Snowblower woods 3pt snowblower design flaw???

[emery]

In an older post about proper gap between impeller paddles and housing it was mentioned that a quarter should be picked up! Well I measured gap in my Woods 5' model and found 3/8 inch which would not pickup a silver dollar!
I can't add to each paddle because it is too tight a fit at the angle needed when inserting the paddles back into the housing.
What's the explanation here, is the Woods design flawed or is such a wide gap normal?

 

[Rick Jay]

Emery,

I've got a new Woods SS60 rear mount snowblower and I also noticed the gap to be a bit larger than suggested by the previous poster to whom you are referring.

I haven't had a chance to use it yet, so I can't say if it will have any major effect on performance. I did read somewhere that a larger gap between the fan and housing helps to minimize damage if a small stone gets picked up and also to decrease the chance of the fan freezing to the housing if a little water collects in the housing during storage.

I did notice my walk-behind snowblower also has a much larger gap than mentioned in that post and it has worked flawlessly through all types of snow.

I believe the post to which you are referring also stated something about a paddle/fan tip speed of at least 5000 ft/min at PTO speed. By my calculation, my Woods will be launching snow at about 3250 ft./min at PTO speed. In order to achieve 5000 ft/min at 540 RPM, the paddle would have to be almost 3' in diameter!

I would like to ask the poster who had stated those specs to expand upon them a bit and tell us where they came from.

When the snow finally starts flying around here, I guess I'll be able to say if the design is sufficient.

Woods generally builds and sells some pretty beefy stuff, so I'm not really concerned. I'm anxiously awaiting a good snowfall. We just had a Nor'easter...unfortunately at our locale it was mostly a rain event.

Have you had the opportunity to use your 'blower yet?

Maybe others can chime in with some specifics about their snowblowers.

~Rick

 

[hwp]

Emery:

<font color=blue>I can't add to each paddle because it is too tight a fit at the angle needed when inserting the paddles back into the housing.</font color=blue>

I'm not sure what you mean by this. To remove or install the impeller on my snowblower, I undo the mounting bolts for the gear box so that I can move it enough to remove or attach the impeller from/to the drive shaft and then put the gear box back in place. There is no way I could remove or install the impeller without moving the gear box. But I doubt that varying the clearance between the impeller and the drum would make a difference in removing or installing the impeller.

In my opinion 3/8" is a very large gap between the impeller and the drum. I would think there will be a lot of leakage between the impeller and the drum which will affect the "throw". The gap on my snowblower had become more than the thickness of a quarter and it wasn't throwing snow as well as I thought it should. Part of the reason for the wide gap was that stones had bowed the drum out away from the impeller blades. The tips of the impeller blades were also worn. I put a sleeve (1/8" steel) in the drum and fit the impeller blades to the new drum diameter (clearance of 0.020") and re-balanced the impeller. This made a big difference in the performance of the machine. It was down to about a 50 foot throw but now it is back up to about a 100 foot throw (in light dry snow).

 

[looch]

Oh yeah. I've had to use mine twice since Sunday. It's a Pronovost Puma. Not sure what the gap is but I'll check it tonight if I remember.

This fan speed thing has got me confused. If I check the specs of my blower, it says

rotor speed: 540 RPM
rotor diameter: 1"
fan diameter: 20"

The calculation that I use is to solve for the ratio and then fan tip speed.

So if...

R is ratio, then
R=Dm/Dd where
Dm is the diameter of the motor and
Dd is the diameter of what is being driven.

R=1"/20"
R=0.05

RPM of Dd (Sd) is RPM of Dm (Sm) divided by R.
Sd=Sm/R
Sd=540/.05
Sd=10800 RPM

I doubt this is the case. Can someone explain what I'm missing. The only thing I can think of is to calculate the speed of the rotor shaft separately. Like the diameter of the PTO shaft is 1.5 (or thereabouts) and the rotor shaft is 1", making the rotor shaft spin at 360RPM and not 540.

My head hurts.

 

[hwp]

Rick:

I think you may be referring to a post I made earlier. My comments were based on what the owner/designer of the company that made my snowblower (a Van Eyl) told me. There are some excellent sites on the web regarding projectile analysis, especially the ubc.ca and richmond.edu sites. Both of these sites show how to calculate the range of a projectile given the launch velocity and launch angle. There are two things to consider. The first is that the velocity of the projectile (snow) leaving the chute is about 70% of the velocity of the tip of the impeller blade. The difference is due to losses from friction and leakage. The second is that the range varies with the square of the velocity. The net result of this is that an impeller tip velocity of 5000 ft/min. and a discharge angle of 45 degrees will result in a throw of about 100 feet.

 

[Rick Jay]

pbenven,

The speed of the paddles at their tips is found by multiplying the diameter of the paddle (in feet) by 3.14 and then by the shaft speed, which I'm assuming for most blowers is 540 rpm.

For yours, the numbers are:

20" diameter = 1.67 feet

1.67 feet x 3.14 x 540 rev./min. = 2831 ft./min.

If your snowblower can launch snow 100 ft. with that, then I would say that 5,000 ft./min figure is a bit out of line.

There is a company that makes flexible tips that mount on the impellers so the impeller "squeegees" the housing. I don't know if they have anything for larger blowers, I saw them advertised mostly for walk-behinds and lawn-tractor snowblowers.

I'm not going to do anything until I get some white-stuff on the ground. Then I'll see how it works. If I can toss it 50', that'll work for me. If I send it 100', my neighbors aren't going to be too happy. /w3tcompact/icons/laugh.gif

~Rick

 

[Von]

I have a LuckNow 3 pt. rear snow blower, and the gap is wider on one side of the drum and gets closer as it gets to the exit. Smallest gap is about 1/8". My guess is that snow will pack in to take up any gaps. This will still give you a tight fit to the fan, but let any small rocks slide under without doing damage. So I wouldn't worry about it, as long as it works!

 

[hwp]

Isn't the fan (impeller) fastened directly to the shaft, so they both rotate at the same angular velocity, i.e. 540 rpm? If so, your discharge velocity is about 2825 ft/min.

 

[Rick Jay]

hwp,

Thanks, I'll do some surfing. Still, a 5000 ft./min velocity seems a bit high. pbenven says he gets about 100' of distance with a much lower velocity.

These numbers get awfully fuzzy because the momentum of the snow and it's wind resistance must also come into play. (Remember the old experiment: Which falls faster? A pound of feathers or a pound of steel? In the absence of air, they fall at the same rate. In the real world, if my foot was in the way, I'd rather it be under the feathers than the steel. /w3tcompact/icons/smile.gif)

And as I calculated, and I think I did it correctly, a 540 rpm PTO driven paddle would need to be almost 3' in diameter to obtain a tip speed of 5000 ft./min. I don't see many blowers with a fan that large.

Again, I've got no real world experience with these larger snowblowers, so I just need to wait for the snow.

Perhaps I believe we have wandered into another world of ambiguous calculations, much like macher's recent post about tractor center of gravity calculations.

Like most engineering problems, what happens on paper is one thing. What happens in the real world can be quite different, and often nearly unpredictable.

Yet, for some strange reason, we keep attaching equations to everything. And enjoy it! It's gotta be a sickness.

I hope there's enough snow for everyone this year! Anyone who doesn't want it, can send it up our way. /w3tcompact/icons/smile.gif


~Rick


Perhaps the type of snowblower design has something to do with this. These Woods units are the typical 2-stage layout. Perhaps the numbers must change for single stage or other design types.

 

[hwp]

<font color=blue>So I wouldn't worry about it, as long as it works!</font color=blue>

Right on. Everybody has different requirements. What is required for one can be quite different than what is required for another.