Mowing 48" Finish Mower Questions

SnowRidge · Jul 9, 2003

48\" Finish Mower Questions

Rob, I'm curious as to why you went with the 48" mower instead of the 60" mower. The 60" seems like a better bang for the buck. We went with the 48" because of clearance issues with out Xmas trees. Did you have similar problems?

On a related note, has anyone noticed that the mower can strike the tilt ram when it is raised up high, unless you are careful to tilt it back first? It seems like some sort of raised bumper mounted on the mower deck might be in order.

The plastic gauge wheels have to go. Anyone got a rubber replacement source?

Last question -- any source for blades, other than PT?

Thanks,

SnowRidge

Bob999 · Jul 9, 2003

Re: 48\" Finish Mower Questions

I have the 72" finish deck and have the same experience with the deck hitting the cylinder rod, and on my machine, with 72" lift, it is not possible to lift the mower to maximum height without hitting the rod--no matter what is done with tilt.

On mower blades--I found PT to be a very reasonable a convenient source. the 3 blades for my deck were $12.15 each plus shipping from PT. This price was comparable or better than alternative sources I could find.

smartguyz · Jul 9, 2003

Re: 48\" Finish Mower Questions

Hi SnowRidge,

I feel like I know you already!

To answer your question, I do have a 2+ acre property, with 2/3 of it landscaped. Of this, about 9000+ SF is fine grass. This is a relatively small amount of lawn, especially compared to guys whose entire property is 90 percent lawn area. My property has a lot of trees, flower and garden beds, etc. I went with the 48" because my JD 318 has a 50" mower, and it works very well in mowing - I consider the 50" an excellent size for this lawn - and the 48" was really close in size. I suppose I should have gone for the extra 12", but I haven't missed it yet. It probably takes a few more minutes to mow my lawn, but the 48" is lighter (I don't know by how much, does anyone know?), was simple to transport in a standard pickup truck, was cheaper, and smaller mower decks usually have a higher rotor speed and achieve a higher blade tip-speed for a finer cut.

For example, one set of mower decks I was looking at had both 48" and 60" variants.

For the 48" deck: rotor speed is 1,042 rpm, 12,560 ft. per minute blade tip speed

For the 60" deck: rotor speed is 794 rpm, 11,948 ft. per minute blade tip speed

Although these are not the PT specs, I would think things would be comparable. The 60" deck is normally a lot heavier, and it usually has a lower blade tip speed. Not a big deal, but choosing the smaller mower deck made it easier to move, easier to store, and easier to justify the purchase of the mini-hoe!!! /forums/images/graemlins/grin.gif

Thanks for asking. How are you enjoying your PT?

Last night I moved the last of the bark mulch (snaking through tight turns, up steep grades, and along grassy spots and through garden beds (with NO damage!!!), and swept the entire courtyard, and the long driveway, and the common-access roadway along the front of the property - took me about 3 hours, and it would have taken me the whole day to do this with any other machine, plus lots of skinned knuckles. Switching implements is SO easy! Love it!

This is one useful tool!

SnowRidge · Jul 9, 2003

Re: 48\" Finish Mower Questions

( Although these are not the PT specs, I would think things would be comparable. The 60" deck is normally a lot heavier, and it usually has a lower blade tip speed. Not a big deal, but choosing the smaller mower deck made it easier to move, easier to store, and easier to justify the purchase of the mini-hoe!!!
Thanks for asking. How are you enjoying your PT? )

Hey, that makes a lot of sense--especially the bit about the mini-hoe. /forums/images/graemlins/smile.gif

We're having fun with ours. We've got 12 hours on it so far, and except for the few rough edges I've posted about, we are really happy. Best of all, we haven't set it on fire again. /forums/images/graemlins/grin.gif

SnowRidge

Schultz · Jul 9, 2003

Re: 48\" Finish Mower Questions

Someone calculated the blade tip speed in another post to be around 17,000 ft/min on the 60" deck. Which is right up there with the commercial mowers. Sorry, I couldn't find the post.

I'm not sure anyone has calculated this for the 48", but I would suspect the blade tip speed to be slower for the 48" since the three blades are about 4" shorter. Just for them to travel at the same speed would require the spindle speed to be 23% faster or 3821 rpm. Dose PT use different hydraulic motors for the two different decks?

My calculations using your rotor speeds only provides tip speeds of

4635 ft/min on a 48" using a 17" blade & 1042 rotor rpm
4346 ft/min on a 60" using a 21"blade & 794 rotor rpm

The formula I used is: (Blade diameter in feet)(3.14)(blade rotor rpm) = blade tip speed in feet/min

smartguyz · Jul 9, 2003

Re: 48\" Finish Mower Questions

Interesting, Shultz -

However, I don't think there is a 1:1 correspondence with RPM of the rotor and how fast the blades spin, as there is also a belt and a series of pulleys on each deck. We need to take into account the ratios of the pulleys and determine the actual spinning speed of the blades.

Therefore, there are more variables, and I don't think your calculations will be valid. We need to know:

Rotor Speed
Diameter of Rotor-Driven pulley
Diameter of belt-driven pulley
Length of blades

Anyone care to chime in?

Sincerely,

Rob /forums/images/graemlins/grin.gif

F350Lawman · Jul 9, 2003

Re: 48\" Finish Mower Questions

I think the calculations are going to be pretty close. If you pick up any brochure for mowers and look at the listed tip speeds for the blades of diff. sized mowers they seem to decrease in accordance with the size of the mower. At least in the brochures I have.

SnowRidge · Jul 9, 2003

Re: 48\" Finish Mower Questions

( Dose PT use different hydraulic motors for the two different decks? )

Don't know--yet. Our 48" has a MGG20020 BB1A3 motor. Anyone with a 60" care to take a look and post what you find?

SnowRidge

Schultz · Jul 10, 2003

Re: 48\" Finish Mower Questions

Is the hydraulic motor coupled directly to the middle blade spindle?

SnowRidge · Jul 10, 2003

Re: 48\" Finish Mower Questions

( Is the hydraulic motor coupled directly to the middle blade spindle? )

As far as I can tell without taking the top cover off, it is. The spindle is directly below the motor.

SnowRidge