repairs cost money. So things were designed not to risk needlessly tearing themselves apart over time in the first place, and be straightforward to maintain and repair when they did need work. Why design a mower deck that will mow like crazy but spin itself to death, when you can design it to run slower and smoother to last longer and still leave a golf-course-quality cut? A different way to look at efficiency. New ways aren't always all they're cracked up to be...

 

OK, old-guy rant over...

[ronhatch 406]

Not to brag, but I have one of the nicest lawn around; no weeds, it's thick, mostly Fine Fescue and Kentucky Blue grass. In the spring I'll mow every 3 to 4 days. I keep it about 2 3/4" to 3" high. I remove the deck twice a year, clean it, paint it with Slip-EZ and sharpen the blades with a Mill file. I just recently increase the blade RPM on my 1966 AC B-10  by about 20% and noticed an improvement in the quality of cut. 

Thank you Racinbob!  Is a double D shaft what I think it is? A round  3/4" shaft with 2 flat sides a 180 degrees from each other and no key way? If so, that's what I have. A 4" to 3 1/2" driven pulley diameter would be perfect. Now all I have to do is find one.

  One last thing, all 4 of my mower decks have rollers instead of wheels for that nice ball park look.

 

Edited June 8, 2017 by ronhatch

[Racinbob 11,041]

Yes, that's what I call a double D shaft.