Hydraulic Motor/Pump Question

[ebme96]

OK so tell me if I am all wrong..... (It has been a long day, and hard)

I was looking online and found the following formula Horsepower=(Torque x RPM)/5252

So 220 ft-lbs (2640 in. lbs. just a guess slightly below the rated numbers)
220x290/5252=12.2 hp take that times 2 for two motors I am showing 24 hp to the ground?

The mower originally had an 18 hp motor on it, that ran a 62" deck as well.......

So is this totally green redneck (me) off his rocker?

I just zapped any remaining brain power I have, so I am headed home for the day...... will check in again tomorrow.


<edit> I know I am throwing questions at you all faster than a...... well I don't know what. So before I go, thanks a million for all the help! I think I am closer than ever to seeing a little light at the end of the tunnel..... lol

 

[Cpjlube]

Cpjlube I used 1800 lbs x 10 inch wheel radius to arrive at 18,000 in lbs torque required to propel this mower. divide the 18000 in lbs by 2 to get 9,000 in lbs required per drive wheel. Hydraulic motor size formula: Torque in ft lbs = displacement (CIR) x pressure drop across the motor x mechanical & hydraulic efficiency ( I typically use between .8 & .9) divided by 75.4 9,000 in lbs dived by 12 = 750 ft lbs 750 x 75.4 divided by (2500 x .9) = 25.13 CIR (Cubic Inch / Revolution) still seems like a large motor for that load but these are the formulas that I have used for figuring propulsion systems on equipment

Now I see......

I wonder what size pumps and motors an Exmark Lazer Z with a 60" deck has? They might have equal weight. I tried to check the specs but can't seem to get the info.
Of course, the tires are larger......

 

[oldnslo]

One thing I have forgotten to mention in all these posts is that I am figuring a direct wheel drive style motor. I believe most if not all ZTR mowers has some form of chain drive reduction between the motor and the driven wheel. Any gear reduction is going to reduce the hydraulic motor size by the same amount as the reduction. I.e. 4;1 reduction means the motor can be 4 times smaller.

ebme96,
No you are not off your rocker. sizing hydraulic drives is a challenge and requires some knowledge and a little luck to get them close to right. Two 1 cu.in motors at 3600 RPM will produce 15.5 GPM each and will require approx 26 HP each to operate at 2500 PSI @ full flow. That is one of the reasons it is so hard to size HST drives that typically you can easily stall the prime mover.

You need to determine what drive speed you want to mow at and the amount of force / torque required to propel your mower at that speed. Possibly look at Eatons website under small hydrostatic trans-axles. The 771 size shows input power of 5 HP and output torque of 600+ ft lbs. It has 23 : 1 gear reduction built in.

 

[Cpjlube]

One thing I have forgotten to mention in all these posts is that I am figuring a direct wheel drive style motor. I believe most if not all ZTR mowers has some form of chain drive reduction between the motor and the driven wheel. Any gear reduction is going to reduce the hydraulic motor size by the same amount as the reduction. I.e. 4;1 reduction means the motor can be 4 times smaller. ebme96, No you are not off your rocker. sizing hydraulic drives is a challenge and requires some knowledge and a little luck to get them close to right. Two 1 cu.in motors at 3600 RPM will produce 15.5 GPM each and will require approx 26 HP each to operate at 2500 PSI @ full flow. That is one of the reasons it is so hard to size HST drives that typically you can easily stall the prime mover. You need to determine what drive speed you want to mow at and the amount of force / torque required to propel your mower at that speed. Possibly look at Eatons website under small hydrostatic trans-axles. The 771 size shows input power of 5 HP and output torque of 600+ ft lbs. It has 23 : 1 gear reduction built in.

I have seen a lot of the older ZTRs use the chain drive reduction. The really old grasshoppers used the Eaton Model 7 hydros with chain driven sprockets. The Eaton Gemini hydros eliminated the sprockets and chains. The newer grasshoppers are pump and direct drive wheel motors.
It's funny how this thread started two days after I started researching this subject. Learning a lot. Thanks.

 

[ebme96]

OK, I think this time I have made up my mind. I am now looking at a dual 1.1 cu. in. pump set-up with 15.68 cu. in. motors. That would give me:
mowing speed of approx. 13.5 (wouldn't mind being slightly less but that works)
torque (from my calcs, could be wrong) 266 ft. lbs. @ 1425psi; 516 ft. lbs. @ 2970 psi; 582 ft.lbs. @ 3480 psi
Tractive Force of: cont. 640 lbs.; peak 1400 lbs.

If speed are too high I can always install bigger pulleys on the pumps and slow them down slowing down speeds or block the handles and cut speeds that way...... I am planning on installing a pressure gauge on at least one of the sides, to see what pressure I am pushing.... (aka BOOST)

I think I can have the motors and pumps for about a quarter of two new hydros for the mower.

For perspective Dixie Chopper (the only mower that lists cu. in . of wheel motors on website) shows that they run 24 cu. in wheel motors on machines up to 54 hp, 74" cut.

and thanks oldnslo for confirming my sanity:laughing:

 

[oldnslo]

ebme96,
Good luck on the rebuild and as you know, pictures are always required..

Interesting that Dixie Chopper kind of matched my guess on motor size required.

 

[ebme96]

OK so I bit the bullet and purchased all the things necessary to start my build. Check out the build thread at http://www.tractorbynet.com/forums/build-yourself/322116-lawn-mower-build.html

Also if you have any questions regarding formulas/calcs, buy this https://www.surpluscenter.com/Hydra...sories/Books/FLUID-POWER-DATA-BOOK-9-1022.axd

I am in no way affiliated with surplus center, just a very happy customer.

 

[J_J]

A 1.1 cu in pump turning at 3600 rpm will pump 17 GPM. If relief set to 1425 psi = 17 HP , and then you have two of the pumps.and will require about 34 HP.

Even if the pumps only pump 15 GPM, that is still 30 HP for both pumps.

The 1425 psi for continuous operation, but will go to 3500 peak. It will stall at this pressure.