[Toolin]

I'm looking to upsize one of the pulleys in the drivetrain of my snowblower attachment. It's a 2.5" ID / 3.75" OD pulley now, want to go up in ID a tad, maybe a 1/2 inch ID more.

This should give some boost in blower speed, but also help with a belt clearance issue I have (belt is getting very close to it's opposite side near the idler pulley when the blower is lowered).

Trouble is, this isn't any ordinary pulley with a circular hole in the center. This one has a 6-pointed star shaped hole, like many mower blades do. This is apparently because this blower uses a standard spindle in its drivetrain with pulleys on both top and bottom of the spindle, and the bottom of the spindle shaft (where this pulley is) has the 6-pointed star for keying.

Are pulleys with 6-pointed star holes fairly common? Do I stand a chance finding a larger pulley with the right type of hole, and any suggestions where I might find one?

See attached pics.

First pic shows the snowblower pulley frame under the tractor with the spindle situated vertically between the two tan belts. The pulley I want to upsize is on the bottom.

And here's the pulley removed from the spindle. Note the star-shaped hole where it mounts to the spindle shaft. The shaft is threaded and has a shoulder that is the positive of this shape, and the nut that holds the pulley on is just a standard threaded nut with flange, shown to the left.

Other options would be to go down a size on the top pulley on the spindle, but that pulley has a 12-point star for a hole, which is probably more common, but this also creates an issue with too much belt that the top idler pulley can't take up.

Last option is to reduce the pulley size at the blower itself, but again that leaves me with a bit too much lower belt that is already close to rubbing itself. I really want to upsize a pulley somewhere to both get speed increase and avoid belts rubbing.

Yeah, I could also downsize a pulley and get a smaller belt at the same time, but I'm lazy.. want to do this with one change not two if possible.

Thanks in advance..

 

[snake99]

Toolin, I found this http://www.psep.biz/store/mtd_pulleys.htm Scroll down to almost the bottom of the page. The one they show is bigger than what you want but maybee they have smaller ones.

 

[99flhr]

Perhaps you could plasma cut the center out of your existing pulley and weld it in to one of the desired size.

 

[Toolin]

Toolin, I found this http://www.psep.biz/store/mtd_pulleys.htm Scroll down to almost the bottom of the page. The one they show is bigger than what you want but maybee they have smaller ones.

Thanks for the link... Yeah, that one has the right type of hole but it is a little big, I think the blower would shake itself apart at that speed. I did some googling based on the "6 slot star" description shown in your link and came up with other sites but all the same size pulley.

So I called MTD customer service to see if they make any other sizes.. answer "Sorry, can't help you. We can only sell the exact replacement part", which didn't really answer my question. I even tried asking a couple different ways, same answer. He recommended I go to a Tractor Supply Co type store (which I have.. they don't make a 6-pt star hole pulley). Sigh.

Perhaps you could plasma cut the center out of your existing pulley and weld it in to one of the desired size.

That's an idea... If only I could do either of those things (plasma cut and weld)! My level of shop cut/paste right now is saw, drill and bolt. But I'm working on it...

But I could avoid the plasma cut step (and avoid destroying the current pulley) by cannibalizing anything that had the 6-pt star hole (or hex) .. such as an old mower blade. Should be easier to cut the hub section out of a mower blade than a pulley anyway. Also I'd have to make sure there's enough thread left on the end of the spindle shaft after adding thickness to the pulley hub.

I guess the other thing I could try is locating a replacement spindle shaft that doesn't have the 6-pt star shoulder on the one end. Then I'd have to either key it or insert a pin or something to lock it to a standard pulley. I might have more luck finding something like that...

 

[LilysDad]

I like your problem solving skills!

 

[bmwe0692]

Just a silly wild a%% guess. Get the larger pulley with flat center, From center of larger pulley, measure and mark star pattern. Drill out the majority of excess metal. Use small dremel tool with grinding stone and/or good file and make your own star pulley. Grind and test fit, might take a while. Don't know if this will work or not. Would have to get pretty well centered,so not much vibration.

Also hope you find another pulley.

T.J.

 

[attorney_barnabas_collins]

Any chance you could just get a new one from the factory?

 

[tophat]

Mower blades are hardned steel-needs to be machined with carbide

 

[Toolin]

@bmwe - yeah, considered that too. Vibration from not being perfectly centered was my main concern, as you mentioned. But I've got drill and dremel so I've got this one in my back pocket.

@abc - been looking for new one, that's my first choice. But the factory (MTD) won't sell me something that doesn't match my original equipment, and the whole point is to change the pulley size so that's out.

@tophat - well my angle grinder seems to cut into mowing blades pretty well, plus I've been surprised what my cordless jigsaw will do with a good metal cutting blade in it, so I think I can make it through a mowing blade if need be, but I'll admit I'd rather not try. This one's farther down the list anyway so hopefully won't need to do, because...


Aside from the probably-too-large 7.25" OD 6-pt star pulley (as compared to 3.75" OD for what I have now) which snake99 referred to and which keeps turning up in my searches (MTF 756-01002), I've found online some pulleys with the 12 pt spline hole to instead replace the top spindle (standard 12pt spline hole) pulley:
- MTD 756-1227 OD 6-5/16"
- MTD 756-0980 OD 5.75"
- MTD 756-0969 OD 5"
If I replace the 7" OD top pulley with one of these instead of upsizing the goofy lower 6-pt star pulley, this should have a similar effect on upping the blower speed, although it won't help my lower belt rubbing problem. I may just have to go buy a smaller belt to address that.

So, before I go ordering a different size pulley, just for a sense check, should I be shooting for 540 RPM at the blower input? I've seen 540 and 1000 RPM in various postings, as well as "it depends". Unfortunately the manual for this blower (MTD 190-823) makes no mention of RPM.

I did order a laser tachometer so I can measure it but it will be a couple weeks before it's here.

I suppose I could also just do the pulley math starting at the PTO and figure out what RPM I end up with at the blower... so here goes...


All dimensions are OD. Because it's the ratio of diameters that matters more than actual diameters I think the ID/OD difference cancels out.

Basically the routing is there is one belt that goes from the tractor PTO backward under the tractor frame to the blower's "undercarriage assembly" (an add-on frame with pulleys to reduce belt speed and reverse it's direction using this funky double-pulley spindle) and a 2nd belt that comes back forward from this assembly to the blower's drive pulley. And a few idler pulleys thrown in but they don't affect speed ratios. I snagged a few shots from the blower manual to illustrate this:

Undercarriage assembly:

UC assembly as seen mounted, viewing from below the tractor, front of tractor to the right. That's the tractor's PTO pulley in the circle at the right side of the figure.

Upper blower belt routing path (viewed from above, front of tractor is to the right)

Lower blower belt routing path:

So..
- Tractor engine PTO pulley (A) is 6", belted to...
- Undercarriage assembly spindle top pulley (B), which is is 7"
- Which is solid shaft (1:1) to spindle lower pulley (C) which is 3.75",
- which is belted to blower's drive pulley (D) which is 7", connected to...
- Right angle drive (MTD 717-1714A), which I think is 1:1 to the impeller input

For any belted pair of Drive (A) to Driven (B) pulleys, the formula is:
RPM(A) x Diameter(A) = RPM(B) x Diameter(B)

I usually run the engine around 3400 RPM. It's a vertical shaft engine with the PTO pulley right on the crankshaft so the PTO should be 1:1 to engine RPM, right? So PTO RPM=3400 (A).

Finally, to the numbers:

Undercarriage assembly spindle (B and C) RPM is
RPM(A) x Diam(A) / Diam(B)
3400 x 6 / 7 = 2914

Therefore blower drive pulley RPM (D) is
RPM(C) x Diam(C) / Diam(D)
2914 x 3.75 / 7 = 1561

Which through a 1:1 right-angle-drive gearbox, is still 1561 RPM at impeller input... (?)

Hmm.. that seems high. I was expecting something below 540 given the number of postings out there stating lawn tractor blower attachments tend to like 540 RPM at the input and that mine (subjectively) seems to run a little anemic.

So the plot thickens... I think I need to wait for my optical tach to arrive and verify this for myself.

 

[Tractor-Holic]

They sell hubs for "Weld-A-Pulleys" that have hexagon bore and some with splined bores,that might be useable...also I have seen shaft collars with hex bore and splined bores..

Leave it to MTD to make a one of a kind "mongrel" pulley & shaft!.....

 

[Toolin]

I like your problem solving skills!

Missed this post until I was looking over the thread again..

Thanks! But I wouldn't call it problem *solving* just yet.. at least not until it's actually solved!

Given that the blower input RPM is already pretty high, perhaps the issue is torque, not speed (maybe I need to go with a slower speed?).

I'm also thinking maybe the "impeller wiper mod" might help.. it seems to be weak on the discharge when the snow is heavier. I thought that was a speed problem but in some other recent snowblower posts I'm reading it may have to do with the heavy stuff slipping past the impeller.

They sell hubs for "Weld-A-Pulleys" that have hexagon bore and some with splined bores,that might be useable...also I have seen shaft collars with hex bore and splined bores..

So would this require welding? I'm not weld-capable (yet).

 

[Tractor-Holic]

Yes,the hubs or shaft collars would need to be welded onto the pulley--but its not a big job,anyone with a welder could do it in ten minutes for a reasonable fee if you had to take it somewhere to be done..

You may want to try some machine shops,I have one local to me who will take on these types of jobs and they dont charge all outdoors..they could probably make a star bore hub that has a flange on it that would let you bolt the pulley to it,if there is enough clearance where it has to go on the blower..

 

[Toolin]

[...] through a 1:1 right-angle-drive gearbox, is still 1561 RPM at impeller input... (?)

Hmm.. that seems high. I was expecting something below 540 given the number of postings out there stating lawn tractor blower attachments tend to like 540 RPM at the input and that mine (subjectively) seems to run a little anemic.

So the plot thickens... I think I need to wait for my optical tach to arrive and verify this for myself.

My new laser tachometer arrived a couple days ago and I recently did some RPM measurements. Since the writeup could also be a reference for the tachometer itself I started a new thread with it: "RPM test of MTD snowblower attach using Laser Tachometer (pics)"

Here's a preview of the pics in that thread: