PTO speed at low engine RPM

[Jerry/MT]

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.

There is a fixed ratio between engine speed and the pto speed.

 

[SSdoxie]

I have never measured my electric mixer but reading this post I was guessing it ran about 30 RPM, which BCP verified in his post. Your PTO ratio of 540 @ 2600 is .207:1 so at 1000 RPM figure about 7 to 1 PTO to drum ratio to get 30RPM drum at 1000 engine RPM. May want to go slower ratio yet then have availblity to rev the engine if more speed is desired. As stated earlier too high of RPM it won't fix, won't fall off the padels at top of the rotation. Too slow will still mix just take longer to mix up good.

 

[zzvyb6]

If you are going to hang the mixer off of the 3pt hitch, be sure to add a LOT of weight to the tractor's front weight bar, better if you have a full loader bucket. My 1070 equiped with a 1/3 yard mixer and a full load of concrete and water will bounce the front wheels off the ground when traveliing to a site. Its good to start the mix off at idle because the powder will flop around and can choke you. But when you are all filled and want a good mix and no lumps, I run it at full rpm. You will also want it moving very slowly when dumping the cement otherwise it will fly all over the place.

Hydraulic dump is VERY handy. I use one side of the loader's curl circuit to run the dump cylinder. Concrete is FUN to work with. I make up a new slab, sidewalk or planter base at every opportunity. I don't bother with the separate gravel, cement and water mix anymore, either. I get the 50 or 80 lb Just-Add-Water bags, dump in a full bucket of water and head out to the form(s). Use all my old crappy cheap tools for reinforcement bar. Even my dogs love to walk across fresh cement !:laughing:

 

[RaydaKub]

Even my dogs love to walk across fresh cement !:laughing:

Oh man, please don't let them do that. A good friend of ours is the resident expert on treating burns at the Mayo Clinic. She had a patient a few years ago who worked in concrete all day long. I just recalled now that he had been kneeling in the wet concrete most of the day troweling it off. He was in for 3rd degree chemical burns. The dogs don't know any better.

But now you do.

 

[zzvyb6]

Thank you for that good advice. I never would have guessed it, but they (as well as YOU) can get a nasty alkaline burn (treat with mild vinegar). In the dogs case, they could rub their paws near their eyes and ears and then you have a serious chemical burn problem on your hands with a very poor outcome.

 

[lostcause]

Thanks for the input everyone. Based on what bcp posted, it looks like low 20's to low 30's is a good rpm range. Since I'm hoping to run mine at around 1000 rpm on the tractor, which should equate to just over 200 rpm at the pto, the original pulley ratio of 3":4" would have yielded 15 rpm, which looks to be a little too slow. If I flip the pulleys and go with a 4:3 ratio the drum speed should be just over 27 rpm, which looks to be in range. Idling the tractor at 800 rpm would put the drum speed at about 22 rpm, so I think I have my starting point for the gearing.

Thanks again everyone. I'm going to abandon this thread, but if anyone is interested in seeing the project as it progresses, I'm going to be updating the thread in the build it yourself section of the forums.