PTO speed at low engine RPM

   / PTO speed at low engine RPM #1  

lostcause

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I'm trying to figure out what my PTO speed is when the tractor's engine is running at near idle. My main question is whether PTO speed is linear with the engine speed. If my tractor (JD 770) is rated for 540 PTO RPM at 2600 engine RPM, can I assume that with the engine at near idle (800-1000 RPM) the PTO speed will be about 165-200 RPM?

What I'm doing is reworking an old cement mixer that is going to be run off the tractor PTO. My tractor is supposed to be about 20HP at the PTO, and I can't see the sense of running the tractor at full throttle when I should be able to accomplish the task with far less power. Since I have a belt drive between the gearbox and the ring gear, I can easily adjust speeds once I figure out the tractor PTO speed.

I am also wondering what the rotational speed of a cement mixer drum should be, if anyone knows.

Thanks.
 
   / PTO speed at low engine RPM #2  
Leave things the way they are, and run your tractor at idle. They already set the pto mixers up to be run at tractor idle as is. At 560rpm, you won't get any mixing done, the cement will be held on the outside of the drum!!!! :)

--->Paul
 
   / PTO speed at low engine RPM
  • Thread Starter
#3  
Leave things the way they are, and run your tractor at idle. They already set the pto mixers up to be run at tractor idle as is. At 560rpm, you won't get any mixing done, the cement will be held on the outside of the drum!!!! :)

--->Paul

all that would imply that i have a functional mixer to start with, which i don't. it's been hacked apart, cobbled together, poorly repaired, and generally mistreated. the pulleys on it don't resemble anything you would see on a commercially made mixer, so i'm confident that one or both have been replaced, and i can't guarantee they were replaced with the correct size.

the mixer may also have been powered by a much older tractor that didn't have a 540 rpm pto, so they could have been changed to compensate for that. also, there is an idler/tensioner from a 1970's european car attached to part of a 4-way tire wrench welded to the frame to keep the belt tight. i don't know if this is because the belt they had on hand was too large, or whether the pulley sizes were changed.

the machine is disassembled and being changed over from a towed axle to a 3 point hitch configuration, so testing is out of the question. i figured it would be simple to get an answer as to what the rough pto speed of a tractor at idle was, but it doesn't seem to be that way. i also figured someone would know the rough speed a cement mixer drum was supposed to rotate at. wrong again i guess.

as it sat, the pto drove a 1:1 ratio gearbox which turned a 3" pulley. the 3" pulley drove a 4" pulley via the belt. the 4" pulley turned an 8 tooth pinion which spun an 81 tooth ring gear. in theory, if i turned the tractor pto at the full 540 rpm, reduced by the 3:4 pulleys and the 8:81 gears, the drum would rotate at 40 rpm. if my pto speed at idle was sub-200, i'd be looking at the drum turning about 12-14 rpm. i'm going to take a guess that a mixer drum should run somewhere between 15 and 50 rpm, but that's a lot of pulley sizes and tractor speeds inbetween.

i have to fabricate a belt guard and install a functional idler, not to mention buy a new belt. even if it was together to test out it wouldn't matter. i mix concrete so infrequently that i'm not a good judge of what speed it's supposed to be done at. i hope to get it right the first time, since re-fabricating costs time and money.
 
   / PTO speed at low engine RPM #4  
to answer the question.. yes.. pto speed is locked to engine rpm via ratio.

look at engine rpm when you are at marked pto speed on the tach.. now do the ratio math to find the pto speed at a reduced rpm.

soundguy
 
   / PTO speed at low engine RPM
  • Thread Starter
#5  
to answer the question.. yes.. pto speed is locked to engine rpm via ratio.

look at engine rpm when you are at marked pto speed on the tach.. now do the ratio math to find the pto speed at a reduced rpm.

soundguy

Thanks, I kind of figured it was linear, but I wanted to hear it from someone else. I've been figuring on building it around a 200 RPM PTO speed, which would be about 1000 RPM on the engine. By going just a little above idle it will allow me to tune the speed down slightly if I ever need (165 PTO @ 800 engine) while not having to run the tractor full bore for a trivial task.
 
   / PTO speed at low engine RPM #6  
Just replied to your other thread on this, I got a 'factory made' pto cement mixer, think it was designed for an N, will be late Friday or Saturday before I get to it during daylingt tho.

Every tractor idles different speed, so it's hard to answer your speeds question _exactly_. :) Getting the pulley & cogs count from mine might help you tho.

Mixed up a fair amount of mud to fix cracks in my rock barn wall foundation. Worked well.

--->Paul
 
   / PTO speed at low engine RPM #8  
Way to post, bcp!
Just awesome.
 
   / PTO speed at low engine RPM #9  
It's been a long time since we used a mixer (maybe 35 years?), but I would have guessed that each revolution would have been 1 1/2 to 3 seconds long, so that would mean 20-30rpm. That matches the advertisements bpc posted.
If it goes too fast, you won't get any mixing action at all, just stationary mud that rides along the outside.

However 78 rpm might be an exception to that rule. :thumbsup:
 
   / PTO speed at low engine RPM #10  
My pto mixer has the 1-1 gearbox, then a 5 inch to 8 inch veebelt pulley, and the cogs is a 8 cog to 95 cog rim.

--->Paul
 
 
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